Assessment of long-term WRF–CMAQ simulations for understanding direct aerosol effects on radiation "brightening" in the United States

Abstract

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 SO 2 and NO x over the past 16 years (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(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 SO 2 and NO x 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 allsky 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.

Extending the Community Multiscale Air Quality (CMAQ) modeling system to hemispheric scales: overview of process considerations and initial applications

“…The WRF configuration over the Northern Hemisphere also used MODIS land use classification with 20 categories, RRTMG shortwave radiation (SWR) and longwave radiation schemes (Iacono et al, 2008), and the ACM2 planetary BL model (Pleim, 2007). WRF's simulation of hourly surface temperature, relative humidity, and wind speed and direction was evaluated by Xing et al (2015a) through comparison with observations from NOAA's National Centers for Environmental Information (NCEI) Integrated Surface Data and no significant bias in the meteorological fields was detected.…”

“…Year-specific emission inputs were derived from the Emission Database for Global Atmospheric Research (EDGAR, version 4.2) database (European Commission, 2011) as discussed earlier in Sect. 2.3.1 and detailed in Xing et al (2015a). Satellite-based tropospheric NO 2 measurements now provide valuable observable information on the changing emission patterns and air quality across the globe (e.g., Richter et al, 2005;van der A et al, 2008).…”

“…Monthly lightning NO x emissions were distributed evenly to each hour of each simulation day. Xing et al (2015a) further describe the processing of global emission inventories for CMAQ, including temporalization of the annual estimates to hourly model inputs, vertical distributions of anthropogenic and lightning emissions, and speciation of PM 2.5 and nonmethane volatile organic compound emissions to model primary aerosol constituents and gas-phase species. Emissions of NO from soil or SO 2 from volcanos are not considered in the applications presented here.…”

“…The hemispheric CMAQ model is now being used for a variety of process-based air pollution studies across the Northern Hemisphere over seasonal (e.g., Mathur et al, 2014;Sarwar et al, 2014) to decadal (Xing et al, 2015a) timescales. In this section we present an overview of these diverse evolving applications with the hemispheric CMAQ model.…”

“…Since EDGARv4.2 provides only PM 10 emissions, PM 2.5 emissions were estimated by deriving the ratio of PM 2.5 to PM 10 from the 2000-2005 EDGAR HTAP (Hemispheric Transport of Air Pollution, version 1) inventory (Janssens-Maenhout et al, 2012) and then applying this ratio to partition EDGARv4.2 PM 10 emissions into PM 2.5 and PM 2.5-10 (Xing et al, 2015a). In applications to date, biogenic volatile organic compound (VOC; Guenther et al, 1995) and lightning NO x (Price et al, 1997) emissions were obtained from GEIA (Global Emissions Inventory Activity; http://www.geiacenter.org).…”