Lingxiao Lu,^ac Buqing Xu,^*ab Gan Zhang ^ab

^aState Key Laboratory of Advanced Environmental Technology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China.

^bGuangdong-Hong Kong-Macao Joint Laboratory for Environmental Pollution and Control, Guangdong Provincial Key Laboratory of Environmental Protection and Resources Utilization, Guangzhou 510640, China

^cUniversity of Chinese Academy of Sciences, Beijing 100049, China

Contact to: Lingxiao Lu (lulingxiao@gig.ac.cn)

ABSTRACT

Low molecular weight monocarboxylic acids (monoacids) are ubiquitous atmospheric organic compounds that influence aerosol hygroscopicity and atmospheric processing. To investigate their molecular distributions, gas–particle partitioning, and sources across contrasting continental and marine environments, paired gaseous and particulate samples were collected from seven Asian cities (n = 106) and during a cruise from the South China Sea to the Indian Ocean (n = 42). Mean gas- and particle-phase monoacid concentrations were 6.90 and 0.34 μg m^–3 in urban air, respectively, compared with 0.77 and 0.030 μg m^–3 in the marine atmosphere. Despite this large concentration difference, monoacids exhibited similar phase and molecular characteristics, with 85–98% occurring in the gas phase. Formic and acetic acids were the dominant species, together accounting for approximately 79% and 86% of gaseous monoacids in urban and marine samples, respectively. Acetic acid dominated the gas phase, whereas formic acid dominated the particle phase, indicating preferential particle-phase enrichment of formic acid, possibly associated with salt formation and aqueous-phase processing. Correlation analyses and temporal variations indicate that urban monoacids were influenced by direct emissions from combustion and vehicle exhaust, with additional contributions from photochemical and aqueous-phase formation. Lower marine concentrations, together with spatial distributions and air-mass trajectories, suggest the influence of continental outflow and subsequent atmospheric processing during transport. The similar molecular and phase-partitioning patterns observed across urban and marine atmospheres, despite their contrasting concentration levels, suggest that source emissions and atmospheric processing jointly regulate the abundance and phase distribution of monoacids from continental source regions to the remote marine atmosphere.