[Effects of high temperature, high relative humidity and rain process on particle size distributions in the summer of Beijing].

On-line measurements of particle number size distribution were conducted during 2004-07-13 - 2004-08-23 by using TDMPS-APS system in Beijing; in between the size distributions of particle mass and chemical compositions were measured by using MOUDI cascade impactor under three kinds of weather conditions: (1) high temperature and high relative humidity night, (2) high temperature and high relative humidity day, and (3) clear days. The results indicate that particulate matter, especially fine particle pollution becomes severe under high temperature and high relative humidity conditions, which results in low visibility (2.5km); the average concentrations of PM1.8 and PM10 are 170.68 microg/m3 and 249.35 microg/m3, respectively, PM1.8 accounts for 68% of PM10; number concentration mainly concentrates in the range of 50 - 100nm, about 2 x 10(4) - 3 x 10(4)cm(-3); rain process has a great effect on scavenging particles, especially fine particles. After the rain process PM10 is 3 times lower and PM1.8 is 6 times lower than the values before the rain; in the following clear day, new particle formation is observed, the newly formed particles (3 - 20nm) grow to 50 - 100nm rapidly. After that, the pollutants start to accumulate and it becomes heavy polluted in few days.