Absorption closure in highly aged biomass burning smoke

Abstract

Abstract. The optical properties of black carbon (BC) are a major source of uncertainty in regional and global climate studies. In the past, detailed investigation of BC absorption has been hampered by systematic biases in the measurement instrumentation. We present airborne measurements of aerosol absorption and black carbon microphysical properties in highly aged biomass burning plumes measured 4–8 d from their source over the southeast Atlantic Ocean during CLARIFY-2017, using a suite of novel photoacoustic spectrometers to measure aerosol absorption at 405, 514, and 655 nm and a single-particle soot photometer to measure the BC mass concentration, size, and mixing state. These measurements are of sufficient quality and detail to provide constraint on optical schemes used in climate models for the first time in biomass burning plumes far from their source – an aerosol environment that is one of the most important climatically. The average absorption Ångström exponents (AAE) were 1.38 over the wavelength range from 405 to 514 nm and 0.88 over the range from 514 to 655 nm, suggesting that brown carbon (BrC) contributed to 11±2 % of absorption at 405 nm. The effective organic aerosol (OA) mass absorption coefficient (MAC) was 0.31±0.09 m2 g−1 at 405 nm. The BC particles were universally thickly coated, and almost no externally mixed BC particles were detected. The average MAC of BC was 20±4, 15±3, and 12±2 m2g−1 at wavelengths of 405, 514, and 655 nm respectively, with equivalent absorption enhancements of around 1.85±0.45 at all three wavelengths, suggesting that the thick coatings acted as a lens that enhanced light absorption by the BC. We compared the measured MAC and AAE values with those calculated using several optical models and absorption parameterisations that took the measured BC mass and mixing state as inputs. Homogeneous grey-sphere Mie models were only able to replicate MAC for some low (real and imaginary) values of the complex BC refractive index (mBC) at the shortest wavelength, but they would have to use unrealistically low values of mBC to accurately replicate the AAE. A core–shell Mie model was able to generate good agreement for MAC in the green–red end of the visible spectrum for most values of mBC. However, there are no possible values of mBC that produce MAC values that agree with our observations at all three wavelengths, due to a wavelength-dependent underestimation of the MAC of the underlying BC core. Four semiempirical parameterisations from the literature were also tested, linking the BC mixing state to either the MAC or absorption enhancement. Two of these schemes produced results that agreed within a few percent with the measured MAC at all three wavelengths, and the AAE agreed well when discounting the effects of BrC. Our results uniquely demonstrate the validity of absorption parameterisations, as well as the failings of Mie calculations, in this highly aged environment. We recommend that future work should conduct similar analyses in environments where BC has different properties; future studies should also investigate the impact of implementing these types of schemes within climate models as well as the impact of developing equivalent schemes for light scattering by soot particles at visible wavelengths.

Characterizing the performance of a POPS miniaturized optical particle counter when operated on a quadcopter drone

“…CC BY 4.0 License. which has been shown to be representative of biomass burning aerosol during the wider CLARIFY-2017 measurement campaign (Wu et al, 2020). That the POPS and SMPS show close overlap at the peak concentrations indicates that the counting statistics and the particle concentrations are similar between the instruments.…”

“…The concentration data also show some correlation with the AODs as measured by a colocated AERONET Cimel sun-photometer (panel five, Figure 3), although this AOD is a column measurement rather than a point measurement so the influence of vertical profile will likely be important (e.g. Wu et al, 2020;Haywood et al, 2020).…”

“…Aerosols >0.7µm diameter that were observed by the POPS that were not present in the CLARIFY-2017 or SAFARI-2000 data may well be generated by dust generation from the arid surface of Ascension Island or by super-micron sea-salt from breaking waves. Taylor et al 2020 We would expect biomass burning aerosols to be associated with an increase in carbon monoxide (CO; Haywood et al, 2003;Wu et al, 2020), and the concentrations measured by both the POPS and SMPS instruments are well correlated with the CO mass mixing ratio (as measured by a co-located CO analyzer - Figure 3d). The concentration data also show some correlation with the AODs as measured by a colocated AERONET Cimel sun-photometer (panel five, Figure 3), although this AOD is a column measurement rather than a point measurement so the influence of vertical profile will likely be important (e.g.…”

“…As part of the CLARIFY-2017 and LASIC campaign, the POPS was deployed at the ARM mobile site on Ascension Island located in the mid-Atlantic (-7.96° N, -14.37° E) alongside an ARM operated SMPS. The time period for sampling for both instruments analyzed here was from 20th August to 9th September 2017 (20 days) continuously, during which time biomass burning aerosol originating from the African continent was frequently present (Zuidema et al, 2018;Haywood, 2020). The SMPS and the POPS were connected to a common aerosol inlet, however, in the case of the SMPS, the sample air was dried before it entered the instrument.…”

“…We first investigated the performance of the POPS instrument in a closely controlled environment on the ground in a three-week comparison of the POPS against reference instruments. The POPS was deployed at the Atmospheric Radiation Measurements (ARM) mobile facility on Ascension Island during colocation of the Layered Atlantic Smoke Interaction with Clouds (LASIC; Zuidema et al, 2016) and CLoud-Aerosol-Radiation Interaction and Forcing: Year-2017 (CLARIFY-2017;Haywood et al, 2020) measurement campaigns. Subsequently we examined the influence of the rotors from the drone on the measured aerosol number concentration and size distribution.…”