second organic aerosol formation by irradiation of α-pinene-nox-h2o in an indoor smog chamber for...

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Second organic aerosol formation by irradiation of α-pinene-NOx-H2O in an indoor smog chamber for atmospheric chemistry and physics

ZHAO Zhe, HAO JiMing†, LI JunHua & WU Shan Department of Environmental Science and Engineering, Tsinghua University, Beijing l00084, China

Ozone(O3) and secondary organic aerosol (SOA) are considered to be the most serious secondary air pollutants of concern in most metropolitan areas, as well as for Beijing. In this study, O3 and SOA for-mation potential of α-pinene, the most abundant biogenic VOCs, is investigated at Tsinghua Indoor Chamber Facility. The experiments were conducted under atmospheric relevant HCs/NOx ratios in both presence and absence of ammonia sulfate seed aerosol. A Scanning Mobility Particle Sizer system (3936, TSI) and a Condensation Particle Counter (3010, TSI) were used to study the SOA formation and a gas chromatograph (GC) equipped with a DB-5 column and a flame ionization detector (FID) was used to measure α-pinene simultaneously. The results show that the presence of ammonia sulfate seed aerosol did not change the formation trend of O3, but significantly contribute to SOA formation. A strong linear relationship (r2 = 0.90) between SOA yield enhancement (ΔY*) and surface concentration of seed aerosol (PMi,s)has been found, denoting that the PMi,s is the control factor for SOA yield en-hancement. And the possible reason for the enhancement is acid-catalyzed heterogeneous reactions.

α-pinene, photo-oxidation, SOA, chamber experiment, seed particles

Fine PM and ozone are considered to be the most seri-ous air pollutants of concern in most metropolitan areas around the world, including Beijing[1,2]. Gas-phase reac-tions of volatile organic compounds (VOCs) associated with photochemical oxidant cycles have been of great interest in predicting ozone (O3) concentrations and, more recently, SOA formation. Laboratory chambers are indispensable in the study of gas phase atmospheric chemistry and atmospheric aerosol formation and growth[3,4]. Because of the difficulty of isolating chemi-cal and microphysical processes in the atmosphere from flow and mixing effects, chamber studies provide the means to develop mechanistic understanding of such processes[5,6].

SOA formation usually occurs in the presence of background particles that are ubiquitous in the ambient atmosphere, therefore, lots of chamber studies focused on the potentially effects of preexisting particles (seed

particles) on SOA formation[7―12]. At a given tempera-ture, the effects of an inorganic species on partitioning are critically changed above and below the deliques-cence and the efflorescence RH[13]. Cocker et al.[14] showed how SOA yields are influenced by inorganic salts, and created different empirical fit parameters for “wet” and “dry” inorganic salts. The study by Czoschke and Jang[13] has demonstrated the presence of wet acidic seed aerosol can increase SOA yield obviously, and the SOA enhancement is due to heterogeneous acid-catalyzed reactions[8,10,15―19]. But the role of dry inorganic seed has not been clarified. In this study, we demon-strates some possible impacts of preexisting dry seed Received January 4, 2008; accepted May 12, 2008 doi: 10.1007/s11434-008-0478-z †Corresponding author (email: [email protected]) Supported by Toyota Central R&D Labs., Inc. (as part of “Study of Photochemical Reaction under High PM Contaminated Condition to Improve Air Quality of Bei-jing”), and the National Natural Science Foundation of China (Grant No. 20637001)

ZHAO Zhe et al. Chinese Science Bulletin | November 2008 | vol. 53 | no. 21 | 3294-3300 3295

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aerosol on SOA formation from irradiated α-pinene/ NOx/H2O reaction system associated with the atmos-pheric NOx cycle in an indoor smog chamber.

1 Experimental

The detailed description of Tsinghua Indoor Chamber System has been reported elsewhere[20] and only the es-sential features are described here. The whole system mainly consists of a temperature-controlled enclosure, a Teflon film-surrounded chamber, the gas and aerosol reactant injection system, and the gas and aerosol sam-pling systems. These systems were integrated to facili-tate the generation and condensation of oxidation prod-ucts onto preexisting particles in a continuous and con-trolled manner.

1.1 Facility overview

The chamber has a volume of 2 m3 and a surface/volume ratio of 5 m−1, and was placed inside a tempera-ture-controlled enclosure (ESPEC, SEWT-Z-120), which provide precisely temperature control at 10℃―60℃, with fluctuating no more than ±0.5℃. The chamber wall material is 54 μm Teflon FEP film (Toray International Inc.), a transparent, nonreactive material. The wall ma-terial is flexible, enabling air extraction from the Teflon chamber without altering the pressure inside the reactor. Forty 40 W GE F40BLB black lights (365 nm) are used to illuminate the reaction chambers. The black lights emission spectrum and the photolysis rate of NO2 esti-mated by steady-state actinometry are shown else-where[20]. To maximize the light intensity in the chamber, reflective aluminum sheeting covers all four walls, the ceiling, and the floor of the enclosure.

1.2 System preparation

Prior to each experiment, the chamber was continuously flushed with purified laboratory-compressed air for at least 40 h and baked with black lamps for at least 1 d. After purification, the chamber was humidified to the desired relative humidity by passing purified air through the saturated steam in the heated tank filled with deion-ized water before entering the chamber. The resulting air contained no detectable reactive hydrocarbons, no parti-cles, and less than 5 ppb NOx.

(i) Reactant injection. Microliter syringes are used to measure and inject known volumes of liquid α-pinene (Aldrich, 98%, boiling point = 160℃) into a stainless

steel heated tank, inside which α-pinene is vaporized into the ultrapure air stream for injection into the cham-ber. Certified mixtures of gas-phase compounds in N2 are introduced into the chamber using a mass-flow con-troller to maintain the flow-rate for a set period of time. The gas is then flushed into the chamber with ultrapure air through a FEP Teflon line and stainless steel port.

(ii) Seed generation. The seed particles were gener-ated by atomizing an aqueous solution (0.002 mol/L) of ammonium sulfate ((NH4)2SO4, AS) using a stainless steel constant rate atomizer (TSI, model 3076). The aerosol was passed through a diffusion dryer (TSI, model 3062), followed by an 85 Kr charge neutralizer (TSI, model 3077) before entering the chamber.

(iii) Gas phase instrumentation. NO, NO2, and NOx mixing ratios are tracked using a chemiluminescent NO-NO2-NOx analyzer (Thermo Environmental Instru-ments Inc. Model 42). The analyzer continuously sam-ples at a flow rate of 0.7 LPM, and the instrument is calibrated weekly using a certified cylinder of NO. Ozone was monitored using a photometric ozone detec-tor (Thermo Environmental Instruments Inc, Model 49), and the zero point is calibrated before irradiation in each experiment. The concentration of α-pinene in the cham-ber was measured using a gas chromatograph (Beifen SP-3420) equipped with a DB-5 column (J&W Scien-tific, Davis, CA) and a flame ionization detector (FID). GC/FID measurements were taken every 15 min, and calibrations were performed prior to each experiment.

The temperature and RH within the chambers were measured using combined temperature and RH probes (Vaisala HMT333 series transmitters). The accuracy of the probe is 0.001℃ and 2% RH.

(iv) Aerosol instrumentation. The particle phase was physically detected by a scanning mobility particle sizer (SMPS, TSI 3936) consisting of a differential mobility analyzer (DMA, TSI 3081) and a Condensation Particle Counter (3010, TSI). The instrument is capable of de-tecting particle in the size range of 7―1000 nm de-pending on the adjusted sheath flow. For all experiments samples with a time resolution of 6min, including 240 s up-scan and 120 s down-scan, were obtained during the reaction time of 300 min, in which the α-pinene was almost totally converted. The used sample flow rate was 0.2 L/min and the sheath flow rate inside the DMA was set to 2.0 L/min during the reactions to match the parti-cle size distribution best with the measuring range of the

3296 ZHAO Zhe et al. Chinese Science Bulletin | November 2008 | vol. 53 | no. 21 | 3294-3300

instrument.

1.3 Correction for the wall loss

A drawback of the use of Teflon film is static charge built up on the FEP Teflon film surface that increases particle deposition to the walls[3]. Particles deposit on the wall at a rate that is proportional to the particle con-centrations and depends on particle size, leading to first-order kinetics:

pdep p p

( )( ) ( ),

dN dk d N d

dt= − (1)

where dep p( )k d is the wall loss coefficient, p( )N d is

the particle number concentration, and dp is the particle diameter. dep p( )k d values are estimated from particle

number concentration versus time data for each experi-ment using the equation above. The experiments should be conducted under dark condition and at low particle concentration (<1000 particles/cm3) to avoid the coagu-lation.

The relationship between kdep (h−1) and dp (nm) was determined by optimization of 4 parameters (a―d) in eq. (2)[21]. The optimized parameters are showed in eq. (3). dep p p p( ) /b dk d ad c d= + , (2)

4 1.19 1.42dep p p1 10 102 /k d d−= × + . (3)

The application of the wall loss correction for typical experiment is illustrated in Figures 1 and 2. The PM suspend and PM correction in the legend denote mass concentration of particles before and after wall loss cor-rection, respectively.

Figure 1 α-pinene photo-oxidation in absence of seed aerosol.

2 Results and discussion

Several key factors may affect the SOA yield, including temperature[22], relative humidity (RH)[14,23―25], the hy-

Figure 2 α-pinene photo-oxidation in presence of seed aerosol.

drocarbon to NOx ratio (HC:NOx)[26], the composition of organic material[27], and the presence/absence and the physical/chemical properties of preexisting aerosol seed. In order to make clear the role of preexisting inorganic seed played during the process of SOA formation, all the other factors are kept consistent except for the seed aerosol-related items. Table 1 shows a summary of ex-perimental conditions. The hydrocarbon to NOx ratio for all experiments ranged from 2.1 to 2.3. And the irradia-tion for each experiment lasted for 5 h, ensuring that the suspended particle matters achieved maximum concen-tration. The relative humidity and temperature inside the chamber were maintained at 40% and 302 K, respec-tively.

2.1 Gas phase components

In each experiment, concentrations of the major inor-ganic parameters, NO, NOx-NO, and O3, were measured continuously. The time-series profiles for these con-stituents show the stability of the chamber. Profiles for these parameters during a typical experiment in the ab-sence or presence of ammonium sulfate seed are shown in Figures 1 and 2, respectively. For both systems in the absence and presence of seed aerosol, initial NO de-creases rapidly to less than 5 ppb after about 60 min. O3 remains at a low level in the beginning and starts to rise after about 20 min, and increases continuously in the 5 hours’ irradiation. The presence of ammonia sulfate seed aerosol does not have measurable effects on the varia-tion trend of gas phase components, including O3.

2.2 Particle phase

(i) Secondary organic aerosol formation. All the experiments conducted in this study are under the condi-tion of 40% relative humidity, lower than the deliques-cence relative humidity (DRH) of ammonia sulfate[28], therefore, none of the water is associated with the seed


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Table 1 Summary of experimental conditions and results

Experiment date PM species NO (ppb)

NOx (ppb)

[PM]i,ma)

(μg/m3)[PM]i,s

b)

(cm2/m3)HCs/NOx(ppb/ppb)

[PM]max Time(h)

ΔHCr (μg/m3)

ΔMo

(μg/m3)

Yield(%)

ΔY*×100(m3/μg)

Jun./15/2007 without 25.04 51.08 0.00 0.0 1.7 2.6 482 39.79 8.3 − Jun./19/2007 AS 24.13 50.60 46.94 19.0 2.3 2.8 635 109.01 17.1 1.18 Jun./21/2007 without 44.81 89.79 0.00 0.0 2.3 2.3 1112 130.59 11.7 − Jun./23/2007 AS 30.76 63.23 48.81 15.2 2.2 2.4 745 87.46 11.7 0.24 Jun./26/2007 without 28.97 58.19 0.00 0.0 2.4 2.7 756 74.36 9.9 − Jun./28/2007 AS 19.44 38.66 53.07 18.3 2.1 2.9 455 72.48 16.0 1.06 Jun./30/2007 AS 30.94 59.25 48.24 16.8 2.3 2.5 750 100.74 13.5 0.51 Jul./2/2007 without 41.07 82.00 0.00 0.0 2.0 2.6 898 107.44 10.9 − Jul./4/2007 AS 40.10 83.05 46.59 17.0 2.2 2.6 1008 151.8 15.1 0.63 Jul./7/2007 AS 28.15 57.17 51.07 16.9 2.2 2.5 679 89.81 13.2 0.49 Jul./31/2007 AS 35.37 68.96 52.82 18.0 2.2 2.8 838 155.76 18.6 1.15 Aug./6/2007 without 36.35 70.27 0.00 0.0 2.3 2.7 865 89.24 10.3 − a) Mass concentration of preexisting seed particle; b) surface concentration of preexisting seed particle.

we injected into the chamber. According to Cocker’s definition[14], the experiments

during which the SOA is produced in the absence of seed aerosol but at elevated RH values, are called humid nucleation experiments. The initial aerosol is produced by nucleation of supersaturated oxidation products; however, the subsequent growth of the aerosol results from absorption of both organics and water. The total aerosol formed, in this case, will depend on organic- organic and organic-water interactions. The mass con-centration of organic aerosol produced is the total aero-sol mass concentration less the mass concentration of water associated with the aerosol. For the experiments in presence of seed aerosol at elevated RH values (but be-low DRH), SOA initially forms via condensation onto the inorganic particles, and subsequent growth occurs via absorption into the organic surface coating the inor-ganic core. The resulting aerosol composition is a mix-ture of organic and inorganic constituents, and organic aerosol mass is equal to total particle mass minus the mass of preexisting inorganic seed and the water associ-ated with the aerosol.

Mass concentration was calculated from the observed particle size distribution assuming spherical particles and unit density, after correcting for wall loss. The total organic aerosol mass concentration produced (Mo) and the total concentration of α-pinene consumed were used to calculate the SOA yield (Y) for each experiment.

(ii) SOA yield. Due to the difficulty associated with the molecular identification and quantification of sec-ondary organic compounds derived from the oxidation of parent hydrocarbons, the aerosol yield has been used as a measure of the overall aerosol forming potential of

various hydrocarbons. Aerosol yield, Y, is defined as the ratio of ΔM (μg/m3),

the aerosol mass concentration produced through oxida-tion of the parent hydrocarbon, to ΔHC (μg/m3), the mass concentration of parent gas phase organic reacted,

/Y M HC= Δ Δ . An absorptive partitioning model proposed by Pankow[29,30] has been demonstrated extensively[31] to describe SOA formation. The aerosol yield Y resulting from the oxidation of a single parent hydrocarbon can be described as the sum of the yields of each organic prod-uct Yi as follows:

,

,1i om i

i oi i om i o

KY Y M

K Mα

= =+∑ ∑ , (4)

where αi is the mass-based stoichiometric fraction of species i formed from the parent hydrocarbon, Kom,i is the gas-particle partitioning coefficient (m3/μg), which is inversely proportional to the compound’s vapor pressure, and Mo (µg/m3) is the total mass concentration of or-ganic material.

One-product model shown in eq. (5) is a mathemati-cally simplified form of eq. (4) by assuming that only one kind of products is formed during oxidation of par-ent hydrocarbons[31]:

1o

o

KY MKM

α=

+ (5)

SOA yield (Y) versus organic mass formed (Mo) for the seed-free experiments are given in Figure 3. The line through the data is generated using one-product model shown in eq. (5), where α and Kom are chosen to fit the data by minimizing the square of the residuals. In Figure


5, Y versus Mo for both seed-free experiments and seeded experiments are shown in the same figure. It can be seen that Y for seeded case (the solid ones) located above the seed-free case (the hollow ones), which de-notes that the presence of seed aerosol enhances the SOA formation. Furthermore, when one-product model was used, the data for the experiments with the same

Figure 3 SOA yields for seed-free experiments as a function of Mo. Values used to generate the one-product model line are 0.1382 and 0.0359 for α and Kom, respectively.

Figure 4 SOA yields for seed-free and seeded experiments as a function of Mo.

Figure 5 Relationship between SOA yield enhancement and surface concentration of seed particle.

surface concentration of seed aerosol are located in the same regression curve, which denotes that the surface concentration of seed aerosol (PMi,s) may be an impor-tant factor for Y enhancement.

For the purpose of quantifying the relationship be-tween SOA yield enhancement and the surface concen-tration of seed aerosol. We define ΔY and ΔY* as ,Y Y Y′Δ = − (6)

,

1*i m

Y YYY PM′ −

Δ = × , (7)

where Y′ and Y are SOA yields for seeded and seed-free cases, respectively; PMi,m (μg/m3) is the mass concen-tration of preexisting seed aerosol. ΔY* is calculated by dividing the difference between Y′ and Y by the mass concentration of seed aerosol (PMi,m) present at the time of SOA begin to generate.

As shown in Figure 5, there is a strong linear rela-tionship (r2=0.90) between ΔY* and PMi,s. Therefore, the surface concentration of seed aerosol is most proba-bly the control factor for SOA yield enhancement.

A possible explanation for the SOA enhancement is the acid-catalyzed heterogeneous reactions. Organic products are initially produced from ozonation in the gas phase partition onto the inorganic seed aerosol and react heterogeneously with an acid catalyst forming low vapor pressure products. These acid-catalyzed heterogeneous reactions are implicated in generating the increased SOA mass observed in seeded systems as they transform pre-dominantly gas phase compounds of high volatility into low vapor pressure predominantly particle phase prod-ucts.

(iii) Size distribution evolution. The change of the size distributions of secondary aerosol formation as a function of time was monitored with an SMPS system. Figure 6(a) and (b) show the example of change in the particle number distribution (dN/dlogDp) for the α-pinene photo-oxidation in the absence and presence of dry ammonium sulfate seed, respectively. The overall shape of the size distribution for seeded experiments and seed-fee experiments are different.

For seed-free system (Figure 6(a)), SOA formation is observed in 42 min by homogenous nucleation. Then the mean geometric particle diameter (Dpg) kept shifting to larger diameters due to the growth by a condensation process, and after 2 hours’ irradiation, the Dpg and the shape of the size distribution were no longer changed. For ammonium sulfate seeded system (Figure 6(b)),


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Figure 6 The evolution of size distribution for the seed-free experiments (a) and ammonium sulfate seeded experiments (b). The legends show the irradiation time. time zero is the initial size distribution of seed particles. The Dpg kept shifting to larger diameters due to growth by condensation, and no significant change in terms of the shape of the size distribution was observed during the whole process of 5 hours’ irradiation.

3 Conclusion

Indoor smog chamber experiments investigating the ef-fects of dry ammonium sulfate seed aerosol on the SOA formation have been performed. SOA produced for seed-free system is 100% organic in content, resulting

from a sufficient supersaturation of low volatility organ-ics to produce homogeneous nucleation followed by condensation to the aerosol. SOA produced in seeded system is a mixture of organic and inorganic constituents, initially forms via condensation onto the inorganic parti-cles, and subsequent growth occurs via absorption into the organic surface coating the inorganic core.

One-product model is used to regress the SOA yield data versus organic mass generated (Mo), and it can be seen that Y for seeded case located above the seed-free case, which denotes that the presence of ammonia sul-fate seed enhances the SOA formation. A strong linear relationship (r2=0.90) between ΔY* and PMi,s has been found, denoting that the surface concentration of seed aerosol is most probably the control factor for SOA yield enhancement. A possible explanation for the SOA en-hancement is the acid-catalyzed heterogeneous reactions. Organic products are initially produced from ozonation in the gas phase partition onto the inorganic seed aerosol and react heterogeneously with an acid catalyst forming low vapor pressure products. These acid-catalyzed het-erogeneous reactions are implicated in generating the increased SOA mass observed in seeded systems as they transform predominantly gas phase compounds of high volatility into low vapor pressure predominantly particle phase products.

This study has shown that the amount of SOA associ-ated with heterogeneous reactions in α-pinene-NOx-H2O irradiated system is directly related to surface concentra-tion of seed aerosol. Further study needs to be done to better understand how other atmospheric VOCs partici-pate in these processes and is linked to SOA production.

The authors would like to thank Hideto Takekawa for helpful guidance of the experiments and discussion of the results.

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