Chemical Characteristics of Aerosols in Coastal and Urban Ambient Atmospheres

ABSTRACT

Chemical characteristics of aerosols (PM₁ (on-line measurement) and PM_2.5 (off-line measurement) were investigated in coastal and urban atmospheres. Organics were the most dominant species in PM₁ at both sites, exhibiting little difference in the relative fractions of chemical constituents in PM₁ (organics, sulfate, black carbon (BC), nitrate, ammonium and chloride) between two sites. However, a clear difference in the types of organics was found between the two sites. The fraction of oxidized (aged) organics was much higher at coastal site than at urban site. The nitrate fraction significantly increased at both sites in PM_2.5 compared with that in PM₁, suggesting that a significant amount of nitrate exists at particle sizes of 1 µm–2.5 µm. Additionally, the aerosols observed at coastal site were acidic. At both sites, photochemical activity played an important role in enhancing sulfate and oxidized organics in the afternoon, thereby overcoming the dilution effect. More distinct diurnal patterns were observed for nitrate, BC and organics at the urban site compared to the coastal site. Chemical characteristics also varied with different air masses. The highest PM concentration was associated with the northwest continental air mass (the air mass passed over heavy industrial areas before arriving at the site and moved slowly compared to other air masses). Three PM events (sulfate-dominant versus organic-dominant) were observed during the sampling periods and were considered long-range transport (LTP) events. The water-soluble organic carbon (WSOC) and oxidized organic contents significantly increased during LTP events, suggesting that organics were highly aged during transport.