Characterisation of particle single scattering albedo with a modified airborne dual-wavelengths CAPS monitor

Abstract. Atmospheric aerosols impact the Earth’s climate system directly by scattering and absorbing solar radiation, and it is important to characterise the aerosol optical properties in detail. This study reports the development and validation of an airborne dual-wavelength cavity-attenuated phase shift-single (CAPS) monitor, named A2S2 (Aerosol Absorption Spectral Sizer) based on the commercial CAPS single scattering albedo monitor (CAPS-PM_SSA, Aerodyne), to simultaneously measure the aerosol optical scattering and extinction at both 450 nm and 630 nm wavelengths. New pressure and temperature sensors and an additional flow control system were incorporated into the A2S2 for its utilization onboard research aircraft measuring within the troposphere. The evaluation of A2S2 characteristics was performed in the laboratory and included the investigation of the signal-to-noise ratio, validation of performance at various pressure levels, optical-closure studies and intercomparing with the currently validated techniques. These laboratory characterisation experiments show that the A2S2 can perform measurements at sample pressures as low as 550 hPa and at sample temperatures as high as 315 K, with an uncertainty of 1 Mm^-1 at 450 nm and 0.3 Mm^-1 at 630 nm for 1 Hz measurements of both scattering coefficients (σ_sca) and extinction coefficients (σ_ext). The optical-closure study with size-selected polystyrene latex (PSL) particles show that the truncation error of the A2S2 is negligible for particles with particle volume diameter (D_p) < 200 nm, while for the larger sub-micrometre particles, the measurement uncertainty of A2S2 increases but remains less than 20 %. The A2S2 shows good agreement with the validated instruments for the σ_sca and σ_ext at 450 nm and 630 nm. The A2S2 was successfully deployed during an aircraft measurement campaign (ACROSS) conducted in the vicinity of Paris and the surrounding regions. The average SSA measured during the entire ACROSS flight campaign is 0.86 and 0.88 at 450 nm and 630 nm, respectively, while the Scattering Ångström Exponent (SAE) varies due to measurements in various pollution conditions. The A2S2 measured σ_sca results exhibit overall good agreement with the nephelometer results, and it successfully produced altitude profile results over the varied background conditions. The results presented in this study indicate that the A2S2 instrument is reliable for measuring aerosol σ_sca and σ_ext at both blue and red wavelengths, and it is suitable to replace the nephelometer onboard for future aircraft campaigns.