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Abstract
Abstract. One of the major uncertainties in the understanding of Earth's climate system is the interaction between solar radiation and aerosols in the atmosphere. Aerosols exposed to high humidity will change their chemical, physical, and optical properties due to their increased water content. To model hydrated aerosols, atmospheric chemistry and climate models often use the volume weighted mixing rule to predict the complex refractive index (RI) of aerosols when they interact with high relative humidity, and, in general, assume homogeneous mixing. This study explores the validity of these assumptions. A humidified cavity ring down aerosol spectrometer (CRD-AS) and a tandem hygroscopic DMA (differential mobility analyzer) are used to measure the extinction coefficient and hygroscopic growth factors of humidified aerosols, respectively. The measurements are performed at 80% and 90%RH at wavelengths of 532 nm and 355 nm using size-selected aerosols with different degrees of absorption; from purely scattering to highly absorbing particles. The ratio of the humidified to the dry extinction coefficients (fRHext(%RH, Dry)) is measured and compared to theoretical calculations based on Mie theory. Using the measured hygroscopic growth factors and assuming homogeneous mixing, the expected RIs using the volume weighted mixing rule are compared to the RIs derived from the extinction measurements. We found a weak linear dependence or no dependence of fRH(%RH, Dry) with size for hydrated absorbing aerosols in contrast to the non-monotonically decreasing behavior with size for purely scattering aerosols. No discernible difference could be made between the two wavelengths used. Less than 7% differences were found between the real parts of the complex refractive indices derived and those calculated using the volume weighted mixing rule, and the imaginary parts had up to a 20% difference. However, for substances with growth factor less than 1.15 the volume weighted mixing rule assumption needs to be taken with caution as the imaginary part of the complex RI can be underestimated.
Highlights
A major uncertainty in understanding Earth’s climate system is the interaction between solar radiation and aerosols in the atmosphere
We explore the validity of the volume weighted mixing rule for water soluble absorbing aerosols, and address the change in the refractive index of absorbing aerosols when exposed to 80 % and 90 % relative humidity values, at wavelengths of 355 nm and 532 nm
The growth factor (GF) values were converted to theoretical f RHext(%relative humidity (RH), Dry) as described for the 532 nm ammonium sulfate (AS) measurement at 80 %RH
Summary
A major uncertainty in understanding Earth’s climate system is the interaction between solar radiation and aerosols in the atmosphere. This interaction is dependent on the physical and chemical properties of the aerosols and the wavelength of the incident light. Aerosols can scatter and absorb shortwave (solar) radiation, and changes in the their atmospheric concentrations and/or their chemical, and physical properties can alter the energy balance of the climate system and are drivers of climate change (IPCC, 2007). The resulting positive or negative changes in the energy balance due to these factors are expressed as radiative forcing; the last report from the Intergovernmental Panel on Climate Change (IPCC) showed that, to a great extent, aerosols have a cooling effect (Forster et al, 2007). Measuring and deriving their optical properties is critical in calculating the effects of aerosols on radiative transfer
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