Accuracy Required in Measurements of Refractive Index and Hygroscopic Response to Reduce Uncertainties in Estimates of Aerosol Radiative Forcing Efficiency

The magnitude of aerosol radiative forcing resulting from the scattering and absorption of radiation is still uncertain. Sources of uncertainty include the physical and optical properties of aerosol, reflected in uncertainties in real and imaginary refractive indices (n and k) and relative humidity (RH). The effect of RH on the geometrical size of aerosol particles is often reported as a hygroscopic kappa parameter (κ). The objective of this study is to explore the sensitivity of radiative forcing efficiency (RFE) to changes in particle properties, with the aim of better defining the accuracy with which optical and hygroscopic measurements must be made to reduce uncertainties in the RFE. Parameterizations of precise values of n and k are considered as functions of relative humidity (RH) for Ammonium Sulphate (AS) and Brown Carbon (BrC). The range of the RFE estimated for typical uncertainties of n and κ for AS of 0.1 µm is less than ±7% and it is not affected by an increase of RH. For typical sizes of AS in the atmosphere (0.35 µm), the range of the RFE increases to ±20% at 90% RH and ±15% at 99% RH. Absorbing small BrC particles (0.1 µm) cause cooling at the top of the atmosphere and, as the RH and hygroscopic kappa parameter increase, the RFE is more negative compared to the usual assumptions of dry unhygroscopic BrC. For larger BrC particles (0.35 µm), The range of values of the RFE for RHs100% compared to dry conditions can take values around -100%.