Science Introduction

Earth's climate is changing as a result of the radiative forcing exerted by several different greenhouse gases and aerosol species. Greenhouse gases such as carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and tropospheric ozone (O3) warm the climate through the absorption of radiation. Aerosols affect climate both directly through the scattering and absorption of radiation, and indirectly through the nucleation of cloud droplets. The net effect of anthropogenic aerosol is estimated by IPCC and other studies to cool the surface climate, and as a result aerosols may have masked as much as 50% of the warming due to greenhouse gases. Climate forcing agents such as ozone and aerosols are also air pollutants, making air quality and climate two issues that are intimately coupled. To develop effective policies, research at the nexus of air quality and climate is urgently needed to guarantee that improvements in air quality are not achieved at a cost to climate, and vice versa.

NOAA has one of the world's leading research programs to improve our understanding of the most important climate forcing agents. The program includes the monitoring of abundances, and the quantification of emissions, the chemical formation of agents such as ozone and aerosols, and the characterization of their climate-relevant properties. NOAA's research program also includes the development of a Global Climate Model (GCM) that combines our understanding of all relevant processes to calculate and predict atmospheric abundances of the climate forcing agents and their impacts on climate and air quality. With this combination of measurement and modeling capabilities, NOAA is poised to make major advances in our understanding of key interactions among climate and air quality, and of the relative importance of natural processes versus human activities in controlling the atmospheric distribution of climate forcing agents.

In the last decade, NOAA has conducted intensive field studies focused on various aspects of the interactions between climate and air quality in the Northeast U.S. (2002, 2004), Texas (2000, 2006), Alaska (2008) and California (2002, 2010). These regions differ in their atmospheric emissions and composition, and the processes that control these. For example, Figure 1 shows the average distribution of ammonium sulfate and organics, two of the main aerosol species, across the U.S. Through the synergy of these targeted field studies, many general insights into the sources and climate impacts of anthropogenic emissions have emerged.

Figure 1
Figure 1: Annual average mass loadings of ammonium sulfate (left) and organic aerosol (right) from IMPROVE and CSN network measurements during 2005-2008. Ammonium sulfate concentrations are highest in the East where many coal-fired power plants are located. Organic aerosol is high along the Pacific coast and in the Southeast.

Here, we propose to do an intensive field study in the Southeast U.S., a region with significant anthropogenic emissions, meteorological conditions that are conducive to active photochemistry and where natural hydrocarbon emissions are the highest in the nation. As a result, the atmospheric abundances of many climate forcing agents and air pollutants are high in the Southeast (Fig. 1). However, the extent to which these abundances are controlled by natural vs. anthropogenic emissions is very poorly understood. For aerosols in particular, the climate radiative forcing, defined as the change in net irradiance at the tropopause between present and pre-industrial, is therefore very uncertain. In addition, the Southeast U.S. has not warmed like other parts of the U.S. in response to global climate change, and the temperature anomaly has been suggested to be related to aerosols derived from a combination of anthropogenic and biogenic precursors.

The Southeast U.S. is chosen here as an (easily accessible) example region of the atmosphere where the interplay between anthropogenic and biogenic emissions to form climate- forcing agents is particularly important. It should be noted that the results of this study will be relevant for many regions of the atmosphere. While the field intensive necessarily has a regional focus, the study is fully intended to advance our description of the global distribution of climate forcing agents and their climate-relevant properties.

The field intensive in the Southeast will build on and extend the Southern Oxidants Study (SOS) that was conducted by NOAA and partners in the Southeast in the 90s. While SOS primarily focused on tropospheric ozone formation, advances in instrumentation warrant a reexamination of this region of the atmosphere with an added focus on aerosols and their climate-relevant properties. Such a study is timely now for several different reasons. First, NOAA can partner with a group of colleagues from academia and other agencies in the so-called Southern Oxidants and Aerosol Study (SOAS), which has the same objectives. Anthropogenic emissions have strongly decreased since the 90s and studying their effects on pollutants will give new and unique insights into their chemical formation.

For further information, download the SENEX White Paper