Presentation on theme: "Atmospheric chemistry"— Presentation transcript:
1Atmospheric chemistry Lecture 3:Tropospheric Oxidation ChemistryDr. David GlowackiUniversity of Bristol,UK
2Yesterday… Today… We discussed photochemistry and kinetics The earth’s atmosphere is a huge low temperature chemical reactor with variable temperature, pressure, and actinic fluxAll of these variables affect the rates of individual chemical reactionsToday…Atmospheric chemistry is largely driven by free radical chain reactionsWe will discuss some of the important individual chemical reactions that are important in the troposphere
3Why is atmospheric chemistry important? Human activity is changing the composition of the atmosphereRegulatory policy requires an understanding of pollutant impactAtmospheric pollutants impact living organismsHealthVegetation (e.g., farming) & animalsClimate changeAtmospheric pollutants & their subsequent chemistry are responsible for:Acid rainPhotochemical smog (e.g., arctic haze)Vegetation & animalsOzone hole
4Atmospheric chemistry and Climate Change Atmospheric chemistry plays an important role in radiative forcing processesSource:IPCC 4th assessment
5Tropospheric Oxidation Starts with OH O3 + h O1D + O2O1D + M O1D + MO1D + H2O 2OHDegradation of atmospheric pollutants starts with the OH radicalOH is often called ‘the detergent of the atmosphere’OH is very reactive because it has an unpaired electron:O-HMeasuring OH is hard! There’s not much of it, and it doesn’t live for longTropospheric oxidation results in ground level O3, which is a greenhouse gas harmful to healthFAGE OH detection instrument in Halley Base, AntarcticaSee:
44General VOC oxidation scheme O3 + h O1D + O2O1D + H2O 2OHOH + RH (+O2) RO2 + H2ORO2 + NO NO2 + RORO + O2 HO2 +R’CHOHO2 + NO OH + NO2NO2 + h NO + O; O + O2 O3OVERALLNOx + VOC + sunlight ozoneThe same reactions can also lead to formation of secondary organic aerosol (SOA)
45PO3 [NO] & independent of [RH] OZONE CONCENTRATIONS vs. NOx AND VOC EMISSIONS Air pollution model calculation for a typical urban airshedNOx limitedPO3 [NO] & independent of [RH]VOC limitedPO3 [NO2]-1; PO3 [RH]
46Polluters: Mobile Transportation: Generates NOx and VOC. Reductions focus on catalytic converters and fuel additives as well as congestion abatement strategiesStationary 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
49NOx sinks & transport NOx lifetime ~1 day NOx sinks – primarily HNO3 HNO3 is water solublePAN allows locally produced NOx to be transported on global scales
50Other oxidizing species NO3NO2 + O3 NO3 + O2NO3 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.O3Ozone reacts with alkenes to form a 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.
51VOC removal by reaction with OH VOC Lifetime with respect to OH:Atmospheric distribution depends on lifetime. The Northern Hemisphere (NH) is a major source of anthropogenic pollutants. CH4 is distributed globally with a slight NH/SH difference. Isoprene is found only close to its sources.The oxidising capacity of the atmosphere refers to its capacity to remove VOCs and depends on [OH] (and the concentrations of other oxidants like O3 and NO3OH + CH × 10-3OH + CO × 10-1OH + isoprene 1.1 × 102OH + ethane 2.4 × 10-1k(298K) in units of10-12 cm3 molecule-1 s-1
52HCHO + OH (+O2) HO2 + CO + H2O CH4 Oxidation SchemeCH OH (+O2) CH3O H2OCH3O NO CH3O NO2CH3O O2 HO HCHOHO NO OH NO2HCHO OH (+O2) HO CO H2OHCHO hn H COHCHO hn (+2O2) 2HO CONote:2 × (NO NO2) conversionsHCHO formation provides a route to HO2 radical formation.
53Global budget for methane (Tg CH4 yr-1) Sources:Natural 160Anthropogenic 375Total 535Natural Sources:wetlands, termites, oceans…Anthropogenic Sources:natural gas, coal mines, enteric fermentation, rice paddiesSinks:Trop. oxidation 445by OHTransfer to 40stratosphereUptake by soils 30Total 515Notes:The rate of oxidation is k5[CH4][OH], where the concentrationsare averaged over the trop.2. Concentrations of CH4 have increased from 800 to 1700 ppb since pre-industrial times3. Methane is a greenhouse gas.
54HISTORICAL TRENDS IN METHANE Historical methane trendRecent methane trendRecent measurements at MaceHead in W Ireland.1mg m-3 = 0.65 ppbNB – seasonal variation –higher in winter
55GLOBAL DISTRIBUTION OF METHANE NOAA/CMDL surface air measurements Seasonal dependence – higher in winter than summer (maximum in NH correlates with minimum in SH).NH concentrations > SH – main sources are in SH; slow transport across the intertropical conversion zone
57Can we model oxidation results of other VOCs? …The MCM Constructed by University of Leeds, in collaboration with Imperial College and UK Met OfficeExplicit mechanism, based on a protocol which describes the chemistry. Includes reactions of OH, NO3 and O3 and photolysis. For development protocol see: M.E.Jenkin et al. Atmos. Env., 1997, 31, 81.Describes the oxidation of 123 VOCs, based on the UK emissions inventory.It can be accessed via the web:The MCM is used by the UK Department of the Energy and Climate Change (DECC) to help develop its air quality strategy.