Chemical characteristics and environmental drivers of nitrogen-containing organic aerosol formation in coastal and inland urban atmospheres in Myanmar

Abstract. Nitrogen-containing organic compounds (NOCs) are important light-absorbing constituents of atmospheric PM_2.5 and can substantially influence aerosol radiative forcing, air quality, and climate. Previous studies have mainly focused on the source apportionment and concentrations levels of NOCs, while the mechanisms governing their formation and particle-phase partitioning remain insufficiently constrained, particularly in tropical regions. Here, we aim to elucidate regional differences in NOCs characteristics in Myanmar, with emphasis on how relative humidity (RH) and precursor species influence their formation pathways. We report the first molecular-level spatio-temporal characterization of NOCs in Myanmar, identifying 1064 organic compounds in ESI^- mode, with NOCs contributing 14–21 % of molecular formulas and 13–35 % of total mass. Organic nitrates (ONs) dominated CHON species across all sites, with higher abundances in Mandalay than in Yangon. Two ubiquitous nitrophenols, nitrocatechol (C₆H₅NO₄) and dimethyl nitrocatechol (C₈H₉NO₄), showed strong covariance but a distinct dependence of their particle-phase C₈H₉NO₄/C₆H₅NO₄ ratio on RH. CHemistry with Aerosol Microphysics in Python (PyCHAM) box model simulations reveal that increasing RH enhances aerosol water content, strengthening Raoult effect and preferentially suppressing condensation of the less soluble C₈H₉NO₄. Seasonal increases in summertime OH further promote C₆H₅NO₄ formation. These two processes explain the observed RH dependence and demonstrate that the C₈H₉NO₄/C₆H₅NO₄ ratio reflects both aerosol liquid water content and oxidative aging, while remaining sensitive to precursor supply. These findings provide new constraints on nitrophenol evolution in humid tropical environments and improve interpretation of NOC sources and aging processes, thereby supporting more accurate assessments of their regional and global radiative impacts.