This paper analyses 320 MOZAIC (Measurements of Ozone aboard Airbus in-service airCraft) flights between Europe and East Asia in the year 2003. Carbon monoxide (CO) measurements in the upper troposphere (UT) clearly show the influence of plumes from boreal forest fires burning around Lake Baikal between April and July. On many flights, CO concentrations were above 300 ppbv over several hundred kilometers, with values above 500 ppbv over 50 kilometers and peaks up to 800 ppbv. To our knowledge, such high UT CO concentrations have never been reported before. A case study reveals that biomass burning plumes were rapidly transported by a warm conveyor belt. On the regional and seasonal scale, MOZAIC monthly-mean concentrations were above 150 ppbv on average, i.e. 30% above the northern hemisphere background as determined over Europe.