Atmospheric chemistry of formaldehyde in the Arctic troposphere at Polar Sunrise, and the influence of the snowpack
Introduction
Over the past few years, interest in the photochemistry of the boundary layer in Polar Regions has increased substantially. This stems from continued developments in our understanding of ozone destruction at the time of polar sunrise in the marine boundary layer (Barrie et al., 1988; Impey et al., 1999; Wennberg, 1999; Foster et al., 2001). Among the important issues is the nature of the formaldehyde (HCHO) sources and sinks, since HCHO is an important source of free radicals in the relatively dry Arctic troposphere, and because it can be an important sink for free radicals such as Br atoms that play a role in Arctic boundary-layer ozone depletion (Shepson et al., 1996; Sumner and Shepson, 1999; Michalowski et al., 2000). Sumner and Shepson (1999) found that HCHO is emitted from the snowpack after polar sunrise, and that this source was likely important to the boundary layer. Although the data were somewhat ambiguous, the authors hypothesized that the HCHO source in the snowpack was photochemical in nature. Hutterli et al. (1999) found that HCHO emission from surface snow at Summit Greenland was an important source of HCHO for the Summit boundary layer, with estimated fluxes ranging from ∼1–10×108 molecules/cm2/s. These investigators discussed evidence that HCHO emission resulted from a temperature-dependent desorption from snow that was enriched relative to equilibrium values. During the Polar Sunrise Experiment 1998 (PSE98) and ALERT2000, we conducted measurements of gas phase HCHO concentrations from the dark through the light period, in conjunction with a variety of supporting chemical and meteorological measurements, which provided an opportunity to investigate the sources and sinks of HCHO in this environment in better detail. Additional experiments were conducted during ALERT2000 in which HCHO was measured in the snowpack interstitial air and in the snowpack condensed phase to examine the role of the snowpack in generating and emitting HCHO into the boundary layer. In this paper we discuss the results of these measurements, and their impact on our understanding of formaldehyde in the Arctic marine boundary layer (MBL).
Section snippets
Methods
Gas phase HCHO determinations were conducted from 15 February to 26 April, 1998, and from 17 February to 5 May, 2000, at the Special Studies Laboratory (SSL) at the Canadian Forces Station at Alert, Nunavut, Canada (82.5°N, 62.3°W). The SSL is located 6 km southwest of the base camp, on a plateau that is 145 m above sea level. During both PSE98 and ALERT2000, HCHO gas phase concentrations were determined using a modified version of the fluorometric method described by Fan and Dasgupta (1994).
Results and discussion
The gas phase HCHO concentrations determined for the full experiment period during ALERT2000 are shown in Fig. 1. The gas-phase concentrations ranged from 100–370 ppt during the dark period, and from ∼35–360 ppt after full sunrise. This is consistent with the observations for PSE98 discussed in Sumner and Shepson (1999). The formaldehyde data are segregated according to the corresponding ozone concentrations. We have identified three conditions that are represented: background conditions in which
Conclusions
This study has shown that, in the dark period, ambient HCHO concentrations are influenced by long-range transport. We observed cases of the transport of polluted air to the measurement site and also air that may have been influenced by photochemistry at lower latitudes. We also find that the gas-phase HCHO concentrations observed during the sunlit period cannot be fully accounted for on the basis of known gas phase chemistry. Our measurements in the snowpack and snowpack interstitial air
Acknowledgements
We thank A. Gallant, J. Deary, the Meteorological Service of Canada, and all the personnel of CFS Alert for technical and logistic support. We gratefully acknowledge the National Science Foundation Office of Polar Programs and Atmospheric Chemistry program (OPP-9818257) for financial support of this work.
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