Measurement report: Variability in ambient BVOC compositions along a latitudinal transect of Chinese forests

Abstract. Forest biogenic volatile organic compounds (BVOCs) affect oxidant cycling and secondary pollution, but coordinated observations of speciated ambient BVOCs remain scarce across China's diverse forests, limiting the evaluation of regional models. We measured ambient isoprene, monoterpenes (MTs), and sesquiterpenes (SQTs) at eight sites spanning tropical to cold-temperate China using a common cartridge-based thermal-desorption gas chromatography-mass spectrometry protocol. We collected ambient samples during summer at all sites and winter at five sites, using fixed daytime and nighttime sampling windows. Summer mean mixing ratios ranged from 30 to 709 ppt for isoprene, 129 to 470 ppt for MTs, and 11 to 27 ppt for SQTs. Isoprene dominated the BVOC mixture at most broadleaf sites, whereas MTs were more prominent at the needleleaf and shrub-steppe sites. Winter concentrations tended to be lower at paired sites, but the seasonal comparison was not significant. Summer daytime isoprene concentrations were higher at all sites. Five sites showed positive associations between daytime, ln-transformed isoprene and air temperature, whereas three sites showed no significant fit. The summer MT day-to-night proxy ratio decreased significantly with latitude. Concentration-weighted metrics indicated that isoprene dominated daytime OH reactivity and ozone formation at broadleaf sites, whereas MTs contributed strongly to nighttime nitrate-radical reactivity and secondary organic aerosol formation. Because we measured in ambient air and collected samples in fixed windows, these concentrations and derived potentials do not represent direct emission fluxes or complete atmospheric production. The dataset provides a cross-ecosystem benchmark for evaluating BVOC composition and sampling-period variability in regional atmospheric chemistry models.