Abstract
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 based on airborne MOZAIC observations of ozone, carbon monoxide, nitrogen oxides and water vapour during the crossing of the southernmost tip of an upper level trough over the Eastern Atlantic on 30 June and on a vertical profile over Washington DC on 30 June, and on lidar observations of aerosol backscattering at Madison (University of Wisconsin) on 28 June. Attribution of the observed CO 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 at isentropic levels (above 335 K) corresponding to those of the downstream MOZAIC observations. It is shown that the order of magnitude of the time needed by the parameterized convective detrainment flux to fill the volume of a model mesh (20 km horizontal, 500 m vertical) above the tropopause with pure boundary layer air would be about 7.5 h, i.e. a time period compatible with the convective diurnal cycle. Over the area of interest, the maximum instantaneous detrainment fluxes deposited about 15 to 20% of the initial boundary layer tracer concentration at 335 K. According to the 275-ppbv carbon monoxide maximum mixing ratio observed by MOZAIC over Eastern Atlantic, such detrainment fluxes would be associated with a 1.4–1.8 ppmv carbon monoxide mixing ratio in the boundary layer over the source region.
Highlights
Biomass burning is a major source of trace gases for the global atmosphere (Crutzen and Andreae, 1990)
We report on MOZAIC observations (Measurements of OZone, water vapour, carbon monoxide and nitrogen oxides by Airbus In-sercice airCraft, Marenco et al, 1998, http: //mozaic.aero.obs-mip.fr/web/) made by airliners at cruise altitudes (10–11 km altitude) over the Eastern Atlantic and during a vertical profile over Washington, DC (USA) on June 30th 2004
We report on in-situ observations during MOZAIC flights of biomass fire plumes injected into the lower stratosphere
Summary
Biomass burning is a major source of trace gases for the global atmosphere (Crutzen and Andreae, 1990). Small-scale mixing processes enhanced by gravity waves, induced by the overshooting convection, increase the irreversible transport of forest fire emissions into the lower stratosphere (Luderer et al, 2007). Additional studies of other events are required to better document the contributions of biomass burning on the composition of the UTLS, as well as assess our modeling capacity in a variety of atmospheric conditions and fire activities. Real et al (2007) describe the processes influencing O3 levels in Alaskan forest fire plumes in 2004 from measurements in several biomass burning plumes over Europe and show that O3 impact of Alaskan fires can be potentially significant over Europe. The goals of the study are to (i) attribute anomalous CO mixing ratios in MOZAIC observations to boreal fire emissions over North America using a Lagrangian dispersion particle model, (ii) evaluate the capacity of a mesoscale model simulation with parameterized convection to vertically transport biomass-fire like emissions over the fire region up to isentropic levels corresponding to MOZAIC observations, (iii) further evaluate the mesoscale model simulation by assessing the CO mixing ratio in biomass fire emissions and comparing it to reported observations in the literature
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