Estimation of the mass absorption cross section of the organic carbon component of aerosols in the Mexico City Metropolitan Area

[摘要] Data taken from the MCMA-2003 and the 2006 MILAGRO field campaigns are usedto examine the absorption of solar radiation by the organic component ofaerosols. Using irradiance data from a Multi-Filter Rotating ShadowbandRadiometer (MFRSR) and an actinic flux spectroradiometer (SR), we deriveaerosol single scattering albedo,ϖ_{0, λ}, as a function ofwavelength, λ. We find that in the near-UV spectral range (250 to400 nm) ϖ_{0, λ} is much lower compared to ϖ_{0, λ} at 500 nm indicating enhanced absorption in the near-UV range.Absorption by elemental carbon, dust, or gas cannot account for this enhancedabsorption leaving the organic carbon component of the aerosol (OA) as themost likely absorber. We use data from a surface deployed Aerodyne AerosolMass Spectrometer (AMS) along with the inferred ϖ_{0, λ} toestimate the Mass Absorption Cross section (MAC) for the organic aerosol. Wefind that the MAC is about 10.5 m²/g at 300 nm and falls close to zeroat about 500 nm; values that are roughly consistent with other estimates oforganic aerosol MAC. These MAC values can be considered as "radiativelycorrect", because when used in radiative transfer calculations, thecalculated irradiances/actinic fluxes match those measured at the wavelengthsconsidered here. For an illustrative case study described here, we estimatethat the light absorption by the "brown" (organic) carbonaceous aerosol canadd about 40% to the light absorption of black carbon in Mexico City. Thiscontribution will vary depending on the relative abundance of organic aerosolrelative to black carbon. Furthermore, our analysis indicates that organicaerosol would slow down photochemistry by selectively scavenging the lightreaching the ground at those wavelengths that drive photochemicalreactions. Finally, satellite retrievals of trace gases that are usedto infer emissions currently assume that the MAC of organic carbon is zero.For trace gases that are retrieved using wavelengths shorter then 420 nm(i.e. SO₂, HCHO, halogenoxides, NO₂), the assumption ofnon-zero MAC values will induce an upward correction to the inferredemissions. This assumption will be particularly relevant in polluted urbanatmospheres and areas of biomass burning where organic aerosols areparticularly abundant.

[学科分类] 大气科学