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Current Research Interests
Reactive nitrogen (NOy) exerts control over the production of tropospheric ozone (O3), the destruction of stratospheric O3, plays an important role in the formation of secondary organic aerosol and represents a critical link between the atmosphere and biosphere. Accurate estimates of the spatial and temporal distribution of nitrogen oxide (NOx) emissions and their subsequent transport and chemical processing are critical to furthering our understanding of these processes. My research focuses on providing accurate measures of speciated NOy from various platforms (ground, aircraft and satellite). These observations are used to directly constrain both chemical mechanisms and transport processes in the atmosphere.
I. In situ Heterogeneous Kinetics
Collaborators: Joel Thornton [University of Washington]
The hydrolysis of N2O5 on aerosol particles plays a critical role in removing NOx from the atmosphere at night, thus setting the initial conditions for the ozone production cycle at sunrise. The rate at which N2O5 is removed by aerosol is a complex function of both aerosol composition and liquid water content. Until recently these processes have been the province of laboratory studies. In this study we directly measure the heterogeneous uptake to real ambient particles using an in situ aerosol flow tube.
Figure: Using an aerosol flow tube we directly measure the loss of N2O5 to ambient aerosol. Co-located measurements of aerosol surface area and aerosol chemical composition permit calculation of the uptake coefficient and the factors that control it's variability in the atmosphere.
Download my 2008 IGAC poster in pdf format [4MB, low res version]
Download our science feature from the IGAC Newsletter in pdf format [3MB]
II. Intercontinental Transport of Pollution
Collaborators: Ronald Cohen [UC Berkeley]
With rapid industrialization occurring throughout the world, the issue of intercontinental transport of air pollution has come to the forefront of the atmospheric science community. In this study we use observations of reactive nitrogen made during the INTEX-B field campaign over the North Pacific to discuss the role of nitrogen partitioning in transporting NOx to the remote North Pacific. These measurements provide constraint on the net flux of nitrogen between Asia and North America.
Figure: Flux of reactive nitrogen [gN m2 day-1] calculated using all available observations of NOy and wind-speed gridded into 5 latitude x 2 km altitude bins.
III. Chemical Constraints on Transport Processes
Collaborators: Ronald Cohen [UC Berkeley], Paul Wennberg [Cal-Tech], Jack Dibb [UNH], Jim Crawford [NASA Langely], Greg Huey [Georgia Tech], Brian Heikes [URI], Alan Fried [NCAR] and many others from the INTEX-NA Science Team
Deep convection represents a highly efficient mechanism for transporting air from near the Earth's Surface (0-2km) to the Upper Troposphere (8-11km). Quantifying the aggregate effects of convection on the chemistry and dynamics of the UT remains a challenging question in atmospheric chemistry. We use chemical observations of the partitioning of reactive nitrogen in the UT to provide new and unique constraints on the chemistry occurring downwind of convection and the rate at which air in the UT is recycled, previously only the province of model analyses. These direct measures of atmospheric rates present a challenge to our thinking about the processes governing UT ozone and its impact on climate.
Figure Time series of measurements taken in the vicinity of recent convective activity between 5 and 9 km from the NASA DC-8 during INTEX-NA. These observations were used to determine the age of air in the upper troposphere.
Read our article in Science, Bertram et al., 2007
IV. Satellite Measurements of NOx Emission Sources
Collaborators: Andreas Richter, Andreas Heckel and John Burrows [University of Bremen]
The current generation of space-based instruments provides a new and exciting avenue to deduce the source strengths, chemistry and transport of various trace gases and aerosol continuously. In our studies, we use observations of NO2 retrieved from the SCIAMACHY instrument to test both the spatial and temporal distribution of soil NOx emissions from heavily fertilized agriculture. We show that the SCIAMACHY observations, over a 2 million hectare agricultural region in Montana, capture the short intense NOx pulses following fertilizer application and subsequent precipitation and we demonstrate that these variations can be reproduced by tuning the mechanistic parameters in an existing model of soil NOx emissions.
Figure Topographical map of North-Central Montana. b-d) April, May and June 2004 monthly averaged NOx emissions as derived from SCIAMACHY NO2 columns.
Read our article in GRL, Bertram et al., 2005
