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. Nitrated aromatic compounds (NACs) are a group of key chromophores for browncarbon (light-absorbing organic carbon, i.e., BrC) aerosol, which affectsradiative forcing. The chemical composition and sources of NACs and theircontributions to BrC absorption, however, are still not well understood. Inthis study, PM2.5-bound NACs in Xi'an, Northwest China, wereinvestigated for 112 daily PM2.5 filter samples from 2015 to 2016. Boththe total concentrations and contributions from individual species of NACsshow distinct seasonal variations. The seasonally averaged concentrations ofNACs are 2.1 (spring), 1.1 (summer), 12.9 (fall), and 56 ng m−3(winter). Thereinto, 4-nitrophenol is the major NAC component in spring(58 %). The concentrations of 5-nitrosalicylic acid and 4-nitrophenoldominate in summer (70 %), and the concentrations of 4-nitrocatechol and4-nitrophenol dominate in fall (58 %) and winter (55 %). The NAC speciesshow different seasonal patterns in concentrations, indicating differencesin emissions and formation pathways. Source apportionment results usingpositive matrix factorization (PMF) further show large seasonal differencesin the sources of NACs. Specifically, in summer, NACs were highly influencedby secondary formation and vehicle emissions (∼ 80 %), whilein winter, biomass burning and coal combustion contributed the most(∼ 75 %). Furthermore, the light absorption contributions ofNACs to BrC are wavelength-dependent and vary greatly by season, with maximum contributions at ∼ 330 nm in winter and fall and∼ 320 nm in summer and spring. The differences in thecontribution to light absorption are associated with the higher massfractions of 4-nitrocatechol (λmax⁡= 345 nm) and 4-nitrophenol(λmax⁡= 310 nm) in fall and winter, 4-nitrophenol in spring,and 5-nitrosalicylic acid (λmax⁡= 315 nm) and 4-nitrophenol insummer. The mean contributions of NACs to BrC light absorption at awavelength of 365 nm in different seasons are 0.14 % (spring), 0.09 %(summer), 0.36 % (fall), and 0.91 % (winter), which are about 6–9 timeshigher than their mass fractional contributions of carbon in total organiccarbon. Our results indicate that the composition and sources of NACs haveprofound impacts on the BrC light absorption.

atmospheric chemistry and physics

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