interaction of internally mixed aerosols with light

Interaction of internally mixed aerosols with light

Naama Lang-Yona,aAli Abo-Riziq,*

aCarynelisa Erlick,

bEnrico Segre,

c

Miri Trainicaand Yinon Rudich

a

Received 2nd July 2009, Accepted 14th September 2009

First published as an Advance Article on the web 12th October 2009

DOI: 10.1039/b913176k

Atmospheric aerosols scatter and absorb solar radiation leading to variable effects on Earth’s

radiative balance. Aerosols individually comprising mixtures of different components (‘‘internally

mixed’’) interact differently with light than mixtures of aerosols, each comprising a different single

component (‘‘externally mixed’’), even if the relative fractions of the different components are

equal. In climate models, the optical properties of internally mixed aerosols are generally

calculated by using electromagnetic ‘‘mixing rules’’, which average the refractive indices of the

individual components in different proportions, or by using coated-sphere Mie scattering codes,

which solve the full light scattering problem assuming that the components are divided into two

distinct layers. Because these calculation approaches are in common use, it is important to

validate them experimentally. In this article, we present a broad perspective on the optical

properties of internally mixed aerosols based on a series of laboratory experiments and theoretical

calculations. The optical properties of homogenously mixed aerosols comprised of non-absorbing

and weakly absorbing compounds, and of coated aerosols comprised of strongly absorbing,

non-absorbing, and weakly absorbing compounds in different combinations are measured using

pulsed and continuous wave cavity ring down aerosol spectrometry (CRD-AS). The success of

electromagnetic mixing rules and Mie scattering codes in reproducing the measured aerosol

extinction values is discussed.

1. Introduction

Atmospheric aerosols, small solid or liquid particles suspended

in the atmosphere, contain organic and inorganic compounds

from anthropogenic as well as natural sources. Aerosols can be

emitted directly into the atmosphere (primary aerosols) from

various sources, such as fossil fuel combustion, biomass

burning, soil and road dust, salt from sea spray, and biological

materials (e.g., pollen, bacteria, etc.).1,2 Secondary aerosols

form in the atmosphere by condensation of gas phase compounds

onto pre-existing particles, homogeneous nucleation of

volatile or semi-volatile compounds to form nanometer-scale

particles, or by heterogeneous and multi-phase reactions.1,2

The latter two processes occur on aerosol surfaces, in the bulk

of the aerosol, as well as within cloud drops.1,3–6 The rate of

secondary aerosol formation is controlled by temperature,

relative humidity (RH), and the concentration of the nucleating

and condensing compounds.7

In the visible range, aerosols affect atmospheric radiation

balance, and hence climate, through three main processes: (1)

direct scattering and absorption of incoming solar radiation

(the direct effect), (2) by acting as cloud condensation nuclei

aDepartment of Environmental Sciences, Weizmann Institute,Rehovot, 76100, Israel

bDepartment of Atmospheric Sciences, The Hebrew University ofJerusalem, Jerusalem, 91904, Israel

c Department of Physical Services, Weizmann Institute, Rehovot,76100, Israel. E-mail: [email protected]

Naama Lang-Yona

Naama Lang-Yona finishedher MSc on optical propertiesof aerosols in the WeizmannInstitute of Science. She didher BSc in EnvironmentalSciences at the Tel-HaiCollege. Naama has juststarted a PhD in atmosphericchemistry in the WeizmannInstitute of Science and willwork on correlating fungi inaerosols with allergies.

Ali Abo-Riziq

Ali Abo Riziq (PhD inChemistry from University ofSanta Barbara California,UCSB, 2005) is a StaffScientist at the WeizmannInstitute of Science. He joinedProf. Yinon Rudich’s group at2005 for postdoctoral work ondeveloping different cavityring down aerosol spectro-meters. His main researchinterests are the opticalproperties of aerosols.

This journal is �c the Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 21–31 | 21

PERSPECTIVE www.rsc.org/pccp | Physical Chemistry Chemical Physics

Publ

ishe

d on

12

Oct

ober

200

9. D

ownl

oade

d on

22/

07/2

015

13:0

7:03

. View Article Online / Journal Homepage / Table of Contents for this issue

http://dx.doi.org/10.1039/B913176K

http://pubs.rsc.org/en/journals/journal/CP

http://pubs.rsc.org/en/journals/journal/CP?issueid=CP012001

(CCN), which control cloud reflectivity, extent, and lifetime

(the indirect effect), and (3) by heating the atmosphere through

absorption of incoming solar radiation (the semi-direct

effect).8–20 While the direct and indirect effects generally lead

to a negative radiative forcing of climate or cooling by

reducing the amount of solar radiation reaching the Earth’s

surface, the semi-direct effect can lead to either cooling or

warming, depending on whether the atmospheric absorption

dominates (cooling the surface) or whether the corresponding

reduction in low cloud cover and liquid water path dominates

(warming the surface).21

Atmospheric aerosols contain a mixture of organic and

inorganic components.22 The chemical constituents that make

up atmospheric aerosols dictate their chemical and physical

properties (chemical reactivity, interaction with radiation,

hygroscopicity, optical properties, etc.). Aerosols that consist

mostly of inorganic matter (such as sea salt, sulfate,

nitrate)23,24 and non-absorbing organic compounds

(OC)25–27 tend to be scattering, affecting climate through the

direct effect. Aerosols that consist of elemental carbon (EC;

soot),10,28–31 mineral dust,32,33 and certain moderately absorbing

organics (‘‘brown carbon’’)27,34–37 tend to be absorbing, affecting

climate through the direct and semi-direct effects.18

The scattering and absorbing properties of aerosol consti-

tuents are characterized by various physical parameters, the

most fundamental of which is the complex refractive index

(RI = n + ik), where the real part (n) describes the ability of

the constituent to reflect or scatter radiation, and the imaginary

part (k) describes the ability of the constituent to absorb

radiation. Both the real and the imaginary parts of the

complex RI are functions of wavelength (l).38 Knowing the

RI of aerosols enables the calculation of other radiative

parameters relevant to the climate and is therefore extremely

valuable for climate change modeling.13,39–43 These radiative

parameters include the scattering, absorption, and extinction

(attenuation) coefficients (describing the e-folding of radiative

intensity with distance: asca, aabs, and aext = asca + aabs,respectively). Other optical parameters include the optical

depth (the extinction coefficient multiplied by the radiation

propagation distance; text), the phase function (describing the

angular pattern of scattered intensity for a single aerosol

scattering event; P(ysca), where ysca is the scattering angle),

the asymmetry parameter (the extent of forward scattering

relative to backward scattering; g), and the single scattering

albedo (describing the amount of scattering relative to total

extinction; $ = asca/(asca + aabs)).44

Carynelisa Erlick

Carynelisa Erlick (PhDin Atmospheric Sciences,University of Chicago) is asenior lecturer at the HebrewUniversity of Jerusalem,Department of AtmosphericSciences. Her main researchinterests are radiative transferin the Earth’s atmosphere andclimate forcing. Her recentresearch involves accountingfor the effect of mixed compo-sition and nonsphericity ofaerosol particles on theirradiative properties. Enrico Segre

Enrico Segre (PhD in Physicsfrom the University of Turin,Italy, 1994), is currentlya Staff Scientist at theWeizmann Institute of Scienceand was an assistant professorat the Polytechnic of Turinbetween 1995 and 2000. Hismain scientific interests are innon-linear aspects of fluiddynamics and in microfluidics.

Miri Trainic

Miri Trainic is a PhD studentin atmospheric chemistry inthe Weizmann Institute ofScience. She did her BSc inChemistry and EnvironmentalSciences in the HebrewUniversity of Jerusalem andher MSc in EnvironmentalDiagnosis, at the ImperialCollege in London. Hercurrent PhD work focuses onthe optical properties oforganic aerosols.

Yinon Rudich

Yinon Rudich received his BScfrom Ben Gurion University(1987) and MSc and PhDdegrees (honors) from theWeizmann Institute (1994).Following postdoctoral workat the National Oceanic andAtmospheric Administrationin Boulder, he joined theWeizmann Institute of Sciencewhere he is currently aProfessor in the Departmentof Environmental Sciencesand Energy Research. Hisresearch interests include thechemistry and physics of

organic aerosols and of mineral dust, optical properties ofaerosols, aerosol–climate interactions and characterizationof ambient atmospheric aerosols.

22 | Phys. Chem. Chem. Phys., 2010, 12, 21–31 This journal is �c the Owner Societies 2010

Publ

ishe

d on

12

Oct

ober

200

9. D

ownl

oade

d on

22/

07/2

015

13:0

7:03

. View Article Online


Related to aext, the extinction efficiency (Qext) is defined as

sext/pr2, where sext is the aerosol extinction cross section

(integrating the extinction cross section over the aerosol size

distribution gives aext).45,46 One way to obtain the complex RI

of an aerosol constituent is by measuring Qext as a function of

particle size for a homogeneous (spherical) aerosol comprised

of the same constituent. The complex RI can then be obtained

by fitting the measured Qext values to a Mie scattering curve,

computed with readily available numerical subroutines that

give the scattering solution to Maxwell’s equations as a

function of particle size and wavelength, following Gustav

Mie’s original derivation (see section 2).45,47

In addition to the RI values of the chemical constituents

that make up atmospheric aerosols, aerosol radiative parameter

values are strongly influenced by the physical configuration of

these constituents within the aerosol, i.e., by the aerosol

microstructure.45,48 As shown in Fig. 1, aerosols can appear

as externally mixed (particles 1 and 2), heterogeneously

internally mixed (i.e., coated particles; particles 3, 4, and 5),

or homogeneously internally mixed (particles 6 and 7).49–61

Homogeneous internally mixed aerosols form by processes

such as evaporation of droplets containing several species with

similar solubility and by simultaneous condensation of

semi-volatile species. They can be found in a variety of

combinations, including: (1) internal mixtures of several

non-absorbing components (e.g., sodium chloride (NaCl)

mixed with non-absorbing organics); (2) internal mixtures of

non-absorbing components (e.g., ammonium sulfate (AS))

mixed with weakly absorbing components (e.g., humic-like

substances (HULIS)); and (3) internal mixtures of non-absorbing

components mixed with strongly absorbing components, such

as mineral elements, metals, or brown carbon.3,62–64

Coated particles can form by processes such as condensation

of semi-volatile species on pre-existing particles, evaporation

of droplets containing both soluble and insoluble components,

dehydration of droplets containing two species with substantially

different solubilities, and heterogeneous chemical reactions

(e.g., oxidation, radical reactions, photochemistry, etc.) on

aerosol surfaces, creating a shell with a different character.

Like homogeneously internally mixed aerosols, coated particles

can be found in a variety of combinations, including: (1)

a non-absorbing core (e.g., NaCl) coated with another

non-absorbing species (e.g., non-absorbing organics); (2) a

non-absorbing core coated with a weakly absorbing species

(e.g., HULIS); or (3) an absorbing core (e.g., soot or dust)

coated with a non- or weakly absorbing species (e.g.,

condensed organics).49,51–61,65 These different aerosol micro-

structures are visualized in Fig. 1.

The ratio of the coating to core diameter in coated particles

can also be an important factor. As shown in Fig. 2(b), two

coated particles with the same external diameter but different

core/coating diameter ratios will have different absorption and

scattering characteristics. Thus, we conducted each experiment

on coated particles twice, once with a constant core diameter

and increasing shell thickness (see Fig. 2(b),i), and again with

increasing core diameter whilst maintaining a constant shell

thickness (see Fig. 2(b),ii).

It is of great interest to understand how these micro-

structures affect aerosol optical properties and how well

theoretical calculations reproduce these characteristics in

order to achieve more reliable climate model simulations.

There are a number of approaches currently employed to

calculate the radiative properties of internally mixed aerosols

in climate models. For homogeneously internally mixed

aerosols, effective medium approximations or ‘‘mixing rules’’

are often used to calculate the particles’ effective RI, and then

a Mie scattering subroutine (for homogeneous spheres;

described above) is applied to calculate the aerosol optical

parameters. Mixing rules are derived assuming that pockets

(‘‘inclusions’’) of an aerosol constituent(s) exist within a

surrounding matrix comprised of another constituent(s). Each

inclusion is subjected to a local quasi-steady electromagnetic

Fig. 1 Complex atmospheric aerosols (the combination of 1 and 2 in

the same environment represents externally mixed aerosols, 3–5

represent different types of coated particles, and 6 and 7 represent

different types of homogeneously mixed aerosols).

Fig. 2 Internally mixed aerosols divided into homogeneously mixed aerosol (a) and heterogeneously mixed aerosols (coated particles) (b), with two

types of coated aerosol, constant core diameter with increasing shell thickness (i), and increasing core diameter with constant shell thickness (ii).


Publ

ishe

d on

12

Oct

ober

200

9. D

ownl

oade

d on

22/

07/2

015

13:0

7:03



field averaged over the configuration of all of the other

inclusions, which contributes to a local polarizability. The

local polarizabilities are summed to compute the macroscopic

polarization of the entire aerosol, and the macroscopic

polarization is related to the effective dielectric constant of

the composite aerosol. (See Chylek et al.48,66 for more detail.)

The combination of any mixing rule and a Mie scattering

subroutine for homogeneous spheres implicitly assumes that

either the constituents are truly homogeneously mixed or that

at least one constituent is randomly distributed throughout a

matrix comprised of the other constituents.

Examples of commonly used mixing rules include: (1) molar

refraction and absorption, in which the effective molar

refraction and effective molar absorption of the aerosol are

calculated by linearly averaging the molar refractions and

molar absorptions, respectively, of the RI of the aerosol matrix

and inclusions, weighted by their molar fractions38,67–69; (2) the

‘‘linear mixing rule’’, in which the effective RI of the aerosol is

calculated by linearly averaging the real and imaginary parts,

respectively, of the aerosol matrix and inclusions, weighted

by their volume fractions;45 (3) the Maxwell–Garnett rule,

according to which second-order effects due to neighboring

inclusions are neglected, and the dielectric constant of

the aerosol matrix without inclusions determines the local

electromagnetic field that acts on the inclusions;45,70 (4) the

Bruggeman rule, according to which the aerosol matrix and

inclusions act symmetrically on one another;45,70 and (5)

extended effective medium approximations, according to

which terms of higher order than the electric dipole of the

inclusions are included.48,66,71,72

For coated particles, two approaches for calculating the

radiative properties are common. For aerosols coated with

water, a growth function—a parameterization describing the

change in scattering coefficient as a function of relative

humidity—is often used.39,73 Alternatively, a Mie scattering

subroutine for coated spheres can be used, which provides a

more explicit calculation, whether the coating consists of

water or of other aerosol constituents. Like Mie scattering

subroutines for homogeneous spheres, Mie scattering subroutines

for coated spheres give the scattering solution to Maxwell’s

equations as a function of particle size and wavelength, but

with extra boundary conditions at the interface between

the two layers.45,74,75 Since such subroutines are more

computationally intensive than mixing rules or growth

functions, they are often applied off-line in a look-up table

fashion,76 but they can also be applied on-line.67

Fig. 3 Illustration of the setup for generating mixed (a) and coated (b) aerosols, coupled to the continuous wave (CW) cavity ring down (CRD)

system (c).


Publ

ishe

d on

12

Oct

ober

200

9. D

ownl

oade

d on

22/

07/2

015

13:0

7:03



Only a few experimental studies of the optical properties of

complex aerosols have so far been conducted.53,66,77–89 Here, we

present new measurements of laboratory-generated aerosols

and theoretical calculations of their optical properties, along

with some of our previous studies of the optical properties

of internally mixed and coated aerosols. With respect to

homogeneously internally mixed aerosols, we consider three

combinations: (1) a mixture of two non-absorbers, glutaric acid

(an organic solid; Glu-A) and NaCl (an inorganic salt);80 (2) AS

(a non-absorbing salt) mixed with Suwannee River fulvic acid

(a weakly-absorbing organic compound with optical properties

similar to those of HULIS; SRFA);27,90 and (3) AS mixed with

Rhodamine-590 (a strong absorber in the visible range with an

absorption peak around 532 nm wavelength; Rh-590).80 Glu-A,

NaCl, and AS are common components of tropospheric aerosols,

while SRFA is a commonly-used proxy for HULIS.27,90 With

respect to coated aerosols, we consider three combinations: (1)

Nigrosin (a strong absorber) coated with Glu-A (a non-absorbing

organic solid); (2) polystyrene latex spheres (a non-absorbing

solid; PSL) coated with Glu-A; and (3) PSL coated with

b-carotene (a weakly absorbing organic compound; b-car).In addition, we present some thoughts about the possible

implications of our results to atmospheric radiation balance

and climate.

2. Methodology

Aerosols were generated by the methods illustrated in Fig. 3,

described in detail in our previous publications.27,53,80,91,92

In short, an aqueous solution of the compound of interest is

nebulized with dry and pure nitrogen, dried in a silica gel

diffusion dryer, and charged by a neutralizer. Size selection

of the resulting polydisperse aerosol is achieved with a

differential mobility analyzer (DMA) (Fig. 3(a,b)), after which

the nearly monodisperse aerosol flow is directed into a pulsed

or continuous wave (CW) cavity ring down (CRD) system for

optical measurements. These systems are described in the next

section.

Homogenously mixed particles are created by nebulizing

aqueous mixtures, as shown in Fig. 3(a). Coated particles are

created by using size-selected aerosols as seeds. The seeds are

directed through a coating system, described in ref. 53 and

illustrated in Fig. 3(b). In short, the coating material is placed

in an oven in which the temperature and the flow rate control

the coating thickness. After exiting the coating oven and

cooling, the coated particles are size-selected in an additional

DMA before entering the CRD system. The second size

selection dictates the coating thickness that will be used.

Aerosol sphericity was checked using atomic force microscopy

in several cases.53

CRD spectroscopy is a sensitive method for spectroscopy of

gas phase species and for measuring optical properties of

aerosols.80,93–108 A pulsed or continuous laser beam enters a

high finesse cavity, performing multiple reflections between

two or three plano-concave mirrors.106 The exponential decay

time of the light exiting the cavity is measured, and differences in

this time are related to the internal losses. The particles exiting

the cavity are directed into a condensation optical counter

(CPC) in order to measure the particle number concentration.

In a CRD aerosol spectrometer, scattering and absorption

by the aerosol particles in the cavity lead to a reduction in the

exponential decay time of the light compared to that in the

empty cavity. From the reduction in decay time and the

particle number concentration, the extinction efficiency (Qext)

can be calculated for a specific wavelength and specific size

parameter x (ratio between the particle diameter, D, and the

laser wavelength, l; x = pD/l).80,100,102

Several pulsed laser CRD systems covering several wave-

length ranges were employed in this study.53,80 Although

pulsed systems often have lower sensitivities than CW systems,

they can achieve quite reliable measurements. Their biggest

advantage is their simplicity, making them easy to handle and

operate.91

In addition to the pulsed laser systems, we use in this study a

recently constructed CRD system employing a CW single

mode laser operating at 532 nm (see Fig. 3(c)).91 In the

portable system, an acousto-optic modulator (AOM) diffracts

the 532 nm single mode laser beam, and the first order beam is

directed into the cavity. The mirror at the light entrance side of

the cavity is mounted on a piezoelectric ring that moves back

and forth. This modifies the cavity’s length and hence its

longitudinal modes. Once mode matching between the CW

laser and the cavity is achieved, the light intensity in the cavity

builds up. This triggers the AOM and the laser beam is

deflected, leading to a single exponential decay of the light in

the cavity which is detected by a photomultiplier (PMT)

placed at one side of the cavity. The modulation frequency

of the AOM is triggered by the light intensity in the PMT and

is around 200 Hz. This new system improves the stability and

sensitivity as compared to pulsed laser systems;80,102 the

sensitivity of our CW-CRD system was 6.64 � 10�10 cm�1,91

compared to 3.77 � 10�9 cm�1 in the pulsed CRD.80 We

present here experimental measurements obtained using both

pulsed and CW laser systems.

The complex RI of aerosols can be obtained using CRD

measurements of the extinction efficiency as a function of

particle size for a homogeneous spherical aerosol with the

same composition throughout the size distribution. The retrieval

algorithm compares the measured extinction efficiency as a

function of the size parameter, with the extinction efficiency

calculated using a Mie scattering subroutine for homogeneous

spheres, while simultaneously varying the real and imaginary

parts of the RI of the aerosols. The algorithm finds the

complex RI of the aerosol particles by minimizing the ‘‘merit

function’’, w2/N2, where w2 is

w2 ¼XN

i¼1

ðQext measured �Qext calculatedÞ2ie2i

; ð1Þ

N is the number of particle sizes, and ei is the standard

deviation of the measurements of particle size

i.27,48,53,80,91,102 The minimization is performed using the

simplex search method109 and can be performed to any desired

resolution in RI values.

In addition to applying the retrieval algorithm to CRD

measurements of single component spherical aerosols, the

algorithm can essentially be applied to any aerosol configuration,


Publ

ishe

d on

12

Oct

ober

200

9. D

ownl

oade

d on

22/

07/2

015

13:0

7:03



with the Mie scattering subroutine for homogeneous spheres

being replaced by some other appropriate theoretical calculation.

For homogeneously internally mixed aerosols, a mixing rule

computation can be inserted before theMie scattering subroutine

calculation, while for coated particles, aMie scattering subroutine

for coated spheres (‘‘core/shell’’) can be put in place of that for

homogeneous spheres. Again, the complex RI of one or more

of the aerosol constituents can be retrieved, or the merit

function can be used simply to ascertain which theoretical

approach best matches the measurements for a given set of

(known) RI.80

3. Laboratory measurements of complex aerosols

Homogeneously internally mixed aerosols

The Qext of laboratory-generated aerosols comprised of

internal mixtures of NaCl and Glu-A with molar ratios 1 : 1

and 1 : 2 was measured in ref. 80 with a pulsed CRD aerosol

spectrometer employing a 532 nm Nd :YAG laser. These

homogeneously internally mixed aerosols mimic aerosols

comprised of inorganic and organic mixtures often detected

above urban and polluted areas.22 The aerosol RI retrieval

algorithm was then used to test which mixing rule provides the

best match to the measurements, given values of the RI of the

pure constituents that make up the mixture as retrieved using

the same system (RI = 1.546 + i0.003 for pure NaCl and

RI = 1.41 + i0.00 for pure Glu-A). The level of agreement

between the mixing rules and the experimental data was

determined using the merit function (eqn (1)); the smaller the

merit function, the better the agreement. The results are shown

in Fig. 4. The linear mixing rule provides the best match to the

experimental Qext values; for a 1 : 1 mixture of Glu-A and

NaCl, the merit function value for the linear mixing rule is 0.14

as compared to 0.15–2.65 for the other mixing rules, and for a

1 : 2 mixture of of Glu-A and NaCl, the merit function value

for the linear mixing rule is 0.14 as compared to 0.14–2.80 for

the other mixing rules, when all particle sizes are included.80

We surmise that the high solubility of both compounds in the

aqueous solution ensured thorough mixing. This, combined

with the fact that the two constituents exhibit very little

absorption, allows such good agreement with the simple linear

mixing rule.

In a new set of mixing experiment, the Qext of aerosols

comprised of internal mixtures of AS (non-absorbing) and

SRFA (weakly absorbing material) with molar ratio 1 : 1 was

measured using an optical parametric oscillator (OPO) pulsed

laser system at 390 nm wavelength. These homogeneously

internally mixed aerosols again mimic aerosols comprised of

inorganic and organic mixtures often detected in urban and

polluted areas.22 The wavelength 390 nm was chosen for

several reasons. First, SRFA absorbs better at shorter

wavelengths than at 532 nm, as implicit in the imaginary

part of its RI (RI = 1.602 + i0.098 at 390 nm, while

RI = 1.634 + i0.004 at 532 nm).27 Second, near ultraviolet

wavelengths are often employed in satellite measurements of

aerosol absorption.110

The mixed aerosol RI retrieval algorithm was then used

to test which mixing rule provides the best match to

the measurements, given values of the RI of the pure

constituents that make up the mixture, which were previously

retrieved using the same system (RI = 1.525 + i0.0008 for

pure AS and RI= 1.602+ i0.098 for pure SRFA). The results

are shown in Fig. 5. The linear mixing rule provides the

best match to the experimental Qext values, but all of the

mixing rules tested provide reasonable results; for a 1 : 1

mixture of AS and SRFA, the merit function value for the

linear mixing rule is 0.11 as compared to 0.11–0.18 for

the other mixing rules. We conclude that for homogeneous

internal mixtures of non-absorbing and weakly absorbing

constituents, Qext is not overly sensitive to how the interaction

between the electromagnetic fields of the constituents is

described.

Fig. 4 Extinction efficiency of mixed particles of NaCl and Glutaric

acid (Glu-A) in mass ratios 1 : 1 and 1 : 2, respectively, along with

the Mie curves corresponding to the effective refractive indices

calculated with the volume-weighted ‘‘linear mixing’’ rule at 532 nm

wavelength.

Fig. 5 Extinction efficiency of mixed ammonium sulfate (AS) and

Suwannee River fulvic acid (SRFA) particles with a 1 : 1 mass ratio,

along with the Mie curves corresponding to the effective refractive

indices calculated with the volume–weighted ‘‘linear mixing’’ rule at

390 nm wavelength.


Publ

ishe

d on

12

Oct

ober

200

9. D

ownl

oade

d on

22/

07/2

015

13:0

7:03



However, when one of the constituents in the homogeneous

internal mixture is strongly absorbing, the situation changes.

In ref. 80 the Qext of laboratory-generated aerosols comprised

of internal mixtures of AS and Rh-590 was also measured

with the Nd :YAG pulsed CRD aerosol spectrometer

running at 532 nm wavelength. Due to the low solubility of

Rh-590 in water, the aqueous solution of the mixture was

diluted with 10% ethanol. The aerosol RI retrieval algorithm

was then used to test which mixing rule provides the best

match to the measurements. We found that for low volume

fractions of Rh-590, the Mie scattering subroutine for

coated spheres, rather than one of the mixing rules, provides

the best agreement with measurements; for a 1 : 500 mixture

of Rh-590 and AS, the merit function value for the Mie

scattering subroutine for coated spheres, including particle

sizes starting from 350 nm, is 3.29 as compared to 3.90–4.26

for the mixing rules. The lack of success of the mixing rules

for low volume fractions of the strong absorber could be

attributed to residue of ethanol, which, unlike water, may

not have completely evaporated in the diffusion drier.

Additionally, AS has a different solubility in water than

Rh-590, which might have caused the final molar ratios of

the two constituents to differ from their original molar ratios

in the solution or might have led to the formation of a more

heterogeneous internal mixture akin to a coated particle.

Alternatively, the coated sphere model might better represent

the interaction between the electromagnetic fields of the

constituents in the case of a strong absorber with low volume

fraction even if they are truly homogeneously mixed. For

higher volume fractions of Rh-590 in the mixture, we found

that the extended effective medium approximation provides

the best agreement with measurements; for a 1 : 10 mixture of

Rh-590 and AS, the merit function value for the extended

effective medium approximation, including particle sizes from

350 nm and inclusion size 10 nm, is 2.95 as compared to

5.85–9.57 for the other mixing rules and as compared to 31.72

for the Mie scattering subroutine for coated spheres. The

success of the extended effective medium approximation for

high volume fractions of the strong absorber is likely connected

to the fact that, among all of the mixing rules, the extended

effective medium approximation retains the highest order

representation of the electromagnetic fields, allowing the highest

order description of the interaction between internal scattering

and absorption.

In addition to testing the theoretical calculation approaches

against extinction measurements, we also performed calculations

of the single scattering albedo (see section 1; $) and the direct

radiative forcing efficiency (RFE; the difference in radiative

flux at the top of the atmosphere with and without aerosols per

unit optical depth) for a pollution-type aerosol containing

ammonium sulfate, absorbing organic carbon (HULIS),

and soot.27 Our calculations suggest that accounting for

absorption by HULIS leads to a significant decrease in $

(more atmospheric absorption) and to a significant increase in

aerosol RFE (heating) as compared to pollution-type aerosols

containing only non-absorbing organic aerosol constituents.

Therefore, an additional conclusion is that HULIS and similarly

absorbing organic constituents are important contributors to

the radiative balance in the atmosphere and to the climate in

polluted environments.

Heterogeneously mixed (coated) aerosols

In ref. 53, measurements of the Qext of laboratory-generated

aerosols comprised of Nigrosin coated with Glu-A performed

with the Nd :YAG pulsed CRD at 532 nm wavelength were

reported. The aerosol retrieval algorithm was then used to test

the accuracy of the Mie scattering subroutine for coated

spheres, and the results are shown in Fig. 6. As can be seen,

the Mie scattering subroutine is successful at predicting the

aerosol Qext for thin coatings (constant shell thickness of

20 nm; compare the green curve to the green dots). However,

it is less successful at predicting the aerosol Qext for thicker

coatings (increasing shell thickness; compare the blue curve to

the blue dots), with the Mie calculation over-predicting the

extinction by a greater extent as the shell thickness increases.

The same discrepancy occurred when the coating was switched

to diethyl-hexyl-sebacate (a non-absorbing organic liquid;

DEHS53), which is liquid at room temperature rather

than solid like Glu-A. In ref. 53, we suggested that these

discrepancies result from either a lack of perfect sphericity in

the generated aerosols or from changes in the dielectric

constant of the coating near the interface between the core

and coating. However, no theoretical representations of

such physical effects that we tested completely resolved the

discrepancy. Since then, we have conducted further sensitivity

tests, systematically reducing the RI of the coating in the

theoretical calculations. A reduction in RI corresponding to

a mixture of air with Glu-A with mass ratio 1 : 5 (representing,

for example, pockets of air in the coating) calculated with the

linear mixing rule does bring the calculations more in line

with the measured extinction. Although Guenther (1984)111

did suggest that small distortions can occur at the interface

between the surface of the core and coating when physical

Fig. 6 Extinction efficiency of Nigrosin coated with glutaric acid

(Glu-A), along with the Mie curves for coated spheres at 532 nm

wavelength. Blue curve and symbols: constant core diameter with

increasing shell thickness; green curve and symbols: increasing core

diameter with constant shell thickness.


Publ

ishe

d on

12

Oct

ober

200

9. D

ownl

oade

d on

22/

07/2

015

13:0

7:03



vapor deposition is used to generate the coating, and that

these distortions can develop into narrow air pockets, we

have no evidence that such pockets exist in our coatings. The

mechanism by which Glu-A vapors condense onto the core

particle in the coating oven needs to be examined more

thoroughly.

To investigate whether coated particles presenting different

optical properties exhibit the same discrepancy, we present

new measurements of Qext of laboratory-generated aerosols

comprised of PSL coated with Glu-A recently measured with

the new CW-CRD system at 532 nm wavelength. The results

are compared with calculations using the same Mie scattering

subroutine for coated spheres, as shown in Fig. 7. As can be

seen, the Mie scattering subroutine is successful at predicting

the aerosol extinction for both thin coatings (constant shell

thickness of 40 nm; compare the green curve to the green dots)

and thicker coatings (increasing shell thickness; compare the

blue curve to the blue dots). Although surface (interface)

effects should be more stable in the case of Nigrosin coated

by Glu-A (due to hydrogen bonds, compared to mostly

van der Waals in the case of PSL coated by Glu-A), we found

that PSL coated by Glu-A can be well described by the core

plus shell model. A possible explanation for these results might

be the existence of a mixed layer in the interface between the

Nigrosin core and the Glu-A shell. In this case, a different

model needs to be applied, and again, a more thorough

investigation on the mechanism by which Glu-A vapors

condense onto the core particle in the coating oven is needed.

In the next experiment, non-absorbing particles with a

weakly absorbing coating were examined. b-Carotene was

chosen as the coating material because it is more stable at

high temperatures than other absorbers such as Nigrosin. The

Qext of laboratory-generated aerosols comprised of PSL

coated with b-car was measured with the new CW-CRD

system at 532 nm wavelength, and the aerosol RI retrieval

algorithm was used to retrieve the RI of b-car, given the

previously retrieved/known RI for PSL (RI = 1.590 + i0.000).

The retrieved value for b-car is RI = 1.455 + i0.090, with a

merit value of 2.38. To the best of our knowledge, the RI of

b-car at 532 nm has not previously been reported in the

literature. The real part of the retrieved RI is in good

agreement with values for typical organic compounds

(B1.4), while the imaginary part was validated by measuring

the absorption of b-car dissolved in dichloromethane (CH2Cl2)

with a spectrophotometer (CARY 100 Bio UV-Visible spectro-

photometer, Varian). The imaginary part of the RI of b-carderived from the spectrophotometer measurements according

to the formulation in Jacobson et al.112 is 0.075 � 0.001, a

difference of 0.025 from our retrieved value. This difference,

though not very large, may be inherent in the different

methods of measurement or could be caused by interactions

of dissolved b-car with the solvent. Either way, we take the

retrieved value of 0.090 to be reliable.

The results of the measurements for PSL coated with b-carare shown in Fig. 8. As can be seen, there are differences

between the measurements and the calculated extinction

(calculated using the retrieved RI for b-carotene and the Mie

scattering subroutine for coated spheres) that increase with

increasing particle size. However, the differences are not

systematic; at times, the theoretical calculations over-predict

the extinction, and at other times they under-predict the

extinction. This is true for the experiment with constant shell

thickness of 40 nm (compare the green curve to the green dots

and see Table 1; the differences are up to 10%) and for the

experiment with increasing shell thickness (compare the blue

curve to the blue dots). The differences between the measurements

and the calculated extinction in the case of PSL coated with

b-car as compared to PSL coated with Glu-A may result from

the more rigid molecular structure of b-car, leading to a less

organized shell around the core PSL.

Fig. 7 Extinction efficiency of PSL coated with glutaric acid (Glu-A),

along with the Mie curves for coated spheres at 532 nm wavelength.

Blue curve and symbols: constant core diameter with increasing shell

thickness; green curve and symbols: increasing core diameter with

constant shell thickness. The red and black lines are Mie fits for

measured points using the same system. The green and blue lines are

the results of the coated sphere Mie calculation, using the retrieved RIs

as input parameters.

Fig. 8 Extinction efficiency of PSL coated with b-carotene (b-car),along with the Mie curves for coated spheres at 532 nm wavelength.

Blue curve and symbols: constant core diameter with increasing shell

thickness; green curve and symbols: increasing core diameter with

constant shell thickness.


Publ

ishe

d on

12

Oct

ober

200

9. D

ownl

oade

d on

22/

07/2

015

13:0

7:03



4. Conclusions

We have summarized and reviewed experiments and theoretical

calculations of optical properties of complex aerosols.

Specifically, we focused on the impact of aerosol microstructure

on the effective refractive index of aerosols composed of

internal mixtures of components with different optical properties.

In the atmosphere, most aerosols are multi-component, and

therefore detailed understanding and modeling of their

complex refractive index will reduce the uncertainty related

to energy balance and climate change modeling.

By controlling the aerosol microstructure and the mass

ratios of the components and knowing the RI of one or more

of the separate components that make up the aerosols at the

wavelength of interest, through our experiments we were able

to validate different theoretical approaches used for evaluating

the optical properties of complex aerosols. Our approach also

allowed us to retrieve the aerosol’s Qext and/or the RI of one

or more of the components.

We conclude that the effective RI of homogeneously mixed

aerosols comprised of two non-absorbing materials with

similar water solubilities can be accurately calculated with

the simple mass or concentration based ‘‘linear mixing’’ rule.

In the case of a non-absorbing material homogeneously mixed

with weakly absorbing compounds, we conclude that all

mixing rules provide a reasonable estimate, but the coated

sphere Mie calculation does not always perform as well. In the

case of a strong absorber ‘‘homogeneously’’ mixed with a

non-absorber (with different solubilities in water), the success

of the different theoretical approaches varies with the mass

fraction of the absorber. For low absorber mass fractions,

surprisingly, the coated sphere Mie calculation provides the

best estimate, while for high absorber mass fractions, the

extended effective medium approximation provides the best

estimate. Given the above, we conclude that the microstructure is

actually dissimilar for high and low mass fractions of the

absorbing compound. In order to generalize this statement for

other absorbers, we first need to eliminate the influence of the

solubility difference on the results.

Regarding coated particles, we find that the coated sphere

Mie subroutine gives an accurate calculation of Qext for

aerosols comprised of a non-absorber coated by another

non-absorber. However, for aerosols comprised of a strong

absorber coated by a non-absorber or of a non-absorber

coated by a weak absorber, discrepancies of up to 10%

between the measurements and theoretical calculations were

documented. These discrepancies await further investigation

with more advanced instrumentation in order to understand

their origin. Several such tools can be considered. Multiwave-

length CRD could provide a comprehensive view of the

wavelength dependence of the refractive index for the internally

mixed particles. Measuring the absorption coefficient with a

photo-acoustic spectrophotometer (PAS) coupled to the CRD

system could provide direct and independent measurements of

the extinction and absorption of aerosols and hence direct

measurement of the single scattering albedo of the aerosol and

more accurate values of the imaginary part of the complex

index of refraction. A new approach, that was recently developed

by Thompson et al., is the albedometer.113 This instrument

combines CRD within an integrating sphere nephelometer.

Table 1 Percentage error between the measured and calculatedQext for PSL coated with glutaric-acid (Glu-A) and with b-carotene (b-car) in bothmodels (constant core diameter with increasing shell thickness and increasing core diameter with constant shell thickness)

Coated diameter/nm Size parameter (x) Qext calculated Qext measured Error/%

PSL–Glu-A: const. core diameter (350 nm)350 2.067 2.510 2.506 0.143370 2.185 2.532 2.511 0.821390 2.303 2.616 2.672 2.170410 2.421 2.768 2.949 6.510430 2.539 2.992 3.050 1.916450 2.657 3.239 3.288 1.531470 2.775 3.421 3.463 1.223490 2.894 3.495 3.679 5.253510 3.012 3.494 3.605 3.197PSL–Glu-A: const. shell thickness (40 nm)280 1.653 1.279 1.165 8.860340 2.008 2.250 1.989 11.593390 2.303 2.616 2.601 0.573440 2.598 3.475 3.371 2.971490 2.894 3.794 3.689 2.767540 3.189 3.916 3.821 2.426PSL–b-car: const. core diameter (350 nm)390 2.303 2.681 2.523 5.875410 2.421 2.843 2.747 3.359440 2.598 3.100 3.102 0.048470 2.775 3.255 3.510 7.837500 2.953 3.296 3.539 7.376PSL–b-car: const. shell thickness (40 nm)340 2.008 2.270 2.094 7.775390 2.303 2.681 2.445 8.807440 2.598 3.430 3.492 1.796490 2.894 3.750 3.917 4.440540 3.189 3.914 3.587 8.335600 3.543 4.309 4.258 1.167


Publ

ishe

d on

12

Oct

ober

200

9. D

ownl

oade

d on

22/

07/2

015

13:0

7:03



The CRD resonator was built using the integrating sphere by

mounting the CRD mirrors on opposite sides of the sphere.

The light scattered by the aerosols inside the sphere is collected

by the inner side of the sphere to a separate PMT from the

CRD PMT. The main advantage of this method is the

simultaneous determination of the extinction and scattering

in a single instrument utilizing two very sensitive methods. In

addition, combining the methods in a single instrument would

minimize the errors in the measurements due to different

sample conditions in different instruments.

Acknowledgements

This work was partially supported by the Israel Science

Foundation (grants 1527/07 and 196/08). Y.R. acknowledges

financial support by the Helen and Martin Kimmel Award for

Innovative Investigation. We thank H. E. Levy for programming

support, and A. Moroz and M. Tzemach for helpful suggestions

regarding surface/interface effects in coated particles.

References

1 B. J. Finlayson-Pitts and J. Pitts, Jr., Chemistry of the Upper andLower Atmosphere, Academic Press, CA, 1999.

2 J. H. Seinfeld and S. N. Pandis, Atmospheric Chemistry andPhysics, 2nd edn, John Wiley & Sons Inc, 2006.

3 P. U. Poschl, Angew. Chem., Int. Ed., 2005, 44, 7520.4 Y. Rudich, N. M. Donahue and T. F. Mentel, Annu. Rev. Phys.Chem., 2007, 58, 321–352.

5 M. C. Jacobson, H. C. Hansson, K. J. Noone and R. Charlson,Rev. Geophys., 2000, 38, 267–294.

6 Y. Rudich, Chem. Rev., 2003, 103, 5097–5124.7 M. Kulmala, H. Vehkamaki, T. Petaja, M. Dal Maso, A. Lauri,V. M. Kerminen, W. Birmili and P. H. McMurry, J. Aerosol Sci.,2004, 35, 143–176.

8 B. T. Johnson, K. P. Shine and P. M. Forster, Q. J. R. Meteorol.Soc., 2004, 130, 1407–1422.

9 U. Lohmann and J. Feichter, Geophys. Res. Lett., 2001, 28, 159.10 S. Menon, J. Hansen, L. Nazarenko and Y. Luo, Science, 2002,

297, 2250–2253.11 U. Lohmann and J. Feichter, Atmos. Chem. Phys., 2005, 5, 715.12 D. Y. Kim and V. Ramanathan, J. Geophys. Res.-Atmos., 2008,

113.13 S. Solomon, D. Qin, M. Manning, R. B. Alley, T. Berntsen,

N. L. Bindoff, Z. Chen, A. Chidthaisong, J. M. Gregory,G. C. Hegerl, M. Heimann, B. Hewitson, B. J. Hoskins,F. Joos, J. Jouzel, V. Kattsov, U. Lohmann, T. Matsuno,M. Molina, N. Nicholls, J. Overpeck, G. Raga,V. Ramaswamy, J. Ren, M. Rusticucci, R. Somerville,T. F. Stocker, P. Whetton, R. A. Wood and D. Wratt, TechnicalSummary, in Climate Change 2007: The Physical Science Basis.Contribution of Working Group I to the Fourth Assessment Reportof the Intergovernmental Panel on Climate Change, CambridgeUniversity Press, Cambridge, 2007.

14 Y. J. Kaufman, I. Koren, L. A. Remer, D. Rosenfeld andY. Rudich, Proc. Natl. Acad. Sci. U. S. A., 2005, 102,11207–11212.

15 V. Ramanathan, C. Chung, D. Kim, T. Bettge, L. Buja,J. T. Kiehl, W. M. Washington, Q. Fu, D. R. Sikka andM. Wild, Proc. Natl. Acad. Sci. U. S. A., 2005, 102, 5326–5333.

16 Y. J. Kaufman and I. Koren, Science, 2006, 313, 655–658.17 I. Koren, L. A. Remer, Y. J. Kaufman, Y. Rudich and

J. V. Martins, Geophys. Res. Lett., 2007, 34.18 I. Koren, J. V. Martins, L. A. Remer and H. Afargan, Science,

2008, 321, 946–949.19 D. Rosenfeld, U. Lohmann, G. B. Raga, C. D. O’Dowd,

M. Kulmala, S. Fuzzi, A. Reissell and M. O. Andreae, Science,2008, 321, 1309–1313.

20 H. Moosmuller, R. K. Chakrabarty and W. P. Arnott,J. Quant.Spectrosc. Radiat. Transfer, 2009, 110, 844–878.

21 C. Erlick, V. Ramaswamy and L. M. Russell, J. Geophys. Res.,2006, 111, D06204.

22 Q. Zhang, J. L. Jimenez, M. R. Canagaratna, J. D. Allan, H. Coe,I. Ulbrich, M. R. Alfarra, A. Takami, A. M. Middlebrook,Y. L. Sun, K. Dzepina, E. Dunlea, K. Docherty,P. F. DeCarlo, D. Salcedo, T. Onasch, J. T. Jayne, T. Miyoshi,A. Shimono, S. Hatakeyama, N. Takegawa, Y. Kondo,J. Schneider, F. Drewnick, S. Borrmann, S. Weimer,K. Demerjian, P. Williams, K. Bower, R. Bahreini, L. Cottrell,R. J. Griffin, J. Rautiainen, J. Y. Sun, Y. M. Zhang andD. R. Worsnop, Geophys. Res. Lett., 2007, 34.

23 K. Chamaillard, S. G. Jennings, C. Kleefeld, D. Ceburnis andY. J. Yoon, J. Quant. Spectrosc. Radiat. Transfer, 2003, 79–80,577–597.

24 J. T. Kiehl, T. L. Schneider, P. J. Rasch and M. C. Barth,J. Geophys. Res., 2000, 105, 1441–1457.

25 C. E. Lund Myhre and C. J. Nielsen, Atmos. Chem. Phys.Discuss., 2004, 4, 3013.

26 S. F. Maria, L. M. Russell, M. K. Gilles and S. C. B. Myneni,Science, 2004, 306, 1921–1924.

27 E. Dinar, A. A. Riziq, C. Spindler, C. Erlick, G. Kiss andY. Rudich, Faraday Discuss., 2008, 137, 279–295.

28 M. O. Andreae, Nature, 2001, 409, 671–672.29 T. C. Bond, D. G. Streets, K. F. Yarber, S. M. Nelson, J. H. Woo

and Z. Klimont, J. Geophys. Res., 2004, 109, D14203.30 B. F. Henson, J. Geophys. Res., 2007, 112, D24S16.31 L. Liua and M. I. Mishchenkoa, J. Quant. Spectrosc. Radiat.

Transfer, 2007, 106, 262–273.32 Z. Li, P. Goloub, L. Blarel, B. Damiri, T. Podvin and

I. Jankowiak, Appl. Opt., 2007, 46, 1548–1553.33 I. N. Sokolik and O. B. Toon, Nature, 1996, 381, 681–683.34 M. O. Andreae and A. Gelencser, Atmos. Chem.Phys. Discuss.,

2006, 6, 3419–3463.35 S. F. S. Decesari, M. C. Facchini, M. Mircea, L. Emblico,

F. Cavalli, W. Maenhaut, X. Chi, G. Schkolnik, A. Falkovich,Y. Rudich, M. Claeys, V. Pashynska, G. Vas, I. Kourtchev,R. Vermeylen, A. Hoffer, M. O. Andreae, E. Tagliavini,F. Moretti and P. Artaxo, Atmos. Chem. Phys., 2006, 6, 375.

36 A. Hoffer, A. Gelencser, P. Guyon, G. Kiss, O. Schmid,G. P. Frank, P. Artaxo and M. O. Andreae, Atmos. Chem. Phys.,2006, 6, 3563–3570.

37 H. L. Sun, L. Biedermann and T. C. Bond, Geophys. Res. Lett.,2007, 34.

38 M. Born and E. Wolf, Principles of optics, 7th edn, CambridgeUniversity Press, 1999.

39 T. S. Bates, T. L. Anderson, T. Baynard, T. Bond, O. Boucher,G. Carmichael, A. Clarke, C. Erlick, H. Guo, L. Horowitz,S. Howell, S. Kulkarni, H. Maring, A. McComiskey,A. Middlebrook, K. Noone, C. D. O’Dowd, J. Ogren,J. Penner, P. K. Quinn, A. R. Ravishankara, D. L. Savoie,S. E. Schwartz, Y. Shinozuka, Y. Tang, R. J. Weber andY. Wu, Atmos. Chem. Phys., 2006, 6, 1657–1732.

40 N. Bellouin, O. Boucher, J. Haywood and M. S. Reddy, Nature,2005, 438, 1138–1141.

41 W.-T. Chen, R. A. Kahn, D. Nelson, K. Yau and J. H. Seinfeld,J. Geophys. Res., 2008, 113.

42 Y. J. Kaufman, Nature, 2002, 419, 215.43 Y. J. Kaufman and R. S. Fraser, Science, 1997, 277, 1636–1639.44 G. W. Petty, A First Course in Atmospheric Radiation, Sundog

publishing, Madison, Wisconsin, 2004.45 C. F. Bohren and D. R. Huffman, Absorption and Scattering of

Light by Small Particles, Wiley, New York, 1983.46 G.-h. Dai, Z.-b. Ren, Q. Sun and J. Liu, Proc. SPIE–Int. Soc.

Opt. Eng., 2006, 602723.47 G. Mie, Ann. Phys., 1908, 25, 377–445.48 C. Erlick, J. Atmos. Sci., 2006, 63, 754–763.49 G. Lesins, P. Chylek and U. Lohmann, J. Geophys. Res., 2002,

107, 4094.50 C. Marcolli, B. P. Luo, T. Peter and F. G. Wienhold, Atmos.

Chem. Phys., 2004, 4, 2593–2599.51 F. Parungo, in Atmos. Aerosol. Nucleat., 1988, p. 269.52 F. P. Parungo, C. T. Nagamoto, J. Rosinski and

P. L. Haagenson, J. Atmos. Chem., 1986, 4, 199.


Publ

ishe

d on

12

Oct

ober

200

9. D

ownl

oade

d on

22/

07/2

015

13:0

7:03



53 A. A. Riziq, M. Trainic, C. Erlick, E. Segre and Y. Rudich,Atmos. Chem. Phys., 2008, 8, 1823–1833.

54 A. H. Falkovich, G. Schkolnik, E. Ganor and Y. Rudich,J. Geophys. Res., 2004, 109, D02208.

55 S. A. Guazzotti, J. R. Whiteaker, D. Suess, K. R. Coffee andK. A. Prather, Atmos. Environ., 2001, 35, 3229.

56 Q. Zhang, M. R. Canagaratna, J. T. Jayne, D. R. Worsnop andJ. L. Jimenez, J. Geophys. Res.-Atmos., 2005, 110.

57 G. B. Ellison, A. F. Tuck and V. Vaida, J. Geophys. Res., 1999,104, 11633–11641.

58 V. Vaida, A. F. Tuck and G. B. Ellison, Phys. Chem. Earth Pt.C-Solar-Terr. Planet. Sci., 2000, 25, 195–198.

59 A. Laskin, D. J. Gaspar, W. H. Wang, S. W. Hunt, J. P. Cowin,S. D. Colson and B. J. Finlayson-Pitts, Science, 2003, 301,340–344.

60 V. H. Grassian, Int. Rev. Phys. Chem., 2001, 20, 467–548.61 M. Posfai, J. R. Anderson, P. R. Buseck and H. Sievering,

J. Geophys. Res., 1999, 104, 21685–21693.62 K. Karar, A. Gupta, A. Kumar and A. Biswas, Environ. Monit.

Assess., 2006, 120, 347.63 U. Panne, R. E. Neuhauser, M. Theisen, H. Fink and

R. Niessner, Spectrochim. Acta, Part B, 2001, 56, 839.64 M. H. Shah, N. Shaheen and M. Jaffar, J. Chem. Soc. Pak., 2007,

29, 598–604.65 H. X. Xue, A. F. Khalizov, L. Wang, J. Zheng and R. Y. Zhang,

Environ. Sci. Technol., 2009, 43, 2787–2792.66 P. Chylek, G. Videen, D. J. W. Geldart, J. S. Dobbie and H. C.

W. Tso, Effective medium approximations for heterogeneousparticles in: Light scattering by nonspherical particles, theory,measurements and application, Academic Press, 2000.

67 M. Z. Jacobson, J. Geophys. Res., 2002, 107.68 A. W. Stelson, Environ. Sci. Technol., 1990, 24, 1676–1679.69 I. N. Tang, J. Geophys. Res., 1997, 102.70 P. Chylek, V. Ramaswamy and R. J. Cheng, J. Atmos. Sci., 1984,

41, 3076–3084.71 A. Sihvola and R. Sharma, Microwave Opt. Technol. Lett., 1999,

22, 229–231.72 M. Z. Jacobson, J. Phys. Chem. A, 2006, 110, 6860–6873.73 M. R. Beaver, R. M. Garland, C. A. Hasenkopf, T. Baynard,

A. R. Ravishankara and M. A. Tolbert, Environ. Res. Lett., 2008,3, 045003.

74 R. E. Danielson, D. R. Moore and H. C. van de Hulst, J. Atmos.Sci., 1969, 26, 1078–1087.

75 T. P. Ackerman and O. B. Toon, Appl. Opt., 1981, 20,3661–3667.

76 J. E. Penner, C. C. Chuang and K. Grant, Clim. Dyn., 1998, 14,839.

77 O. Mohler, S. Benz, H. Saathoff, M. Schnaiter, R. Wagner,J. Schneider, S. Walter, V. Ebert and S. Wagner, Environ. Res.Lett., 2008, 3, 025007.

78 O. Mohler, S. Buttner, C. Linke, M. Schnaiter, H. Saathoff,O. Stetzer, R. Wagner, M. Kramer, A. Mangold, V. Ebert andU. Schurath, J. Geophys. Res., 2005, 110, D11210.

79 M. Schnaiter, C. Linke, O. Mohler, K. H. Naumann, H. Saathoff,R. Wagner, U. Schurath and B. Wehner, J. Geophys. Res., 2005,110, D19204.

80 A. A. Riziq, C. Erlick, E. Dinar and Y. Rudich, Atmos. Chem.Phys., 2007, 7, 1523–1536.

81 K. Adachi and P. R. Buseck, Atmos. Chem. Phys., 2008, 8,6469–6481.

82 P. J. Sheridan, W. P. Arnott, J. A. Ogren, E. Andrews,D. B. Atkinson, D. S. Covert, H. Moosmuller, A. Petzold,B. Schmid, A. W. Strawa, R. Varma and A. Virkkula, AerosolSci. Technol., 2005, 39, 1–16.

83 A. F. Khalizov, H. Xue, L. Wang, J. Zheng and R. Zhang,J. Phys. Chem. A, 2009, 113, 1066–1074.

84 P. Chylek, V. Srivastava, R. G. Pinnick and R. T. Wang, Appl.Opt., 1988, 27, 2396.

85 G. Videen, D. Ngo and P. Chlek, Opt. Lett., 1994, 19, 1675.86 G. Videen and P. Chlek, Opt. Commun., 1998, 158, 1.87 M. J. Wolff, G. C. Clayton and S. J. Gibson, Astrophys. J., 1998,

503, 815–830.88 J. E. Spanier and I. P. Herman, Phys. Rev. B., 2000, 61, 10437.89 L. Kolokolova and B. S. Gustafsonm, J. Quant. Spectrosc.

Radiat. Transfer, 2001, 70, 611.90 E. R. Graber and Y. Rudich, Atmos. Chem. Phys. Discuss., 2006,

5, 9801.91 N. Lang-Yona, Y. Rudich, E. Segre, E. Dinar and A. Abo-Riziq,

Anal. Chem., 2009, 81, 1762–1769.92 C. Spindler, A. Abo Riziq and Y. Rudich, Aerosol Sci. Technol.,

2007, 41, 1011–1017.93 A. O. Keefe and D. A. G. Deacon, Rev. Sci. Instrum., 1988, 59,

2544–2555.94 D. Romanini, A. A. Kachanov, N. Sadeghi and F. Stoeckel,

Chem. Phys. Lett., 1997, 264, 316–322.95 M. D. Wheeler, S. M. Newman and A. J. Orr-Ewing, J. Chem.

Phys., 1998, 108, 6594–6605.96 S. S. Brown, H. Stark, S. J. Ciciora, R. J. McLaughlin and

A. R. Ravishankara, Rev. Sci. Instrum., 2002, 73, 3291–3301.97 M. Mazurenka, A. J. Orr-Ewing, R. Peverallb and and G. A.

D. Ritchieb, Annu. Rep. Prog. Chem., Sect. C, 2005, 101, 100–142.98 M. Pradhan, R. E. Lindley, R. Grilli, I. R. White, D. Martin and

A. J. Orr-Ewing, Appl. Phys. B, 2008, 90, 1–9.99 G. Berden, R. Peeters and G. Meijer, Int. Rev. Phys. Chem., 2000,

19, 565.100 V. Bulatov, M. Fisher and I. Schechter, Anal. Chim. Acta, 2002,

466, 1–9.101 H. Moosmuller, R. Varma and W. P. Arnott, Aerosol Sci.

Technol., 2005, 39, 30–39.102 A. Pettersson, E. R. Lovejoy, C. A. Brock, S. S. Brown and

A. R. Ravishankara, J. Aerosol Sci., 2004, 35, 995–1011.103 A. D. Sappey, E. S. Hill, T. Settersten and M. A. Linne, Opt.

Lett., 1998, 23, 954–956.104 J. D. Smith and D. B. Atkinson, Analyst, 2001, 126, 1216–1220.105 A. W. Strawa, R. Castaneda, T. Owano, D. S. Baer and

B. A. Paldus, J. Atmos. Oceanic Technol., 2003, 20, 454–465.106 T. J. A. Butler, J. L. Miller and A. J. Orr-Ewing, J. Chem. Phys.,

2007, 126, 174302.107 J. J. Scherer, J. B. Paul, A. O’Keefe and R. J. Saykally, Chem.

Rev., 1997, 97, 25–52.108 S. M. Ball and R. L. Jones, Chem. Rev., 2003, 103, 5239–5262.109 J. C. Lagarias, J. A. Reeds, M. H. Wright and P. E. Wright,

SIAM J. Optim., 1998, 9, 112–147.110 O. Torres, A. Tanskanen, B. Veihelmann, C. Ahn, R. Braak,

P. K. Bhartia, P. Veefkind and P. Levelt, J. Geophys. Res., 2007,112, D24S47.

111 K. H. Guenther, Appl. Opt., 1984, 23, 3806.112 M. Z. Jacobson, J. Geophys. Res., 1999, 104, 3527–3542.113 J. E. Thompson, N. Barta, D. Policarpio and R. DuVall, Opt.

Express, 2008, 16, 2191–2205.


Publ

ishe

d on

12

Oct

ober

200

9. D

ownl

oade

d on

22/

07/2

015

13:0

7:03



interaction of internally mixed aerosols with light

Documents