plumes emitted into dry, clean air. It is sometimes not true for plumes emitted into a background with high moisture or ozone levels. I t also may not apply to plumes emitted from smelters or urban areas.

Since visibility is a sensitive political issue and since VISTTA is sponsored jointly by government and industry, it is very important to us that the results not be generalized to conditions for which they might not apply. Aside from these minor problems, you have performed a great service by clearly summarizing the status of the visibility issue.

Donald L. Blumenthal VISTTA Program Coordinator L. W. Richards VISTTA Project Manager Meteorology Research, Inc. Santa Rosa, Calif. 95401 PAHs

Dear Sir: I am writing this in regard to your article “Polynuclear aromatic hydrocarbons,” which appeared in a recent issue of ES& T (January 198 1, p. 20). This article describing impor- tant new developments in the analysis of hydrocarbons and their metabolites is well-written and of general in- terest.

1 was rather pleased to see that our recent accomplishments in the sepa- ration and analyses of benzo[a] pyrene metabolites from living organisms were reviewed in some detail on p. 22 . Regrettably, however, one sen- tence-in the third paragraph under the subheading “Metabolite analy- sis”-describing our technique is in- correct. The paragraph should read as follows:

“In order to separate BaP metabo- lites more clearly-in this case from liver lipids-Varanasi developed two-dimensional thin-layer chroma- tography (TLC). This involved suc- cessive developments of a TLC plate containing the sample in two different solvent systems.”

Usha Varanasi Northwest & Alaska Fisheries Center Seattle, Wash. 98 1 12

ES&T features Dear Sir: I am certain you get more

letters of criticism or “helpful sug- gestions” than compliments con- cerning Environmental Science and Technology, since most of us are moved to action only when we are displeased. However, I have found the improvement in the “Features” section

of ES& T so dramatic in recent months that I feel a letter of thanks is very much in order. I’m sure there are many people who, like me, wish to keep abreast of the state of the art in diverse aspects of environmental engineering and science. However, it is difficult to find the time to read all the relevant research papers, and often the level of detail in such papers is beyond our needs or interest. Recent feature arti- cles, which provide an extensive over- view of a topic of broad interest com- bined with up-to-date experimental results and an appraisal of “where we go from here,” have been excellent a t addressing these needs. The authors consistently have been recognized leaders in their fields, and the articles have been uniformly well-written. You can be assured that, though you may not receive many tangible expressions of thanks, your efforts a t improving ES&T are neither unnoticed nor unappreciated.

Mark M. Benjamin University of Washington Seattle, Wash. 98195

Correction February 1981, p. 124. The cover

credits should have included David E. Patterson of Washington University.

Air analysis of 0.1 pm particles for a more healthy environment

The ability to monitor minute airborne particles is critical to the successful control and protection of environmental quality. HiadRoyco comes to the rescue with individual particle count- ing at the 0.1 p m level, a level lower than you can get from any other manufacturer. And in our Model 226, you get it in a pack- age that’s compact and completely portable, yet highly versa- tile as well. This unique laser-based instrument provides 16 accumulating memory channels with a dynamic size range from 0.1 pm up to 6 p.m. All this and a concentration capability of up to 50,000,000 particles per cubic foot. Operating controls are front-panel mounted for ease of use. There’s a selectable 6-digit display for channel particle counts and a built-in data printer. We’d like to tell you more about our Model 226. Please call or write. 141 Jefferson Drive, Menlo Park, California 94025. Telephone (415) 325-7811.

HlaC/ROYCO I - - - - ~-

Where technology counts for excellence

insTRumEnTs !? PaciFic DlVlSlOn O F SCIEnTIFIC

Paclllc Sclenllllc Inc Allmend Cenler-CH4460 Gelferkinden Base1 Switzerland 8 Cambridge Road-Brighlon Sussex BN3-1 DF England I Pacific Scienlillc GmbH HertlQslrasse 51 7250 Leonberg 2 Wesl Germany

CIRCLE 17 ON READER SERVICE CARD

488 Environmental Science & Technology