| dc.description | Aerosol physical properties and chemical composition and concurrently measured gasphase species and meteorological data at a rural site in Egbert, about 70 km to the north of Toronto in southern Ontario, in the spring of 2003, were analyzed to gain insights into the properties of rural aerosols and the processes governing their changes. The properties of rural aerosol were compared with the same of urban aerosol measured at Toronto. The intercomparisons among co-located aerosol instruments showed reasonable agreement, given the upper cut-off size of different instruments were considered. Aerosol chemical composition measured with the aerosol mass spectrometer revealed that exceptionally high nitrate mass concentrations were observed at Egbert, and the organic material contributed, on average, the highest fraction to aerosol mass for the sampling period. The ammonium, nitrate, sulfate and the organics at both sites had a mass modal vacuum aerodynamic diameters around 400-500 nm, however, a small organic mass mode at 100-200 nm was occasionally observed at Egbert, when the site was exposed to fresh urban air masses. Organic material dominated the composition of urban aerosol. The total organic material had two modes centered at 400 nm and 150 nm, with a significant amount of organic mass in the small mode. At both sites, the small organic modes were composed of mainly hydrocarbon-like substances, indicating that the combustion related emissions were the likely sources of small organic particles, whereas the larger mode organic particles were composed of mainly oxygenated materials. The nitrate mass concentrations averaged diurnally exhibited higher values during dark hours and a minimum in the afternoon, whereas sulfate and the oxygenated organics were highest during the afternoon period, indicative of photochemical processing. Total organics in the aerosol from the north showed a gradual increase with increasing temperature, likely associated with increasing biogenic emissions. The oxygenated organic fraction increased rapidly within 48 hours of photochemical age, after which there was little change, while the mass of sulfate increased continuously with time, suggesting that the condensation of sulfates onto organic particles impeded the increase in oxygenated organic fraction. Thus, within 1-2 day of photochemical age, the ability of these particles to act as cloud condensation nuclei (CCN) is controlled by inorganic species such as sulfates. | en_US |
