Atmospheric Processes. Hypothesis 3.1 Aerosol nucleation (biogenic precursors or SO2) can be a major source of aerosol number in both urban and rural areas in the study region.
Atmospheric Processes. The SOA contribution to the total organic PM concentration in Pittsburgh varies from around 10% during the winter months to around 50% of the total OC concentration in the summer months (Xxxxxx et al., 2002; Xxxxxx et al., 2004a; Xxxxxx et al., 2005). The SOA was estimated to be 35±15% of the OC during July 2001 (Xxxxxx et al., 2004a; Xxxxxx et al., 2005; Xxxxxxxx et al., 2005). On a daily basis the SOA contribution is quite variable from almost zero during the low OC days to more than 50% during the high PM2.5 episodes (Xxxxxx et al., 2004a). [HYPOTHESIS 3.5] • The annual average SOA contribution to the organic PM was estimated to be around 30% (Xxxxxxxx et al., 2005). [HYPOTHESIS 3.5] • The ammonium nitrate concentrations in Western Pennsylvania are quite sensitive to the ammonia levels and reductions in PM2.5 concentrations may be assisted by reductions in ammonia emissions during both the summer and winter (Xxxxxxxx et al., 2004). [HYPOTHESIS 3.4] • Regional scale formation of ultrafine particles (nucleation) takes place in Western Pennsylvania on 30% of the days of the year, during all seasons but it is most frequent in fall and spring and least frequent in winter. Regional nucleation is most common on sunny days with below average PM2.5 concentrations. Twenty- four-hour average number concentrations were approximately 40% higher on days with nucleation compared to those without. (Xxxxxxx et al., 2004a). [HYPOTHESIS 3.1] • The available aerosol thermodynamic models can reproduce the observed nitrate partitioning within experimental error (Xxxxxxxx et al., 2004). • The frequent nucleation events can be explained by a ternary sulfuric acid- ammonia-water nucleation model. The results are consistent with the composition of the fresh ultrafine particles of Xxxxx et al., (2004). Reductions of ammonia emissions are predicted to decrease the frequency of nucleation events during both summer and winter, with a more dramatic effect during the summer. Reductions of sulfur dioxide and the resulting sulfate by up to 40% are expected to increase the frequency of nucleation during the summer and decrease it during the winter (Xxxxxx et al., 2005a). [HYPOTHESIS 3.1]
