Abstract
We present the evolution of multispectral optical properties through urban aerosols that have aged and interacted with biogenic emissions, resulting in stronger short wavelength absorption and the formation of moderately brown secondary organic aerosols. Ground-based aerosol measurements were made in June 2010 within the Sacramento urban area (site T0) and at a 40-km downwind location (site T1) in the forested Sierra Nevada foothills area. Data on black carbon (BC) and non-refractory aerosol mass and composition were collected at both sites. In addition, photoacoustic (PA) instruments with integrating nephelometers were used to measure spectral absorption and scattering coefficients for wavelengths ranging from 355 to 870 nm. The daytime absorption Ångström exponent (AAE) indicated a modest wavelength-dependent enhancement of absorption at both sites throughout the study. From 22 to 28 June 2010, secondary organic aerosol mass increased significantly at both sites, which was due to increased biogenic emissions coupled with intense photochemical activity and air mass recirculation in the area. During this period, the median BC mass-normalized absorption cross-section (MAC) values for 405 nm and 532 nm at T1 increased by ~23% and ~35%, respectively, compared with the relatively less aged urban emissions at the T0 site. In contrast, the average MAC values for the 870 nm wavelength were similar for both sites. These results suggest the formation of moderately brown secondary organic aerosols in biogenically-influenced urban air.
Highlights
Atmospheric aerosols impact the Earth’s radiation budget by scattering and absorbing solar radiation, which is referred to as the aerosol direct effect [1,2]
During the same period, βsca and βabs at 870 nm increased by ~5% and ~11%, respectively. These results suggest that the SOA formed from the oxidation of mixed biogenic and anthropogenic emissions in the forested region developed a modest apparent or intrinsic absorption at 355 nm, which is much less than the strong intrinsic SOA ultraviolet (UV) absorption observed for biomass burning aerosols [18]
As part of the CARES field campaign carried out in June 2010 in Sacramento Valley, California, the evolution of multispectral aerosol optical properties was characterized for the Sacramento urban area and a 40-km downwind location in the forested Sierra Nevada foothills area, as the urban aerosols aged and interacted with biogenic emissions
Summary
Atmospheric aerosols impact the Earth’s radiation budget by scattering and absorbing solar radiation, which is referred to as the aerosol direct effect [1,2] This can lead to heating or cooling, depending on the ratio of aerosol scattering and extinction coefficients (single scattering albedo, or SSA), the particle scattering asymmetry parameter, and the albedo of the underlying surface [3,4,5,6]. OA constitutes a large fraction of the atmospheric aerosol mass; typical measurements range from ~20% to 90% of the submicron-diameter aerosol mass [22] These aerosols scatter light efficiently because their size is comparable to the wavelength of the visible solar radiation, and typical values of the real part of their refractive index are fairly high. The role of OA on the Earth’s radiation budget is still poorly understood [23]
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