Observations of nucleation-mode particle events and size distributions at a rural New England site

Particle number (PN) concentrations, size distributions, and composition were measured continuously at the University of New Hampshire atmospheric observatory at Thompson Farm from 9 February to 4 April, 2007. A total of thirteen PN events, identified by PN concentration statistics, were observed during this two-month period. Events were subdivided into five different event types based on event duration and secondary measurements. Four of the five event types were observed, with the majority of events consisting of long-lived episodes, greater than 5 h in duration, associated with air mass backward trajectories from the clean northwest. Particle size distributions indicated increased number concentrations of nucleation-mode particles during event periods and particle growth of the nucleation-mode fraction. Particle diameter growth rates, determined from the geometric mean diameter of the 6–60 nm particle fraction, ranged between 0.9 and 5.5 nm h−1. All events displayed steady particle growth rates, indicating the regional nature of PN events. An Aerodyne quadrupole Aerosol Mass Spectrometer indicated that pre-existing aerosol concentrations were decreased on PN event days (compared to non-event days), as were temperature and absolute humidity; in contrast, solar radiation was increased on event days. Finally, theoretical particle growth rates from oxidation products of SO2, toluene, isoprene, and α-pinene were calculated and compared to measured growth rates. These species were able to account for up to 42.4% of the observed growth.