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Abstract A three-dimensional chemical transport model, PMCAMx, was used to quantify the contribution of biomass burning organic aerosol (bbOA) to total organic aerosol (OA) concentrations in the continental U.S. Simulations were performed during three seasonally-representative months (April, July, and September 2008). Biomass burning is predicted to contribute 9% of the domain-averaged OA in April, 28% in July and 13% in September. Secondary OA from semivolatile and intermediate volatility organic compounds is the largest contributor to bbOA concentrations for all months simulated, followed by fresh primary OA. PMCAMx bbOA predictions using the Volatility Basis Set were compared to the results of CAMx version 6.10 that assumes that bbOA is nonvolatile and inert. PMCAMx predicts 66%, 113%, and 108% higher average concentration values of bbOA for April, July, and September, respectively. Predicted OA concentrations at biomass-impacted sites were compared to observed values from the STN and IMPROVE networks. The treatment of biomass burning emissions as semivolatile and reactive improved model performance during the spring and summer. During the fall both models overpredicted the OA levels, suggesting that the fire emissions may have been overestimated. In this case the additional SOA produced in PMCAMx resulted in worse performance. Highlights • Treatment of biomass burning emissions as semivolatile improved model performance. • Biomass burning contributes 9% of the OA in April, 28% in July, 13% in September. • Secondary biomass burning OA can be more important on average than primary. [ABSTRACT FROM AUTHOR]

Source contributions to ultrafine particle number concentrations for a summertime period in the Eastern U.S. are investigated using the chemical transport model PMCAMx-UF. New source-resolved number emissions inventories are developed for biomass burning, dust, gasoline automobiles, industrial sources, non-road and on-road diesel. According to the inventory for this summertime period in the Eastern U.S., gasoline automobiles are responsible for 40% of the ultrafine particle number emissions, followed by industrial sources (33%), non-road diesel (16%), on-road diesel (10%), and 1% from biomass burning and dust. With these emissions as input, the chemical transport model PMCAMx-UF reproduces observed ultrafine particle number concentrations (N 3-100 ) in Pittsburgh with an error of 12%. For this summertime period in the Eastern U.S., nucleation is predicted to be the source of more than 90% of the total particle number concentrations. The source contributions to primary particle number concentrations are on average similar to those of their source emissions contributions: gasoline is predicted to contribute 36% of the total particle number concentrations, followed by industrial sources (31%), non-road diesel (18%), on-road diesel (10%), biomass burning (1%), and long-range transport (4%). For this summertime period in Pittsburgh, number source apportionment predictions for particles larger than 3 nm in diameter (traffic 65%, other combustion sources 35%) are consistent with measurement-based source apportionment (traffic 60%, combustion sources 40%). [ABSTRACT FROM AUTHOR]

A three-dimensional chemical transport model, PMCAMx-SR, was applied in the continental U.S. to investigate the contribution of prescribed burning and other biomass burning sources to the total organic aerosol (OA) concentrations. Simulations were performed during three seasonally-representative months (April, July and September 2008). Prescribed biomass burning organic aerosol (bbOA) emission rates were estimated to be approximately 300, 80 and 250 tn d−1 during April, July and September respectively. Prescribed burning was limited during July due to the prevailing weather conditions. Prescribed burning is responsible for 45% of the total bbOA emissions during the simulated spring period, 20% during the fall, and just 3% during the summer. The predicted maximum hourly prescribed bbOA concentration reached approximately 100 μg m−3 during April in South Carolina and during September in New Mexico, whereas it was less than 15 μg m−3 during July in Arkansas. The average contribution of prescribed burning to OA in the continental U.S. is quite low (less than 5%) and 60–70% is predicted to be secondary OA. Biomass burning secondary organic aerosol (bbSOA) levels far away from the prescribed fires are predicted to exceed 2 μg m−3. In states with intensive prescribed burning activities this source is predicted to contribute 20–40% of the OA on a monthly average basis. In these states (for example South Carolina during April), the predictions reveal that people can be exposed for 20 h per month on average to bbOA levels exceeding 5 μg m−3 and for 200 h to levels above 0.5 μg m−3. This should be contrasted to 220 h of exposure to bbOA above 5 μg m−3 and 650 h to bbOA above 0.5 μg m−3 for states and periods with intense wild fires (for example California during July). • Prescribed burning contributes 20–40% of organic aerosol in some US states. • The average contribution of prescribed burning to OA in the US is less than 5%. • On average 60–70% is predicted to be secondary OA. • Exposures to OA from prescribed burning are significant but lower than wildfires. [ABSTRACT FROM AUTHOR]