Nitrogen cycling in a northern forest: Gases to clouds to rain

Kimberly Ann Hill, Purdue University

Abstract

In a forest environment, many processes alter the amount and type of nitrogen that deposits by wet and dry deposition to the forest floor. Gas-phase chemistry changes what compounds are available to be scavenged by cloud and rain droplets as well as to dry-deposit to the leaves. Cloud chemistry and microbes in the cloud water alter which compounds may go back into the gas-phase or wet deposit to the surface. Processing of dry-deposited nitrogen species on leaf surfaces changes the nitrogen washed off by rainwater as it passes through the forest canopy. In this thesis, measurements of the inorganic and organic nitrogen composition of precipitation both under the forest canopy and in an open field at the University of Michigan Biological Station are described. The data collected showed that the nitrate and organic nitrogen concentrations were substantially enhanced by interaction with the forest canopy, while ammonium concentrations were slightly decreased. Wash off of dry-deposited nitric acid and particulate nitrate was determined to be the likely source of the enhanced nitrate concentration, although some of the deposited nitric acid was either taken up by the canopy or revolatilized back into the atmosphere. Cloud water collected over the forested area of the northern section of the lower peninsula of Michigan and analyzed for inorganic and organic nitrogen content showed a significant fraction of the nitrogen was organic. Although relatively large concentrations of bacteria existed in the clouds, they contributed insignificant amounts of nitrogen to the cloud water. However, the bacteria have the potential to alter the nitrogen composition, as nitrifiers were identified in the samples. Finally, the development of an NO chemiluminescence analyzer with NO2 and NOy inlets and thermal dissociation inlets to study total reactive odd-nitrogen (NO y) and the individual species that compose NOy(NOx, organic nitrates, peroxyacyl nitrates, and nitric acid) are described. Insufficient limits of detection of the chemiluminescence analyzer and problems with the conversion efficiency prevent the instrument and inlets from being useful for field measurements in clean air environments. However, the instrument and NO2 and NOy inlets were successfully compared against a thermal dissociation laser induced fluorescence instrument in laboratory experiments.

Degree

Ph.D.

Advisors

Shepson, Purdue University.

Subject Area

Biogeochemistry|Analytical chemistry|Atmosphere

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