Absorbing aerosols at high relative humidity: linking hygroscopic growth to optical properties

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 (f RH ext (%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 f RH(%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.

Broadband measurements of aerosol extinction in the ultraviolet spectral region

“…Suwannee River fulvic acid), measurement of multiple diameters can be used to determine n and k for each wavelength. This is similar to the approach used in conventional CRDS measurements (Pettersson et al, 2004;Abo Riziq et al, 2007;Flores et al, 2012), although used for multiple wavelengths in this case. The implementation of these approaches is described in further detail below.…”

“…In addition, the optical cross sections of the multiple-charged particles are typically greater than that of the singly charged particles, with a fractional contribution to the optical cross section that is greater than their fractional abundance. A common method used in CRDS retrievals of complex refractive index is to calculate and subtract the extinction due to doubly and triply charged particles from the total measured extinction (Fierz-Schmidhauser et al, 2010;Cappa et al, 2011;Flores et al, 2012). The disadvantage of this method is that it requires an assumed refractive index for the multiple-charged particles.…”

“…(5) conceals a complex dependence on the variables λ, D p , n, and k. However, for given values of λ, D p , n, and k and particles that are assumed to be spherical, Eq. (5) may be readily solved using numerical codes to determine the optical cross section (Bohren and Huffman, 1983) as demonstrated previously (Pettersson et al, 2004;Lack et al, 2006;Abo Riziq et al, 2007;Flores et al, 2009Flores et al, , 2012Lang-Yona et al, 2009;Nakayama et al, 2010;Miles et al, 2010;Chartier and Greenslade, 2012;Bluvshtein et al, 2012). The goal of this work is to determine n and k for different aerosol types by measuring the wavelength dependence of σ for particles of known N and D p .…”

A new method for estimating aerosol mass flux in the urban surface layer using LAS technology

“…Figure 2d shows the temporal variations in relative humidity. As we know, relative humidity is an important factor that controls aerosol particle growth (Flores et al, 2012;Winkler, 1973). Therefore, to study the transport flux of dry aerosol mass from different surface aerosol sources, only measurements with a relative humidity less than 60 % were used in the analysis to minimize the influence of the water content in the aerosols.…”

Assessing the accuracy of complex refractive index retrievals from single aerosol particle cavity ring-down spectroscopy

“…Cavity ring-down spectroscopy (CRDS) is a technique which enables high precision measurements of s ext for aerosol particles and, consequently, precise determinations of RI (Miles et al 2011a). Ensemble cavity ring-down spectroscopy (E-CRDS) has become a commonplace technique in aerosol science for measuring the extinction coefficient, a ext , for a sample consisting of a cloud of particles (Attwood and Greenslade 2012;Mason et al 2012;Michel Flores et al 2012). A flow of atomized aerosol is introduced into an optical cavity in which the aerosol ensemble is probed by CRDS.…”