Improving chemical mechanisms for ozone and secondary organic carbon

This report explores multiple strategies to improve the accuracy of predictions for secondary organic aerosol (SOA), nitrate, and ozone formation potential within regional chemical transport models. A statistical oxidation model (SOM) was used to explore the role of multigenerational oxidation chemistry and vapor wall loss corrections on predicted SOA concentrations. The SOM framework was incorporated into the UCD/CIT air quality model and tested for the conditions in Southern California with 8km resolution from July 20 to August 2, 2005, and in the eastern half th nd of the US with 36 km resolution from August 20 to September 2 , 2006. Results show that SOA concentrations predicted by the UCD/CIT-SOM model are very similar to those predicted by the standard two-product model used in CMAQ4.7 when both models use parameters that are derived from the same chamber data. Since the two-product model does not explicitly resolve multi-generational oxidation reactions, this finding suggests that the chamber data used to parameterize the models captures the majority of the SOA mass formation from multigenerational oxidation under the conditions tested. It was further observed that the use of low and high NOx yields perturbs SOA concentrations by a factor of two. This issue is probably a much stronger determinant of SOA concentrations in 3-D models than multi-generational oxidation. SOM calculations were also performed to quantify the effects of vapor wall losses that were not accounted for previously in chamber studies. Revised SOM fits were derived for chamber data under “low” and “high” vapor wall-loss rates to bound the range of possible values and compare with the results using the base case “no” vapor wall loss parameterization. Accounting for vapor wall losses substantially increased the simulated SOA concentrations in both the Southern California and East Coast domains, with predicted increases ranging from a factor of 2-10. Lower concentrations experienced the greatest increase. In Southern California, the predicted SOA fraction of total OA increases from ~0.2 (no) to ~0.5 (low) and to ~0.7 (high), with the high vapor wall loss simulations providing best general agreement with observations. The predicted absolute values and diurnal variability in the O:C and H:C atomic ratios also agreed better with observations for the high vapor wall loss simulations. In the eastern US, the SOA fraction is large in all cases but increases further when vapor wall losses are accounted for. Explicit reactions between biogenic VOCs and NOx were added to the base SAPRC11 mechanism to determine how they influence the predicted formation of secondary organic aerosol (including organic nitrates). These simulations used the CMAQ4.7 base two product model framework to predict SOA formation resulting from the additional gas-phase reactions. Simulations at 24/4 km resolution were conducted for conditions in Southern California during May 19-June 14, 2010 during the CALNEX field campaign and during Jan 16 – Feb 10, 2013 during the DISCOVER-AQ field campaign. The simulation results show that reactions between NOx and biogenic VOCs produce negligible SOA during winter conditions but may produce up to ~1 μg m -3 during summer conditions. The majority of the SOA produced through these pathways is monoterpene nitrates and glyoxal / methylglyoxal. First order estimates for the efficiency of control strategies suggest that a 25% reduction in NOx emissions would produce a