Investigating three patterns of new particles growing to the size of cloud condensation nuclei in Beijing's urban atmosphere

摘要

The growth of newly formed particles with diameters from ～ ?10?nm to larger sizes was investigated in Beijing's urban atmosphere during 10–23?December?2011, 12–27?April?2012, and June–August 2014. In 11 out of 27 new particle formation (NPF) events during June–August, the maximum geometric median diameter ( D pgmax ) of newly formed particles exceeded 75?nm, and the grown new particles may contribute to the population of cloud condensation nuclei. In contrast, no apparent growth in new particles with D pgmax ? ?20?nm was observed in all of the events in December, in approximately half of the NPF events in April, and in only two events during June–August. New particles observed in the latter NPF events were too small to be activated as cloud condensation nuclei. Apparent new particle growth with D pgmax ? ≤ ?50?nm was observed in the remaining 18 NPF events. The 11 NPF events during June–August with D pgmax exceeding 75?nm were analyzed in detail. The particle growth patterns can be clearly classified into three types: one-stage growth and two-stage growth-A and growth-B patterns. The one-stage growth pattern is characterized by a continuous increase in D pg with D pgmax ? ≥ ?80?nm (4 out of 11 NPF events), and the two-stage growth-A and growth-B patterns are characterized by no apparent growth and shrinkage of particles, respectively, in the middle 2–4?h of the growth period (7 out of 11 NPF events). Combining the observations of gaseous pollutants and measured (or modeled) concentrations of particulate chemical species, the three growth patterns were discussed in terms of the spatial heterogeneity of NPF, formation of secondary aerosols, and evaporation of semivolatile particulates. Secondary organic species and NH 4 NO 3 were argued to be two major contributors to the growth of new particles, but NH 4 NO 3 likely contributed to growth only in the late afternoon and/or at nighttime.