The impact of biogenic carbon sources on aerosol absorption in Mexico City

Abstract

Abstract. In order to determine the wavelength dependence of fine (<1 micron) atmospheric aerosol absorption in the Mexico City area, the absorptionÅngstrom exponents (AAEs) were calculated from hourly averages of aerosol absorption measured at seven wavelengths (370, 450, 520, 590, 660, 880, and 950 nm) The AAE values determined in the afternoon were consistently higher than the corresponding morning values, suggesting the photochemical aging of the aerosols leading to the formation of more highly UV absorbing organic aerosol species in the afternoon.The AAE values were compared to stable and radiocarbon isotopic measurements of the 12-h aerosol samples to determine the sources of the aerosol carbon. The fraction of modern carbon (fM) in the aerosol samples, as determined from 14 C analysis, showed that an average of 70% of the carbonaceous aerosols in Mexico City were from modern biomass sources during both field campaigns. The 13 C/ 12 C ratios of

Spectral absorption of biomass burning aerosol determined from retrieved single scattering albedo during ARCTAS

“…Thus, the greater OS C in the 17 April plume is likely due to the oxidation of POA, either heterogeneously, or via evaporation-oxidation-recondensation. This is consistent with Marley et al (2009) who found increases in the AAE with age without corresponding SOA formation.…”

“…Enhanced spectral absorption on 17 April may be explained by changes to the OA in biomass burning plumes as they age, and/or possibly by differences in the original OA emissions. The oxidation of biomass burning primary organic aerosol (POA) may result in the formation of compounds with stronger spectral absorption signatures than the biomass burning POA emitted at the source, as has been observed in previous studies (Decesari et al, 2002;Marley et al, 2009). Additionally, secondary organic aerosol (SOA) generated in several laboratory experiments via both the light (photochemical) and dark (e.g., reaction with O 3 ) oxidation of gas-phase organics exhibited spectral absorption (Noziere et al, 2007;Bones et al, 2010;Zhong and Jang, 2011).…”

“…While aerosol absorption measurements at a few discrete UV wavelengths exist, high resolution absorption measurements below 400 nm are lacking. Recent studies have determined aerosol absorption and the AAE from aerosol single scattering albedo (SSA) which describes the contribution of scattering (b scat,λ ) to aerosol extinction (b ext,λ ) (Bergstrom et al, 2007;Marley et al, 2009;Russell et al, 2010). However, measurements of SSA have been largely limited to the visible due to few scattering and absorption measurements at UV wavelengths.…”

“…While BC is known to absorb strongly at all wavelengths, OA absorption is much more spectrally dependent, with greatest OA absorption at shorter (≤ 400 nm) wavelengths. This spectral dependence has been summarized by the absorption Angstrom exponent (AAE), the fit to an exponential relationship between the absorption coefficient (b abs,λ ) and wavelength (Kirchstetter et al, 2004;Lewis et al, 2008;Marley et al, 2009;Chen and Bond, 2010).…”

An overview of the MILAGRO 2006 Campaign: Mexico City emissions and their transport and transformation

“…Marley et al (2009a) report that 45-78% of the total carbon at T0 (TC=EC+OC) arises from modern sources. Aiken et al (2010) report TC modern fractions that range from an average of 46% during the high BB period to 30% during the low BB period.…”