Spatial and Temporal Variations in Characteristic Ratios of Elemental Carbon to Carbon Monoxide and Nitrogen Oxides across the United States

A ratio-based method is used to characterize anthropogenic elemental carbon (ECa) using in situ measurements and emissions of carbon monoxide (CO) and nitrogen oxides (NOx). We use long-term records of ground-based measurements from the U.S. Environmental Protection Agency (EPA) Air Quality System and Interagency Monitoring of Protected Visual Environments to assess the patterns in anthropogenic combustion ratios (ΔECa/ΔCO and ΔECa/ ΔNOx) across the U.S. Petroleum Administration for Defense Districts (PADD) regions for the years 2000−2015. We investigate the change in these ratios between the periods 2000−2007 and 2008−2015. Overall, ΔECa/ΔCO ratios increase by 0.7−82% and ΔECa/ΔNOx by 6.8−104% across the East and West PADD regions. The urban West showed the largest increase relative to other regions. This is mainly attributed to a 13−23% increase in ΔECa during the winter and fall seasons and significant reductions in urban ΔNOx (except in winter). We also find that emission ratios derived from the EPA’s National Emission Inventory (NEI) overestimate (underestimate) the increase in the observed enhancement ratios in the East (West). Analyses of changes in NEI emissions in the West reveal (a) smaller reductions in NEI emissions for NOx from the off-road sector and (b) an increase in PM2.5 (particulate matter 2.5 μm or less in diameter) emissions from commercial/residential combustion and smaller reductions in nonroad emissions.