Although it has been known for more than half a century that plants release a plethora of non-methane hydrocarbons (NMHCs), it was not until the mid-1980s that scientists firmly established the links between ozone formation/destruction and ambient NMHCs. Owing to their high reactivity, it is now well known that NMHCs can augment ozone formation in environments rich in nitrogen oxides. In jurisdictions such as the northeastern part of North America, this is a serious environmental issue as summer-time ozone levels frequently exceed the National Ambient Air Quality Standard set by the U.S. Environmental Protection Agency. I have conducted research to separate the contribution of biogenic precursors to ozone formation. My most active research area concerns field and theoretical investigations involving turbulent transport theory and chemical processing of biogenic NMHCs inside and above forests. My early work on this topic involved the development of measurement protocols to investigate the environmental and vegetation controls on hydrocarbon emissions at foliage and plant canopy levels. The fieldwork I conducted in the temperate and boreal forests of Canada led me to develop and test one-dimensional canopy models to scale hydrocarbon emissions from single leaves to the canopy dimension. Theoretical studies showed that in remote environments (where nitrogen oxide levels are negligible) these scaling processes yield credible results provided that the variation of the physical environment within the modeling domain is correctly prescribed. Hydrocarbon emissions depend on leaf temperature and light levels impinging on foliage control. I have contributed with research to develop and test models to describe the physical environment inside forest canopies, and thus estimate hydrocarbon emisions and chemical processing. These models are based on the state-of-the-science turbulent transport and radiative transfer theory, which has been integrated and adopted to address my NMHC research. Much of my recent research involves the understanding of the seasonal controls on biogenic production from deciduous forests. Lately, we have been investigating the aerosol yields from hydrocarbon oxidation. The impetus of this latest research is to quantify the radiative forcing by phytogenic aerosols and to discern the influence of phytogenic aerosols on cloud formation processes. The hydrocarbon research is being carried in places such as Canada, Brazil, the Piedmont of Virginia, the Florida Everglades and New Mexico.
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.