Trends in Seasonal Mean Speciated Aerosol Composition in Remote Areas of the United States From 2000 Through 2021

Abstract

AbstractLarge reductions in anthropogenic emissions of particulate matter and its precursor emissions have occurred since the enactment of the Clean Air Act Amendments of 1990. The Interagency Monitoring of Protected Visual Environments network has measured PM2.5 gravimetric mass (mass of particles with aerodynamic diameters less than 2.5 μm, also referred to here as fine mass, “FM”) and speciated PM2.5 aerosol composition at remote sites since 1988. Measured species include inorganic anions such as sulfate, nitrate, and chloride, carbonaceous aerosols such as organic (OC) and elemental carbon (EC), and elemental concentrations used to estimate fine dust (FD). Trends in seasonal and annual mean mass concentrations were calculated from 2000 through 2021, a period that includes the largest reductions in emissions. On average, annual mean FM at remote sites in the continental United States has decreased at a rate of −1.8% yr−1. This reduction is largely due to annual mean trends in sulfate (−6.1% yr−1), nitrate (−2.7% yr−1), EC (−2.2% yr−1), FD (−1.3% yr−1), and OC (−0.9% yr−1), although the OC annual mean trend was insignificant. Seasonal and regional mean FM trends varied significantly, with strong reductions in the East in all seasons due to sulfate reductions, and flat and insignificant trends in summer and fall in the West due to the influence of biomass burning emissions on OC trends. Evaluating regional and seasonal mean trends in aerosol composition helps identify sources that continue to adversely impact air quality and hinder progress in FM reductions due to successful regulatory activity.