l.a. photo smog london smog at daylight - ?· the london smog in 1952 • 1)london ... letʼs look...

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    L.A. photo smog

    London smog at daylight

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    smog: 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.

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    SO2 in otherwise clean air causes health problems for > 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.

    Freshurban

    Agedurban

    rural

    remoteWarneck [1999]

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

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

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

    D.J. Jacob

    PM2.5 (aerosol particles < 2.5 m diameter)

    Site PM 2.5 [g m-3] Urban / rural

    Ume (S) 8 / 7

    Malm (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 standard2005:

    PM10 = 40 g m-3 (annual mean)

    50 g m-3 (daily mean, 35 days/yr.)

    2010:PM10 = 20 g m-3

    50 g m-3 (daily mean, 7 days/yr.)

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    Schaap, 2003

    Chin et al. [2000]

    DIESEL

    DOMESTICCOAL BURNING

    BIOMASSBURNING

    D.J. Jacob

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    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

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    Information threshold human health risk

    European Environment Agency, 2005

    European Environment Agency, 2005

    Worlds record: Mexico City with a maximum of 441 ppbv ( 850 g m-3)

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    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 + H2ORCO + O2 RO.O2RO.O2 + NO2 PAN

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    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.

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    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

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    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, lets 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 H2O2O3

    O3

    O3PHOx

    45

    67

    89

    RH

    NO

    O2

    NONO2

    h

    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 + h NO + O(3P) R3: O(3P) + O2 O3

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    Reactions in the VOC NOx HOx O3 system:R1-R3 on previous slide (O3 formation from NO2).R4: RH + OH (+ O2) RO2 + H2OR5: RO2 + NO RO + NO2R6: RO + O2 RCHO + HO2R7: HO2 + NO OH + NO2R8: HO2 + HO2 H2O2 + O2R9: 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

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    NO NO2 HNO3hv

    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).

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