Abstract
Abstract. During the CONCERT 2011 field experiment with the DLR research aircraft Falcon, an enhanced aerosol layer with particle linear depolarization ratios of 6–8% at 532 nm was observed at altitudes above 10 km over northeast Germany on 16 September 2011. Dispersion simulations with HYSPILT suggest that the elevated aerosol layer originated from the Pagami Creek forest fire in Minnesota, USA, which caused pyro-convective uplift of particles and gases. The 3–4 day-old smoke plume had high total refractory black carbon (rBC) mass concentrations of 0.03–0.35 μg m−3 at standard temperature and pressure (STP) with rBC mass equivalent diameter predominantly smaller than 130 nm. Assuming a core-shell particle structure, the BC cores exhibit very thick (median: 105–136 nm) BC-free coatings. A large fraction of the BC-containing particles disintegrated into a BC-free fragment and a BC fragment while passing through the laser beam of the Single Particle Soot Photometer (SP2). In this study, the disintegration is a result of very thick coatings around the BC cores. This is in contrast to a previous study in a forest-fire plume, where it was hypothesized to be a result of BC cores being attached to a BC-free particle. For the high-altitude forest-fire aerosol layer observed in this study, increased mass specific light-absorption cross sections of BC can be expected due to the very thick coatings around the BC cores, while this would not be the case for the attached-type morphology. We estimate the BC mass import from the Pagami Creek forest fire into the upper troposphere/lower stratosphere (UTLS) region (best estimate: 25 Mg rBC). A comparison to black carbon emission rates from aviation underlines the importance of pyro-convection on the BC load in the UTLS region. Our study provides detailed information on the microphysics and the mixing state of BC in the forest-fire aerosol layer in the upper troposphere that can be used to better understand and investigate the radiative impact of such upper tropospheric aerosol layers.
Highlights
Light-absorbing aerosol species, such as black carbon (BC), contribute to a large part to the positive radiative forcing of aerosols (Jacobson, 2001)
We use the more specific term refractory black carbon, whenever we refer to quantitative measurements of the rBC mass with the SP2 (Schwarz et al, 2006)
Averaged 10 s rBC mass concentrations were observed as high as 0.67 μg m−3 (STP)
Summary
Light-absorbing aerosol species, such as black carbon (BC), contribute to a large part to the positive radiative forcing of aerosols (Jacobson, 2001). The terminology used in the literature for carbonaceous particles from combustion emissions is often ambiguous (Petzold et al, 2013). F. Dahlkötter et al.: The Pagami Creek smoke plume after long-range transport use the term “BC” exclusively for the most refractory and light-absorbing component of carbonaceous combustion particles, which is essentially pure carbon, not including organic carbon, brown carbon or inorganic aerosol components. The term “BC-containing” particle is used for referring to combustion particles with some BC content. We use the more specific term refractory black carbon (rBC; see Petzold et al, 2013), whenever we refer to quantitative measurements of the rBC mass (or quantities inferred from rBC mass measurements) with the SP2 (Schwarz et al, 2006)
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