[Observation Analyses of Aerosol Size Distribution Properties from February to March, 2019 in Tianjin Urban Area]

The aerosol size distribution is an important physical parameter reflecting the source, formation process, and pollution characteristics of aerosol particles. In order to study the properties of aerosol number concentration and size distributions in the Tianjin urban area,the aerosol number concentration and size distributions ranging from 10-600 nm were detected using a scanning mobility particle sizer (SMPS) during February and March, 2019. The results showed that in the Tianjin urban area, the aerosol number concentration,surface area concentration. and volume concentration in the size range of 10-600 nm were 22188.22 cm^-3, 1581.08 μm²·cm^-3, and 70.76 μm³·cm^-3,respectively, in late winter and early spring. The aerosol number concentration,surface area concentration, and volume concentration spectrum were all unimodally distributed,and the peak value sizes were 109.40, 269.00, and 429.40 nm. The number concentrations of the nucleation mode (10-20 nm),Aitken mode (20-100 nm), and accumulation mode (100-600 nm) aerosols accounted for 1.40%, 52.44%, and 46.16% of the total number concentration. The diurnal variation in aerosol number concentration showed three peaks (06:00-08:00, 12:00-14:00, and 18:00-20:00) on work days and two peaks (07:00-08:00 and 19:00-21:00) on weekends. The peaks appeared 1-2 hours later on weekends,and the increment of aerosol number concentration was attributed to vehicle exhaust emissions. Meteorological factors had a significant influence on the aerosol size distribution in Tianjin; aerosol number concentration values were high in east and southwest wind. On non-precipitation days,the aerosol number size distribution moved to larger size ranges with the increment of relative humidity (RH); as the RH increased from <20% to 50%-60%,the size peak increased from 50 nm to 131 nm. The precipitation removed 100-200 nm aerosol particles discernibly,which resulted in the size peak decreasing to 98 nm.