Characterizing chemical composition and light absorption of nitroaromatic compounds in the winter of Beijing

Nitroaromatic compounds (NACs) were believed as one of the major light absorption components in brown carbon (BrC) and important fractions of organic aerosols. To understand the chemical compositions and light absorption of NACs in the highly polluted atmospheres, a field campaign was conducted in the winter of urban Beijing. Ambient PM2.5 samples were collected and a total of 12 NACs were identified and quantified using ultrahigh performance liquid chromatography coupled to Orbitrap mass spectrometer. Light absorption of methanol-extracted BrC and individual NAC in PM2.5 were also measured using UV-Vis spectrophotometer. The average concentration of quantified NACs was 173 +/- 137 ng/m(3), ranging from 19 to 585 ng/m(3), which was much higher than those in summer in Beijing or in winter in other areas. Nitrophenols and nitrocatechols are the dominant species among the identified NACs in the winter of Beijing. Four pollution episodes were observed during the observation. The NAC concentrations increased from 62 ng/m(3) during clean days to 224 ng/m(3) during pollution episodes, with both secondary formation and biomass burning emissions as important sources. Particulate BrC was estimated to contribute as high as 45% of the total light absorption of carbonaceous aerosols at 370 nm based on the online aethalometer measurement. The identified NACs contributed about 17% of the total absorption by methanol-extracted BrC at 370 nm, though the mass concentrations of quantified NACs only accounted for 0.6% of the organic matter. This indicates the high absorption capability of NACs, which needs to be taken into consideration when studying the aerosol light absorption in urban environments. Moreover, the maximum absorption values of NACs among the range of 300-400 nm made them important to atmospheric photochemistry reactions. The linkage between chemical compositions and light absorption of NACs deserves to be further investigated to understand the light absorption mechanisms and environmental impacts of organic aerosols.