~aosphcric hiromvnr Vol. 19. No. 12, pp. 2153-2158. 1985 ooo44981/85 $3.00 + 0.00 Printed in Great Britain. Pergamon Press Ltd. zyxwvutsrqpo INTRUSIONS OF STRATOSPHERIC AIR INTO ALASKA’S TROPOSPHERE, MARCH 1983 WOLFGANGE.RAATZ, RUSSELLC.SCHNELL*, MELVYNA.SHAPIRO~,SAMUELJ.OLTMANS and BARRY A.BODHAINE Geophysical Monitoring for Climatic Change, Air Resources Laboratory, NOAA, Boulder, CO 80303, U.S.A., *Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309, U.S.A. and +Wave Propagation Laboratory, NOAA, Boulder, CO 80303, U.S.A. (First receiued 5 November 1984 and zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONM infind fo rm 16 May 1985) Abstract-During the Arctic Gas and Aerosol Sampling Program (AGASP) in March 1983, two distinctly different mechanisms for transporting stratospheric air into the Arctic troposphere were documented. A tropopause folding event, associated with an Arctic front, injected ‘perturbed’ polar stratospheric air into the troposphere. This perturbed polar stratospheric air was characterized by enhanced condensation nuclei concentrations (up to 1800 cmT3), enhanced aerosol light scattering (up to 90 x 10M6 m-i), and crustal aerosol particles of probable volcanic origin. The second mechanism, large-scale anticyclonic subsidence, transported relatively ‘clean stratospheric air into the Arctic troposphere. This clean stratospheric air was characterized by relatively low condensation nuclei concentrations (maximum of 300 cm-s), low aerosol light scattering ([S-7] x low6 m-i), and the absence of detectable crustal particles. Key word index: Stratospheric aerosols, stratospheric intrusions, Arctic stratosphere. Injections of stratospheric air into the troposphere have been observed to occur during so-called tropo- pause folding events (Kleinschmidt, 1955; Reed, 1955). A tropopause fold is characterized by extrusions of stratospheric air into an upper-tropospheric baroclinic zone, which slopes downward from a normal tropo- pause level (near 300 mb) to the middle troposphere (500-700 mb). In midlatitudes, tropopause folds as- sociated with the polar front have been documented extensively (e.g. Danielsen, 1968; Danielsen and Mohnen, 1977; Shapiro, 1980; Johnson and Viezee, 1981); however, tropopause folds present in the Arctic have been little studied until recently (Shapiro et al., 1984). According to Diitsch (1969) the Arctic stratosphere contains maximum concentrations of ozone during March and April, mainly due to stratospheric circu- lation patterns. This is supported by measurements taken at Resolute, Canada (AES, 1978). As shown by Danielsen and Mohnen (1977) for midlatitudes, spring is a time when stratospheric ozone intrusions, due to tropopause folding events, occur about SO-100% more often than during the rest of the year because of the enhanced baroclinicity of the atmosphere. During the Arctic Gas and Aerosol Sampling Program (AGASP) in March 1983, research flights over Alaska provided an excellent opportunity to *To whom correspondence should be sent. study stratosphere-troposphere exchange in the Arctic. It is the purpose of this paper to present two different mechanisms of stratospheric intrusions into the Arctic troposphere documented in the spring of 1983: a tropopause folding event and a large-scale anticyclonic subsidence. The tropopause folding brought large amounts of El Chichon volcanic debris into the troposphere, whereas the subsidence event produced the cleanest air observed during the entire AGASP project. 2.DESCRIPTIONOFDATA SET In the Alaskan Arctic portion of the AGASP program, four research flights were conducted by the NOAA WP-3D (P-3) aircraft flying from Anchorage via Fairbanks to the vicinity of Barrow, then north over the ice (Schnell, 1984). Meteorological observations for this study were obtained from (1) conventional rawinsonde soundings at Anchorage (ANC), Fairbanks (FAI). and Barrow (BRW). (2) NOAA P-3 _. , inertial navigation wind speed and direction, pitot static pressure, and Rosemont temperature probes, and (3) Omega navigation dropwindsondes for vertical profiling of wind velocity and direction, air temperature and moisture. A continuous record of ozone concentrations was obtained with a DASIBI ozone analyzer (Oltmans, 1981) condensation nuclei (CN) concentrations with a GE automatic CN counter (Bodhaine and Murphy, 1980). and aerosol light scattering coefficient (a,,) with an MRI integrating nephelometer (Ruby and Waggoner, 1981). In-situ aerosol concentrations were obtained with Particle Measuring Systems ASASP-100X and FSSP probes. Aerosols were coll&ed on a variety of filter and cascade impactor substrates using the P-3 aerosol sampling system (Schnell, 1984). 2153