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Abstract:
Long-term simulations with the coupled WRF-CMAQ (Weather Research and
Forecasting-Community Multi-scale Air Quality) model have been conducted
to systematically investigate the changes in anthropogenic emissions of
SO2 and NOx over the past 16 years (1995-2010) across the United States
(US), their impacts on anthropogenic aerosol loading over North America,
and subsequent impacts on regional radiation budgets. In particular,
this study attempts to determine the consequences of the changes in
tropospheric aerosol burden arising from substantial reductions in
emissions of SO2 and NOx associated with control measures under the
Clean Air Act (CAA) especially on trends in solar radiation. Extensive
analyses conducted by Gan et al. (2014a) utilizing observations (e.g.,
SURFRAD, CASTNET, IMPROVE, and ARM) over the past 16 years (1995-2010)
indicate a shortwave (SW) radiation (both all-sky and clear-sky)
"brightening" in the US. The relationship of the radiation brightening
trend with decreases in the aerosol burden is less apparent in the
western US. One of the main reasons for this is that the emission
controls under the CAA were aimed primarily at reducing pollutants in
areas violating national air quality standards, most of which were
located in the eastern US, while the relatively less populated areas in
the western US were less polluted at the beginning of this study period.
Comparisons of model results with observations of aerosol optical depth
(AOD), aerosol concentration, and radiation demonstrate that the coupled
WRF-CMAQ model is capable of replicating the trends well even though it
tends to underestimate the AOD. In particular, the sulfate concentration
predictions were well matched with the observations. The discrepancies
found in the clear-sky diffuse SW radiation are likely due to several
factors such as the potential increase of ice particles associated with
increasing air traffic, the definition of "clear-sky" in the radiation
retrieval methodology, and aerosol semi-direct and/or indirect effects
which cannot be readily isolated from the observed data.
