Urban fine particulate matter: Chemical composition and possible origins.

摘要

Fine aerosol plays an important role in the atmosphere's radiative transfer. Elevated fine aerosol concentration in urban areas can result in visibility reduction and human respiratory diseases. To study the chemical composition and possible origins of fine particulate matter (PM _2.5) in the Baltimore-Washington (B-W) corridor, chemically speciated PM_2.5 and trace gases (including NH₃, HNO₃, CO, SO₂, NO_y) have been measured at Fort Meade (FME: 39.10°N, 76.74°W; elevation 46 m MSL), Maryland over a three-year period (1999–2002) including nine seasonally-representative months. FME is suburban, located in the middle of the B-W corridor, and generally downwind of the highly industrialized Midwest. The PM_2.5 shows an annual mean concentration of ∼13 μg m−3. On average, over 90% of the PM_2.5 mass can be attributed to sulfate, nitrate, ammonium, carbonaceous material, and crustal material. The PM _2.5 reconstructed mass is found ±15% different from its gravimetric mass. Ammonium sulfate dominates (>50%) in the summertime high PM episodes while organic matter also contributes (∼30%). By comparing the FME data with concurrent measurements at upwind and downwind sites, sulfate and crustal material are found to be more regional than other major components.; The inorganic fraction of fine aerosol is studied using a thermodynamic model, ISORROPIA. FME changes from an ammonia-poor environment in summer to an ammonia-rich environment in winter. ISORROPIA also estimates the aerosol water content; water can contribute to >50% of the atmospheric extinction in the visible region on humid days.; The elemental carbon (EC)/CO ratio observed at FME is compared to those from individual vehicles, tunnel studies, and biomass burning to estimate the contribution from each potential source. Ambient temperature appears to influence emission factors. Based on a well-established CO emission inventory, the EC/CO ratio leads to an estimate of EC emission at 0.32 ± 0.12 Tg (EC) yr−1 for North America.; Using a factor analysis module, UNMIX, six factors that contribute to the PM_2.5 mass are resolved; these include regional sulfate, local sulfate, wood smoke, mobile, secondary nitrate, and copper/iron processing industry. The six factors are studied further using an ensemble back trajectory method to identify the possible source locations. Regional sulfate and wood smoke are more regional than other factors and associated with westerly and southerly transport, respectively. This study suggests that the local contribution to the PM_2.5 mass can vary from 30% in summer to >60% in winter.