Pollutant Deposition Processes

Ground-level ozone is formed through complex photochemical reactions entailing nitrogen oxides, carbon monoxide, and hydrocarbons. Ozone is an important atmospheric constituent because at elevated concentrations it can perniciously impact human health and vegetation, and influence the energy balance of the troposphere.  In my laboratory, we have investigated the mechanisms controlling ozone deposition to forest ecosystems. The field studies examined how atmospheric processes such as turbulence and stability regimes affect the transfer of ozone from the lower atmosphere to forests. These investigations were the first to employ micrometeorological methods to derive in situ ozone fluxes to terrestrial surfaces. One of the features uncovered in my research was that the ozone deposition process was enhanced when receptors were wet due to condensation or precipitation. Subsequent research, under laboratory conditions, revealed that the water-foliage interactions dictated how readily ozone can be taken up by surface wetness. In acid-rain prone environments with forests dominated by red maples (whose leaves exude ascorbic acid, which reacts instantaneously with ozone), substantial ozone flux could be experienced due to the co-deposition of sulfur dioxide (which, once in solution, can become a strong ozone sink). Results of these field and laboratory studies were later introduced in a one-dimensional modeling system to investigate ozone deposition to forests. Prior to this research, most numerical models ignored ozone deposition in response to surface wetness. This paradigm was changed as a result of my investigations. Recently, I have participated in the field campaigns of the Large scale Biosphere-Atmosphere (LBA) experiment in Brazil (1999) and the Polar Sunrise Experiment (2000) in Canada to continue my research on the processes driving surface ozone deposition. In Brazil, my graduate students and I investigated the influence of deforestation on ozone dynamics and deposition. The overall conclusion of such studies  was that if the Amazon rainforest is eliminated and transformed into pastures, then the ozone sink can be reduced by as much as 30% (meaning that ozone levels close to the ground could increase with time). And in the Arctic, where ozone in the atmospheric boundary layer can be completely but episodically removed during the polar sunrise, results indicate that the snowpack represents an important ozone sink. This is in response to the snowpack serving as storage of materials that, after the polar sunrise, become photochemically active and produce ozone-scavenging species.  Ozone deposition studies are being pursued at remote environments such as the boreal forests of Canada, mangrove forests in the Florida Everglades, and the remi-arid regions of New Mexico.

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