Drivers of Ambient Nitrophenol Formation and Evolution in Fine Particulate Matter (PM2.5): Coupled Roles of NOx, Temperature, and Aerosol Physicochemical Properties.
Abstract
Nitrogen-containing aromatic compounds (NACs), such as nitrophenols, are disproportionately strong contributors to brown carbon (BrC) light absorption and aerosol toxicity, yet their sources, formation pathways, and controlling factors in tropical coastal megacities remain poorly constrained. Here, the occurrence and formation of 2-nitrophenol (2-NP) in PM2.5 were investigated at an urban site in Mumbai, India, during the 2023-2024 winter using ultra-high-performance liquid chromatography coupled to Orbitrap mass spectrometry (UHPLC-Orbitrap MS), together with other co-located measurements, and thermodynamically derived aerosol acidity (pH) and aerosol liquid water content (ALWC). The mean 2-NP concentration was 25.7 ± 12.3 ng m-3 and co-varied most strongly with WSOC (r = 0.92) and with K+ (r = 0.67), NO3- (r = 0.66), and the sulfur and nitrogen oxidation ratios, indicating a secondary, combustion-influenced origin. The campaign comprised two contrasting periods: a more polluted January period with elevated NOx, biomass tracers, WSOC, and 2-NP, and a cleaner February period with lower precursor loadings and higher ozone. 2-NP was weakly and negatively associated with temperature (r = -0.48) and negatively associated with aerosol pH (r = -0.50) and RH, but not with aerosol liquid water. The particles were acidic (median pH 2.7), and 2-NP accounted for only about 1% of light absorption at 365 nm. These results indicate that, at this site, 2-NP formation is primarily governed by precursor availability and NOx-driven nitration of aromatic VOCs. To our knowledge, this is the first study from a coastal South Asian megacity to combine optical and molecular measurements with thermodynamic modelling to examine the drivers of 2-NP, and it underscores that NOx and aromatic-VOC control offer a targeted lever for reducing toxic, light-absorbing aerosol.
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