Strong impact of wildfires on the abundance and aging of black carbon in the lowermost stratosphere

Wildfires inject large amounts of black carbon (BC) particles into the atmosphere, which can reach the lowermost stratosphere (LMS) and cause strong radiative forcing. During a 14-month period of observations on board a passenger aircraft flying between Europe and North America, we found frequent and widespread biomass burning (BB) plumes, influencing 16 of 160 flight hours in the LMS. The average BC mass concentrations in these plumes (similar to 140 ng.m(-3), standard temperature and pressure) were over 20 times higher than the background concentration (similar to 6 ng.m(-3)) with more than 100-fold enhanced peak values (up to similar to 720 ng.m(-3)). In the LMS, nearly all BC particles were covered with a thick coating. The average mass equivalent diameter of the BC particle cores was similar to 120 nm with a mean coating thickness of similar to 150 nm in the BB plume and similar to 90 nm with a coating of similar to 125 nm in the background. In a BB plume that was encountered twice, we also found a high diameter growth rate of similar to 1 nm.h(-1) due to the BC particle coatings. The observed high concentrations and thick coatings of BC particles demonstrate that wildfires can induce strong local heating in the LMS and may have a significant influence on the regional radiative forcing of climate.