Gaseous carbonyls in China's atmosphere: Tempo-spatial distributions, sources, photochemical formation, and impact on air quality

Carbonyls are an important class of oxygenated volatile organic compounds that play a crucial role in tropospheric chemistry as intermediates in the formation of ozone (O-3) and secondary organic aerosols. Over the last two decades, China's severe air pollution has led atmospheric chemists to devote substantial efforts to investigate the contribution of carbonyl compounds to the observed phenomena. This study reviews the major findings with regard to the gas-phase atmospheric chemistry of carbonyls in China, including their chemical compositions, temporal and spatial distributions, source apportionments, photochemical formation mechanisms, and impact on tropospheric oxidative capacity, air quality, and human health. Extremely high levels of carbonyls have frequently been observed in China's most rapidly developing regions, such as the North China Plain, the Yangtze River Delta, and the Pearl River Delta, but observational data from other regions are relatively scarce. Primary emissions and photochemical formation are major sources of carbonyls. Alkenes, aromatics, and isoprene have been identified as major precursors on a national scale. In addition, an increasing number of studies has focused on the effects of carbonyls on O-3 formation, radical chemistry, the formation of secondary organic aerosols, and human health. The photolysis of oxygenated volatile organic compounds was recognized as a dominant pathway to ROx production, which further influences O-3 formation, mainly via HO2 + NO or RO2 + NO. Dicarbonyls (such as glyoxal and methylglyoxal) make important contributions to secondary organic aerosol formation via irreversible uptake by aqueous particles. Indoor and outdoor carbonyls often pose a significant threat to human health. This review also includes recommendations from the perspective of emissions, observations, photochemical formation mechanisms, and the effects of carbonyls to guide future research and provide scientific support for the formulation of mitigation policies to address photochemical air pollution.