Effects of relative humidity on aerosol light scattering in the Arctic

Abstract

Abstract. Aerosol particles experience hygroscopic growth in the ambient atmosphere. Their optical properties -especially the aerosol light scattering -are therefore strongly dependent 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-Ålesund, Svalbard. The aerosol light scattering coefficient σ sp (λ) was measured at three distinct wavelengths (λ=450, 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 factor f (RH, λ) is the key parameter to describe the RH effect on σ sp (λ) and is defined as the RH dependent σ sp (RH, λ) divided by the corresponding dry σ sp (RH dry ,λ). During our campaign the average f (RH=85%, λ=550 nm) was 3.24±0.63 (mean ± standard deviation), and no clear wavelength dependence of f (RH, λ) was observed. This means that the ambient scattering coefficients at RH=85% were on average about three times higher than the dry meaCorrespondence to: E. Weingartner (ernest.weingartner@psi.ch) sured in-situ scattering coefficients. The RH dependency of the recorded f (RH, λ) can be well described by an empirical one-parameter equation. We used a simplified method to retrieve an apparent hygroscopic growth factor g(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 = 85%) values to be 1.61±0.12 (mean±standard deviation). No clear seasonal shift of f (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 of f (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.

Observations of relative humidity effects on aerosol light scattering in the Yangtze River Delta of China

“…The similarity results from the low scattering enhancement factor (e.g., f (80 %) = 1.44 ± 0.12) at Lin'an, which was similar to the value (f (80 %) = 1.33 ± 0.07) obtained by Gassó et al (2000) for a dust event. The f (RH) in other studies was much higher than that at Lin'an, ranging from 2.04 (polluted marine aerosols in Gassó et al, 2000) to 3.77 (arctic aerosols in Zieger et al, 2010), and therefore their parameter g was much higher.…”

“…The wavelength dependence of scattering enhancement factor f (RH,λ) varies with generalized aerosol types. Kotchenruther and Hobbs (1998) and Zieger et al (2010Zieger et al ( , 2011 found no pronounced wavelength dependence of f (RH,λ) for biomass burning aerosols and arctic aerosols, respectively; Zieger et al (2013) found small variations (< 5 %) of f (RH,λ) at 450, 550, and 700 nm for several European sites; Kotchenruther et al (1999) and Magi and Hobbs (2003) reported significant wavelength dependence of f (RH,λ) for urban/industrial aerosols off the east coast of the United States. In this study, the wavelength dependence of enhancement factors was also investigated.…”

“…The scattering enhancement factor f (RH) can be parameterized with empirical equations (Kotchenruther and Hobbs, 1998;Kotchenruther et al, 1999;Gassó et al, 2000;Carrico et al, 2003;Liu et al, 2008;Zieger et al, 2010;Zieger et al, 2014). Humidograms from Lin'an were fitted with two empirical equations and the fitting results are shown below.…”

“…η is a nonnegative number. Zieger et al (2010) has defined an index describing the magnitude of deliquescence transitions based on fitting equation f (RH) = (1−RH) −g (see Sect. 3.6.1), while the steepness index η proposed in this study provided a way of quantitatively describing the steepness of humidograms that are well described by the equation f (RH) = 1+a× RH b .…”

Seasonal variation of aerosol water uptake and its impact on the direct radiative effect at Ny-Ålesund, Svalbard

“…Numerous experimental and modeling studies have investigated the influence of RH on optical properties of aerosol particles, which is often described with the enhancement factor f (RH), defined as the ratio of aerosol scattering coefficient at a given RH and the scattering coefficient at dry conditions (see e.g., Zieger et al, 2010). f (RH) has been investigated in a number of studies at various locations (see Table 1), typically by comparing the signal of a nephelometer operated at a given RH to a corresponding instrument at dry conditions.…”