Hygroscopicity of aerosol particles at low temperatures. 1. New low-temperature H-TDMA instrument: setup and first applications

Interaction of cesium bound fission product compounds (CsI and CsOH) with abundant inorganic compounds of atmosphere: Effect on hygroscopic growth properties

Cesium compounds if present in atmosphere, can affect human health as well as the ecosystem due to their highly hazardous nature. Interaction of cesium compounds with abundantly available atmospheric salts can modify the hygroscopic behavior in sub-saturation relative humidity (RH) domain. Any marked modification in growth factor (GF) for the mixed particle state in comparison to the single particles ultimately affects the settling rates and hence the deposition flux. This work studies the hygroscopic behavior of two important cesium bound fission product aerosols (CsI, CsOH) internally mixed with some common atmospheric particles viz. [Formula: see text] and NaNO₃ for a fixed dry particle size of 100 nm. Experimental measurements, performed with Hygroscopic tandem differential mobility analyzer in the range of 20-94% RH, have been compared with the predictions made from Zdanovskii-Stokes-Robinson (ZSR) approach. Apart from the single/pure particle state for the constituents (i.e. mixing ratios 1:0 and 0:1), three other mixing ratios 1:4, 1:1 and 4:1 have been considered. The results show that the GF vs RH pattern for mixed particles is different from that for single CsI and CsOH particles. The intrinsic water uptake behavior for these cesium compounds was found to be perturbed for some of the chosen combinations as well. Deliquescent transition for the mixed particles was observed at lower RH compared to the single electrolytes. Relative differences noticeable for the chosen mixing ratios could be related to the available fractions in the mixed state. Overall, ZSR method was found to be capturing the trend of increasing GFs with increasing RH. Terminal gravitational settling velocities calculated from the measured GFs were also found to be different for single and mixed particles. The relative difference was significant for some combinations and test conditions. Any modification in settling velocity ultimately impacts the deposition flux estimations. Hence neglecting the presence of atmospheric salts affects the accuracy of the source term estimates for a postulated nuclear reactor accident scenario.

Synergetic technique combining elastic backscatter lidar data and sunphotometer AERONET inversion for retrieval by layer of aerosol optical and microphysical properties

We present a so-called lidar and almucantar (LidAlm) algorithm that combines information provided by standard elastic backscatter lidar (i.e., calibrated attenuated backscatter coefficient profile at one or two wavelengths) and sunphotometer AERONET inversion of almucantar like measurements (i.e., column-integrated aerosol size distribution and refractive index). The purpose of the LidAlm technique is to characterize the atmospheric column by its different aerosol layers. These layers may be distinct or partially mixed, and they may contain different aerosol species (e.g., urban, desert, or biomass burning aerosols). The LidAlm synergetic technique provides the extinction and backscatter coefficient profiles, particle size distributions, and backscatter-to-extinction ratios for each aerosol layer. We present the LidAlm procedure and sensitivity studies. The applications are illustrated with examples of actual atmospheric conditions encountered in the Paris area.

Adaptation of dry nephelometer measurements to ambient conditions at the Jungfraujoch

In a numerical study the influence of relative humidity (RH) on aerosol scattering coefficients sigma was investigated. Based on a core/coating aerosol model, RH enhancement factors for scattering, xi(RH) = sigma(RH)/sigma(RH = 0), were calculated for the wavelengths lambda = 450, 550, and 700 nm for a summer and a winter case. The investigation was adapted to the situation (e.g., chemical composition, particle size distributions, hygroscopic behavior) of the high-alpine site Jungfraujoch (JFJ, 3580 m asl), where long-term measurements of dry aerosol scattering coefficients are performed at these wavelengths. The presented results are therefore representative of the lower free troposphere above a continent. The RH enhancement factors at a specific RH strongly depend on the average particle size. For example, at RH = 85% they vary between approximately 1.2 and approximately 2.7 in summer and between approximately 1.4 and approximately 3.8 in winter. It is shown that there is a strong relationship between xi and the Angstrom exponent å (based on scattering only) of the dry aerosol, which is directly derived from the dry scattering measurements. This allows for parametrizing xi for a specific wavelength and season with å and RH. The parametrization is applicable for RH up to approximately 90%--for higher RH the underlying hygroscopic models become unreliable--and for å between approximately -0.25 and approximately 2.75, which covers the range observed at the JFJ. Also addressed is a systematic error in the dry scattering coefficients measured with a nephelometer previously discussed in the literature, which arises from nonidealities in the angular intensity distribution of the light inside the instrument. This effect also depends strongly on the particle size and can be described by a correction factor C that can be parametrized with å. The scattering coefficient corrected for measurement artifacts at ambient RH for specific wavelength and season therefore can be estimated from the uncorrected dry nephelometer scattering coefficient sigma(neph) as sigma(å, RH) = C(å) x xi(å, RH) x sigma(neph). As additional information only ambient RH data are needed. The 95% confidence bound of this total correction ranges from less than 5% for low RH and large å up to approximately 40% for high RH and small å.

Hygroscopicity of aerosol particles at low temperatures. 2. Theoretical and experimental hygroscopic properties of laboratory generated aerosols

A Hygroscopicity Tandem Differential Mobility Analyzer (H-TDMA) system has been used to measure hygroscopic growth curves and deliquescence relative humidities (DRH) of laboratory generated (NH4)2SO4, NaCl, and NaNO3 particles at temperatures T= 20 degrees C and -10 degrees C. Good agreement (better than 3.5%) between measured growth curves and Köhler theory was found using empirical temperature and concentration dependent values for water activity, solution density, and surface tension. The measured growth curves only experience a small temperature dependence in the observed temperature range. Therefore, to a first approximation, it is possible to neglect the temperature dependence of the water activity for theoretical calculations in the temperature range -10 degrees C < T < 25 degrees C. The small differences between experiment and theory, which were predominantly observed for NaCl particles, are probably caused by a small amount of water adsorbed on the "dry" crystals. It was also observed that these particles experience a significant restructuring at relative humidity RH < DRH, which was also taken into account for a comparison with theoretical curves. If salt particles are used for instrument calibration, precautions regarding the dry particle diameter have to be taken.