Development of a low-pressure 'counterflow exchanging virtual impactor' for aerosol analysis, and, Measurement of ozone mixing ratios and meteorological parameters through the boundary layer at Summit, Greenland.

摘要

Trace atmospheric gases may provide significant interferences for in situ chemical analyses of atmospheric particles. Hence, a continuous technique to selectively remove the aerosol fraction from ambient air is desirable. A low-pressure “counterflow exchanging virtual impactor” (LP-CEVI), based upon the principle of inertial particle impaction, was developed to exchange aerosol particles larger than a certain aerodynamic size into a stream of inert gas while excluding atmospheric gases. Impactor particle transmission was characterized by two experimental techniques, one utilizing condensation particle counting of size-selected, nebulized salt particles and the other utilizing filter collection and extraction of monodisperse fluorescent microspheres. The impactor was also characterized for the ability to exclude ambient air, using the detection of nitric oxide by gas-phase chemiluminescence resulting from its reaction with ozone. Within certain pressure and counterflow regimes, the LP-CEVI was found to reproducibly transmit particles greater than approximately 0.2 μm in diameter while excluding more than 99.9% of ambient gas from the particle stream. Potential applications of the impactor to laboratory and field studies are discussed.; Chemical processes occurring in firn air, interstitial air within the top several centimeters of snowpack, are important to understanding the composition of the Arctic troposphere. Research was conducted at Summit, Greenland in June 2000 to measure vertical profiles of the ozone mixing ratio and various meteorological parameters through the top of the boundary layer from tethered helium balloon platforms. Ozone mixing ratio profiles were measured using electrochemical ozonesondes in addition to a compact, lightweight, single-beam UV absorbance ozone instrument designed for field use. Profiles of meteorological parameters such as temperature, water vapor pressure, wind speed, and wind direction were measured using radiosondes and tethersondes. Continuous ozone and meteorological measurements were also made at the surface. Variations in the ozone mixing ratio are discussed in the context of meteorological processes occurring over diurnal cycles. Results included the observation of ozone-rich stratospheric air intruding into the boundary layer due to localized atmospheric settling. Additionally, a slight but persistent ozone gradient, increasing with altitude within a very stable and shallow boundary layer, indicated the possible occurrence of ozone loss processes at the surface.