Reversal of Trends in Black Carbon Concentrations across the United States

Black carbon is both an important climate forcer and an air pollutant hazardous to human health. Following the implementation of the Clean Air Act Amendments, dramatic reductions in anthropogenic emissions of black carbon in the United States due to regulations of power plant and automobile emissions resulted in a decrease in black carbon concentrations. However, decreasing trends in black carbon concentrations in the contiguous United States (CONUS) have been disrupted in recent years, changing from a significant decreasing trend of -0.13 μg m^-3 decade^-1 (2006-2015) to a significant increasing trend of +0.13 μg m^-3 decade^-1 (2015-2022). This trend reversal occurs even when accounting for the influence of wildfire smoke. The trend reversal is widespread geographically and occurs in all seasons. Anthropogenic black carbon emissions from the EPA’s National Emissions Inventory (NEI) decrease from 2016 to 2022 despite increasing black carbon concentrations, suggesting an underestimate in black carbon emissions. We use the GEOS-Chem global chemical transport model and surface observations from the Chemical Speciation Network (CSN) and the Interagency Monitoring of Protected Visual Environments (IMPROVE) network to analyze the discrepancies between the black carbon emissions and observations. We conduct simulations for 2002-2022 to examine the relative contributions of source-specific emissions from transportation, residential wood combustion (RWC), and biomass burning emissions, as well as long-range transport of non-domestic black carbon emissions. We investigate how these sources may contribute to the reversal of trends in black carbon concentrations.