Understanding Different HULIS Sources on the Basis of Absorption and Fluorescence Spectral Measurements

Abstract Humic‐like substances (HULIS) are important brown carbon components that affect global climate change. In this study, sources of acidic and neutral HULIS (HULIS‐a and HULIS‐n) as well as their relationships with light‐absorbing properties were analyzed on the basis of atmospheric particle samples collected in Qingdao, China. The results clearly revealed that the mass concentration of HULIS‐n was greater than that of HULIS‐a except in summer, whereas light absorption by HULIS‐n was greater than that by HULIS‐a across all seasons. The light absorption contribution of HULIS to water‐soluble organic carbon (WSOC) was 40%–85% over a spectral range of 250–500 nm, which was greater than the proportional mass contribution of HULIS‐C to WSOC. The sources of HULIS‐a and HULIS‐n were predicted by multiple methods. The absorbance and fluorescence parameters as well as the results from positive matrix factorization (PMF) analysis suggest that HULIS‐n may be more related to combustion emissions, whereas HULIS‐a may be derived mainly from secondary generation/aging processes. An analysis by the PMF model coupled with light‐absorption spectra suggests that significant absorption only below 350 nm may be due to secondary generation, whereas higher absorbance at shorter and longer wavelengths may be related to primary combustion and aging of combustion products. Additionally, we propose that the Peak T/C ratio in fluorescence spectra can be used to characterize the relative contribution by primary combustion emissions and secondary generation. This study increases the understanding of different HULIS sources with simplified HULIS source identification methods.