Measurement Report: Optical and structural properties of atmospheric water-soluble organic carbon in China: Insights from multi-site spectroscopic measurements

Abstract. To understand the spatial variation of optical and structural properties of water-soluble brown carbon and its influencing factors in China, the light absorption, fluorescence, and Fourier transform infrared (FTIR) spectrum of water-soluble organic carbon (WSOC) in different regions of China are measured following the same analytical methods. The average light absorption coefficients and mass absorption efficiencies of WSOC at 365 nm (Abs₃₆₅ and MAE₃₆₅) rank from high to low as northwest China > southwest China > north China > east China > regional site, with higher values in Northern China than Southern China and regional sites, and higher values in inland areas than coastal areas. The light absorption factors resolved by positive matrix factorization model combined with light absorption spectra and abundant aromatic O-H and C=C functional groups determined by FTIR both indicate that aromatic compounds are important light-absorbing substances in WSOC, which also have a significant impact on fluorophores. Multiple linear regression analysis shows that the identified fluorophores by fluorescence spectra combined with parallel factor analysis contribute to about 62–93 % of the WSOC light absorption at all sites, in which humic-like substance (HULIS) contributes the most, especially highly‑oxygenated HULIS (29 %–50 %) with long emission wavelengths. Combustion source emissions and atmospheric chemical processes have significant impacts on the WSOC light absorption at some sites. Moreover, relative humidity (RH) can also affect MAE₃₆₅ of WSOC with MAE₃₆₅ values decreasing with the increase of RH when RH < 60 % and remaining relatively unchanged when RH > 60 %. Taken together, this study promotes a better understanding of the spatial heterogeneity of optical and structural properties of WSOC and their influencing factors in China.