U. Dusek

A closure study of sub-micrometer aerosol particle hygroscopic behaviour

Abstract The hygroscopic properties of sub-micrometer aerosol particles were studied in connection with a ground-based cloud experiment at Great Dun Fell, in northern England in 1995. Hygroscopic diameter growth factors were measured with a Tandem Differential Mobility Analyser (TDMA) for dry particle diameters between 35 and 265 nm at one of the sites upwind of the orographic cloud. An external mixture consisting of three groups of particles, each with different hygroscopic properties, was observed. These particle groups were denoted less-hygroscopic, more-hygroscopic and sea spray particles and had average diameter growth factors of 1.11–1.15, 1.38–1.69 and 2.08–2.21 respectively when taken from a dry state to a relative humidity of 90%. Average growth factors increased with dry particle size. A bimodal hygroscopic behaviour was observed for 74–87% of the cases depending on particle size. Parallel measurements of dry sub-micrometer particle number size distributions were performed with a Differential Mobility Particle Sizer (DMPS). The inorganic ion aerosol composition was determined by means of ion chromatography analysis of samples collected with Berner-type low pressure cascade impactors at ambient conditions. The number of ions collected on each impactor stage was predicted from the size distribution and hygroscopic growth data by means of a model of hygroscopic behaviour assuming that only the inorganic substances interacted with the ambient water vapour. The predicted ion number concentration was compared with the actual number of all positive and negative ions collected on the various impactor stages. For the impactor stage which collected particles with aerodynamic diameters between 0.17–0.53 μm at ambient relative humidity, and for which all pertinent data was available for the hygroscopic closure study, the predicted ion concentrations agreed with the measured values within the combined measurement and model uncertainties for all cases but one. For this impactor sampling occasion, the predicted ion concentration was significantly higher than the measured. The air mass in which this sample was taken had undergone extensive photochemical activity which had probably produced hygroscopically active material other than inorganic ions, such as organic oxygenated substances.

Enhanced organic mass fraction and decreased hygroscopicity of cloud condensation nuclei (CCN) during new particle formation events

In a forested near-urban location in central Germany, the CCN efficiency of particles smaller than 100 nm decreases significantly during periods of new particle formation. This results in an increase of average activation diameters, ranging from 5 to 8% at supersaturations of 0.33% and 0.74%, respectively. At the same time, the organic mass fraction in the sub-100-nm size range increases from approximately 2/3 to 3/4. This provides evidence that secondary organic aerosol (SOA) components are involved in the growth of new particles to larger sizes, and that the reduced CCN efficiency of small particles is caused by the low hygroscopicity of the condensing material. The observed dependence of particle hygroscopicity (k) on chemical composition can be parameterized as a function of organic and inorganic mass fractions (forg, finorg) determined by aerosol mass spectrometry: k = korg forg + kinorg finorg. The obtained value of korg ~ 0.1 is characteristic for SOA, and kinorg ~ 0.7 is consistent with the observed mix of ammonium, sulfate and nitrate ions. (Less)

Soluble Mass, Hygroscopic Growth, and Droplet Activation of Coated Soot Particles during LACIS Experiment in November (LExNo)

The LACIS Experiment in November (LExNo) campaign was conducted in November 2005 at the Atmospheric Composition Change the European Network of Excellence (ACCENT) site Leipzig Aerosol Cloud Interaction Simulator (LACIS). The goal of LExNo was to provide deeper insight into the activation properties of coated soot particles imitating aged combustion aerosol particles. The aerosols were prepared by starting with spark-generated soot particles. In some experiments the soot particles were compacted by exposure to propanol vapor; in others this step was bypassed. The soot was thermally coated with ammonium sulfate, levoglucosan, or a mixture of both ammonium sulfate and levoglucosan. The synthesized particles were investigated using aerosol mass spectrometry, a Hygroscopicity Tandem differential mobility analyzer, two Wyoming static diffusion cloud condensation nuclei (CCN) instruments, a Droplet Measurement Technologies continuous flow CCN instrument, and LACIS. A close correlation between the hygroscopic growth factor at 98% relative humidity and the critical supersaturation of CCN activation was observed. Closure between hygroscopic growth, CCN activation, and chemical composition of the investigated particles was achieved with two different single-parameter Kohler model approaches and with a third approach, a standard Kohler model using as input parameter the soluble mass as determined by aerosol mass spectrometry. (Less)

Black carbon measurements using an integrating sphere

An integrating sphere was used to determine the black carbon (BC) content of aerosol filter samples dissolved in chloroform (method originally described by Heintzenberg [1982)). The specific absorption coefficient B a (equal to absorption per mass) of the samples was also measured using the sphere as an integrating detector for transmitted light. Comparing the B a of ambient samples taken in Vienna, Austria to the BC concentrations measured on the dissolved filters, a value of approximately 6 m 2 /g was found to bea a reasonable value for the B a of the black carbon found at the site. The size dependence of B a of a nebulized suspension of soot was measured using a rotating impactor, and a reasonable agreement between measured and calculated values was found.

Humidity-Dependent Growth of Size-Segregated Aerosol Samples

ABSTRACT This paper presents first results obtained with a measurement system designed to measure the humidity-dependent water uptake by deposited aerosol samples. Aerosol in the size range 0.06–16 μm aerodynamic equivalent diameter is sampled with a nine stage cascade impactor. The individual impaction foils are exposed to elevated relative humidities in a chamber containing an aqueous solution of CaCl2 of specified concentration. The mass increase of the deposits is measured with a balance above the chamber. Only equilibrium values of m/m0 (i.e., wet mass/dry mass) for increasing humidities can be obtained. The two samples discussed here showed a strongly size-selective growth pattern. Deposits with accumulation range particles have larger m/m0 than those with coarse mode particles (e.g., sample 1: 2.6 vs 1.33 at 90% humidity; sample 2: 2.35 vs 1.16). Although the two samples had been obtained at the same time of the year, the growth patterns are different, which might have been caused by the meteorolog...

The Great Dun Fell Experiment 1995: an overview

Abstract During March and April of 1995 a major international field project was conducted at the UMIST field station site on Great Dun Fell in Cumbria, Northern England. The hill cap cloud which frequently envelopes this site was used as a natural flow through reactor to examine the sensitivity of the cloud microphysics to the aerosol entering the cloud and also to investigate the effects of the cloud in changing the aerosol size distribution, chemical composition and associated optical properties. To investigate these processes, detailed measurements of the cloud water chemistry (including the chemistry of sulphur compounds, organic and inorganic oxidised nitrogen and ammonia), cloud microphysics and properties of the aerosol and trace gas concentrations upwind and downwind of the cap cloud were undertaken. It was found that the cloud droplet number was generally strongly correlated to aerosol number concentration, with up to 2000 activated droplets cm−3 being observed in the most polluted conditions. In such conditions it was inferred that hygroscopic organic compounds were important in the activation process. Often, the size distribution of the aerosol was substantially modified by the cloud processing, largely due to the aqueous phase oxidation of S(IV) to sulphate by hydrogen peroxide, but also through the uptake and fixing of gas phase nitric acid as nitrate, increasing the calculated optical scattering of the aerosol substantially (by up to 24%). New particle formation was also observed in the ultrafine aerosol mode (at about 5 nm) downwind of the cap cloud, particularly in conditions of low total aerosol surface area and in the presence of ammonia and HCl gases. This was seen to occur at night as well as during the day via a mechanism which is not yet understood. The implications of these results for parameterising aerosol growth in Global Climate Models are explored.

Examination of laboratory-generated coated soot particles: An overview of the LACIS Experiment in November (LExNo) campaign

In the suite of laboratory measurements described here and in companion articles we deal with the hygroscopic growth and activation behavior of coated soot particles synthesized to mimic those of an atmospheric aerosol originating from biomass combustion. The investigations were performed during the measurement campaign LACIS Experiment in November (LExNo) which took place at the Leipzig Aerosol Cloud Interaction Simulator (LACIS). The specific goals of this campaign were (1) to perform a critical supersaturation measurement intercomparison using data sets from three different cloud condensation nucleus (CCN) instruments (two static thermal gradient type, one stream-wise thermal gradient type) and LACIS, (2) to examine particle hygroscopic growth (hydrated particle size as function of relative humidity) for particle characteristics such as aerosol mass spectrometer (AMS) measured soluble mass and particle morphology, and (3) to relate critical supersaturations derived from both measurements of soluble mass and high humidity tandem differential mobility analyzer (HH-TDMA) determined growth factors to critical supersaturations measured by means of the CCN instruments. This paper provides information on the particle synthesis techniques used during LExNo, an overview concerning the particle characterization measurements performed, and, by proving relations between measured composition, hygroscopic growth, and activation data, lay the foundations for the detailed investigations described in the companion studies. In the context of the present paper, excellent agreement of the critical supersaturations measured with three different CCN instruments and LACIS was observed. Furthermore, clear relations between coating masses determined with AMS and both hygroscopic growth factors at 98% RH and measured critical supersaturations could be seen. Also, a strong correlation between measured hygroscopic growth (growth factors at 98%) and measured critical supersaturation for all of the differently coated soot particles (coating substances being levoglucosan and/or ammonium (hydrogen) sulfate) was found. This is clearly indicative of the possibility of predicting the critical supersaturation of coated soot particles based on hygroscopic growth measurements using Kohler theory. (Less)

Modelling of aerosol modification resulting from passage through a hill cap cloud

Abstract Data collected at the Great Dun Fell site are used to provide input and comparative output for a computer model of the hill cap cloud system. The aim of the report is to investigate the effect of in-cloud chemical processing on aerosols and trace gases, focusing on the production of S(VI) via the oxidation of SO 2 by H 2 O 2 . Both airflow, cloud chemistry and cloud microphysics are modelled and compared to upwind, downwind, and summit observations made during the Great Dun Fell field campaign in 1995. The results indicate that there exists a broader droplet size distribution than predicted due to the mixing of separate parcels of air with different trajectories and humidities. Modification of the aerosol size distribution is predominantly due to sulphate production at the expense of sulphur dioxide gas oxidised by hydrogen peroxide. Predicted nucleation scavenging resulted in the loss of the more hygroscopic particles of diameters 0.05–0.13 μm, which by the addition of soluble mass grew to between 0.13 and 0.3 μm in diameter. The less hygroscopic mode comprised approximately 2% of the total mass input and thus did not significantly contribute to the modified sections of the aerosol spectrum. The modified particles were of a size suitable for nucleation scavenging, increasing the number of CCN available for future droplet activation. The hygroscopic properties of the modified particles were also affected by the addition of soluble mass, such that they would require a lower critical supersaturation for activation ( Swietlicki et al., 1999 ). The level of aerosol augmentation is dependent upon the activation history of the cloud droplets, the concentrations of interstitial gas species, and the partitioning of the aerosol ion species.

Comparison of recent (1994) black carbon data with those obtained in 1985 and 1986 in the urban area of Vienna, Austria : implications for future environmental policy decisions

Abstract For a short period in May 1994 and from 15 October to 15 December, 1994, day-time and night-time filter samples of the atmospheric aerosol were taken in an enclosed courtyard in central Vienna. The filters were subsequently analyzed for black carbon (BC) concentrations and the specific absorption coefficient Ba (methods: [1,2]). The 1994 data are compared with the data obtained in 1985 and 1986 [3]. The mean BC content increased by 27% (outside the heating period, i.e. May, 106%), while the total aerosol mass concentration decreased by 44% (May: 36%). The mean BC concentration decreased by 33% (May: increase by 35%). Fuel sale data show that the sale of diesel fuel increased by 89%, while sales of gasoline, fuel oil and coal decreased. This paper presents possible measures to reduce the BC emissions of the major sources (i.e. traffic and space heating).

Aerosol number to volume ratios in Southwest Portugal during ACE-2

Past studies have indicated that long-term averages of the aerosol number to volume ratios (defined as the number ofparticles larger than a certain diameter divided by the particle volume over some range less than 1 μm) show littlevariability over the Atlantic. This work presents number to volume ratios (R) measured during the ACE-2 experiment onthe land-based Sagres field site located in Southwest Portugal. The values of R measured in Sagres compare reasonablywell with previous measurements over the Atlantic. The main emphasis of this work is therefore to investigate moreclosely possible reasons for the observed stability of the number to volume ratio. Aerosol number size distributionsmeasured in Sagres are parametrized by the sum of two log-normal distributions fitted to the accumulation and to theAitken mode. The main factor that limits the variability of R is that the parameters of these log-normal distributionsare not always independent but show some covariance. In polluted air mass types correlations between parameters ofthe Aitken and accumulation mode are mostly responsible for stabilizing R. In marine air mass types the variabilityof R is reduced by an inverse relationship between the accumulation-mode mean diameter and standard deviation, consistent with condensational processes and cloud processing working on the aerosol. However, despite this reduction, the variability of R in marine air mass types is still considerable and R is linearly dependent on the number concentrationof particles larger than 90 nm. This partly due to a tail of Aitken-mode particles extending to sizes larger than90 nm.