Ozone depletion's new environmental threat - increase of smog and acid rain

Science News, Dec 6, 1986 by Janet Raloff

Ozone Depletion's New Environmental Threat

The seeds of future stratospheric ozone depletion have already been sown. Not only have many of the ozone-attacking chemicals -- such as chlorofluorocarbons -- been irretrievably dispersed into the atmosphere, but the global rate at which such chemicals are emitted continues to grow annually. As a result, atmospheric chemists no longer talk about whether stratospheric ozone depletion will occur, but rather how much will be lost. In fact, the first apparent evidence that measurable depletion has already occured was reported earlier this year (SN: 6/28/86, p.404).

Until recently, most predictions of the environmental hazards posed by this ozone depletion focused on the direct effects--such as human skin cancers--of bathing the earth's living things in a more intense field of solar ultraviolet radition, or on the climate change that might occur as many of the ozone-destroying "green-house gases" initiated a global warming (SN: 5/18/85, p.308).

But sthe recent findings of a pair of atmospheric chemists have added two new dimensions. Declines in stratospheric ozone, they say, could exacerbate not only smog but also the acid rain with which urban areas may have tok contend. Details of their preliminary estimates of such effects -- based on air quality data from Philadelphia, Nashville and New York -- were circulated in a discussion paper last week in Amsterdam at a meeting of the United Nations Coordinating Committee on the Ozone Layer.

One of the chief benefits of earth's stratospheric ozone layer is its ability to filter out much of the sun's biologically harmful ultraviolet radiation. But as air pollutants such as the chlorofluorocarbons CFC-11 and -12 reach the stratosphere and begin destroying that ozone, increased ultraviolet levels will penetrate to the lower atmosphere, the troposphere. There, the higher ultraviolet levels will begin driving subtle perturbations in tropospheric chemistry. A computer model to simulate those tropospheric chemistry changes and their potential impacs on the human environment is being developed by Gary Z. Whitten and Michael Gery of Systems Applications Inc. in San Rafael, Calif.

"We see about a 2 percent increase in smog ozone from a 1 percent decrease in stratospheric ozone," says Whitten. Ultraviolet increases the rate of ozone formation -- a process that occurs only during daylight hours. Whitten says this means not only that there is the potential for an overall increase in how much smog ozone is produced from reactions involving combustion poluttants such as hydrocarbons and nitrogen oxides (NO.sub.x.), but also that ozone production will peak earlier in the day.

Tha latter could have serious implications for human exposure to ozone -- the primary irritant in smog. Both smog production and human activity tend to be concentrated around industrial urban centers. If smog production peaked after many of the urban workers had commuted home to the suburbs, relatively few individuals would be exposed to the most intense smog. However, if, as the new Whitten-Gery simulations suggest, smog peaks in early to mid-afternoon, far more people could be exposed to serious smog-ozone pollution than most future projections would indicate.

But smog ozone is not the only hazard. Another potentially serious effect of more efficient smog ozone production is a dramatic increase in the production of hydrogen peroxide, a key chemical precursor to acid rain. The generation of hydrogen peroxide--also a product of ultraviolet-driven reactions between derivatives of hydrocarbons and NO.sub.x -- only occurs after ozone production shuts down, explains Whitten. "If the ozone process is still going when the sun goes down, you won't make any hydrogen peroxide. But if the ozone process finishes at noon," he says, "you have the whole rest of the day to make hydrogen peroxide."

Based on preliminary analyses of their data for Nashville, Gery says it appears there could be "about an 80 percent increase in hydrogen peroxide production for each 1 percent decrease in stratospheric ozone formation, but the ultraviolet-induced changes in hydrogen peroxide are nonlinear -- for each successive unit of stratospheric ozone depletion there is a disproportionately larger increase in hydrogen peroxide generation.

In their computer modeling calculations, Whitten and Gery tried to account for whether the city being studied would be enacting major controls on the emission of hydrocarbons and NO.sub.X in the future to limit smog and acid rain production. But when they accoudnted for the increased tropospheric ultraviolet levels that would correspond to an 8 percent depletion of stratospheric ozone, Gery says, these cities all but lost the benefit of the expensive hydrocarbon- and NO.sub.x.-control measures in controlling acid rain precursors. Their analyses indicate that levels of hydrogen peroxide would increase dramatically -- roughly to levels that match those forming under today's lower tropospheric-ultraviolet levels and no emissions controls.