Simultaneous measurements of the relative-humidity-dependent aerosol light extinction, scattering, absorption, and single-scattering albedo with a humidified cavity-enhanced albedometer

Hygroscopic aerosols take up water and grow with increasing relative humidity (RH), giving rise to large changes in light extinction (b(ext)), scattering (b(scat)), absorption (b(abs)), and single scattering albedo (SSA, omega). The optical hygroscopic growth factors for each parameter (f (RH)(ext,cat,abs,omega)) are thus important for assessing aerosol effects on regional air quality, atmospheric visibility, and radiative forcing. The RH dependence of aerosol scattering and extinction has been studied in many laboratory and field studies. However, owing partly to the absence of suitable instrumentation, there are few reports of the RH dependence of aerosol absorption and omega. In this work, we report the development of a humidified cavity-enhanced albedometer (H-CEA) for simultaneous measurements of f (RH)(ext,sct,abs,omega) at lambda = 532 nm from 10% to 88% RH. The instrument's performance was evaluated with laboratory-generated ammonium sulfate, sodium chloride, and nigrosin aerosols. Measured hygroscopic growth factors for different parameters were in good agreement with model calculations and literature-reported values, demonstrating the accuracy of the H-CEA for measuring RH-dependent optical properties.