Humidified single-scattering albedometer (H-CAPS-PM\u3csub\u3eSSA\u3c/sub\u3e): Design, data analysis, and validation

We report the development and validation of a new humidified aerosol single-scattering albedometer to quantify the effects of water uptake on submicrometer particle optical properties. The instrument simultaneously measures in situ aerosol light extinction (σ ) and scattering (σ ) using a cavity-attenuated phase shift-single scattering albedo particulate matter (PM) monitor (CAPS-PM , Aerodyne Research, Inc., Billerica, MA, USA). It retrieves by difference aerosol light absorption (σ ) and directly quantifies aerosol single-scattering albedo (SSA), the aerosol “brightness.” We custom built a relative humidity (RH) control system using a water vapor-permeable membrane humidifier and coupled it to the CAPS-PM to enable humidified aerosol observations. Our humidified instrument (H-CAPS-PM ) overcomes problems with noise caused by mirror purge-flow humidification, heating, and characterizing cell RH. Careful angular truncation corrections in scattering, particularly for larger particles, were combined with empirical observations. Results show that the optimal operational size to be D \u3c 400 nm. The H-CAPS-PM was evaluated with several pure single-component aerosols including ammonium sulfate ((NH ) SO ), absorbing nigrosin, and levoglucosan, an organic biomass smoke tracer. The measured σ , σ , and the derived optical hygroscopicity parameter (κ) for size-selected ammonium sulfate are in good agreement with literature values. For dry size-selected nigrosin in the 100 \u3c D \u3c 400 nm range, SSA values increased from ∼0.3 to 0.65 with increasing D . The enhancement in nigrosin σ at RH = 80% was a factor of 1.05–1.20 relative to dry conditions, with the larger particles showing greater enhancement. SSA increased with RH with the largest fractional enhancement measured for the smallest particles. For polydisperse levoglucosan, we measured an optical κ of 0.26 for both light extinction and scattering and negligible absorption. Our new instrument enables reliable observations of the effects of ambient humidity on mixed aerosol optical properties, particularly for light-absorbing aerosols whose climate forcing is uncertain due to measurement gaps