Understanding of the Fate of Atmospheric Pollutants Using a Process Analysis Tool in a 3-D Regional Air Quality Model at a Fine Grid Scale

Yang Zhang, Shiang-Yuh Wu

Department of Air Quality and Environmental Management, Las Vegas, USA.
Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, USA.

DOI: 10.4236/acs.2013.31004                

Abstract

The process analysis is performed for August and December, 2002 using the process analysis tool embedded in the Community Multiscale Air Quality (CMAQ) modeling system at a fine horizontal grid resolution of 4-km over an area in the southeastern U.S. that is centered at North Carolina. The objectives are to qunatify the contributions of major atmospheric processes to the formation of major air pollutants and provide the insights into photochemistry that governs the fate of these pollutants at a fine grid scale. The results show that emissions provide a dominant source for gases including ammonia (NH₃), nitric oxide (NO), nitrogen dioxide (NO₂), and sulfur dioxide (SO₂) and Particulate Matter (PM) species including fine PM (PM_2.5) and its composition such as sulfate, elemental carbon, primary organic aerosol, and other inorganic fine PM in both months. While transport acts as a major sink for NH₃, NO, and SO₂ at most sites and PM_2.5 and most of PM_2.5 composition at urban sites, it provides a major source for nitric acid (HNO₃) and ozone (O₃) at most sites in both months, and secondary PM species in August and most PM species in December at rural and remote sites. Gas-phase chemistry serves as a source for NO₂ and HNO₃ but a sink for O₃ at urban and suburban sites and for NO and SO₂ at all sites. PM processes contribute to the formation of PM_2.5 and nitrate () at the urban and suburban sites and secondary organic aerosol (SOA) at most sites in December and ammonium () in both months. They reduce formation at most sites in August and at rural and remote sites in December and the formation of PM_2.5 and SOA at most sites in August. Dry deposition is an important sink for all these species in both months. The total odd oxygen (O_x) production and the total hydroxyl radical (OH) reacted are much higher at urban and suburban sites than at rural sites. Significant amounts of OH are consumed by biogenic volatile organic compounds (BVOCs) in the rural and remote areas and a combination of anthropogenic VOCs (AVOCs) and BVOCs in urban and subareas areas in August and mainly by AVOCs in December. The amount of NO₂ produced by the reactions of hydroperoxy radical (HO₂) is similar to that of organic peroxy radical (RO₂) at all sites in August but higher than that by the reactions of RO₂ inDecember. The production rate of HNO₃ due to the reaction of OH with NO₂ dominates in both months. The ratio of the production rates of hydrogen peroxide (H₂O₂) and HNO₃ (PH₂O₂/PHNO₃) is a more robust photochemical indicator than the ratios of their mixing ratios (H₂O₂/HNO₃) and the afternoon mixing ratios of NO_y in both months, and it is highly sensitive to the horizontal grid resolution in August. The use of PH₂O₂/PHNO₃ simulated at 4-km indicates a VOC-limited O₃ chemistry in urban and suburban areas in August that was not captured in previous model simulations at a coarser grid resolution.

Keywords

Air Pollutants; Process Analysis; Photochemical Indicator; MM5; CMAQ

Conflicts of Interest

The authors declare no conflicts of interest.

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