Water soluble organic matter (WSOM) as a significant fraction of atmospheric organic aerosols affects issues ranging from visibility impairment to aerosol effects on the Earth’s radiation balance. Our understanding of the chemical composition of WSOM and its humic-like (HULIS) fraction at the individual component level is still limited. Chemical characterization of just recently detected and potentially toxic nitro-aromatic compounds (NAC) in aerosols is an emergent issue to better understand their impact to the atmospheric chemistry and air quality. The AERONAR represents a first attempt to study the aerosol size segregated WSOM over the seasons with an emphasis on NAC through investigating their extraction efficiency, speciation and quantification, (trans)formation in aqueous phase, surfactant nature and interactions with environmentally relevant micro constituents. Project will contribute to fundamental understanding of the chemical nature, sources and the mechanisms underlying WSOM transformation in the atmosphere.