An intercomparison of aerosol absorption measurements conducted during the SEAC⁴RS campaign

Abstract

During the SEAC 4 RS campaign in 2013, inflight measurements of light-absorption by aerosol in biomass burning and agriculture fire plumes were collected along with concomitant measurements of aerosol extinction, scattering, and black carbon mass concentration. Here, we compare three measurements of aerosol absorption coefficients: from a photoacoustic spectrometer (PAS), a particle soot absorption photometer (PSAP), and a continuous light absorption photometer (CLAP). Each of these absorption measurements was collected in three visible spectral regions: red, green, and blue (although the precise wavelength and bandwidth vary with each instrument). The absorption measurements were compared during the plumes, in the boundary layer, and in the free troposphere. The slopes from the comparison ranged from 0.6 to 1.24. For biomass burning plumes, the uncertainty in the absorption measurements translates into a range in single scattering albedos of 0.93-0.94 at a wavelength of 660 nm, 0.94-0.95 at 532 nm and 0.92-0.95 at 405 nm. Overall, the aerosol absorption instruments agreed within their stated accuracies. Comparisons with simultaneous measurements of refractive black carbon mass concentration (collected by a single particle soot photometer), were used to derive the mass absorption coefficients (MAC). For all wavelengths, the MAC was high by greater than a factor of three compared to the expected MAC for black carbon.

Assessing relative humidity dependent photoacoustics to retrieve mass accommodation coefficients of single optically trapped aerosol particles

“…5 Photoacoustic (PA) spectroscopy is a standout technique widely used for ensemble absorption measurements of atmospheric aerosols, both in laboratories and environmental campaigns. [6][7][8][9][10][11][12][13] The physical basis of photoacoustic spectroscopy is the conversion of modulated light energy into thermal energy. During a photoacoustic measurement, the modulated excitation laser is partially absorbed by the studied system leading to periodic heating and cooling of the system.…”

Radiative effect and climate impacts of brown carbon with the Community Atmosphere Model (CAM5)

“…Both the GM and SA are located in latitudes near the descending branch of the Hadley circulation but are marked by different regional circulations: SA has an annual average upwelling (Figs. 11e, 12a) influenced by leeside cyclogenesis (Mendes et al, 2007), while GM has annual average downwelling influenced by the Hadley circulation (Figs. 11e, 12a).…”

Evaluating biases in filter-based aerosol absorption measurements using photoacoustic spectroscopy

“…Aerosol-radiation interactions are estimated to contribute a global mean effective radiative forcing of −0.45 (−0.95 to +0.05) W m −2 , offsetting a potentially significant but poorly constrained fraction of the positive effective radiative forcing associated with greenhouse gases (2.26 to 3.40) W m −2 (Myhre et al, 2013a). One of the major factors governing the uncertainty in estimates of aerosol direct radiative forcing is the poorly constrained aerosol single-scattering albedo (SSA), defined as the ratio of aerosol scattering to total extinction (Loeb and Su, 2010;McComiskey et al, 2008;Sherman and McComiskey, 2018). Accurate determination of aerosol SSA is limited by uncertainties in aerosol absorption estimates, which could potentially be underestimated by up to a factor of 2 in global climate models Stier et al, 2007).…”