Correlation of aerosol nucleation rate with sulfuric acid and ammonia in Kent, Ohio: An atmospheric observation

Abstract

New particle formation (NPF) occurs in various atmospheric environments, and these newly formed particles have the potential to grow to cloud condensation nuclei. But at present it is unclear which chemical species are involved in aerosol nucleation and growth, in part because there are only a limited number of simultaneous measurements of aerosol precursors and aerosol size distributions. Observations of ambient aerosol size distributions, sulfuric acid, and ammonia were made for over a year in Kent, Ohio, a relatively less polluted continental environment. Particle sizes in the diameter range from 3 to 1000 nm were measured continuously through the whole year, while sulfuric acid and ammonia were measured seasonally with two chemical ionization mass spectrometers. Strong NPF events were more frequently found during the spring and fall and less frequently during the summer and winter. The median of measured sulfuric acid was higher in spring (5.2 × 106 cm−3) and summer (2.9 × 106 cm−3) than in winter (6 × 105 cm−3) and fall (5 × 105 cm−3). We have used the inverse model Particle Growth and Nucleation to derive aerosol nucleation and growth rates from the measured aerosol size distributions. Nucleation rates derived during the NPF events ranged from 1.4 to 12.9 cm−3 s−1 and were proportional to the measured sulfuric acid concentration with a power of 0.6–2.3. Our results show that sulfuric acid is an important aerosol nucleation precursor, but only a small fraction of the aerosol growth rates could be explained by the condensation of sulfuric acid alone. Ammonia mixing ratios did not have a diurnal trend but had some seasonal variations, higher in spring than in fall and winter, typically at sub‐ppbv level; aerosol nucleation rates did not show a clear correlation with ammonia at least at this sub‐ppbv level at sub‐ppbv level, mostly because the ammonia mixing ratios were nearly constant. Our observations also indicate that the role that ammonia plays in aerosol nucleation is more complicated than is currently understood by the aerosol nucleation theories.