## Doctoral Dissertations

Fall 1997

Dissertation

Earth Sciences

#### Degree Name

Doctor of Philosophy

Robert C Harriss

#### Abstract

A unique high resolution ambient air methane data set consisting of approximately 125,000 independently measured data points for the years 1991-1995 has been collected at a site in the northeastern United States. This data base is used to examine the long term trend, seasonal and diurnal cycles, and the frequent pollution events that affect the site on a year round basis.

The annual median mixing ratio of methane for all measurements was 1808 ppbv in 1992, increasing at a variable rate to 1837 ppbv in 1995. The lower 10-30% of the data from each month was defined as representative of background air and was compared to the global CMDL data set. The background data exhibit a variable upward trend of 5.5 $\pm$ 2 ppbv/year during the 4-year time period, with most of the increase observed during 1993 and 1994.

The seasonal cycle for the background data set is similar to what is observed by CMDL stations and varies from 24 to 35 ppbv. The amplitude of the seasonal cycle for the full data set was larger, ranging from 35 to 44 ppbv. Differences between the full and background mixing ratios vary on a seasonal basis and are largest in the winter and smallest in the summer. These differences appear to be controlled by changes in atmospheric stability and changes in emissions from local and regional sources throughout the year.

Wind roses of chemical species are examined for annual and seasonal time periods with enhancements in anthropogenic species corresponding to the location of large cities and landfills. Methane is strongly correlated to species that have an anthropogenic component, including acetylene, propane, ethane, and hexane. The southwest quadrant is subjected to the most severe pollution events and is impacted by outflow from large cities in that sector, including Northampton and Springfield, MA. Emissions from cities in other quadrants, including Boston and Worcester, MA., Providence, RI., and the near by town of Petersham, MA, also affect the site, but to a lesser degree.

Case studies are used to identify atmospheric conditions that lead to high concentrations of methane and other species. The co-occurrence of a persistent wind direction, light wind speed, and stable atmospheric conditions is the ideal scenario in which emissions from nearby cities and landfills are advected to the site. Emissions from local and regional, rather than distant sources, are the primary cause of elevated events.

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