Measurement report: PM<sub>2.5</sub>-bound nitrated aromatic compounds in Xi'an, Northwest China – seasonal variations and contributions to optical properties of brown carbon

Abstract. Nitrated aromatic compounds (NACs) are a group of key chromophores for brown carbon (light-absorbing organic carbon, i.e., BrC) aerosol, which affects radiative forcing. The chemical composition and sources NACs their contributions to BrC absorption, however, still not well understood. In this study, PM2.5-bound in Xi'an, Northwest China, were investigated 112 daily PM2.5 filter samples from 2015 2016. Both the total concentrations individual species show distinct seasonal variations. seasonally averaged 2.1 (spring), 1.1 (summer), 12.9 (fall), 56 ng m?3 (winter). Thereinto, 4-nitrophenol is major NAC component spring (58 %). 5-nitrosalicylic acid dominate summer (70 %), 4-nitrocatechol fall %) winter (55 different patterns concentrations, indicating differences emissions formation pathways. Source apportionment results using positive matrix factorization (PMF) further large NACs. Specifically, summer, highly influenced by secondary vehicle (? 80 while winter, biomass burning coal combustion contributed most 75 Furthermore, light absorption wavelength-dependent vary greatly season, with maximum at ? 330 nm 320 spring. contribution associated higher mass fractions (?max?= 345 nm) 310 spring, 315 summer. mean wavelength 365 seasons 0.14 % 0.09 0.36 0.91 (winter), about 6–9 times than fractional carbon. Our indicate that have profound impacts on absorption.