Measurement report: PM_2.5-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 aerosol (light absorbing organic carbon, i.e., BrC), which affects radiative forcing. The chemical composition and sources of NACs and their contributions to BrC absorption, however, are still not well understood. In this study, PM_2.5-bound NACs in Xi'an, Northwest China, were investigated for 112 daily PM_2.5 filter samples from 2015 to 2016. Both the total concentrations and contributions from individual species of NACs show distinct seasonal variations. The seasonally averaged concentrations of NACs are 2.1 (spring), 1.1 (summer), 12.9 (fall), and 56.3 ng m⁻³ (winter). Thereinto, 4-nitrophenol is the major NAC component in spring (58 %). The concentrations of 5-nitrosalicylic acid and 4-nitrophenol dominate in summer (70 %), and the concentrations of 4-nitrocatechol and 4-nitrophenol dominate in fall (58 %) and winter (55 %). The NAC species show different seasonal patterns in concentrations, indicating differences in emissions and formation pathways. Source apportionment results using positive matrix factorization (PMF) further show large seasonal differences in the sources of NACs. Specifically, in summer, NACs were highly influenced by secondary formation and vehicle emissions (~ 80 %), while in winter, biomass burning and coal combustion contributed the most (~ 75 %). Furthermore, the light absorption contributions of NACs to BrC are wavelength dependent and vary greatly by seasons, with maximum contributions at ~ 330 nm in winter and fall and ~ 320 nm in summer and spring. The differences in the contribution to light absorption are associated with the higher mass fractions of 4-nitrocatechol (λ_max = 345 nm) and 4-nitrophenol (λ_max = 310 nm) in fall and winter, 4-nitrophenol in spring, and 5-nitrosalicylic acid (λ_max = 315 nm) and 4-nitrophenol in summer. The mean contributions of NACs to BrC light absorption at the wavelength 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 times higher than their mass fractional contributions of carbon in total organic carbon. Our results indicate that the composition and sources of NACs have profound impacts on the BrC light absorption.