Speciation of “brown” carbon in cloud water impacted by agricultural biomass burning in eastern China

Abstract

Despite growing interest in the visible light‐absorbing organic component of atmospheric aerosols, referred to as “brown” carbon, our knowledge of its chemical composition remains limited. It is well accepted that biomass burning is one important source of “brown” carbon in the atmosphere. In this study, cloud water samples heavily affected by biomass burning were collected at Mount Tai (1534 m, ASL), located in Shandong province in the North China Plain in summer 2008. The samples were analyzed with high performance liquid chromatography equipped with a UV/Vis absorbance detector immediately followed by electrospray ionization and analysis using a time‐of‐flight (ToF) mass spectrometer. The high mass resolution and accuracy provided by the ToF mass spectrometer allow determination of the elemental composition of detected ions. Using this approach, the elemental compositions of 16 major light‐absorbing compounds, which together accounted for approximately half of measured sample absorption between 300 and 400 nm, were determined. The most important classes of light‐absorbing compounds were found to be nitrophenols and aromatic carbonyls. Light absorption over this wavelength range by reduced nitrogen compounds was insignificant in these samples.