Evaluation of the 365 nm CAPS PM SSA monitor and its use in both laboratory and field measurements

The newly designed 365 nm Cavity Attenuated Phase-Shift Single Scattering Albedo Monitor (CAPS PM SSA monitor; Aerodyne Research, Inc) is evaluated. The instrument measures the extinction and scattering coefficients of suspended particles. It has a precision of 0.66 Mm⁻¹ and 0.45 Mm⁻¹ (1σ) in the extinction and scattering channels, respectively with a time resolution of 1 s. The truncation effect in the scattering channel was evaluated using monodisperse polystyrene nanospheres of varying sizes, yielding correction factors of 0.90 ± 0.002 for 200 nm and 0.80 ± 0.002 for 300 nm particles. Mie theory was used for retrieving the complex refractive index (RI) from the SSA monitor and aerosol size distribution measurements. Fresh and aged indole and naphthalene secondary organic aerosols (SOA) generated using a potential aerosol mass (PAM) oxidation flow reactor (OFR) under varying oxidative conditions with and without the presence of NOx were also measured. For indole, the imaginary part of the RI decreased with oxidative aging involving OH radicals, whereas it increased for naphthalene. The retrieved RI values were consistent with previously reported data. Measurements conducted during a field measurement campaign revealed a mean SSA of 0.76 ± 0.05. Several particle burst events were observed during the campaign, characterized by a significant decrease in SSA (<0.65) and an increase in absorption coefficients (from <2 Mm⁻¹ to >10 Mm⁻¹). These events coincided with enhanced organics-to-sulfate ratios (>2.5), indicating the contribution of BrC aerosols. This study demonstrates the application of the newly designed SSA monitor at 365 nm for measuring the optical properties of laboratory and field BrC aerosols.