How Different Calculations of the Refractive Index Affect Estimates of the Radiative Forcing Efficiency of Ammonium Sulfate Aerosols

Abstract

Calculations of the radiative properties of hydrated ammonium sulfate (AS) aerosols often employ the conventional volume mixing rule, in which the refractive indices of AS and water are linearly averaged, weighted by their respective volume fractions in solution, and the real part of the refractive index of pure AS is taken to be 1.52-1.55, based on measurements of dry crystalline AS. However, there are significant differences between the refractive indices of AS-water solutions calculated using the conventional volume mixing rule and empirically derived refractive indices. The authors use a simple model for calculating the direct solar radiative forcing efficiency (RFE; radiative forcing divided by optical depth) of an optically thin layer of aerosols to investigate the magnitude of these differences. The difference between the conventional volume mixing rule and empirically derived refractive indices amounts to a modest difference in the direct solar RFE of AS aerosols at the top of the atmosphere at 0.550-mm wavelength and at relative humidities of 37%-99.9%. Without black carbon, the difference in RFE is up to 20.42 W m 22 for relative humidities less than around 66% and up to 0.25 W m 22 for relative humidities greater than 66%, whereas with 2% black carbon by volume, the range of difference in RFE is up to 20.59 W m 22 for relative humidities less than 66% and up to 0.30 W m 22 for relative humidities greater than 66%. Although modest, this difference in RFE may become important when investigating regional aerosol forcing in areas with a high concentration of urban and industrial pollution.

Simultaneous retrieval of effective refractive index and density from size distribution and light-scattering data: weakly absorbing aerosol

“…7). The observed underestimation of retrieved characteristics under humid conditions is consistent with results from previous studies (e.g., Erlick et al, 2011) and can be explained as follows. The aerosol characteristics (n and ρ) have smaller values for the pure water than for other chemical components, and the fraction of aerosol water increases with an increase of the RH due to water uptake.…”

Retrievals of aerosol optical and microphysical properties from Imaging Polar Nephelometer scattering measurements

“…Alternative methods for estimating the refractive index of hygroscopic particles exist, but their deviation from the volume mixing rule is less than 1 % for solutions that are made up of more than 50 % water (Erlick et al, 2011;Schuster et al, 2009). …”

A complete parameterisation of the relative humidity and wavelength dependence of the refractive index of hygroscopic inorganic aerosol particles

“…Reducing the large uncertainty associated with aerosol radiative forcing (RF) is crucial to improving the representation of aerosol in climate models. Top-of-the-atmosphere radiative forcing (RF TOA ) is a common metric for assessing the contribution of different aerosol particles to the warming or cooling of Earth's atmosphere (Alexander et al, 2013;Erlick et al, 2011;Haywood and Boucher, 2000;Ravishankara et al, 2015). RF TOA can be estimated for a uniform, optically thin layer of aerosol in the lower troposphere using (Dinar et al, 2008) …”

“…By comparing the measured light scattering traces to calculations from Mie theory, n was parameterised as a function of RH for a range of inorganic aqueous solution droplets, key components of nonabsorbing atmospheric aerosol. Tang's parameterisations of n 633 for aqueous inorganic solutes have become a benchmarking standard for new techniques measuring refractive index (Cotterell et al, 2015b;Hand and Kreidenweis, 2002;Mason et al, 2012Mason et al, , 2015 and are used as reference RI data for RF calculations of aqueous inorganic aerosol (Erlick et al, 2011). However, Tang's measurements were limited to λ = 633 nm and knowledge of the optical dispersion is required to calculate spectrally weighted values of the singlescattering albedo and backscatter function.…”