Characteristics of carbonaceous aerosols at a pair of suburban and downtown sites in Nanjing, China

Abstract:

Carbonaceous species in PM_2.5 were measured in three seasons (except winter) in downtown and around a year in suburban in Nanjing. In particular, the OC and EC concentrations were continuously measured with 1-h interval at both sites. The mean concentrations of PM_2.5, OC, EC were 85.9 μg/m³, 10.4 μg/m³, 4.2 μg/m³ and 63.4 μg/m³, 8.1 μg/m³, 5.3μg/m³ for downtown and suburban respectively. At the suburban site, compared to the lowest average concentration in spring, the relatively higher concentration of carbonaceous aerosol in summer was caused by biomass-burning activities. Significant increasing peaks of OC (up to 122.9 ug/m³) and EC (up to 35.8 ug/m³) in suburban were recorded in the June 10^th night, and it is indicated using back trajectory that the air mass came through nearby agriculture areas. On the other hand, the OC and EC concentrations during the 2^nd AYG (Asian Youth Games, Aug 16^th - 24^th in 2013) compared to that in rest days in August reduced by 21% and 10%, respectively, attributing to the effective control measures on reduction of industrial point sources, temporary control of motor vehicle and fugitive dusts. Rations of OC/EC in suburban were much lower than that in downtown. This might be due to the fact that the sampling point in suburban is quite close (in 400 meters) to a main highway. Both OC and EC concentrations in suburban were higher in nighttime than daytime, except OC in summer, indicating the existence of strong SOC and supported by high OC/EC and O₃ at noon. The annual average SOC estimated by EC-tracer methods occupied 34.5% and 36.7% of the total OC for the downtown and suburban areas, respectively. OC/EC correlations in suburban were much weaker than those from downtown areas, which shows suburban areas is much more affected by long-range transport of pollution and/or complex sources including domestic coal combustion, biomass burning, industrial and transportation, especially in three other seasons except winter, which is also supported by poor correlations between carbonaceous species and gaseous pollutants (NO₂, SO₂, CO) in spring at suburban site.