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

Abstract

Large 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 PM _2.5 gravimetric mass (mass of particles with aerodynamic diameters less than 2.5 μm, also referred to here as fine mass, “FM”) and speciated PM _2.5 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.

Plain Language Summary

Particulate matter in the atmosphere is made up of many species that have both anthropogenic and natural sources. Thanks to the Clean Air Act Amendments of 1990, anthropogenic emissions that lead to some particulate matter (PM) have decreased, which has resulted in measurable improvements in air quality in remote regions of the United States. Evaluating trends in aerosol measurements from a large‐scale monitoring network over the past two decades has shown that at remote sites in the United States, some aerosol species, like sulfates, nitrates, and some carbonaceous aerosols, have decreased significantly due to the emission reductions; but others, like mineral dust and carbonaceous aerosols from wildfire smoke, have not. In order to continue to make progress in improving air quality in the United States, targeting future sources for emission reductions will require accurate assessments of the contributions from these sources. Dust and wildfire smoke contributions to PM in remote locations across the United States are now larger fractions compared to two decades ago and will likely continue to grow with climate change.

Key Points

• Fine mass seasonal mean concentrations have significantly decreased in remote regions of the U.S. in response to regulatory activity

• Sulfate aerosols have decreased at the highest rate, followed by nitrate, elemental carbon, fine dust, and organic carbon

• Flat and insignificant trends in organic carbon and fine mass at western sites in summer/fall were influenced by biomass smoke emissions