Ambient nitro-aromatic compounds – biomass burning versus secondary formation in rural China

Abstract

Abstract. Nitro-aromatic compounds (NACs) were measured hourly at a rural site in\nChina during wintertime to monitor the changes due to local and regional\nimpacts of biomass burning (BB). Concurrent and continuous measurements of\nthe concentrations of 16\xa0NACs in the gas and particle phases were performed\nwith a time-of-flight chemical ionization mass spectrometer (CIMS) equipped\nwith a Filter Inlet for\nGases and AEROsols (FIGAERO)\xa0unit using iodide as the\nreagent ion.\xa0NACs accounted for <2\u2009% of the mass concentration of\norganic matter (OM) and total particulate matter (PM), but the total\nparticle mass concentrations of these compounds can reach as high as 1000\u2009ng\u2009m−3 (299\u2009ng\u2009m−3 avg), suggesting that they may contribute\nsignificantly to the radiative forcing effects of atmospheric particles.\nLevels of gas-phase\xa0NACs were highest during the daytime (15:00–16:00\xa0local\ntime, LT), with a smaller night-time peak around 20:00\u2009LT. Box-model\nsimulations showed that this occurred because the rate of NAC production\nfrom gas-phase sources exceeded the rate of loss, which occurred mainly via\nthe OH reaction and to a lesser degree via photolysis. Data gathered during\nextended periods with high contributions from primary BB sources (resulting\nin 40\u2009%–60\u2009% increases in NAC concentrations) were used to characterize\nindividual\xa0NACs with respect to gas–particle partitioning and the\ncontributions of regional secondary processes (i.e. photochemical smog). On\ndays without extensive BB, secondary formation was the dominant source of\nNACs, and NAC levels correlated strongly with the ambient ozone\nconcentration. Analyses of individual\xa0NACs in the regionally aged plumes\nsampled on these days allowed precursors such as phenol and catechol to be\nlinked to their NAC derivatives (i.e. nitrophenol and nitrocatechol).\nCorrelation analysis using the high time resolution data and box-model\nsimulation results constrained the relationships between these compounds and\ndemonstrated the contribution of secondary formation processes. Furthermore,\n13 of 16\xa0NACS were classified according to primary or secondary formation\nprocess. Primary emission was the dominant source (accounting for 60\u2009%–70\u2009%\nof the measured concentrations) of 5 of the 16\xa0studied\xa0NACs, but secondary\nformation was also a significant source. Photochemical smog thus has\nimportant effects on brown carbon levels even during wintertime periods\ndominated by primary air pollution in rural China.\n