Characterizing CO and NOy sources and relative ambient ratios in the Baltimore area using ambient measurements and source attribution modeling

Modeled source attribution information from the Community Multiscale Air Quality model was coupled with ambient data from the 2011 Deriving Information on Surface conditions from Column and Vertically Resolved Observations Relevant to Air Quality Baltimore field study. We assess source contributions and evaluate the utility of using aircraft measured CO and NOy relationships to constrain emission inventories. We derive ambient and modeled CO:NOy ratios that have previously been interpreted to represent CO:NOy ratios in emissions from local sources. Modeled and measured CO:NOy are similar; however, measured CO:NOy has much more daily variability than modeled values. Sector-based tagging shows that regional transport, on-road gasoline vehicles, and nonroad equipment are the major contributors to modeled CO mixing ratios in the Baltimore area. In addition to those sources, on-road diesel vehicles, soil emissions, and power plants also contribute substantially to modeled NOy in the area. The sector mix is important because emitted CO:NOx ratios vary by several orders of magnitude among the emission sources. The model-predicted gasoline/diesel split remains constant across all measurement locations in this study. Comparison of CO:NOy to emitted CO:NOy is challenged by ambient and modeled evidence that free tropospheric entrainment, and atmospheric processing elevates ambient CO:NOy above emitted ratios. Specifically, modeled CO:NOy from tagged mobile source emissions is enhanced 5-50% above the emitted ratios at times and locations of aircraft measurements. We also find a correlation between ambient formaldehyde concentrations and measured CO:NOy suggesting that secondary CO formation plays a role in these elevated ratios. This analysis suggests that ambient urban daytime CO:NOy values are not reflective of emitted ratios from individual sources.