Vol.8. pp.773-775. Pergamon Press1974.Printed inGreatBritain
In their paper, Hutcheson and Hall use an interesting series of arguments to infer a mechanism of sulfate formation on the basis of long-term integral washout data. Their assumption that plumeborne sulfate particles stay primarily in the sub-micron range during wet conditions is questionable, although this is not necessarily important to their overall argument. One would, for example, expect less washout of particulate material (per unit precipitation volume) by larger raindrops simply on the basis of geometric effects. The authors’ findings to the contrary of this are indeed interesting and have prompted us to reevaluate some of our own data, which were obtained under similar circumstances but included raindrop size measurements as well. These showed no strong or obvious relationship between mean drop size and plumeborne sulfate washout, although they did exhibit a strong inverse correlation between washout and background acidity of the rain. It would be interesting in view of this to hear the authors’ views on the effect of background rain acidity on plumeborne sulfate formation and washout, and whether any inverse correlation between background acidity and rain rate may have contributed to the findings that they report. JEREMYM. HALES
Atmospheric Sciences Department, Battelle Laboratories, Richland, Washington 99352, U.S.A.
If, as is done by the authors, “plumeborne sulfate formation and washout” is assumed to include all sulfates produced and collected below the cloud base from a given plume, it is our view that plumeborne sulfate formation and washout must increase markedly with decreasing background acidity. This view is based on the fact that: (1) SO2 absorption by raindrops is strongly inhibited by increasing acidity in the droplets, and (2) conversion to sulfate of absorbed SOz will continue after the droplet falls below the plume and may even continue until the sulfate concentration is measured, long after the raindrop reaches the surface. Thus, a greater mass of sulfur is removed from the plume in the form of SO1 by a low acid rain than by a strongly acid rain and catalytically oxidized to sulfate. While the total sulfate deposition on the ground beneath the plume may not be increased, that portion which originated from the plume is significantly increased. The data available for this study did not provide any information regarding the effect of background rain acidity on plumeborne sulfate formation within the plume. In view of the known difference in magnitude of background sources of acidifying agents between Pennsylvania and Alabama, it is the authors’ judgement that the background acidity of the rain in the study area was relatively low. This leads to the view that an inverse correlation between background acidity and rain rate did not contribute significantly to our findings. This view is additionally supported by the lack of a high correlation between the background sulfate washout obtained prior to the start of power production and the sulfate washout coefficients obtained from measurements of the washout attributed to the generating station. MAIN HUTCHE~ON
Department of Meteorology, University of Oklahoma, Norman, Oklahoma 73069, U.S.A.
* HUTCHE~ONM. R.
HALL F. P. (1974) Atmospheric Enuironrncnt 8,2328.