New Constraints on the Sources Contributing to PAN in the North American Free Troposphere from TES Observations and GEOS-Chem Simulations

Peroxyacetyl nitrate (PAN) is the most important reservoir for nitrogen oxide radicals (NO_x = NO + NO₂) in the troposphere, and it plays a critical role in the redistribution of NO_x to remote regions. We present an analysis of Tropospheric Emission Spectrometer (TES) peroxyactyl nitrate (PAN) and co-located carbon monoxide (CO) observations over North America during summer (July) 2005 to 2010.  We segregate and examine the abundance of PAN relative to CO in TES observations located within smoke plumes identified by the NOAA Hazard Mapping System (HMS) or in close proximity to active fires. Though CO is enhanced in retrievals impacted by smoke, we find no systematic difference in the PAN when all successful PAN retrievals are segregated by the presence of smoke in the atmospheric column. We compare the PAN enhancement relative to CO in smoke-impacted samples and find that a large fraction of the enhancement ratios calculated from the TES data are larger than pseudo-emission factors for PAN derived from prior aircraft observations. PAN is a notoriously difficult compound to simulate due to the complexity of PAN chemistry and its sensitivity to vertical transport. We compare summertime TES PAN data over North America to the GEOS-Chem PAN simulation to show that the model appears to systematically underestimate the contribution of fires to elevated PAN during summer months over western North America, and it also overestimates the amount of PAN observed by TES over the southeastern U.S. Improved model agreement over the southeastern U.S. is achieved by reducing NO_x emissions in this region and making a series of updates to the treatment of isoprene oxidation chemistry. We also present a series of simulations varying the amount of biomass burning NO_x emissions that are immediately partitioned to PAN, and we find that the model continues to be unable to reproduce TES PAN observations over many regions/years with active wildfires. This appears to be due in part to missing fires in the Global Fire Emissions Database (GFED).