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

Nitrogen-containing organic compounds (NOCs) are important light-absorbing constituents of atmospheric PM2.5 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 relationship of their particle-phase C8H9NO4/C6H5NO4 ratio with RH. CHemistry with Aerosol Microphysics in Python (PyCHAM) box model simulations reveal that increasing RH enhances aerosol water content, to which C8H9NO4 and C6H5NO4 respond differently because of differences in their partitioning thermodynamics. Increased photochemistry in summertime further promotes C6H5NO4 formation. These two processes, in addition to gas-phase precursor concentration, can explain the observed RH relationship and demonstrate that the C8H9NO4/C6H5NO4 ratio is sensitive, by comparable extents, to: partitioning thermodynamics, photochemistry and 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.