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Abstract. The major air pollutant emissions have decreased, and the overall air quality has substantially improved across China in recent years as a consequence ofactive clean air policies for mitigating severe air pollution problems. Askey precursors of formaldehyde (HCHO) and ozone (O 3 ), the volatileorganic compounds (VOCs) in China are still increasing due to the lack ofmitigation measures for VOCs. In this study, we investigated the drivers ofHCHO variability from 2015 to 2019 over Hefei, eastern China, by using ground-based high-resolution Fourier transform infrared (FTIR) spectroscopyand GEOS-Chem model simulation. Seasonal and interannual variabilities ofHCHO over Hefei were analyzed and hydroxyl (OH) radical production rates from HCHO photolysis were evaluated. The relative contributions of emittedand photochemical sources to the observed HCHO were analyzed by using ground-level carbon monoxide (CO) and O x (O 3 +  nitrogen oxide(NO 2 )) as tracers for emitted and photochemical HCHO, respectively.Contributions of emission sources from various categories and geographicalregions to the observed HCHO summertime enhancements were determined byusing a series of GEOS-Chem sensitivity simulations. The column-averaged dryair mole fractions of HCHO ( XHCHO ) reached a maximum monthly mean valueof 1.1  ±  0.27 ppbv in July and a minimum monthly mean value of 0.4  ±  0.11 ppbv in January. The XHCHO time series from 2015 to 2019over Hefei showed a positive change rate of 2.38  ±  0.71 % per year.The photochemical HCHO is the dominant source of atmospheric HCHO over Hefeifor most of the year (68.1 %). In the studied years, the HCHO photolysiswas an important source of OH radicals over Hefei during all sunlight hours of both summer and winter days. The oxidations of both methane (CH 4 )and nonmethane VOCs (NMVOCs) dominate the HCHO production over Hefei andconstitute the main driver of its summertime enhancements. The NMVOC-related HCHO summertime enhancements were dominated by the emissions withineastern China. The observed increasing change rate of HCHO from 2015 to 2019over Hefei was attributed to the increase in photochemical HCHO resultingfrom increasing change rates of both CH 4 and NMVOC oxidations, which overwhelmed the decrease in emitted HCHO. This study provides a valuableevaluation of recent VOC emissions and regional photochemical capacity in China. In addition, understanding the sources of HCHO is a necessary stepfor tackling air pollution in eastern China and mitigating the emissions ofpollutants.

atmospheric chemistry and physics

Mapping the drivers of formaldehyde (HCHO) variability from 2015 to 2019 over eastern China: insights from Fourier transform infrared observation and GEOS-Chem model simulation