The Characteristics Of Number Concentration And Size Distribution Of Aerosols In The Urban Area Of Guangzhou

Atmospheric aerosols have great effects on human health, ambient air quality and global climate change. With the rapid urbanization and development of industries, Guangzhou experienced severe atmospheric aerosol pollutions, which lead to visibility degradation and deteriorate air quality. The environmental and health effects of atmospheric aerosols depend on their physical and chemical properties, such as number concentrations, size distributions and chemical components. The investigation on the number concentration and size distribution of atmospheric aerosols could help us understanding the source and formation mechanism of particles in urban area of Guangzhou..In this study, we carried out a measurement of atmospheric submicron particles in spring(from April 11 to May 05, 2013), summer(from June 02 to July 1 5, 2013) and fall(September 29 to October 19, 2013) by scanning mobility particle sizer(SMPS). Particle number concentrations, size distributions, their diurnal variations and pollution characteristics were investigated. Main results are as follows:1) Hourly PM2.5 was the highest in spring with 49.4 μg/m3, followed by fall(38.6 μg/m3) and summer(30.4 μg/m3). Aerosol volume concentrations and the mass concentrations of gases including CO, SO2, NOx and NO2 showed similar seasonal variation trend, but the highest mass concentrations of O3 occurred in fall. There were more hazy events in spring and fall. The average ambient visibility was 8.8 Km in spring, 12.1 Km in fall, and 15.1 Km in summer.2) During the whole sampling period, the highest particle number concentration was in spring with 17790 cm-3 and the lowest in fall. The particle numbers were dominated by Aitken mode particles in all seasons. However the highest number concentration of nucleation mode particles occurred in fall with 3166 cm-3. The number concentration of Aitken mode particles was similar in spring and summer, and the lowest in fall. The number concentrations of accumulation mode particles were higher in spring and fall than in summer.3) The median number size distributions of particles showed unimodal in spring and summer, exhibiting a peak at 63 nm and 55 nm respectively, and bimodal in fall, with peaks at 22 nm and 137 nm. The median size distributions of volume and surface concentrations exhibited unimodal with a peak at about 300-400 nm and 200-300 nm respectively in different seasons.4) The median number size distributions in different seasons were fitted with two log-normal distributions. The geometric mean diameters of two modes were at 56 nm and 144 nm in spring, 51 nm and 177 nm in summer, 21 nm and 127 nm in fall. The number concentration of the first mode was higher in spring and summer, about 104 cm-3. The number concentration of the second mode was the highest in fall, and the lowest in summer.5) Precipitation process can remove accumulation mode particles effectively. After rain, the number concentration of accumulation mode particles decreased by 59%, while Aitken mode particles decreased by 30%. Nucleation mode particle number concentration and it’s percentage increased after rain. New particle formation may also take place after rain. During hazy period, the number concentration of accumulation mode particles increased from 3876 cm-3 to 6440 cm-3. The percentage of accumulation mode particles increased from 21.6% to 35.8%. Number size distributions shift to larger particles during haze.6) The diurnal variations of number concentration and size distribution in spring and fall were influenced by vehicle emissions, with obvious number concentration peaks at rush hours, about 15000 cm-3, particularly in the evening when the atmospheric dispersion and dilution ability was weak. The diurnal variation of particle number concentrations was more influenced by photochemical reactions in summer with strong solar radiation. The number concentration of Aitken mode particles often increased up to 14000 cm-3 during afternoon.7) There were two new particle formation events in October 2 and 4.The formation rates of 11 nm particles were 4.3、3.0 cm-3s-1, and growth rates were 9.0、2.3 nm h-1 respectively. Strong solar energy and low RH can lead to new particle formation event.