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Elevated pollution levels, combined with abundant vegetation uniquely affect the climate and air quality in the southeastern United States. While elsewhere in the nation and world, effects of climate change have resulted in increased average temperatures, the Southeast has experienced a cooling trend. Additionally, the U.S. Southeast tends to have air quality issues resulting from chemical reactions occurring between organic compounds emitted from vegetation (biogenic volatile organic carbons or BVOCs) and human-made pollution. Wanting to better understand the dynamics of BVOCs and pollution in the atmosphere and the related effects on climate and air quality, the Southern Oxidant and Aerosol Study (SOAS) launched on June 1st, 2013 as part of an unprecedented 5-project air quality field campaign.
Included as part of SAS – the Southeast Atmosphere Study – SOAS addresses various components of air quality and chemical- and aerosol-constituent evolution over the southeastern United States. The largest U.S. air quality study in decades, SAS is jointly taken on by the National Science Foundation, the Environmental Protection Agency, the National Center for Atmospheric Research (NCAR), and 30 other U.S. and international research institutions. The atmospheric chemistry community, via the synergy and collaborative analysis of data from SAS, is poised to uncover the controlling processes of biosphere-atmosphere interactions that affect regional climate and air quality in the U.S. Southeast.
Atmospheric chemists have known for a decade that human-made emissions have the potential to interact with plant-emitted (biogenic) VOCs, turning these hydrocarbons into aerosols that may then affect air quality and human health. More typically, however, regions have either high pollution levels and low BVOC levels or high BVOC levels and less pollution, says Alex Guenther, an atmospheric chemist at NCAR and co-Principal Investigator on SOAS. This is less the case in the Southeast, where the aerosols resulting from the mixing of BVOCs and pollution also affect the region’s climate.
“Black aerosols, which have been in the media quite a lot lately, are warming aerosols; they absorb incoming solar radiation, increasing global temperatures,” explains Guenther. “However, many of the lighter-colored aerosols, such as those composed of sulfates or organics, which form from reactions between BVOCs and pollution, have a cooling effect on the planet because they reflect some amount of incoming light back to space.”
Presence of high concentrations of light-colored aerosols is good from a climate perspective because it helps reduce average temperature, but bad from an air quality perspective, continues Guenther.
Many different kinds of VOCs are emitted to the atmosphere and their interactions with chemicals already in the atmosphere, such as water vapor, nitrogen, and ozone, and with each other, varies greatly, depending on the compound. In addition, VOC, when reacting in the presence of NOx from car emissions or other pollution sources, and sunlight can result in formation of ozone in the troposphere, the region of the atmosphere just above the Earth’s surface, which provides the air that people breath and to which plants are exposed.
Complicating things further, organic aerosols affect cloud formation and cloud opacity, both of which can, in turn, have impacts on incoming solar radiation (radiative forcing), potentially affecting the amount of greenhouse gases trapped in Earth’s atmosphere. Moreover, because cloud formation is complex, many unknowns exist related to the specifics of aerosols’ effects. Today’s climate and atmospheric chemistry models have difficulty capturing these effects accurately. While models can replicate some atmosphere-aerosol observations, they don’t achieve this at the level of detail that is required for a complete understanding of the chemical and atmospheric dynamics or to make accurate predictions of future air quality and climate.
To address some of these questions and improve observations that may be used to validate climate and chemistry models, the team of SAS/SOAS investigators will bring an unprecedented suite of filter sampling equipment and in-situ sensors to characterize the atmosphere and chemical processes across the Southeast U.S. this summer. From June 1-July 15, university and federal scientists will investigate why and how the Southeast has not warmed similarly to the rest of the continental United States. A combined investment of more than $20 million will include deployment of major measurement facilities, including the NSF/NCAR C-130 and NOAA P3 sampling across the region from the Mississippi River to the Atlantic Ocean, and from the Ohio River Valley to the Gulf of Mexico. Additionally, an Integrated sounding system (ISS) measuring boundary layer winds, moisture and temperature gradients, flux and chemical platform towers with instrumentation to take high resolution measurements from the surface up to 45 meters within the forest canopy and in the air above the forest will be deployed as part of these set of experiments.
Among the goals of SOAS – and SAS – are to develop a better understanding of the magnitudes, variations, and controlling processes for biosphere-atmosphere fluxes of oxidants and reactive carbon and nitrogen across spatial scales relevant to air quality and climate. The scientists expect to develop a better understanding of the chemical and physical processes that control the chemical reactions of BVOCs in the atmosphere. Additionally, study will determine how human-made emissions alter distribution of BVOC oxidation products, how aerosol formation affects clouds, and what the implications of oxidation are for ozone formation and reactive nitrogen.
For more information, a web site dedicated to the suite of SAS projects, including SOAS, deployment and measurement plans, as well as observation summaries and preliminary data products can be found at: http://www.eol.ucar.edu/projects/sas/.