Abstract
The NASA Global Tropospheric Experiment (GTE) Transport and Atmospheric Chemistry Near the Equator‐Atlantic (TRACE A) expedition was conducted September 21 through October 26, 1992, to investigate factors responsible for creating the seasonal South Atlantic tropospheric ozone maximum. During these flights, fine aerosol (0.1–3.0 μm) number densities were observed to be enhanced roughly tenfold over remote regions of the tropical South Atlantic and greater over adjacent continental areas, relative to northern hemisphere observations and to measurements recorded in the same area during the wet season. Chemical and meteorological analyses as well as visual observations indicate that the primary source of these enhancements was biomass burning occurring within grassland regions of north central Brazil and southeastern Africa. These fires exhibited fine aerosol (N) emission ratios relative to CO (dN/dCO) of 22.5 ± 9.7 and 23.6 ± 15.1 cm−3parts per billion by volume (ppbv)−1over Brazil and Africa, respectively. Convection coupled with counterclockwise flow around the South Atlantic subtropical anticyclone subsequently distributed these aerosols throughout the remote South Atlantic troposphere. We calculate that dilute smoke from biomass burning produced an average tenfold enhancement in optical depth over the continental regions as well as a 50% increase in this parameter over the middle South Atlantic Ocean; these changes correspond to an estimated net cooling of up to 25 W m−2and 2.4 W m−2during clear‐sky conditions over savannas and ocean respectively. Over the ocean our analyses suggest that modification of CCN concentrations within the persistent eastern Atlantic marine stratocumulus clouds by entrainment of subsiding haze layers could significantly increase cloud albedo resulting in an additional surface radiative cooling potentially greater in magnitude than that caused by direct extinction of solar radiation by the aerosol particles themselves.
Highlights
Estimatendetcoolingofupto25W m-2 and2.4W m-2 duringclear-skcyonditionosver savannasandoceanrespectively.Over theoceanouranalysessuggesthatmodificationof CCN concentrationwsithinthepersistenetasternAtlanticmarinestratocumulucsloudsby entrainment of subsidinghazelayerscouldsignificantlyincreasecloudalbedoresultingin an additional surfaceradiativecoolingpotentiallygreaterin magnitudethanthatcausedby directextinctionof solarradiationby theaerosopl articlesthemselves
The imagi- comparisonof Figure 2 data suggeststhat valuesat altitudes nary term,ni, reportedlyvariesfrom 0.01 to 0.04 [Li and Mao, above the marine boundary layer were at least an order of 1990;Lenoble1, 991;WestphaalndToon1, 991].Wehave,based magnitudegreateroverthesouthernregions.Theseloadings, on thereportedfractionof elementacl arbonin tropicalbiomass which are alsoapproximatelya factorof 10 higherthanseen burningaerosol[sAndreaet al., 1988],chosentouse0.03in our duringthewet seasonovercoastaal ndinteriorBrazil [Gregory calculationT. hisgivesa single-scattaelrbedoO, J('afiwreh, ere:a etal., 1990], are consistenwt ith previousSouthAtlanticdry is theaerosolabsorptionandXeis theaerosolextinction),of about season observations [Anderson et al, 1993a; Andreae et al, 0.83
Smolderingburn tionsovertheeasternSouthAtlanticbasin,applicationof theRH scarsobservedover Brazil were, in general,widely distributed correctionmadelittle differenceto thecalculatedopticalproper- and relatively small in aerial extent,whereas tiesof the African aerosolbut, dueto the greaterprevalenceof over Africa, fire lineshundredsof meterslong attachedto large wet convectionandmoistureassociatedwith the endingof the scars(manysquarekilometers)weregreatlyin evidence.In both dry seasonin the westerntropics,increasedcolumn-integrated regions the whitish gray smokefrom the fires producedwideextinctionbsy upto40% for someof theBraziliancases
Summary
Aerosolsfrom biomassburning over the tropical South Atlantic region: Distributions and impacts.
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