Letters▼ Larger PAHs pose big risk In reading the article “Persistent Organic Pollutants in the Dusts that Settled Across Lower Manhattan after September 11, 2001” (Environ. Sci. Technol. 2003, 37, 502–508), I am heartened that the authors extended their analysis of the polycyclic aromatic hydrocarbons (PAHs) upward in molecular weight range to include coronene, as well as more isomers and alkylated PAHs. Many of the past assumptions about PAHs in the environment have been proven wrong in research during the past decade. Previously, little work was done on PAHs with more ring carbons than the 22 found in benzo[ghi]perylene, indeno[1,2,3-cd]pyrene, and dibenz[a,h]anthracene—the largest ones in the U.S. EPA’s list of 16 priority pollutant PAHs. This limitation was mainly due to a lack of widely available standard compounds. Several, with up to 24 carbons, are now available, and the potential health impacts and occurrences of these higher molecular weight PAHs are becoming known. The assumption that these larger PAHs would have much less biological activity and be less prevalent than the smaller ones has been found to be wrong. As shown in this article, coronene is found in levels similar to many of the smaller ones. Schubert, Schantz, Sander, and Wise recently reported (Anal. Chem. 2003, 75, 234–246) that the 24-carbon PAHs are found in all of the environmental-matrix standard reference materials derived from air particulates, marine sediments, and coal tars. Other studies have found them in other similar materials, such as diesel particulates and carbon blacks. The levels are not much lower than for many of the currently targeted smaller PAHs.
© 2003 American Chemical Society
The health impacts of the 24carbon PAHs can be very significant, comprising 20–50% of the carcinogenic activity in many samples. This is not surprising since four species— dibenzo[a,e]pyrene (naphtho[1,2,3,4def]chrysene), dibenzo[a,h]pyrene (dibenzo[b,def]chrysene), dibenzo[a,i]pyrene (benzo[rst]pentaphene), and dibenzo[a,l]pyrene (dibenzo[def,p]chrysene)—are already listed as probable human carcinogens by the U.S. Department of Health and Human Services. The last isomer is 1000 times more active than benzo[a]pyrene in some mutagenicity tests. Minute amounts of this compound may be so significant as to dwarf the effects of all of the smaller PAHs combined. There also are reports of biological activities for PAHs with 26- and 28-ring carbons. The extension of environmental analyses to include these larger PAHs is both logical and the right thing to do in light of these newer findings. JOHN FETZER Fetzpahs Consulting Pinole, Calif.
Planting trees not the answer As someone who formerly had a professional interest in the science, technology, and policy aspects of atmospheric-carbon sources and sinks, I read “Taking Credit for Forest Carbon Sinks” (Environ. Sci. Technol. 2003, 37, 58A–63A) with great interest. To be sure, policies “to grow our way out” of problems related to fossil-fuel consumption and deforestation are rhetorically compelling: They sound natural, solve a real problem, and let us continue “business as usual”, which is the most important goal to many. Unfortunately, there are logistical aspects of such “solutions”, not considered in the article, which re-
quire careful consideration irrespective of the scientific merits. A simple analysis can illustrate some logistical problems. As implied in the article, there is an annual gap of about 5.5 Gt (gigaton = 1012 kg) carbon between the amount of carbon released annually by fossil-fuel consumption and deforestation (8 Gt) and that absorbed annually by plant growth on land (2.5 Gt). Suppose that we implement a policy of planting forests to address this gap, everything else being equal. Given a maximum carbon-sequestration intensity of about 0.5 kilograms (kg) per square meter per year for forests under an elevated-CO2 atmosphere (550 parts per million volume) (1), about 1.1 billion hectares of new forest—about threefold larger than Canada’s 404 million hectares of forest—would have to be planted. In terms of trees at a carbon-sequestration intensity of about 5 kg per tree per year, this would mean planting about 1.1 trillion trees. Who is going to plant these trillion new trees and where? Who is going to fertilize and to water them for maximum sequestration benefits and how? How much of a sequestration penalty do the fertilizing and watering processes (e.g., fertilizer production and water transport) incur? Besides such logistical questions, here is a chemistry question to consider: If carbon-sequestration processes are first order in CO2 kinetics, does it not make more sense to sequester at a CO2 concentration of about 10 mol%—a postcombustion concentration—than at a CO2 concentration of about 0.055 mol%—an elevated atmospheric concentration long after combustion and subsequent atmospheric mixing? DAVID L. WAGGER Signum Environmental, Inc. Rockville, Md.
APRIL 1, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 121 A
