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

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.

[1]  S. Solomon.  The Physical Science Basis : Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change , 2007 .

[2]  R. Betts,et al.  Changes in Atmospheric Constituents and in Radiative Forcing. Chapter 2 , 2007 .

[3]  R. Robinson,et al.  Interactions in Aqueous Nonelectrolyte Solutions. I. Solute-Solvent Equilibria , 1966 .

[4]  P. Barber.  Absorption and scattering of light by small particles , 1984 .

[5]  Y. Rudich,et al.  Complex refractive indices of aerosols retrieved by continuous wave-cavity ring down aerosol spectrometer. , 2009, Analytical chemistry.

[6]  Yinon Rudich,et al.  Optical properties of absorbing and non-absorbing aerosols retrieved by cavity ring down (CRD) spectroscopy , 2006 .

[7]  P. Zieger,et al.  Measurement of relative humidity dependent light scattering of aerosols , 2009 .

[8]  Influence of uncertainties in the diameter and refractive index of calibration polystyrene beads on the retrieval of aerosol optical properties using cavity ring down spectroscopy. , 2010, The journal of physical chemistry. A.

[9]  Jianqi Shen,et al.  Improved algorithm of light scattering by a coated sphere , 2007 .

[10]  O. Toon,et al.  Potential climatic impact of organic haze on early Earth. , 2011, Astrobiology.

[11]  P. Hobbs,et al.  Airborne measurements of carbonaceous aerosols on the East Coast of the United States , 1997 .

[12]  A. R. Ravishankara,et al.  Key factors influencing the relative humidity dependence of aerosol light scattering , 2006 .

[13]  U. Baltensperger,et al.  Hygroscopicity of aerosol particles at low temperatures. 2. Theoretical and experimental hygroscopic properties of laboratory generated aerosols. , 2002, Environmental science & technology.

[14]  Yinon Rudich,et al.  Hygroscopic growth of atmospheric and model humic-like substances , 2007 .

[15]  A. Ravishankara,et al.  Parameterization for the relative humidity dependence of light extinction: Organic-ammonium sulfate aerosol , 2007 .

[16]  David G. Streets,et al.  Two‐decadal aerosol trends as a likely explanation of the global dimming/brightening transition , 2006 .

[17]  A. R. Ravishankara,et al.  Measurement of aerosol optical extinction at 532 nm with pulsed cavity ring down spectroscopy , 2004 .

[18]  S. Sjogrena,et al.  Hygroscopic growth and water uptake kinetics of two-phase aerosol particles consisting of ammonium sulfate, adipic and humic acid mixtures , 2007 .

[19]  M. Daimon,et al.  Measurement of the refractive index of distilled water from the near-infrared region to the ultraviolet region. , 2007, Applied optics.

[20]  P. Buseck,et al.  Absorbing Phenomena , 2000, Science.

[21]  Corinne Le Quéré,et al.  Climate Change 2013: The Physical Science Basis , 2013 .

[22]  A. Slingo,et al.  General Circulation Model Calculations of the Direct Radiative Forcing by Anthropogenic Sulfate and Fossil-Fuel Soot Aerosol , 1997 .

[23]  O. Boucher,et al.  Aerosol optical depths and direct radiative perturbations by species and source type , 2005 .

[24]  Sonia M. Kreidenweis,et al.  Reduction in biomass burning aerosol light absorption upon humidification: roles of inorganically-induced hygroscopicity, particle collapse, and photoacoustic heat and mass transfer , 2009 .

[25]  S. Tripathi,et al.  Aerosol black carbon radiative forcing at an industrial city in northern India , 2005 .

[26]  S. Kreidenweis,et al.  Water uptake by particles containing humic materials and mixtures of humic materials with ammonium sulfate , 2004 .

[27]  T. Bond,et al.  Light Absorption by Carbonaceous Particles: An Investigative Review , 2006 .

[28]  Yan Feng,et al.  Uncertainties in global aerosol simulations: Assessment using three meteorological data sets , 2007 .

[29]  A. Stohl,et al.  Perturbation of the European free troposphere aerosol by North American forest fire plumes during the ICARTT-ITOP experiment in summer 2004 , 2007 .

[30]  A. Wiedensohler,et al.  An approximation of the bipolar charge distribution for particles in the submicron size range , 1988 .

[31]  Y. Rudich,et al.  The complex refractive index of atmospheric and model humic-like substances (HULIS) retrieved by a cavity ring down aerosol spectrometer (CRD-AS). , 2008, Faraday discussions.

[32]  Yoram J. Kaufman,et al.  On the twilight zone between clouds and aerosols , 2007 .