Seasonal variations of concentrations and optical properties of water soluble HULIS collected in urban environments

Abstract. Major contributors to the organic aerosol include water-soluble macromolecular compounds (e.g. HULIS_WS: Water Soluble Humic LIke Substances). The nature and sources of HULIS_WS are still largely unknown. This work is based on a monitoring in six different French cities performed during summer and winter seasons. HULIS_WS analysis was performed with a selective method of extraction complemented by carbon quantification. UV spectroscopy was also applied for their chemical characterisation. HULIS_WS carbon represent an important contribution to the organic aerosol mass in summer and winter, as it accounts for 12–22% of Organic Carbon and 34–40% of Water Soluble Organic Carbon. We found strong differences in the optical properties (specific absorbance at 250, 272, 280 nm and E2/E3 ratio) and therefore in the chemical structure between HULIS_WS from samples of summer- and wintertime. These differences highlight different processes responsible for emissions and formation of HULIS_WS according to the season, namely biomass burning in winter, and secondary processes in summer. Specific absorbance can also be considered as a rapid and useful indicator of the origin of HULIS_WS in urban environment.