Effects of relative humidity on aerosol light scattering in the Arctic

Abstract

Aerosol particles experience hygroscopic growth in the ambient atmosphere. Their optical properties-especiallytheaerosollightscattering- arethereforestronglydependent on the ambient relative humidity (RH). In-situ light scattering measurements of long-term observations are usually performed under dry conditions (RH>30-40%). The knowledge of this RH effect is of eminent importance for climate forcing calculations or for the comparison of remote sensing with in-situ measurements. This study combines measurements and model calculations to describe the RH effect on aerosol light scattering for the first time for aerosol particles present in summer and fall in the high Arctic. For this purpose, a field campaign was carried out from July to October 2008 at the Zeppelin station in Ny-Ã..lesund, Svalbard. The aerosol light scattering coefficient σsp(λ) was measured at three distinct wavelengths (ÎCombining double low line450, 550, and 700 nm) at dry and at various, predefined RH conditions between 20% and 95% with a recently developed humidified nephelometer (WetNeph) and with a second nephelometer measuring at dry conditions with an average RH<10% (DryNeph). In addition, the aerosol size distribution and the aerosol absorption coefficient were measured. The scattering enhancement factorf(RH, λ) is the key parameter to describe the RH effect on σsp(λ) and is defined as the RH dependent σsp(RH, λ) divided by the corresponding dry σsp(RHdry, λ). During our campaign the averagef(RHCombining double low line85%, ÎCombining double low line550 nm) was 3.24±0.63 (mean ± standard deviation), and no clear wavelength dependence off(RH, λ) was observed. This means that the ambient scattering coefficients at RHCombining double low line85% were on average about three times higher than the dry measured in-situ scattering coefficients. The RH dependency of the recordedf (RH, λ) can be well described by an empirical one-parameter equation. We used a simplified method to retrieve an apparent hygroscopic growth factorg(RH), defined as the aerosol particle diameter at a certain RH divided by the dry diameter, using the WetNeph, the DryNeph, the aerosol size distribution measurements and Mie theory. With this approach we found, on average,g(RH)Combining double low line85%) values to be 1.61±0.12 (mean±standard deviation). No clear seasonal shift off(RH, λ) was observed during the 3-month period, while aerosol properties (size and chemical composition) clearly changed with time. While the beginning of the campaign was mainly characterized by smaller and less hygroscopic particles, the end was dominated by larger and more hygroscopic particles. This suggests that compensating effects of hygroscopicity and size determined the temporal stability off(RH, λ). During sea salt influenced periods, distinct deliquescence transitions were observed. At the end we present a method on how to transfer the dry in-situ measured aerosol scattering coefficients to ambient values for the aerosol measured during summer and fall at this location. © 2010 Author(s).