Influence of water uptake on the aerosol particle light scattering at remote sites (Invited)
Abstract
Since ambient aerosol particles experience hygroscopic growth at enhanced relative humidity (RH), their microphysical and optical properties - especially the aerosol light scattering - are also strongly dependent on RH. The knowledge of this RH effect is of importance for climate forcing calculations or for the comparison of remote sensing with in-situ measurements because in the field aerosol in-situ measurements are often performed under dry conditions. The scattering enhancement factor f(RH,λ) is the key parameter to describe this effect of water uptake on the particle light scattering. It is defined as the particle light scattering coefficient σ(RH) at a certain RH and wavelength λ divided by its dry value. Here, we will present results from two remote sites: the Jungfraujoch located at 3580 m a.s.l. in the Swiss Alps and from Zeppelin station located at 78.5°N in the Arctic (Fierz-Schmidhauser et al., 2010; Zieger et al., 2010). Various aerosol optical and microphysical parameters were recorded at these sites using in-situ and remote sensing techniques. The scattering enhancement varied largely from very low values of f(RH=85%,λ=550 nm) around 1.28 for mineral dust transported to the Jungfraujoch to 3.41 for pristine Arctic aerosol. Compensating effects of size and hygroscopicity were observed in the Arctic, i.e. small but less hygroscopic particles eventually had the same magnitude in f(RH) as large but more hygroscopic particles like sea salt. Closure studies and Mie simulations showed that both size and chemical composition determine the magnitude of f(RH). The f(RH)-values from the two remote sites will also be related to values measured at other maritime, rural, and continental sites in Europe (Zieger et al., 2013). Active and passive remote sensing techniques were used to study the vertical distribution of aerosol optical properties around Jungfraujoch. Part of these in-situ measured parameters, together with the RH-dependent σ(RH) were used to retrieve the particle light extinction coefficient at ambient RH, which were compared to remote sensing measurements of the LIDAR (light detection and ranging) instruments. However, the comparison was clearly affected by orographic effects due to the exposed location of the Jungfraujoch (Zieger et al., 2012). Finally, the measurement results were compared to the widely used aerosol model OPAC (Hess et al., 1998). Significant discrepancies were seen especially at intermediate RH ranges, which were mainly attributed to the inappropriate implemented hygroscopic growth within OPAC. Replacement of the hygroscopic growth with recent literature values showed a clear improvement (Zieger et al., 2013). References: Fierz-Schmidhauser et al.: Measured and predicted aerosol light scattering enhancement factors at the high alpine site Jungfraujoch, Atmos. Chem. Phys., 10, 2319-2333, 2010 Hess, M. et al.: Optical properties of aerosols and clouds: The software package OPAC, Bull. Amer. Meteor. Soc., 79, 831-844, 1998 Zieger et al.: Effects of relative humidity on aerosol light scattering in the Arctic, Atmos. Chem. Phys., 10, 3875-3890, 2010 Zieger et al.: Spatial variation of aerosol optical properties around the high-alpine site Jungfraujoch (3580 m a.s.l), Atmos. Chem. Phys., 12, 7231-7249, 2012 Zieger et al.: Effects of relative humidity on aerosol light scattering: results from different European sites, Atmos. Chem. Phys. Discuss., 13, 8939-8984, 2013
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.A13B0184Z
- Keywords:
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- 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE;
- 0305 ATMOSPHERIC COMPOSITION AND STRUCTURE Aerosols and particles;
- 0360 ATMOSPHERIC COMPOSITION AND STRUCTURE Radiation: transmission and scattering;
- 0365 ATMOSPHERIC COMPOSITION AND STRUCTURE Troposphere: composition and chemistry