Drivers of Ambient Nitrophenol Formation and evolution in fine particulates: Influence of NOx, pH, and Relative Humidity

Nitrophenols are important contributors to light absorption and toxicity in atmospheric aerosols, yet their sources and formation pathways remain poorly constrained. In this study, we investigate the occurrence and formation mechanisms of 2-nitrophenol (2-NP) in PM_2.5 at an urban site in Mumbai, India, using ultra-high performance liquid chromatography mass spectrometry coupled with Orbitrap detector (UHPLC–MS-O). The mean concentration of 2-NP was 22.03 ± 13.45 ng m^-3. Concurrent measurements of major water-soluble inorganic ions, organic and elemental carbon fractions, and water-soluble organic carbon (WSOC) were employed to examine sources and atmospheric processing. 2-NP exhibited strong positive correlations with WSOC (r = 0.92), K⁺ (r = 0.73), organic carbon (r = 0.62), and NO₃^- (r = 0.61), while negative correlations were observed with Cl^-. Principal component analysis indicates that 2-NP is predominantly associated with secondary organic aerosol formation under nitrate-rich conditions, with additional influence from biomass-burning emissions. The co-variation of 2-NP with WSOC, carbon fractions, NH₄⁺, and SO₄^2- further suggests that photochemical aging and multiphase processing of phenolic precursors under elevated oxidant and NO_x levels are key drivers of its formation in fine particles. Together, these results provide molecular-level evidence that NO_x-driven secondary processing is a dominant pathway for ambient nitrophenol formation in a humid, polluted urban environment. Our findings show that controlling NO_x emissions can directly suppress the formation of toxic, light-absorbing nitro-aromatic aerosols, offering a targeted strategy for improving air quality and climate-relevant aerosol properties.