Exploring the Formation of High Levels of Hydroxyl Dicarboxylic Acids at an Urban Background Site in South China.

Hydroxyl dicarboxylic acids (OHDCA) are ubiquitous in the atmosphere as an important constituent of secondary organic aerosol, yet the formation mechanisms remain unclear. At an urban background site on the coast of South China, we observed notable levels of OHDCA, with the highest concentration of malic acid (a typical OHDCA species) reaching 533 ng m−3. In the coastal air, the correlation between OHDCA and sulfate was better (R2 = 0.48) in the period when the relative humidity was higher and the sulfate size distribution was in a droplet mode, fitting the features of aqueous formation. In the short‐range continental air, a significant rise in OHDCA levels from morning through early afternoon (588 ng m−3) was observed under marked daytime increment of ozone that was corrected for titration loss (O3_corr, sum of ozone and nitrogen dioxide). In addition, good correlation between OHDCA and O3_corr was identified in this period, illuminating the role for gas‐phase photochemistry in regulating OHDCA formation. Therefore, the elevated OHDCA was likely attributed to aqueous photooxidation, and the dominant factors varied under different atmospheric conditions. The precursors of OHDCA could be derived from biogenic emissions, as indicated by the correlations of OHDCA with 2‐methylglyceric acid (bihourly data) and isoprene and monoterpenes (daily average data). However, anthropogenic aromatics might also be involved in OHDCA formation, especially in the short‐range continental air. The formation mechanisms probed through observational evidence will be an important reference for rectifying simulations of OHDCA and its impact on air quality and climate. Plain Language Summary: Organic aerosol, a vital component of fine particulate matter, is suspended in the atmosphere. It is of widespread concern due to health and climate effects. In a coastal area generally considered clean, we observed remarkably high levels of climate‐relevant hydroxyl dicarboxylic acids in organic aerosol. Different factors dominated the aqueous photochemical formation of hydroxyl dicarboxylic acids between air masses arriving at the measurement site. This provides a scientific basis for mitigating organic aerosol and its climate effects. Key Points: The concentrations of hydroxyl dicarboxylic acids (OHDCA) in Hong Kong's air were among the highest in the worldAqueous and photochemical processes dominated OHDCA formation in the coastal and short‐range continental air, respectivelyAromatic compounds were likely one of the precursors of OHDCA in continental outflows [ABSTRACT FROM AUTHOR]