1. Thanks to Colette Heald for many of the slides in this lecture
ATS 621 Fall 2012
Lecture 15
Thanks to Colette Heald for many of the slides in this lecture

2. A “typical” smoggy day

5. OZONE PRODUCTION IN TROPOSPHERE
Photochemical oxidation of CO and volatile organic compounds (VOCs)
catalyzed by hydrogen oxide radicals (HOx)
in the presence of nitrogen oxide radicals (NOx)
HOx = H + OH + HO2 + RO + RO2
NOx = NO + NO2
OH can also add to double bonds of unsaturated VOCs
Oxidation of VOC:
Oxidation of CO:
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 HOx radicals (branching reaction)

6. OXIDATION OF HYDROCARBONS CONTRIBUTE TO OZONE FORMATION IN POLLUTED AIR
Generic Alkane OH Oxidation Scheme (no longer just CO and CH4!)
org nitrates
alkyl nitrates
RONO2
RO2NO2 RH
RO2
ROOH RO
R’CHO OR
R’C(O)R” NO
HO2 O2
isom
decomp hv R
alkyl
radical
alkylperoxy
alkoxy OH
aldehyde
ketone
HO2
> C4:
NO2
NO2
< C4: NO
Can photolyze to produce HOx or react with OH to continue chain…
Additional oxidation by NO3 (but only at night!)
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)

7. 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 O2 (if they are small) or decompose (if they are large); O2 addition produces peroxy radicals.
Organic peroxy radicals (RO2) react with NO and HO2 (dominant), other RO2 (minor); they also react with NO2 but the products decompose rapidly (except in the case of peroxyacyl radicals which produce peroxyacylnitrates or PANs)
RO2+HO2 produces organic hydroperoxides ROOH, RO2+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
RO2+R’O2 reactions produce a range of oxygenated organic compounds including carbonyls, carboxylic acids, alcohols, esters…

8. OZONE PRODUCTION: BASIC CHAIN MECHANISM
OH, M
VOC limited hn
HNO3
NO2 NO O3
hn, H2O
NOx limited OH
HO2
H2O2
CO, CH4, RH
CO, HC, NOx

9. DEPENDENCE OF OZONE PRODUCTION ON NOx AND HYDROCARBONS
NET: RH + 4O2  R’CHO + 2O3 + H2O
Argue that chain propagation is fast, so R4=R5=R6=R7
PO3 = R5+R7 = 2R7
PHOx = R8+R9 O3
HOxfamily NO
RO2 RO 5 RH O2 4 6
PHOx 7 O3 OH
HO2 NO
NO2, M 9 8
Low VOC: R9 major loss
Low NOx: R8 major loss
HNO3 O3
H2O2
“NOx- saturated” or
“hydrocarbon-limited” regime
“NOx-limited” regime

10. Examine the limiting regimes:
High VOC, low NOx: peroxide formation is a major radical sink
Notice that the production rate of O3 scales directly with the NOx level
Also there is NO explicit dependence on the concentration of VOCs (“RH”)
Little dependence on speciation of VOCs, either
Low VOC, high NOx: formation of HNO3 is a major radical sink
Notice that the production rate of O3 scales directly with the VOC level
BUT the production rate of O3 scales INVERSELY with the NOx level!
Less NOx  more O3 ?!
In this regime, there is so much NOx that NO2 effectively competes with VOCs for OH; the HOX radical pool cannot build up. When the NOx is reduced, the radical pool increases and more O3 is formed.
“NOx-limited” regime
“NOx- saturated” or
“hydrocarbon-limited” regime

11. OZONE CONCENTRATIONS vs
OZONE CONCENTRATIONS vs. NOx AND VOC EMISSIONS Air pollution model calculation for a typical urban airshed
NOx-limited
Ridge
NOx-saturated
(or HC limited)

12. EKMA Diagram (Empirical Kinetic Modeling Approach)
Free radicals scavenged by NOx
urban?
O3 consumed by alkenes;
NO2 removed by radical reactions; radical-radical termination important
rural?

15. LATEST INVENTORIES OF BIOGENIC vs. ANTHROPOGENIC VOCs
…notice difference in scale!
isoprene
pinene
Millet et al. [2007]

16. BTEX
BTEX is an acronym for benzene, toluene, ethylbenzene, and xylene. This group of volatile organic compounds (VOCs) is found in petroleum hydrocarbons, such as gasoline, and other common environmental contaminants.
BTEX have in recent years attracted much attention, since they constitute one of the most common and serious threats to groundwater reservoirs and indoor climate deriving from contaminated sites. This is mainly due to the potential effects of benzene, which is considered a strong carcinogen, and which is highly mobile in the soil and groundwater environment, which is also the case for the other BTEX.
They are found in numerous sites, including areas used for fuel operations, refineries, gasoline stations, and gasification sites.

17. Incremental reactivity

19. Have prior control strategies been effective?

20. Have prior control strategies been effective?
isoprene
atmospheric lifetime less than 1 hour
Inventories now show that isoprene emission in the United States is larger than the sum of all anthropogenic hydrocarbon emissions; even without anthropogenic hydrocarbons, isoprene emission would be sufficient to make O3 production NOx-limited everywhere in the United States except in large urban centers.

21. LARGE SUPPLY OF BIOGENIC VOCs – unrecognized until the 1990s
Switches polluted areas in U.S. from NOx-saturated to NOx-limited regime!
recognized in Revised Clean Air Act of 1999
Anthropogenic VOCs
Isoprene (biogenic VOC)
Jacob et al., [1993]
Isoprene (C5H8) and monoterpenes (C10H16) are oxidized by OH, O3 and NO3 (generally analogous to alkene rxn)  secondary organic aerosol (SOA)

22. U.S. EMISSIONS OF OZONE PRECURSORS and trends over past 20 years
Fuel combustion
vehicles
power plants
Vehicles
Fires
Down 40%
Flat/down
Anthropogenic VOCs
Isoprene (biogenic VOC)
Fuel combustion & transport
Solvents
Vegetation
Flat
Down 30%

23. Present-day ozone background at northern midlatitudes
HEMISPHERIC OZONE POLLUTION: IMPLICATIONS OF ENHANCED OZONE BACKGROUND FOR MEETING AIR QUALITY STANDARDS (AQS)
Europe AQS
(8-h avg.)
Europe AQS
(seasonal)
U.S. AQS
(8-h avg.)
U.S. AQS
(1-h avg.)
Was here until 2008!
ppbv
Preindustrial
ozone
background
Present-day ozone background at northern midlatitudes

24. Extras

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

26. EFFECT OF CLIMATE CHANGE ON OZONE AIR QUALITY
Probability of max 8-h O3 > 84 ppbv
vs. daily max. T
Ozone exceedances of 90 ppbv,
summer 2003
Lin et al. [Atm. Env ]
Correlation of high ozone with temperature is driven by
stagnation, (2) biogenic hydrocarbon emissions, (3) chemistry

27. EFFECT OF CLIMATE CHANGE ON REGIONAL STAGNATION
GISS GCM simulations for 2050 vs. present-day climate using pollution tracers with constant emissions
weather map illustrating
cyclonic ventilation of the eastern U.S.
summer
Pollution episodes double in duration in 2050 due to decreasing frequency of cyclones ventilating the eastern U.S; expected result of greenhouse warming.
Mickley et al. [2004]