Abstract
A global three‐dimensional model of tropospheric chemistry is used to investigate the changes in tropospheric O3 and OH since preindustrial times as a result of fuel combustion and industry, biomass burning, and growth in atmospheric CH4. Model results indicate a 63% increase of the global tropospheric O3 burden from preindustrial times to present (80% and 50% in the northern and southern hemispheres, respectively). Anthropogenic emissions of NOx and of CO and hydrocarbons make comparable contributions to the global O3 increase (60% and 40% respectively), even though the local rate of tropospheric O3 production is generally NOχ limited. The rise in O3 production parallels closely the rise in the emissions of CO and hydrocarbon because the O3 yield per mole of CO or hydrocarbon oxidized has remained constant at 0.7–0.8 mol/mol since preindustrial times. In contrast, the O3 production efficiency per mole of NOχ emitted has decreased globally by a factor of 2. We find a 9% decrease in the global mean OH concentration (mass‐weighted) since preindustrial times. A linear relationship is found in the model between the global mean OH concentration and the SN/SC3/2 ratio, where SN and SC are the sources of NOχ and of CO and hydrocarbons, respectively. The relative constancy of the global mean OH concentration since preindustrial times reflects the conservation of the SN/SC3/2 ratio despite large increases in both SN and SC. Comparisons of model results with reconstructed nineteenth century observations of O3 at continental sites indicate a systematic overestimate of about 5 ppbv. Correcting this overestimate would require either a large missing chemical sink for O3 or a downward revision of the natural NOχ source from lightning (3 Tg N yr−1 in our model). The nineteenth century observations of O3 over France show no vertical gradient between the boundary layer and the free troposphere, which is inconsistent with our current understanding of tropospheric O3. The model underestimates preindustrial CO concentrations derived from polar ice cores; these measurements are difficult to reconcile with any reasonable CO emission inventories.
Highlights
A numberof modelstudieshaveattemptedto describethePreindustriaml easurementosf 0 3 concentrationast surface changesof tropospheri0c3 andOH concentrationssincepresites in western Europe and South America indicate an industrialtimes: one-dimensionaml odelsbefore the early increaseof severalfold from preindustriatlimesto present 1990s [Thompson, 1992] and two- and three-dimensional [Volzand Kley, 1988;Anfossiet al., 1991; Sandroniet al., models more recently [Crutzen and Zimmermann, 1991; 1992;Marencoet al., 1994].Thisincreaseis likely drivenby anthropogeneicmissionosf NOx(NO+NO2),CO, andhydro-Thompsonet al., 1993; Martinerie et al, 1995; Lelieveld and VanDorland,1995;Kasibhatlaet al., 1996;Levyet al., 1997; carbonsfrom fossilfuel combustioanndbiomassburning
The lower estimateof largersourceof NOx from soils.Model resultsindicatethat chemical lossby Crutzen [1994] is due to a 40% higher 03
Anfossi emissionsin the nineteenthcenturythan at presentin the et al [1991] found in the nineteenthcenturyMoncalieri data southernhemisphereand comparableemissionsin the norththat 03 concentrationsassociatedwith downslopewindsfrom em hemisphereS. ucha scenariois inconsistenwt ith historical theAlpswere5-10 ppbvhigherthanaverage,suggest- trends in fossil fuel combustion and is inconsistent with ing an increaseof 0 3 concentrationswith altitudewhichis at the observedincreasein troposphericCO over Jungfraujoch odds with the Pic du Midi data
Summary
Preindustriaml easurementosf 0 3 concentrationast surface changesof tropospheri0c3 andOH concentrationssincepresites in western Europe and South America indicate an industrialtimes: one-dimensionaml odelsbefore the early increaseof severalfold from preindustriatlimesto present 1990s [Thompson, 1992] and two- and three-dimensional [Volzand Kley, 1988;Anfossiet al., 1991; Sandroniet al., models more recently [Crutzen and Zimmermann, 1991; 1992;Marencoet al., 1994].Thisincreaseis likely drivenby anthropogeneicmissionosf NOx(NO+NO2),CO, andhydro-. Increasingconcentrationosf 03, NOx,CO, andhydrocarbons modelsimulationssuggesatnincreasein global concentramayhavealsoinducedsignificancthangesin theabundancoef tionssincepreindustriatlimesrangingfrom 40% [Levyet al., tropospheriOcH, the mainatmospheriocxidant[Thompson, 1997; Roelofset al., 1997] to 70% [Lelieveldand VanDot-. Evaluatepdreviouslwy ithobservationfosr thepresenattmo- Naturael missionasrefromlightning(3.0Tg N yr-1of NOx), spher[eWangetal., 1998b]a, ndweextendin sectio4n this soils(4.7TgN yr- ofNOx),andvegetatio(n600TgC yr- of evaluationto reconstructeodbservationosf 03, CO, andOH isoprenaend15Tg C yr- of acetoneD).rydepositioinscomproxiefsromthenineteencthenturyIn. sectio5n,weinterpret putedwith a resistance-in-seriesscheme. 2. Model Description acetylnitrate(PAN), HNO3, C2H6,acetone,andH202 in different regions of the troposphere. Versus Present assumein our standardsimulationthat preindustriabl iomass Simulatedmeanconcentrationosf 03 andCO nearthe surburningemissionasre10%of thepresent-davyalueandretain facein Julyare shownin Figure for thepreindustriaalnd
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