A study led by scientists at the US Department of Energy’s Pacific Northwest National Laboratory found that, in the western United States, increases in aerosols from East Asia decreased the radiative warming effect induced by reductions in US emissions by 25% from 1980-2014. The open-access study appears in the AGU journal Earth’s Future.

Because they can either absorb or reflect energy, aerosols are important components influencing air quality and atmospheric changes. Quantifying the source of aerosols and their influence on energy balance is necessary for predicting future air quality and Earth system changes. As industrial aerosols decrease in the United States, foreign emissions become increasingly influential on the energy balance over the country.

Long‐term (1980–2014) trends and aerosol source apportionment are quantified in this study using a global aerosol‐climate model equipped with an explicit aerosol source tagging technique. Due to US emission control policies, the annual mean near‐surface concentration of particles, consisting of sulfate, black carbon, and primary organic aerosol, decreases by about −1.1 (±0.1)/−1.4 (±0.1) μg/m³ in western United States and −3.3 (±0.2)/−2.9 (±0.2) μg/m³ in eastern United States during 2010–2014, as compared to those in 1980–1984.

Meanwhile, decreases in US emissions lead to a warming of +0.48 (±0.03)/+0.46 (±0.03) W/m² in western United States and +1.41 (±0.07)/+1.32 (±0.09) W/m² in eastern United States through changes in aerosol direct radiative forcing. Increases in emissions from East Asia generally have a modest impact on US air quality but mitigated the warming effect induced by reductions in US emissions by 25% in western United States and 7% in eastern United States.

—Yang et al.

In the study, the researchers quantified the relative roles of multi-decadal US pollution control programs and changes in emissions in other regions of the world on aerosol concentrations and energy balance over the continental United States for the time period of 1980-2014.

To do this, they used an aerosol source tagging capability in the Community Atmosphere Model (version 5), a global aerosol-climate model. Researchers found that increases in emissions from East Asia decreased the warming effect (e.g., radiative forcing) induced by reductions in US aerosol emissions by 25% in the western United States, but had a more modest impact on US air quality through changes in surface concentrations.

East Asia has a much larger contribution to aerosols in the free troposphere over the United States, where they have offset much of the warming forcing in western United States that would otherwise have been associated with reductions in US domestic emissions. As US domestic emissions continue to decrease due to current polices, the relative impact of foreign emissions may well continue to increase. It is, therefore, important for future projections to consider the potential impact of foreign emissions on radiative forcing, but also on surface concentrations, particularly in western United States.

—Yang et al.

This research was supported by the National Aeronautics and Space Administration’s Atmospheric Composition Modeling and Analysis Program and by the U.S. Department of Energy (DOE) Office of Science, Biological and Environmental Research, as part of the Regional and Global Climate Modeling program.

Resources

• Y. Yang, H. Wang, S.J. Smith, R. Zhang, S. Lou, H. Yu, C. Li, P.J. Rasch (2018) “Source Apportionments of Aerosols and Their Direct Radiative Forcing and Long-Term Trends Over Continental United States.” Earth’s Future 6 doi: 10.1029/2018EF000859