Spectra Aerosol Light Scattering and Absorption for Laboratory and Urban Aerosol
AuthorGyawali, Madhu Sudhan
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Atmospheric aerosols considerably influence the climate, reduce visibility, and cause problems in human health. Aerosol light absorption and scattering are the important factors in the radiation transfer models. However, these properties are associated with large uncertainties in climate modeling. In addition, atmospheric aerosols widely vary in composition and size; their optical properties are highly wavelength dependent. This work presents the spectral dependence of aerosol light absorption and scattering throughout the ultraviolet to near-infrared regions. Data were collected in Reno, NV from 2008 to 2010. Also presented in this study are the aerosol optical and physical properties during carbonaceous aerosols and radiative effects study (CARES) conducted in Sacramento area during 2010. Measurements were made using photoacoustic instruments (PA), including a novel UV 355 nm PA of our design and manufacture. Comparative analyses are presented for three main categories: (1) aerosols produced by wildfires and traffic emissions, (2) laboratory-generated and wintertime ambient urban aerosols, and (3) urban plume and biogenic emissions. In these categories, key questions regarding the light absorption by secondary organic aerosols (SOA), so -called brown carbon (BrC), and black carbon (BC) will be discussed. An effort is made to model the emission and aging of urban and biomass burning aerosol by applying shell-core calculations. Multispectral PA measurements of aerosols light absorption and scattering coefficients were used to calculate the Ångström exponent of absorption (AEA) and single scattering albedo (SSA). The AEA and SSA values were analyzed to differentiate the aerosol sources. The California wildfire aerosols exhibited strong wavelength dependence of aerosol light absorption with AEA as for 405 and 870 nm, in contrast to the relatively weak wavelength dependence of traffic emissions aerosols for which AEA varied approximately as . By using a shell-core model, we verified, for the first time, that AEA can be as high as 1.6 even for non-absorbing coating on BC, suggesting that the organic coating need not be intrinsically brown to observe effects commonly attributed to BrC absorption. Additionally, for laboratory generated incense burning aerosols, AEA varied as for wavelengths ranging from 355 to 1047 nm. In contrast, the wood smoke aerosols during winter had a much weaker wavelength dependence ( ), comparable to that of traffic emission aerosols. During these observations, the multispectral SSA decreased with the wavelength for traffic-related emissions, yet it increased for biomass and incense burning aerosol. The strong spectral dependence was due to the enhanced light absorption by BrC at UV and blue wavelengths. In all cases, results of this analysis suggested that inefficient smoldering combustion processes can emit predominantly BrC, in comparison to high-temperature and flaming burning processes. During the CARES field campaign, aerosols were dominated by biogenic emissions. Aerosol light absorption was modestly enhanced ( ) at shorter wavelengths (355, 375, 405, and 532 nm) compared to 870 and 1047 nm, likely due to the spectral dependence of coating on BC. The secondary organic aerosol (SOA) mass concentration steadily increased in the latter half of the campaign, with strong 355 nm aerosol light scattering. Overall, results of this field campaign showed that the biogenic SOA was not BrC, i.e. it didn't have intrinsic characteristics near UV absorption. These results should be further tested and analyzed to assess the full implications of BrC aerosol light absorption.