Air Pollution as a Climate Forcing: A Workshop
Day 1 Presentations
Radiative Forcing and Photochemical Effects of Regional Aerosol Pollution
G. Stenchikov*, R. Park+, K. Pickering°, R. Dickerson°, G. Miguez-Macho*, A. Robock*, A. Miretzky*
* Dept. Environmental Sciences, Rutgers University, New Brunswick, NJ, U.S.A.
+ Div. Engineering and Applied Science, Harvard University, Oxford, MA, U.S.A.
° Dept. Meteorology, University of Maryland, College Park, MD, U.S.A.
During the period July 12-15, 1995, high O3 mixing ratios were registered in the Baltimore-Washington area and throughout the eastern United States [Dickerson et al., 1997; Ryan et al., 1998]. Ozone observations had a peak hourly-average O3 mixing ratio of 110 ppbv on July 12, increasing to 160 ppbv on July 15. A number of monitors observed values above the National Ambient Air Quality Standard (NAAQS) for O3 throughout the eastern part of United States during this period. Meteorological conditions on July 12-15 were hot and stable, typical for high smog episodes in this area. Synoptic-scale subsidence accompanying the high pressure system inhibited cloud formation and intensified atmospheric stability so that vertical mixing of low-level emissions was suppressed and therefore photochemical activity was enhanced. Late on July 15 a weak trough moved in, bringing a weak surface front southward and terminating the high ozone episode.
This pollution episode was also characterized by high concentrations of aerosols. The aerosol optical depth reached 0.7 in the visible and 2 in the near-UV. Sulfate aerosol particles scatter solar radiation back to space and cool the surface, but can increase the UV actinic flux. Observations and theoretical calculations show that UV-scattering particles in the boundary layer accelerate photochemical reactions (Fig. 1) and ozone production, but UV-absorbing aerosols inhibit photochemical processes. Our calculations with a Single Column Model [Park et al., 2001] and Chemical Transport Model [Dickerson et al., 1997] showed that for the conditions of July 12-15, 1995 aerosols caused an increase of ozone mixing ratio by about 20 ppbv. The increased ozone was transported north and by July 15 reached New Jersey, Delaware, and New York. At the same time aerosol solar radiative forcing was as large as -100 W/m2 (Fig. 2). This caused a cooling of the land surface and stabilized the atmospheric column. This example demonstrates that the regional impacts of aerosols on radiation need to include the effects of aerosols on photochemistry and resulting changes in tropospheric ozone. In addition, aerosols probably strongly affect the regional circulation, and we are currently conducting experiments with Regional Atmospheric Modeling System (RAMS) to quantify this effect.
- Dickerson, R. R., S. Kondragunta, G. Stenchikov, K. L. Civerolo, B. G. Doddridge, and B. N. Holbel, The impact of Aerosols on solar ultraviolet radiation and photochemical smog, Science, 278, 827-830, 1997.
- Park, R. J., G. L. Stenchikov, K. E. Pickering, R. R. Dickerson, D. J. Allen, and S. Kondragunta, Regional air pollution and its radiative forcing: Studies with a single-column chemical and radiation transport model, J. Geophys. Res., 106, 28,751-28,770, 2001.
- Ryan, W. F., B. G. Doddridge, R. R. Dickerson, R. M. Morales, K. A. Hallock, P. T. Roberts, D. L. Blumenthal, J. A. Anderson, and K. L. Civerolo, Pollutant transport during a regional O3 episode in the mid-Atlantic states, J. Air & Waste Manage. Assoc., 48, 786-797, 1998.