Background
The authors note that "anthropogenic aerosols have more efficient optical extinction for light than natural aerosols (Carrico et al., 2003; Seinfeld and Pandis, 1998) and are responsible for about half of the extinction and scattering of light by particles globally (Houghton et al., 2001)."  These facts are extremely important, because it has recently been demonstrated that increases in atmospheric aerosols, although reducing the total flux of solar radiation received at the earth's surface, increase the flux of diffuse light received there, such that the volume of shade within forest canopies is so greatly decreased that it enhances whole-canopy photosynthesis to the point that more CO2 is extracted from the atmosphere under high-aerosol-content conditions than during more pristine conditions.  See, for example, our reviews of the papers of Roderick et al. (2001), Law et al. (2002) and Gu et al. (2003).

What was done
In a study that further explores this subject, Misson et al. (2005) investigated the effects of anthropogenic aerosols on the net ecosystem exchange of Blodgett Forest, which is situated on the western slope of California's Sierra Nevada Mountains downwind of significant anthropogenic pollution sources in Sacramento and the agricultural Central Valley of California.

What was learned
It was determined that the recurrent influx of air pollution from California's Central Valley to the Blodgett Forest site during summer afternoons decreased total irradiance there by 11% from what it was at comparable times relative to solar noon in the mornings.  Diffuse radiation, on the other hand, was 24% higher in the afternoons; and this effect predominated to the extent, in the words of the authors, that "aerosol loading caused net uptake of CO2 by the forest to increase by 8% in the afternoon."

What it means
This study provides yet another independent verification of the fact that the presence of atmospheric aerosols not only reduces the receipt of solar radiation at the earth's surface, providing thereby an impetus for cooling, it also enhances forest rates-of-removal of CO2 from the atmosphere, which over the long term provides yet another cooling effect, while simultaneously providing all of the biological benefits associated with enhanced forest productivity.

References
Carrico, C.M., Bergin, M.H., Xu, J., Baumann, K. and Maring, H.  2003.  Urban aerosol radiative properties: measurements during the 1999 Atlanta supersite experiment.  Journal of Geophysical Research 108: 10.1029/2001JD001222.

Gu, L., Baldocchi, D.D., Wofsy, S.C., Munger, J.W., Michalsky, J.J., Urbanski, S.P. and Boden, T.A.  2003.  Response of a deciduous forest to the Mount Pinatubo eruption: Enhanced photosynthesis.  Science 299: 2035-2038.

Houghton, J.T., Ding, Y., Griggs, D.J., Noguer, M., van der Linden, P.J., Dai, X., Maskell, K. and Johnson, C.A.  (Eds.).  Climate Change 2001. Contribution of working group I to the third assessment report of the IPCC.  Cambridge University Press, Cambridge, UK/New York, NY, USA, p. 881.

Law, B.E., Falge, E., Gu,. L., Baldocchi, D.D., Bakwin, P., Berbigier, P., Davis, K., Dolman, A.J., Falk, M., Fuentes, J.D., Goldstein, A., Granier, A., Grelle, A., Hollinger, D., Janssens, I.A., Jarvis, P., Jensen, N.O., Katul, G., Mahli, Y., Matteucci, G., Meyers, T., Monson, R., Munger, W., Oechel, W., Olson, R., Pilegaard, K., Paw U, K.T., Thorgeirsson, H., Valentini, R., Verma, S., Vesala, T., Wilson, K. and Wofsy, S.  2002.  Environmental controls over carbon dioxide and water vapor exchange of terrestrial vegetation.  Agricultural and Forest Meteorology 113: 97-120.

Roderick, M.L., Farquhar, G.D., Berry, S.L. and Noble, I.R.  2001.  On the direct effect of clouds and atmospheric particles on the productivity and structure of vegetation.  Oecologia 129: 21-30.

Seinfeld, J. and Pandis, S.  1998.  Atmospheric Chemistry and Physics.  John Wiley, New York, NY, USA, p. 1326.