Characterizing North American biomass burning layers in the free troposphere with a single-particle soot photometer

Kurzfassung

Biomass burning is the largest natural source of black carbon, the most absorbing particle type in the atmosphere. Layers containing biomass burning aerosol that are lifted into the upper troposphere have an increased atmospheric lifetime which can enhance their effect on climate. Biomass burning layers were measured during the Deep Convective Clouds and Chemistry (DC3) campaign with an airborne SP2 in the free and upper troposphere. Two case studies show the properties of black carbon particles in biomass burning layers at different altitudes. On June 11, 2012, biomass burning layers in Oklahoma and Missouri at 3-8 km altitude contained high concentrations, up to 2 µg/m³, of large, thickly coated black carbon particles. Biomass burning layers measured on June 17, 2012, over Kansas and Arkansas were located in the upper troposphere, at about 11 km altitude. The mass size distribution of black carbon particles and the rBC/CO ratio indicate that the biomass burning layers underwent cloud processing by a large thunderstorm on the previous day. Washout was less strong than for anvil outflow, leading to the conclusion that the layer was not transported in the main updrafts but in weaker updrafts that mixed with stratospheric air in the anvil region. Black carbon in layers up to 9 km altitude contain large particles with a mass median diameter of about 160-200 nm with 60-80% of the particles being thickly coated. Concentrations of large particles, possibly dust, were elevated above background in biomass burning layers up to 1 cm-3. Biomass burning layers in the upper troposphere (UT-BB) are vertically often only thin, several hundred meters thick, but can horizontally stretch over about thousand kilometers. The estimated total amount of black carbon in one biomass burning layer is about 2 Mg, which corresponds to roughly one eighth of daily global aviation emissions. Black carbon particles in upper tropospheric biomass burning layers showed a nearly constant, probably steady-state, mass size distribution with mass median diameters of 145 nm, (160-200 nm in the middle troposphere). The O3 concentrations in the UT-BB layers were elevated, with values above 200 ppb. The lower rBC/CO ratio of the UT-BB layers, 1.43 (ng/m³)/ppb in contrast to 4.05 (ng/m³)/ppb in the middle troposphere, indicates that up to two thirds of black carbon particles were removed during transport.