Injection in the lower stratosphere of biomass fire emissions followed by long-range transport: a MOZAIC case study
1Université de Toulouse, UPS, LA (Laboratoire d'Aérologie), 14 avenue Edouard Belin, 31400 Toulouse, France and CNRS, LA (Laboratoire d'Aérologie), 31400 Toulouse, France
2Chemical Sciences Division, Earth Science Research Laboaratory, NOAA, Boulder, Colorado, USA
3Forschungszentrum, Jülich, Germany
4Norwegian Institute for Air Research (NILU), Kjeller, Norway
5Naval Research Laboratory, Washington, DC, USA
Abstract. This paper analyses a stratospheric injection by deep convection of biomass fire emissions over North America (Alaska, Yukon and Northwest Territories) on 24 June 2004 and its long-range transport over the eastern coast of the United States and the eastern Atlantic. The case study is done using MOZAIC observations of ozone, carbon monoxide, nitrogen oxides (NOx+PAN) and water vapour during the crossing of the southernmost tip of an upper level trough over the Eastern Atlantic on 30 June 03:00 UTC and 10:00 UTC and in a vertical profile over Washington DC on 30 June 17:00 UTC, and by lidar observations of aerosol backscattering at Madison (University of Wisconsin) on 28 June. Attribution of the plumes to the boreal fires is achieved by backward simulations with a Lagrangian particle dispersion model (FLEXPART). A simulation with the Meso-NH model for the source region shows that a boundary layer tracer, mimicking the boreal forest fire smoke, is lofted into the lowermost stratosphere (2–5 pvu layer) during the diurnal convective cycle. The isentropic levels (above 335 K) correspond to those of the downstream MOZAIC observations. The parameterized convective detrainment flux is intense enough to fill the volume of a model mesh (20 km horizontal, 500 m vertical) above the tropopause with pure boundary layer air in a time period compatible with the convective diurnal cycle, i.e. about 5 h. The maximum instantaneous detrainment fluxes deposited about 15–20% of the initial boundary layer tracer concentration at 335 K, which according to the 275-ppbv carbon monoxide maximum mixing ratio observed by MOZAIC over eastern Atlantic, would be associated with a 1.4–1.8 ppmv carbon monoxide mixing ratio in the boundary layer over the source region.