Martina Krämer

Control of solute concentrations in cloud and fog water by liquid water content

We measured the concentrations of Cl^−, NO_3^−, and SO_4^(2−) in atmospheric cloudwater collected with a passive collector from a remote marine site and the concentrations of Cl^−, NO_3^−, SO_4^(2−), Na^+ and NH_4^+ in cloudwater from a continental site. At the continental site, an active rotating arm collector was employed for cloudwater collection. At both locations, we observed an inverse relationship between the measured ion concentrations and the liquid water content of the clouds. The product of the observed concentrations and the liquid water content, the cloudwater loading, was found to be nearly constant. A study of published cloudwater data showed that at the majority of the examined locations this product varied little at any given site, whereas large differences exist between sites. The finding of the small variation in cloudwater loadings could be explained by the hypothesis that nucleation scavenging is the dominant process in the acquisition of solutes into hydro-meteors during fog and cloud formation.

The water‐soluble fraction of atmospheric aerosol particles and its influence on cloud microphysics

The water-soluble fraction of atmospheric aerosol particles is a major property dividing the particle population into cloud condensation nuclei and interstitial particles during cloud formation. Likewise, this property influences cloud microphysics and chemistry as well as direct and indirect aerosol forcing of climate. Up to now this important parameter has been only poorly determined experimentally. Here, we present SoFA, a new method to determine the water-Soluble Fraction of large and giant Aerosol particles in five narrow size bands with geometrical radii of 0.4, 0.6, 0.9, 1.3, 1.8 μm and one band for particles with radius larger than 2.3 μm. First results show three different types of aerosol particles (AP I, II, III). AP I were characterized by 9% water-soluble material, and AP II by 50% and correspond to those found in earlier studies for small particles. The new AP III consists of 88% water-soluble material. In those size bands, where all three particle types could be detected (0.9, 1.3, and 1.8 μm; because of detection limits of SoFA, only two types of particles could be detected in the remaining size bands), about 50% of the total number of analyzed particles belong to AP III, whereas about 25% belong to both AP II and I. These numbers lead to 59% water-soluble material on average. Particles of AP III are assumed to be cloud-processed particles, those of AP I might be of biological origin. The impact of the water-soluble fraction of particles on cloud microphysics is investigated by comparing two cloud situations using an entraining air parcel model with detailed microphysics. The model simulations show an influence on nucleation scavenging, cloud interstitial aerosol, and in particular, drop size population. Most notable is that the same aerosol particle number distribution causes either a precipitating or a non precipitating cloud, depending only upon different water-soluble fractions of the particles.

Rainwater Composition Over a Rural Area with Special Emphasis on the Size Distribution of Insoluble Particulate Matter

The rainwater composition in the vicinity of Mainz, FRG, has been investigated with special emphasis on insoluble constituents. The number size distribution was determined in the range from 0.1 μm up to 100 μm radius. For particles with r>0.5 μm radius the shape of the size distribution of insoluble particles in rain follows the shape of the average urban and rural aerosol. In this particular size range no major size selective removal processes could be seen. For r<0.5 μm the number size distribution tends to flatten compared to the average aerosol. This might be the indication of a size selective removal process (Greenfield Gap).

A method to determine rainwater solutes from pH and conductivity measurements

Abstract An established method to calculate the acid fraction (AF= [H+]/[An]) of substances dissolved in an aqueous solution solely based on the measurement of the pH-value and the electrical conductivity is extended to determine in addition the total solute concentration. The loading of samples of atmospheric hydrometeors can be assessed from these four parameters (total dissolved material, acid fraction, pH and electrical conductivity). Mean electrical conductivities, the fundamental parameters of the method, are presented for rainwater from marine, coastal, rural and mountain sampling sites. The accuracy of the method is confirmed by comparing solute concentrations and acid fractions of various samples of rainwater, determined on the one hand by the method proposed here and on the other hand by ion chromatography. Annual average values of pH, electrical conductivity, total dissolved material and acid fraction are presented for rainwater from coastal, rural and mountain sampling sites located in Germany as an example of an application of the new method proposed here. The observed solute concentration decreases from the North to the South. The acid fraction is lowest at the coast (10% on average), rising to about 20% at the rural and 25% at the mountain sites.

On the collection efficiency of a rotating ARM collector and its applicability to cloud- and fogwater sampling

A Rotating Arm Collector (RAC) belonging to the class of (virtualy) wide stream impactors is designed and operated to sample cloud or fog droplets. The applicability of RACs to sample atmospheric aerosol particles or fog/cloud droplets larger than about 5 μm is demonstrated by discussing their working principle in detail and comparing their sampling characteristics with impactors requiring an inlet. The specific properties of the flow past the sampling units of RACs, especially the variation of the flow velocity along the sampling area results in a dependence of the collection efficiency on the distance to the axis of rotation. This is remarkable when operating a RAC to collect cloud or fog droplets, because drops flow together after sampling. A method is developed which determines the overall collection efficiency of the sampling units experimentally. The resulting collection efficiency curve is compared with simplified theoretical evaluations of the collection efficiency of flat plates (Langmuir and Blodgett, Collected Work of Irving Langmuir10, pp. 348–393. Pergamon Press, Oxford, 1946) and the collection efficiency of the gutter-formed sampling areas used in this study (Lesnic et al., J. Aerosol Sci.24, 163–180, 1993). The agreement between experiment and theory is good, showing that the main characteristics of wide stream impaction are not affected by the complex flow field past the sampling units of the RAC and thus the simplified theory is an acceptable approach to describe the real sampling conditions.

Quality assessment of MOZAIC and IAGOS capacitive hygrometers: insights from airborne field studies

In 2011, the MOZAIC (Measurement of Ozone by AIRBUS In-Service Aircraft) successor programme IAGOS (In-service Aircraft for a Global Observing System) started to equip their long-haul passenger aircraft with the modified capacitive hygrometer Vaisala HUMICAP® of type H. The assurance of the data quality and the consistency of the data set during the transition from MOZAIC Capacitive Hygrometers to IAGOS Capacitive Hygrometers were evaluated within the CIRRUS-III and AIRTOSS-ICE field studies. During these performance tests, the capacitive hygrometers were operated aboard a Learjet 35A aircraft together with a closed-cell Lyman-α fluorescence hygrometer, an open-path tunable diode laser (TDL) system and a closed-cell, direct TDL absorption hygrometer for water vapour measurement. For MOZAIC-typical operation conditions, the comparison of relative humidity (RH) data from the capacitive hygrometers and reference instruments yielded remarkably good agreement with an uncertainty of 5% RH. The temperature dependence of the sensors response time was derived from the cross-correlation of capacitive hygrometer data and smoothed data from the fast-responding reference instruments. The resulting exponential moving average function could explain the major part of the observed deviations between the capacitive hygrometers and the reference instruments.

Number size distribution of insolubleatmospheric aerosol particles in fog/cloud-water

AbstractThe design and the efficiency — curve of a newly developed fog/cloud-water sampler (rotating arm collector)is discussed. Fog/cloud-water samples were taken during Oct./Nov. 1990 on the mountain ‘Kleiner Feldberg’/Taunus, FRG. The number size distribution of the insoluble atmospheric aerosol particles in water is determined in the size range 0.025μm p . The size distribution of activated aerosol particles in air is derived from this distribution. Comparison of the spectrum of the activated aerosol particles with the total distribution of aerosol particles in air (measured during the same campaign, but in the size range 0.008μm p ) indicates, that in the size range r p > 0.06μm the major part of atmospheric aerosol particles becomes activated during fog/clouds. The variations of the size distribution in fog/cloud-water during an event is low.

A new method for measurements of insoluble submicron particles in water

Abstract A new method was developed to determine the size distribution of insoluble particles in e.g., fog water in the particle size range 50 nm p nm. First the water was nebulized and than the droplets dried to form residual aerosol particles. Using a Tandem Differential Mobility Analyser (TDMA), soluble and insoluble particles were separated. The system was calibrated with monodisperse latex particles to determine the loss factor for insoluble particles.

A new method to measure the size distribution of insoluble submicron particles in water

Abstract In the atmosphere, cloud and fog droplets usually contain insoluble material. The role of these insoluble particles is still unknown today, and is of interest to study. To determine the size distribution and number concentration of these particles in water, different techniques are available. The instrumentation, however, to measure nanometer-sized particles down to 50 nm diameter is not known. A new instrument, the Liquid Tandem Differential Mobility Analyser (LTDMA), was developed to measure size distributions of insoluble particles in water in the size range 50–300 nm in diameter. The new method is based on nebulising, e.g. cloud water and forming a residue aerosol consisting of both, insoluble particles with a soluble shell, and pure soluble particles. The insoluble, hydrophobic particles can be separated from soluble, hygroscopic residue particles with a Tandem Differential Mobility Analyser. The system is calibrated with monodisperse latex particles to determine the size-dependent transmission factor of insoluble particles of the Liquid Tandem Differential Mobility Analyser. A size distribution of insoluble particles in cloud water is presented as an application of this new measuring method. The cloud water sample was taken during the field campaign of the EUROTRAC sub project Ground-based Cloud Experiments (GCE) on the Kleiner Feldberg mountain, Germany, in November 1990.