Understanding the Sources and Light Absorption Properties of Brown Carbon Aerosols via Source Apportionment Analysis of Combined AMS and UV-vis Measurements

WENQING JIANG, Christopher Niedek, Lan Ma, Cort Anastasio, Qi Zhang, University of California, Davis

     Abstract Number: 539
     Working Group: Source Apportionment

Abstract
Atmospheric brown carbon (BrC) which absorbs sunlight efficiently in the near-UV and visible ranges can significantly impact the radiative balance of the earth. However, the chemical and light absorption properties of BrC from different sources remain poorly understood. To address this knowledge gap, in this study, PM_2.5 samples collected at Davis, CA from Nov 2019 to Oct 2020 were analyzed using high-resolution time-of-flight aerosol mass spectrometry (HR-AMS), ion chromatography (IC), total organic carbon (TOC) analyzer, and ultraviolet−visible spectroscopy (UV-vis). Positive matrix factorization (PMF) was applied to the combined HR-AMS mass spectra and UV-vis spectra. Five organic aerosol (OA) factors with distinctive mass spectra and UV-vis spectra profiles were identified, including a fresh biomass burning OA (BBOA) factor with enhanced m/z 60 and 73 signals, an oxygenated OA (OOA) tightly correlating with secondary inorganic aerosol species, and a wintertime OOA influenced by aqueous reactions. All the OA factors are moderately oxidized with O/C ratios in the range of 0.46 – 0.60, but their light absorption properties are considerably different. Among the five OA factors, the fresh BBOA is the most light-absorbing, with a mass absorption efficiency (MAE) of 1.6 m² g^-1 at 365 nm, consistent with previously reported MAE values for BB-influenced aerosols from residential combustion. This result suggests biomass burning is a significant source of light-absorbing aerosols at Davis. The MAE values of the 4 different OOAs are in the range of 0.3 – 0.8 m² g^-1 at 365 nm and their AAE_300-400nm values are in the range of 7.2 – 8.3, higher than that of BBOA (AAE_300-400nm = 5.9). The aqueous-phase OOA resolved from the winter months appears to be more absorbing than the summertime OOAs. These results may help interpret the climate forcing of ambient aerosols as well as the photosensitized production of oxidants in condensed phases.