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

Abstract

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 (∼140 ng·m^-3, standard temperature and pressure) were over 20 times higher than the background concentration (∼6 ng·m^-3) with more than 100-fold enhanced peak values (up 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 ∼120 nm with a mean coating thickness of ∼150 nm in the BB plume and ∼90 nm with a coating of ∼125 nm in the background. In a BB plume that was encountered twice, we also found a high diameter growth rate of ∼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.

Keywords: biomass burning; black carbon; climate change; high altitude; mixing state.

Fig. 1.

Spatial distribution of M_rBC between Europe and North America in the LMS as obtained by 22 CARIBIC flights flying between Munich and San Francisco, between Munich and Los Angeles, and between Munich and Mexico City from August 2014 through October 2015. Flights mainly cover the region of 123.75°W–11.25°E and 35°N–77°N. The M_rBC data have been converted to STP conditions (273.15 K, 1,013.25 hPa) on a 1° × 1° grid.

Fig. 2.

Time series of observed M_rBC (A), N_rBC (B), CO mixing ratio (C), and CH₃CN mixing ratio (D) for seven IAGOS-CARIBIC flights. Individual flights are marked with white or pale yellow backgrounds. The whole dataset is divided into three air-mass regimes: (i) background (gray dots), (ii) BB-affected air (purple dots), and (iii) BB plumes (yellow dots). The numbers in brackets on the x axis show the day of the year (DOY). The complete series of 22 flights is shown in SI Appendix, Fig. S2.

Fig. 3.

PV-based vertical profiles of CO along the flight tracks Munich ↔ San Francisco (flights 472 and 473) and from Munich to Los Angeles (flight 515). The color coding shows the rBC loadings as M_rBC. The gray shading indicates the TP; the area below is classified as UT, and the area above is classified as LMS.

Fig. 4.

Concentration-weighted frequency distributions of M_rBC (A), CO (B), and CH₃CN (C) for background air (gray), BB-affected air (purple), and BB plumes (yellow). The gray and light red shaded boxes represent the corresponding concentration ranges for background and BB plumes in the UT/LMS observed by previous studies (refs. , , , , , and –; see also SI Appendix, sections S2 and S14, Fig. S2, and Table S7).

Fig. 5.

M_rBC as a percentage of total accumulation-mode aerosol mass concentration (Left) and N_rBC as a percentage of total accumulation-mode aerosol number concentration (Right) in the three air-mass regimes: background, BB-affected air, and BB plumes. The white boxes mark the respective median values (middle lines) and the 25th and 75th percentile (top and bottom box edges). The shaded areas show the probability density of the three data populations.

Fig. 6.

Number size distributions of coated rBC particles (A), rBC cores (B), and all aerosol particles (C). (D) Volume distribution of all aerosol particles. D_total and D_core denote the particle diameter and rBC core diameter, respectively. The dashed lines are Gaussian distributions fitted to the mass distribution (shown in SI Appendix, section S3 and Fig. S3 A, a) and converted to the corresponding number distributions. Colors represent the three air-mass regimes.

Fig. 7.

Distribution of coating thickness of rBC particles at different core diameters for the three types of air mass. (Left) Background air. (Center) BB-affected air. (Right) BB plume air. The gray triangles denote rBC particles with a total particle size <180 nm.

Fig. 8.

Aging of rBC particles in an LMS-BB plume. Backward (77-h and 96-h) trajectories and the forward trajectories (88 h) for the marked flight segments (orange color in Insets) for the August 2014 flights. The color code denotes the altitude of the air mass. Smoke transported from the BB source area on 17 August 2014 over Greenland toward Iceland and the British Isles on 20 August 2014 was visualized in satellite imagery from MODIS Aqua Corrected Reflectance (True Color). Red dots mark fires and thermal anomalies obtained from MODIS Terra and Aqua (day and night). Satellite images courtesy of the NASA Worldview application (https://worldview.earthdata.nasa.gov/) operated by the NASA/Goddard Space Flight Center Earth Science Data and Information System (ESDIS) project.

Fig. 9.

Evolution of coating thickness for the range of core diameters of rBC particles between the two encounters of the BB plume intercepted by flights 472 and 473 (as shown in Fig. 8). The middle line denotes the median, and the box limits mark the 25th and 75th percentiles.