Abstract
Two elevated particle number/mass growth events associated with Aitken-mode particles were observed during a sampling campaign (13-29 September 2004) at the Duke University Free-Air CO2 Enrichment facility, a forested field site located in suburban central North Carolina. Aerosol growth rates between 1.2 and 4.9 nm hr(-1) were observed, resulting in net increases in geometric mean diameter of 21 and 37 nm during events. Growth was dominated by addition of oxidized organic compounds. Campaign-average aerosol mass concentrations measured by an Aerodyne quadrupole aerosol mass spectrometer (Q-AMS) were 1.9 +/- 1.6 (sigma), 1.6 +/- 1.9, 0.1 +/- 0.1, and 0.4 +/- 0.4 mu g m(-3) for organic mass (OM), sulfate, nitrate, and ammonium, respectively. These values represent 47%, 40%, 3%, and 10%, respectively, of the measured submicron aerosol mass. Based on Q-AMS spectra, OM was apportioned to hydrocarbon-like organic aerosol (HOA, likely representing primary organic aerosol) and two types of oxidized organic aerosol (OOA-1 and OOA-2), which constituted on average 6%, 58%, and 36%, respectively, of the apportioned OM. OOA-1 probably represents aged, regional secondary organic aerosol (SOA), while OOA-2 likely reflects less aged SOA. Organic aerosol characteristics associated with the events are compared to the campaign averages. Particularly in one event, the contribution of OOA-2 to overall OM levels was enhanced, indicating the likelihood of less aged SOA formation. Statistical analyses investigate the relationships between HOA, OOA-1, OOA-2, other aerosol components, gas-phase species, and meteorological data during the campaign and individual events. No single variable clearly controls the occurrence of a particle growth event.