Application of Satellite NO2 Retrievals to the Uncertainty Analysis of Sector-Specific NOx Emissions

Having a reliable nitrogen oxides (NO_x) emission inventory is very important because nitrogen dioxide (NO₂) is a criteria air pollutant and NO_x participate in the formation of other pollutants such as ozone (O₃) and particulate matter. The uncertainties in the 2011 US National Emissions Inventory (NEI) have been assessed in several studies, and the on-road mobile sector has been identified as a major source of uncertainty, with its NO_x emissions overestimated probably by as much as a factor of two. In this study, satellite retrievals of NO₂, more specifically Ozone Monitoring Instrument (OMI) NO₂, were used to evaluate NO_x emissions over the Great Lakes Region. Slant column NO₂ measurements were converted to vertical column NO₂ using vertical NO₂ profiles derived from simulations by the Community Multiscale Air Quality (CMAQ) model. When compared to the NO₂ columns simulated by CMAQ, it was found that CMAQ tended to overestimate NO₂ column in rural areas while underestimation occurred in urban areas. A sensitivity simulation with a 50% reduction of NO_x emissions from on-road mobile sources reduced the discrepancies between simulated NO₂ columns and satellite retrievals. In this presentation, we will present results from a detailed sensitivity analysis using the Decoupled Direct Method (DDM) and relate the differences in NO₂ columns to NO_x emissions from different sectors. DDM yields the contribution of various NO_x emission sectors to NO₂ concentrations. The correlation of the column differences between the simulations and satellite retrievals with the contributions will be used to evaluate the uncertainties in NO_x emissions by source sector, particularly those from mobile sources. The results will be compared to those of similar analyses conducted by using ground-based measurements of NO₂.