1

L.A. photo smog

London smog at daylight

2

ʻsmogʼ: view reduction and bad air quality caused by traffic, industrial and further anthropogenic emissions, appearing like fog but consisting of gases and particles (e.g. Los Angeles, London, Mexico City, Athens)

Relation to death rate during the London smog in 1952

•  1)London – type smog, consists mainly of aerosol particles (soot & sulfates from combustion) + elevated SO2 levels, known since 1800s.

–  Particles and SO2 both cause respiratory problems –  Centralized heating, catalytic converters and cleaner fuels have significantly

reduced the occurrence of this type of smog in industrial countries. –  However, some measures may lead to an increase in aerosol particle

number even if the particle mass goes down (”more but smaller particles”), this may have adverse health effects.

–  SO2 and particle emission developing world are increasing ⇒ more smog problems there.

•  2)Photochemical (Los Angeles – type) smog, characterised by high O3 and PAN levels as well as particulate matter, discovered around 1940.

–  Actually not a new phenomena, ”blue haze” smog is formed also from biogenic terpene emissions (R. Reagan: ”trees pollute more than cars do”).

–  Gas-phase chemistry more complicated than London smog. –  Harder to regulate because of nonlinear feedbacks in the NOx/ VOC/O3

system ⇒ still a problem in many western cities, e.g. Los Angeles.

3

•  SO2 in otherwise clean air causes health problems for ξ > 1 ppm. –  Below about 25 ppm, problems confined to upper respiratory tract. –  Effects include chronic and acute bronchitis, pleurisy and

ephysema (chronic obstructive lung disease). •  The simultaneous presence of aerosol particles makes the effects of

SO2 much worse ⇒ health problems at lower concentrations. •  Aerosol particles from combustion often contain carcinogens (e.g. PAH;

polyaromatic hydrocarbons); linked to e.g. lung cancer. •  British coal (used for heating) was high in sulfur, and also contained

tars & other hydrocarbons which produced a lot of particles when burned. Hence the infamous London smog.

•  Due to new legislation, SO2 concentrations e.g. in London dropped by a factor of 6 between 1974-1994.

Fresh urban

Aged urban

rural

remote Warneck [1999]

Note: Concentrations especially of larger particles decrease rapidly with height. D.J. Jacob

4

PM10 (particles > 10 µm) PM2.5 (particles > 2.5 µm)

Red circles indicate violations of national air quality standard: 50 µg m-3 for PM10 15 µg m-3 for PM2.5

D.J. Jacob

PM2.5 (aerosol particles < 2.5 µm diameter)

Site PM 2.5 [µg m-3] Urban / rural

Umeå (S) 8 / 7

Malmö (S) 14 / 10

Oslo (N) 11 / 7

Kuopio (FIN) 11 / 8

Amsterdam (NL) 25 / 27

Berlin (D) 31 / 26

Katowice (PL) 36 / 44

Cracow (PL) 35 / 34

Prague (CZE) 32 / 30

Teplice (CZE) 45 / 20

Pisa (I) 38 /42

Athens (GR) 59 / 30

EU air quality standard 2005:

PM10 = 40 µg m-3 (annual mean)

50 µg m-3 (daily mean, 35 days/yr.)

2010: PM10 = 20 µg m-3

50 µg m-3 (daily mean, 7 days/yr.)

5

Schaap, 2003

Chin et al. [2000]

DIESEL

DOMESTIC COAL BURNING

BIOMASS BURNING

D.J. Jacob

6

•  Main pollutants of photochemical smog: –  Ozone

•  Respiratory problems (aggravation of asthma, reduced lung function, respiratory infections, inflammation).

•  Plant damage. –  PAN

•  Powerful lacrymator (causes tears) already at low levels. •  Respiratory damage at higher levels. •  Very phytotoxic (plant damage).

–  Particles and/or SO2 often also involved. •  Often, total oxidant (O3 + PAN) levels are used instead of O3 alone.

–  Adverse health effects for short-term exposure are observed around 150-200 ppb for e.g. asthmatics, 250-300 ppb more generally. WHO guidelines are < 76-100 ppb for 1 h exposure and < 50-60 ppb for 8-hour exposure.

–  These limits are often greatly exceeded in polluted urban areas!

A - tobacco

B - birch

Loreto et al., 2001

1 – ozone fumigation 2 – ozone and isoprene (VOC) 3 – before treatment

1

3

2

7

Information threshold human health risk

European Environment Agency, 2005

European Environment Agency, 2005

World’s record: Mexico City with a maximum of 441 ppbv (≅ 850 µg m-3)

8

European Environment Agency, 2005

Asia

Europe

•  Basically a ”grotesquely exaggerated form of the oxidation and transformation chemistry of the unperturbed troposphere” (Wayne).

•  Starts with NO and hydrocarbon emissions from e.g. traffic. •  Hydrocarbon oxidation (mainly by OH) produces peroxyradicals RO2

and HO2, also acylperoxyradicals RO.O2. •  NO reacts with these radicals to form NO2; NO2 photolysis leads to

ozone formation and reaction with RO.O2 leads to PAN formation. •  Typically NO and hydrocarbon concentrations build up during early-

morning rush hours, followed by NO2, O3 and PAN formation later in the day (when photolysing radiation is more intensive).

•  Alkenes and aldehydes more effective than alkanes in producing smog. –  Recall that RO.O2 is produced from the oxidation of aldehydes: RCHO + OH• RCO• + H2O RCO• + O2 RO.O2• RO.O2• + NO2 PAN

9

Blue haze is a bluish appearance similar to smog caused by newly formed particles, formed over forests.

F.W. Went (1960): … caused by terpenes released from the vegetation and ozone.

10

•  Main termination reactions for HOx radicals in the troposphere: OH + NO2 + M HNO3 + M (1) HO2 + HO2 H2O2 + O2 (2) RO2 + HO2 ROOH + O2 (3) •  Unless NOX is low, reaction 1 is the main termination step for radicals. •  In effect, NO2 and VOC compete for OH. Reaction 1 is about 5.5 times

faster than the average VOC + OH reaction ([VOC] given in ppmC). •  If [VOC]/[NO2] > 5.5, OH will react mainly with VOC.

–  The VOC + OH reaction chain generates more radicals than it consumes (e.g. due to photolysis of intermediate products). More radicals ⇒ more O3 formation. O3 production increases with [NO2] ⇒ ”NOx – limited” conditions.

•  If [VOC]/[NO2] < 5.5, OH will react mainly with NO2. –  NO2 thus acts to remove radicals, and an addition of NOX therefore

decreases radical concentrations and thus ozone formation. Thus, O3 production decreases with [NO2] ⇒ ”VOC – limited” conditions.

NOx- saturated/

Hydrocarbon limited

NOx-limited Ridge

D.J. Jacob

11

•  In general an increase in VOC will always lead to an increase in O3 (or have no effect).

•  However, an increase in NOX may increase or decrease O3 depending on the VOC concentration.

•  For each VOC concentration there exists an ”optimal” NOX concentration that leads to a maximum production of O3. –  This corresponds to the ridge in the ozone isopleth plots shown

earlier. –  The optimal ratio naturally depends on the precise VOC mixture

present (the value 5.5 given earlier is a rough average for typical urban air).

•  City centers and sites downwind of them tend to have low [VOC]/[NO2] ratios (due to high NOX emissions)

•  Rural environments usually have high [VOC]/[NO2] ratios because of the longer lifetimes of VOCs compared to NOX.

•  Next, letʼs look at a derivation of the ozone production rate as a function of [VOC] and [NOX] in a highly simplified system.

HOxfamily

OH

RO2 RO

HO2

HNO3 H2O2 O3

O3

O3 PHOx

4 5

6 7

8 9

RH

NO

O2

NO NO2

hν

RH or hydrocarbons = Volatile Organic Compounds (VOCs)

RO2 = peroxy radicals formed e.g. by a RH reaction with OH (- H), adding oxygen (+O2)

RO = alkoxy radical. from which a oxygen atom is abstracted by reaction (NO)

D.J. Jacob

R1: RO2• + NO → RO• + NO2 R2: NO2 + hν → NO + O(3P) R3: O(3P) + O2 → O3

12

•  Reactions in the VOC – NOx – HOx – O3 system: R1-R3 on previous slide (O3 formation from NO2). R4: RH + OH• (+ O2) RO2• + H2O R5: RO2• + NO RO• + NO2 R6: RO• + O2 RʼCHO + HO2• R7: HO2• + NO OH• + NO2 R8: HO2• + HO2• H2O2 + O2 R9: NO2 + OH• + M HNO3 + M •  In polluted air, cycling is fast, and we can assume that chain propagarion is

efficient: •  Rate(R4) =Rate(R5) = Rate(R6) = Rate(R7) •  Rate of O3 production P(O3) = k5[RO2][NO] + k7[HO2][NO] = 2k7[HO2][NO] •  Steady state for OH:

k4[RH][OH] = k7[HO2][NO] ⇒ [OH] = k7[HO2][NO]/k4[RH] •  Steady state for HOx family: source P(HOx) must equal sink:

P(HOX) = k8[HO2]2 + k9[NO2][OH][M]

Low NOx: k8[HO2]2 > k9[NO2][OH][M]; P(HOX) ≈ k8[HO2]2

High NOx: k8[HO2]2 << k9[NO2][OH][M]; P(HOX) ≈ k9[NO2][OH][M]

•  Collecting the results and expressing P(O3) in terms of VOC and/or NOX: P(O3) = 2k7[HO2][NO] [OH] = k7[HO2][NO]/k4[RH]

Low NOx: P(HOX) ≈ k8[HO2]2 ⇒ [HO2] ≈ (P(HOX)/k8)0.5

•  O3 production increases with increasing [NOX]

High NOx: P(HOX) ≈ k9[NO2][OH][M] ⇒ [OH] ≈ P(HOX)/k9[NO2][M] = k7[HO2][NO]/k4[RH] ⇒ [HO2] ≈ P(HOX)k4[RH]/k7k9[NO2][NO][M]

•  O3 production decreases with increasing [NOX]

( ) [ ]NOkP

kOP xHO ⋅

⋅=⇒

21

873 2

( )[ ]

[ ][ ]MNOkRHPk

OP xHO

29

43

2=⇒

13

NO NO2 HNO3 hv

HO2,RO2,O3 OH, O3

P(O3) L(NOx)

3 7 2 4

9 2 9 2

( ) 2 [ ][ ] 2 [ ]( ) [ ][ ] [ ]

OPE = x

P O k HO NO k RHL NO k NO OH k NO

= =

Emission Deposition

Assuming NOx steady state, efficient HOx cycling, and loss of NO2 by reaction with OH:

OPE as NOx strong nonlinearity$

Define ozone production efficiency (OPE) as the total number of O3 molecules produced per unit NOx emitted.

D.J. Jacob

Ozone production efficiency

•  Necessary factors for the formation of photochemical smog:

1.  NOx emissions (high NOx concentrations)

2.  VOC emissions (high hydrocarbon concentrations, anthropogenic (traffic, heating, industry) or biogenic (vegetation))

3.  Sunlight

Furthermore, any area surrounded by hills increases the probability of temperature inversions, which prevent dilution of the polluted air and enhances the occurrence of smog remarkably (L.A., London, Mexico city).

14

Lin et al. [2001] Jonson et al., 2005

Lin et al. [2001] Jonson et al., 2005

15

Jonson et al., 2005

D.J. Jacob

16

•  In the 1960-80s, legislation focused on reducing VOC emissions. E.g. VOC emissions decreased 12% in the US from 1980-1995, but NOx emissions stayed constant. –  Some control measures (e.g. ”lean-burn” in California, 1966)

actually increased NOx emissions while decreasing VOC. –  Less smog in city centers (VOC-limited) but more pollution

downwind! •  Newer atmospheric chemistry models indicate that most of the U.S.

(even urban regions) is actually NOx – limited, not VOC – limited. –  Older models underestimated automobile VOC emissions and also

did not account for natural VOC sources. –  More efficient NOx emission reductions needed to decrease ozone

pollution. –  Modern three-way catalytic converters are pretty good at removing

both VOC and NOx from vehicle exhaust but this requires very accurate control of the fuel:air ratio.