Abstract
Abstract. Nitro-aromatic compounds (NACs) were measured hourly at a rural site in China during wintertime to monitor the changes due to local and regional impacts of biomass burning (BB). Concurrent and continuous measurements of the concentrations of 16 NACs in the gas and particle phases were performed with a time-of-flight chemical ionization mass spectrometer (CIMS) equipped with a Filter Inlet for Gases and AEROsols (FIGAERO) unit using iodide as the reagent ion. NACs accounted for <2 % of the mass concentration of organic matter (OM) and total particulate matter (PM), but the total particle mass concentrations of these compounds can reach as high as 1000 ng m−3 (299 ng m−3 avg), suggesting that they may contribute significantly to the radiative forcing effects of atmospheric particles. Levels of gas-phase NACs were highest during the daytime (15:00–16:00 local time, LT), with a smaller night-time peak around 20:00 LT. Box-model simulations showed that this occurred because the rate of NAC production from gas-phase sources exceeded the rate of loss, which occurred mainly via the OH reaction and to a lesser degree via photolysis. Data gathered during extended periods with high contributions from primary BB sources (resulting in 40 %–60 % increases in NAC concentrations) were used to characterize individual NACs with respect to gas–particle partitioning and the contributions of regional secondary processes (i.e. photochemical smog). On days without extensive BB, secondary formation was the dominant source of NACs, and NAC levels correlated strongly with the ambient ozone concentration. Analyses of individual NACs in the regionally aged plumes sampled on these days allowed precursors such as phenol and catechol to be linked to their NAC derivatives (i.e. nitrophenol and nitrocatechol). Correlation analysis using the high time resolution data and box-model simulation results constrained the relationships between these compounds and demonstrated the contribution of secondary formation processes. Furthermore, 13 of 16 NACS were classified according to primary or secondary formation process. Primary emission was the dominant source (accounting for 60 %–70 % of the measured concentrations) of 5 of the 16 studied NACs, but secondary formation was also a significant source. Photochemical smog thus has important effects on brown carbon levels even during wintertime periods dominated by primary air pollution in rural China.
Highlights
Nitro-aromatic compounds (NACs) are aromatic compounds containing at least one nitro (–NO2) functional group attached directly to a benzene ring
The exact positioning and assignment of the functional groups of the nitro-aromatic compounds could not be determined because ToF-chemical ionization mass spectrometer (CIMS) cannot differentiate between isomers, i.e. compounds with the same molecular formulas
The modelled concentration to the observed concentration (Model/Obsavg = 1.25) and the coefficient of determination (r2 = 0.51) between the two data sets. These results clearly show the explicit dependence of the secondary formation of NACs such as nitrocatechol on the oxidation of thermal degradation and pyrolysis products of lignins in aged plumes
Summary
Nitro-aromatic compounds (NACs) are aromatic compounds containing at least one nitro (–NO2) functional group attached directly to a benzene ring. Oxidation of precursors is initiated by hydroxyl (OH) and nitrate (NO3) radicals, which can cause both daytime and night-time formation of NACs. Wood burning processes emit significant quantities of aromatic compounds with OH substituents such as phenol and catechol, which can be transformed into NP and NC under high NOx conditions (Finewax et al, 2018). Oxidation of mononitrates generates nitrophenoxy radicals and similar compounds that produce DNP and other dinitrates In most of these processes, the ambient concentration of NO2 is the main determinant of the rate of NAC formation until the system/site reaches a NOx-saturated regime under which further increases in NO2 levels do not increase NAC formation (Yuan et al, 2016; Wang et al, 2019). Expected that emissions due to BB events would be captured during the study period
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