Environmental context Nitroaromatic compounds constitute an important portion of brown carbon and thereby contribute to the light-absorbing properties of atmospheric aerosols. We report their abundance in Hong Kong over 3 years and show that they were mainly associated with aged biomass burning particles. Knowledge of the abundance and sources of nitroaromatic compounds could assist in evaluating their contribution to brown carbon and in apportioning secondary organic aerosols from biomass burning sources. Abstract Biomass burning is a major source of atmospheric aerosols on both global and regional scales. Among the large number of unidentified organic compounds related to biomass burning, nitroaromatic compounds (NACs) have drawn attention because of their UV light-absorbing ability. In this study, an analytical method based on liquid chromatography–mass spectrometry was used to quantify a group of NACs (nitrophenol, methylnitrophenols, dimethylnitrophenol, nitrocatechol and methylnitrocatechols) in aerosol samples. The nitrocatechol–metal complex interference, sample matrix effects, sample stability, precision and reproducibility were investigated. The method detection limits ranged from 0.10 to 0.23ngmL–1 and the recoveries for the target NACs were in the range of 96–102%. The method was applied to a total of 184 ambient PM2.5 samples (particulate matter of 2.5µm or less in aerodynamic diameter) collected at an urban site in Hong Kong over 3 years (2010–2012). The NACs quantified showed a distinct seasonal variation with higher concentrations in autumn and winter (3.6–21.0ngm–3), coinciding with more biomass burning activities coming from the regions west and north-east to Hong Kong, and lower levels during spring and summer (0.3–3.8ngm–3). The good correlations between NACs and levoglucosan (R=0.82), a known biomass burning tracer compound, support the common origin from biomass burning. Moderate to good correlations between NACs and nitrate suggest that they might be products of secondary formation processes involving the same precursor gases (e.g. NOx). Additional lines of circumstantial evidence were also found and presented in the paper to support secondary formation derived from biomass burning as the main contributing source of NACs.