QUESTIONS 1.If CO emission to the atmosphere were to double, would you expect CO concentrations to (a) double, (b) less than double, (c) more than double? 2.What fraction of NOx emissions are natural? 3.Peroxynitric acid (HNO 4 ) is produced in the atmosphere by addition of HO 2 to NO 2. In the atmosphere it may either (1) thermolyze or photolyze back to the original reactants, (2) react with OH, or (3) deposit to the Earth’s surface. What are the implications of each of these three sinks for tropospheric NO x and OH?

CHAPTER 12: OZONE AIR POLLUTION

SMOG (Sulfurous vs. Photochemical Pollution) Example: London (also Eastern US) smoke + fog SO 2 characterized by inversions, cool weather, coal burning Example: LA UV, hydrocarbons, NOx characterized by hot dry sunny weather, reduced visibility and high oxidant levels  We will be focusing on this kind!

GREAT LONDON SMOG OF 1952 Dec 5-9, 1952 Cold fog + (high sulfur) coal + diesel buses = “pea souper” Killed 4000 people (young & elderly), and over 8000 died subsequently Clean Air Acts of 1956 and 1968 and City of London Act of Smog = fog intensified by smoke (Henry Antoine Des Voeux, who first used it in 1905 to describe British urban areas) Monet painting showing Victorian smog (1900) [Baker and Thornes, 2006]

GOOD AND BAD OZONE Stratosphere: O 2 + UV sunlight = O + O O + O 2  O 3 Troposphere: CO + NOx + sunlight  O 3 VOCs

75 ppb (new standard, set in 2008) OZONE AND PARTICULATE MATTER (PM): THE TOP TWO AIR POLLUTANTS IN THE U.S. 15  g m -3 (annual), 35 (daily)

THE NEW OZONE STANDARD CAUSES MORE U.S. AREAS TO BE OUT OF COMPLIANCE

OZONE POLLUTION IN THE UNITED STATES 90 th percentiles of summer afternoon (1-5 pm) surface ozone concentrations (ppbv), Fiore et al. [1998]

OZONE/PM EXPOSURE DEGRADES HEALTH Unscheduled hospital visits for asthma medication increases with ozone [Environmental Working Group Report, 2005] In California alone… In 1997 EPA estimated that particulate air emissions cause 35,000 premature deaths each year.

OZONE IMPACTS ON VEGETATION SUM06 Index: ozone dose in excess of 60 ppb during growing season Documented negative effects 1995 data

TRANSPORT PATTERN ON HIGH-OZONE DAYS 90 th percentile O 3 concentrations for summers and mean resultant 850hPa winds on days when O 3 > 90 th percentile

NON-METHANE VOC EMISSIONS Vegetation ~ 600 Tg C yr -1 Isoprene, terpenes, oxygenates… Biomass burning ~ 50 Tg C yr -1 Alkenes, aromatics, oxygenates… ~ 200 Tg C yr -1 Alkanes, alkenes, aromatics… Industry Largest global flux is from isoprene ( Tg C yr -1 )

OZONE PRODUCTION IN TROPOSPHERE Photochemical oxidation of CO and volatile organic compounds (VOCs) catalyzed by hydrogen oxide radicals (HO x ) in the presence of nitrogen oxide radicals (NO x ) HO x = H + OH + HO 2 + RO + RO 2 NO x = NO + NO 2 Oxidation of CO: Oxidation of VOC: RO can also decompose or isomerize; range of carbonyl products Carbonyl products can react with OH to produce additional ozone, or photolyze to generate more HO x radicals (branching reaction) OH can also add to double bonds of unsaturated VOCs

OXIDATION OF HYDROCARBONS CONTRIBUTE TO OZONE FORMATION IN POLLUTED AIR Alkenes: OH oxidation adds to double bond (does not abstract H as with alkanes). With double bond, alkenes can also be oxidized by ozone Aromatics (with benzene rings): reactive with OH, via either addition or abstraction  source of secondary organic aerosol (SOA) RH RO 2 ROOH RO R’CHO OR R’C(O)R” NO HO 2 O 2 isom decomp hv R O2O2 alkyl radical alkylperoxy radical alkoxy radical OH aldehyde ketone Additional oxidation by NO 3 (but only at night!) > C4: RONO 2 NO < C4: alkyl nitrates NO 2 HO 2 Generic Alkane OH Oxidation Scheme (no longer just CO and CH 4 !) Can photolyze to produce HOx or react with OH to continue chain… NO 2 RO 2 NO 2 org nitrates

General rules for atmospheric oxidation of hydrocarbons Attack by OH is by H abstraction for saturated HCs (alkanes), by addition for unsaturated HCs (alkenes) Reactivity increases with number of C-H bonds, number of unsaturated bonds Organic radicals other than peroxy react with O 2 (if they are small) or decompose (if they are large); O 2 addition produces peroxy radicals. Organic peroxy radicals (RO 2 ) react with NO and HO 2 (dominant), other RO 2 (minor); they also react with NO 2 but the products decompose rapidly (except in the case of peroxyacyl radicals which produce peroxyacylnitrates or PANs) RO 2 +HO 2 produces organic hydroperoxides ROOH, RO 2 +NO produces carbonyls (aldehydes RCHO and ketones RC(O)R’) Carbonyls and hydroperoxides can photolyze (radical source) as well as react with OH Unsaturated HCs can also react with ozone, producing carbonyls and carboxylic acids RO 2 +R’O 2 reactions produce a range of oxygenated organic compounds including carbonyls, carboxylic acids, alcohols, esters…

OZONE PRODUCTION: BASIC CHAIN MECHANISM O3O3 OHHO 2 h, H 2 O NO H2O2H2O2 CO, CH 4, RH NO 2 h HNO 3 OH, M VOC limited NOx limited CO, HC, NOx

DEPENDENCE OF OZONE PRODUCTION ON NO x AND HYDROCARBONS HO x family OH RO 2 RO HO 2 HNO 3 H2O2H2O2 O3O3 O3O3 O3O3 P HOx “NO x - saturated” or “hydrocarbon-limited” regime “NO x -limited” regime RH NO O2O2 NO 2, M NET: RH + 4O 2  R’CHO + 2O 3 + H 2 O

OZONE CONCENTRATIONS vs. NO x AND VOC EMISSIONS Air pollution model calculation for a typical urban airshed NO x -saturated (or HC limited) NO x -limitedRidge