HR: 1340h
AN: A43C-0061    [Abstracts]
TI: Measurements of NO$_3$ and N$_2$O$_5$ in the Polluted Subarctic Atmosphere: A Seasonal Perspective From Multi-year Observations in Fairbanks, AK
AU: * Ayers, J D
EM: ffjda1@uaf.edu
AF: Geophysical Institute and Department of Chemistry and Biochemistry University of Alaska Fairbanks, 903 Koyukuk Drive, Fairbanks, AK 99775 United States
AU: Simpson, W
EM: ffwrs@uaf.edu
AF: Geophysical Institute and Department of Chemistry and Biochemistry University of Alaska Fairbanks, 903 Koyukuk Drive, Fairbanks, AK 99775 United States
AB: Nitrate radical is both an important oxidant in the nighttime atmosphere and an important intermediate in the removal of NO$_x$ from the atmosphere. Although its source is understood (the reaction of NO$_2$ with ozone), questions still surround the budget of nitrate resulting from its many possible sinks. Further complicating the situation is the equilibrium between NO$_2$, NO$_3$, and N$_2$O$_5$, which becomes important at low temperatures when the equilibrium shifts toward N$_2$O$_5$. We report measurements of the sum of NO$_3$ and N$_2$O$_5$ made with a cavity ring-down spectroscopy-based instrument over the past two years in Fairbanks, AK, along with auxiliary CO, O$_3$, and meteorological data. The sum of NO$_3$ plus N$_2$O$_5$ is detected by heating ambient air to dissociate N$_2$O$_5$ to NO$_3$; absorption of NO$_3$ at 662 nm is then used to detect the total NO$_3$ radical concentration. We term the sum of NO$_3$ + N$_2$O$_5$ the total nitrate radical species. We observe a clear correlation between the total nitrate radical species mixing ratio and the type of airmass situated over Fairbanks. Values below the detection limit are measured for both clean and very polluted airmasses. Measurable amounts are observed at night in moderately polluted airmasses that result from the mixing of background, ozone-rich air masses with polluted, NO$_x$-rich airmasses. With measurements in mid-winter, late winter, and spring, we can begin to construct a picture of the seasonal role nitrate radical plays in the chemistry of Subarctic regions. The highest values measured (250 pptv total nitrate radical species) were observed in late winter, with lower values observed in both winter and spring. Both mid-winter and springtime low levels can be understood by considering meteorological conditions. In mid-winter strong inversions dominate, trapping nitrate radical species close to the ground where surface losses are likely large. In springtime, significant daytime solar heating causes vertical mixing and dilutes pollutants rapidly, slowing production of nitrate radical species. Estimates of the nitrate radical species lifetimes in these various environments are discussed.
DE: 0345 Pollution--urban and regional (0305)
DE: 0365 Troposphere--composition and chemistry
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting