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Atmospheric chemistry. Lecture 3: Tropospheric Oxidation Chemistry. Dr. David Glowacki University of Bristol,UK david.r.glowacki@bristol.ac.uk. Yesterday…. We discussed photochemistry and kinetics
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Atmospheric chemistry Lecture 3: Tropospheric Oxidation Chemistry Dr. David Glowacki University of Bristol,UK david.r.glowacki@bristol.ac.uk
Yesterday… • We discussed photochemistry and kinetics • The earth’s atmosphere is a huge low temperature chemical reactor with variable temperature, pressure, and actinic flux • All of these variables affect the rates of individual chemical reactions Today… • Atmospheric chemistry is largely driven by free radical chain reactions • We will discuss some of the important individual chemical reactions that are important in the troposphere
Why is atmospheric chemistry important? • Human activity is changing the composition of the atmosphere • Regulatory policy requires an understanding of pollutant impact • Atmospheric pollutants impact living organisms • Health • Vegetation (e.g., farming) & animals • Climate change • Atmospheric pollutants & their subsequent chemistry are responsible for: • Acid rain • Photochemical smog (e.g., arctic haze) • Vegetation & animals • Ozone hole
Atmospheric chemistry and Climate Change • Atmospheric chemistry plays an important role in radiative forcing processes Source: IPCC 4th assessment
Tropospheric Oxidation Starts with OH O3 + h O1D + O2 O1D + M O1D + M O1D + H2O 2OH • Degradation of atmospheric pollutants starts with the OH radical • OH is often called ‘the detergent of the atmosphere’ • OH is very reactive because it has an unpaired electron: O-H • Measuring OH is hard! There’s not much of it, and it doesn’t live for long • Tropospheric oxidation results in ground level O3, which is a greenhouse gas harmful to health FAGE OH detection instrument in Halley Base, Antarctica See: http://www.atmos.bham.ac.uk/chablis.htm
O3 Photolysis makes OH O3 + hn g O2 + O(1D)
OH sinks OH Sinks: oxidation of reduced species CO + OH g CO2 + H CH4 + OH g CH3 + H2O HCFC + OH g H2O + … Major OH sinks GLOBAL MEAN [OH] ~ 1.0x106 molecules cm-3
High NOx Initiation O3 sunlight NO2 NO OH HO2 VOC RO2 RO NO NO2
High NOx Initiation NO2 NO OH HO2 VOC RO2 RO NO NO2
High NOx Initiation NO2 NO OH HO2 VOC RO2 RO O2 NO NO2
High NOx Propagation NO2 NO OH HO2 VOC RO2 RO NO NO2
High NOx Ozone Formation NO2 NO OH HO2 VOC RO2 RO NO NO2 O2 O3 sunlight
High NOx Propagation NO2 NO OH HO2 oxidation product VOC RO2 RO O2 NO NO2 O3
High NOx Propagation NO2 NO OH HO2 oxidation product VOC RO2 RO NO NO2 O3
High NOx Ozone Formation O3 sunlight O2 NO2 NO OH HO2 oxidation product VOC RO2 RO NO NO2 O3
High NOx O3 NO2 NO OH HO2 oxidation product VOC RO2 RO NO NO2 O3
High NOx Run Cycle O3 NO2 NO OH HO2 oxidation product VOC RO2 RO NO NO2
High NOx O3 sunlight NO2 NO OH HO2 oxidation product VOC RO2 RO NO NO2
High NOx NO2 NO OH HO2 oxidation product VOC RO2 RO NO NO2
High NOx NO2 NO OH HO2 oxidation product VOC RO2 RO O2 NO NO2
High NOx NO2 NO OH HO2 oxidation product VOC RO2 RO NO NO2
High NOx NO2 NO OH HO2 oxidation product VOC RO2 RO NO NO2 O2 O3 sunlight
High NOx NO2 NO OH HO2 oxidation product VOC RO2 RO O2 NO NO2 O3
High NOx NO2 NO OH HO2 oxidation product VOC RO2 RO NO NO2 O3
High NOx O3 sunlight O2 NO2 NO OH HO2 oxidation product VOC RO2 RO NO NO2 O3
High NOx O3 NO2 NO OH HO2 oxidation product VOC RO2 RO O2 NO NO2 O3
High NOx O3 NO2 NO OH HO2 oxidation product VOC RO2 RO NO NO2 O3
High NOx O3 NO2 NO OH HO2 oxidation product VOC RO2 RO NO NO2 O2 O3 sunlight O3
High NOx O3 NO2 NO OH HO2 oxidation product VOC RO2 RO O2 NO NO2 O3 O3
High NOx O3 NO2 NO OH HO2 oxidation product VOC RO2 RO NO NO2 O3 O3
High NOx O3 sunlight O3 O2 NO2 NO OH HO2 oxidation product VOC RO2 RO NO NO2 O3 O3
High NOx O3 O3 NO2 NO OH HO2 oxidation product VOC RO2 RO O2 NO NO2 O3 O3
High NOx O3 O3 NO2 NO OH HO2 oxidation product VOC RO2 RO NO NO2 O3 O3
High NOx O3 O3 NO2 NO OH HO2 oxidation product VOC RO2 RO NO NO2 O2 O3 sunlight O3 O3
High NOx O3 O3 NO2 NO OH HO2 oxidation product VOC RO2 RO O2 NO NO2 O3 O3 O3 O3
High NOx O3 O3 NO2 NO OH HO2 oxidation product VOC RO2 RO NO NO2 O3 O3 O3 O3
High NOx O3 O3 sunlight O3 O2 NO2 NO OH HO2 oxidation product VOC RO2 RO NO NO2 O3 O3 O3 O3
High NOx O3 O3 O3 NO2 NO OH HO2 oxidation product VOC RO2 RO O2 NO NO2 O3 O3 O3
High NOx Ozone Production O3 O3 O3 NO2 NO OH HO2 oxidation product VOC RO2 RO NO NO2 O3 O3 O3
Chemistry of ozone formation O3 sunlight O2 sunlight NO2 NO OH HO2 oxidation product VOC RO2 RO O2 O2 NO NO2 O2 O3 sunlight
Low NOx Initiation O3 O3 O3 sunlight OH
Low NOx Initiation O3 O3 OH VOC RO2
Low NOx Termination O3 O3 OH VOC RO2 HO2 ROOH
General VOC oxidation scheme • O3 + h O1D + O2 • O1D + H2O 2OH • OH+ RH (+O2) RO2+ H2O • RO2+ NO NO2+ RO • RO + O2 HO2+R’CHO • HO2+ NO OH +NO2 • NO2+h NO + O; O + O2 O3 • OVERALL • NOx + VOC + sunlight ozone • The same reactions can also lead to formation of secondary organic aerosol (SOA)
OZONE CONCENTRATIONS vs. NOx AND VOC EMISSIONSAir pollution model calculation for a typical urban airshed NOx limited PO3 [NO] & independent of [RH] VOC limited PO3 [NO2]-1; PO3 [RH]
Polluters: Mobile Transportation: Generates NOx and VOC. Reductions focus on catalytic converters and fuel additives as well as congestion abatement strategies Stationary industrial sources of VOC and NOx: Reductions involve scrubbing of pollutants from chimney stacks. Biogenic Emissions: Generate VOCs, no feasible reduction strategy, Can propose urban landscapes that reduce emissions
NOx sinks & transport • NOx lifetime ~1 day • NOx sinks – primarily HNO3 • HNO3 is water soluble • PAN allows locally produced NOx to be transported on global scales
Other oxidizing species NO3 NO2 + O3 NO3 + O2 NO3 is rapidly lost in the day by photolysis and reaction with NO ( NO2), so that its daytime concentration is low. It is an important night time oxidant. It adds to alkenes to form nitroalkyl radicals which form peroxy radicals in the usual way. O3 Ozone reactswith alkenes to forma carbonyl + an energised Criegee biradical. The latter can be stabilised or decompose. One important reaction product is OH: O3 reactions with alkenes can act as a source of OH, even at night.