Sources And Atmospheric Transformation Processes Of Secondary Aerosols In Basin-and Plateau-type Regions Of China

Recent studies have demonstrated the importance of secondary aerosols in atmospheric aerosol pollution and haze events.Regional differences of secondary aerosols were suggested in the previous studies due to the complexity of the sources,formation pathways and mechanisms.However,the research on secondary aerosols in the plateau and basin regions was limited.In this paper,we investigated the source and formation pathways of the secondary aerosols,the impact of long-range transport and emission reduction on the secondary aerosol formation in the basin area.Furthermore,we compared the differences in the source and formation of secondary aerosol between plateau and basin,and analyzed the gaseous-particle transformation processes of secondary organic aerosols in plateau.This research is novel and innovative,revealing the formation mechanism of secondary aerosols in different environments and conditions.Analysis in Xi’an showed that biomass burning organic aerosol and oxygenated organic aerosol contributed>50%of total organic aerosol in both autumn and winter in urban region of the Guanzhong Basin.A large increase in oxygenated organic aerosol concentration was observed during polluted days,by a factor of～4 in autumn and～6in winter compared to clean days.In both seasons,oxygenated organic aerosol formation was most likely dominated by photochemical oxidation when aerosol liquid water content was<30μg m^-3 or by aqueous-phase processes when O_x was<35 ppb.Furthermore,compared with organic aerosol fraction in non-refractory PM₁ in other regions of China,biomass burning organic aerosol（17–19%）and coal combustion organic aerosol（10–20%）were major emission sources in the GZB and the Beijing-Tianjin-Hebei region,respectively,whereas oxygenated organic aerosol（10–34%）was an important source in all studied regions.The regional transport analysis in the Guanzhong Basin showed that the fraction of secondary inorganic aerosol and more-oxidized oxygenated organic aerosol increased significantly after transport.The sulfur oxidation rate（0.49-0.58）,nitrogen oxidation rate（0.25-0.29）,f44/f43 ratio（1.6-2.1）and O/C ratio（0.72-0.87）were significantly higher than those investigated locally,indicating that long-distance transport largely enhanced the secondary inorganic aerosol formation,the organic aerosol oxidation and ageing.The formation rate of sulphate(0.25 hr^-1versus 0.1 hr^-1),nitrate(0.17 hr^-1 versus 0.03 hr^-1)and more-oxidized oxygenated organic aerosol(0.14hr^-1 versus 0.06 hr^-1)in the Beijing-Tianjin-Hebei transport sector was much higher than that in the Guanzhong Basin transport sector,indicating the stronger sulphate,nitrate and more-oxidized oxygenated organic aerosol formation efficiency in the Beijing-Tianjin-Hebei transport sector.During the emission reduction period,oxygenated organic aerosol increased from 24%of total OA(3.2±1.6μg m^-3)before lockdown to54%of total OA(4.5±1.3μg m^-3)during lockdown,likely due to substantial decrease of NO_x emissions during lockdown which resulted in large increase of O₃ and thus atmospheric oxidizing capacity.oxygenated organic aerosol showed higher mass concentrations and fractional contributions during lockdown than before lockdown,and increased with the increase of O_x in both periods.In comparison,aqueous secondary organic aerosol showed high mass concentrations and fractional contributions in both polluted periods before and during lockdown with the increase of aerosol liquid water content.The increase of aqueous secondary organic aerosol under high aerosol liquid water content conditions is very likely the reason of pollution events during the emission reduction period.The observation in the Qinghai-Tibet Plateau showed that the fraction of sulphate（22.5%）was significantly higher than that of nitrate（3.4%）in the plateau compared with the basin.The fraction of secondary aerosol in the non-refractory PM_2.5 showed different variations from clean period to polluted period in the basin and plateau,indicating different pollution pathways in these two landforms.The source apportionment result of volatile organic compounds showed that there were 12 volatile organic compounds source factors.Acetone and acetic acid related products,nighttime related acetone and acetic acid products,monoterpenes,isoprene and isoprene oxidized products and anthropogenic emissions are major gaseous precursor of less-oxidized oxygenated organic aerosol,accounting for 77%,10%,8%,4%and 1%of less-oxidized oxygenated organic aerosol source,respectively.For more-oxidized oxygenated organic aerosol,acetone and acetic acid related products and nighttime related acetone and acetic acid products are the major gaseous precursor of more-oxidized oxygenated organic aerosol,accounting for 86%and 14%of more-oxidized oxygenated organic aerosol source,respectively.This result showed the gaseous-particle transformation processes of secondary organic aerosols in the Qinghai-Tibet Plateau.