Real-time geochemistry of urban aerosol during a heavy dust episode by single-particle aerosol mass spectrometer: Spatio-temporal variability, mixing state and spectral distribution

A heavy dust episode occurred from May 3 to 8,2017 in China,with an influenced area exceeding 1.63 million km2.In this work,the mixing state of aerosols and their spectral distributions were simultaneously observed in the sand source region ofHohhot and the long-range dust transport regi on of Nanji ng by using a single-particle aerosol mass spectrometer(SPAMS).The duration time of this dust episode was 37-40 h in Hohhot and prolonged to 51-104 h in Nanjing.Totals of 336,135(Hohhot)and 235,840(Nanjing)particles in PM2.5 were successfully ionized to identify 10 main particle classes.During this episode,aerosol particles including OCEC(30.65%),K(22.42%),K-CN(17.03%),sodium(9.46%),heavy metal(8.96%),EC-sulfate(3.46%)and Al(3.22%)were prevailing in Hohhot and were dominated by EC-sulfate(22.26%),OCEC(15.21%),heavy metal(11.96%),K(13.68%),sodium(12.27%),Al(10.54%)and EC(9.02%)in Nanjing.The spectral distribution peaked at 0.66μm during the dust episode in Nanjing,0.12μm larger than the peak size in the non-dust episode.Strong signals at-62[NO3)and-61[HCO3]/-61[C5H]for aerosol particles were observed in Hohhot and Nanjing respectively.The proportions of K-CN,sodium,heavy mental,OCEC and EC-sulfate particles in the dust episode of Hohhot were 2.75,1.41,1.80,1.22 and 1.28 times as large as the values in the non-dust episode.For Nanjing,the fractions of EC-sulfate,EC,Al particles in the dust episode were 10.55,4.65 and 1.46 times higher than values in the non-dust episode.The proportions of EC-secondary and EC-nitate particles were found to decrease in the dust episode in the two regions.