Calibration Aerosol Research Articles

Cracking the code—Matching a proprietary algorithm for a low-cost sensor measuring PM1 and PM2.5

Some manufacturers of low-cost particle sensors use proprietary algorithms to estimate particle mass concentrations such as PM2.5. Often little or no information is given regarding the calibration aerosol, how the algorithm was created or tested, or how the mass was estimated from the particle number counts. If the algorithm is faulty in some way, researchers have little ability to correct it in a fundamental way, although they can multiply the output by some calibration factor to match the particular aerosol combination they are studying. However, the adjustment still requires the use of the proprietary algorithm, which may have quirks that make it impossible to fix completely using a single calibration factor. It might be possible in some cases to avoid using the proprietary algorithm at all. That is the approach of this study. The low-cost sensor studied is the Plantower PMS 5003, and the algorithm is the CF_1 algorithm offered by the manufacturer. Data from a six-month study of four collocated PurpleAir PA-II monitors, each containing two independent Plantower PMS 5003 sensors, were collected. Two of these monitors had previously been calibrated against research-grade monitors. The best-fitting model for PM1 was found to be of the form PM1 = a*(N1 + N2) + d, where N1 and N2 are the particle numbers in the size categories 0.3–0.5 μm and 0.5–1 μm, and d is an additive constant. The best-fitting model for PM2.5 was of the form a*(N1 + N2) + b*N3 + d, where N3 is the number of particles in the third size fraction (1–2.5 μm). The individual models for all 8 sensors matched the reported CF_1 values for both PM1 and PM2.5 with R2 values exceeding 0.99, intercepts near zero, and slopes in the 0.99–1.01 range. The proposed models may also explain why the CF_1 algorithm reports values of zero for a substantial portion of PM1 and PM2.5 estimates. General models capable of being applied to other datasets were developed and estimated to have mean absolute errors (MAEs) <1 μg/m3.

Analysis of Surface Reflectance Retrieval Over Four Typical Surfaces Via Gaofen-1 Satellite WFV4 Imagery

As a basis for the quantitative application of satellite remote sensing, surface reflectance can be retrieved through atmospheric correction methods. Currently, most studies have focused on developing or comparing atmospheric correction methods. However, few studies have quantitatively analyzed the effects of input parameters in an atmospheric correction method on retrieved surface reflectance. In this study, we evaluated the effects of the calibration coefficient, aerosol optical depth (AOD), aerosol type, and satellite zenith angle over four typical surfaces using wide field-of-view sensor four data of the Gao Fen-1 satellite. The results showed that (1) the relative errors of shrub, corn, grass, and soil reflectance increased as the calibration coefficient error increased; (2) the calibration coefficient, AOD, aerosol type, and satellite zenith angle affected corn reflectance retrieval the most, whereas they had the smallest effect on soil reflectance retrieval; and (3) the accuracy of the satellite zenith angle on the retrieved surface reflectance was the least pronounced, whereas the accuracy of aerosol type was the most pronounced.

Characterization of the Miniaturized Inverted Flame Burner as a Combustion Source to Generate a Nanoparticle Calibration Aerosol

Soot generators are able to produce carbonaceous nanoparticles purposefully and can therefore play a vital role in the calibration of particle instruments with an actual combustion aerosol. Condensation particle counters (CPCs) have become the instrument of choice for legislative measurements of the particle number (PN) concentration. The Euro 5B standard introduced by the European Union in 2011 was the first regulation that introduced a PN limit for the exhaust emission from light duty diesel vehicles. Since then, several other PN regulations for soot-emitting vehicles and combustion engines have been introduced or are currently in draft stages, all with similar requirements for the periodic calibration of the particle counter’s measurement accuracy. It is for this reason that combustion particles produced under laboratory conditions have become an attractive choice of calibration aerosol. Nonetheless, it is often difficult to generate a large amount of particles in the small nanometer range from a laboratory combustion source. In this study, we evaluated the performance of a recently introduced soot generator and its suitability for calibrating the counting efficiency and lower cut-off size of CPCs in the nanometer size range. We first characterized the soot generator’s warm up time to determine when it reaches a stable output when using propane as its fuel. We then investigated the influence of a dozen propane-to-air flow settings on the resulting particle size distribution of the combustion aerosol. Finally, we determined the resulting nanoparticle concentrations for 13 size classes below 20 nm in order to achieve a high size resolution at or near the lower detection limit of common CPCs. We performed our measurements under low-pressure conditions as our group operates CPCs onboard commercial passenger aircraft that are used as an atmospheric measurement platform. Another consideration is that CPCs are also operated elsewhere at much less than standard sea-level pressure. Examples include high-altitude research stations as well as engine test rigs used for vehicle exhaust emission testing or certification operated at elevated locations, e.g., in the USA, Mexico, and China. From these experiments, we concluded that when operating this novel soot generator with 7.5 SLPM airflow and 62.5 SCPM propane flow, it is possible to generate a realistic combustion aerosol for tests and calibrations that is still adequate even in the small nanometer size range. The concentrations measured under these operating conditions were just sufficient for detection with an aerosol electrometer, which is the concentration reference typically used in CPC calibrations.

Characterization of black carbon particles generated by a propane-fueled miniature inverted soot generator

The miniature inverted soot generator is a novel soot particle generator, capable of producing black carbon (BC) particles with a wide range of properties. The main advantages of this device are its portability, simplicity, low fuel consumption, and relatively low cost compared to other soot generators. In this study, the performance of the miniature inverted soot generator was evaluated using propane as the fuel over a range of air and fuel flow rates. The stability and repeatability of the soot generator to produce BC particles of controlled size distribution, and number and mass concentration was demonstrated. Furthermore, optical absorption, single scattering albedo (SSA), and elemental carbon (EC) and organic carbon (OC) concentrations of the generated particles were measured. When fueled with propane, the miniature inverted soot generator was capable of repeatedly generating particles with average mode mobility diameter ranging from 140 nm to 200 nm, number concentrations ∼4 × 106–∼2.7 × 107 #/cm3, and mass concentrations ∼8–∼140 mg/m3. For many of the investigated operating conditions, the elemental to total carbon ratio (EC/TC) of the generated soot particles was greater than 90%. The mass absorption cross section of generated particles ranged from 3 to 5 m2/g at a wavelength of 870 nm. The results obtained from this study indicate that the miniature inverted soot generator is ideal for producing BC particles with high EC fraction, which can be used as calibration aerosol for BC diagnostic instruments.

Scanning DMA data analysis II. Integrated DMA-CPC instrument response and data inversion

Analysis of scanning electrical mobility spectrometer (SEMS) or SMPS data requires coupling the scanning differential mobility analyzer (DMA) transfer function with the response functions for the instrument plumbing and the detector. In the limit of plug flow (uniform velocity) within the DMA, the scanning DMA transfer function has the same form as that for constant voltage. Most SEMS/SMPS data analysis uses this model, though previous studies have shown that boundary layers distort the transfer function during scanning DMA measurements. Part I determined the instantaneous transfer function during scanning of the TSI Model 3081 A long column DMA by modeling the flows, fields, and particle trajectories within the actual DMA geometry. This study (Part II) combines that transfer function with empirical data on the efficiencies and delay time distributions of the plumbing and detector of the SEMS/SMPS to determine the instantaneous rate at which particles are counted, and integrates the count rate over the finite counting time interval to obtain the integrated SEMS/SMPS response function. Simulations using this geometrical model are compared with those obtained using traditional, idealized DMA models for scan rates ranging from slow (240 s) to very fast (10 s), and with measurements of monodisperse calibration aerosols. Data inversion studies show that both increasing and decreasing voltage scans can be used to determine the particle size distribution, even with fast scans.

Development of polydisperse aerosol generation and measurement procedures for wind tunnel evaluation of size-selective aerosol samplers

Accurate development and evaluation of inlets for representatively collecting ambient particulate matter typically involves use of monodisperse particles in aerosol wind tunnels. However, the resource requirements of using monodisperse aerosols for inlet evaluation creates the need for more rapid and less-expensive techniques to enable determination of size-selective performance in aerosol wind tunnels. The goal of recent wind tunnel research at the U.S. EPA was to develop and validate the use of polydisperse aerosols which provide more rapid, less resource-intensive test results which still meet data quality requirements necessary for developing and evaluating ambient aerosol inlets. This goal was successfully achieved through comprehensive efforts regarding polydisperse aerosol generation, dispersion, collection, extraction, and analysis over a wide range of aerodynamic particle sizes. Using proper experimental techniques, a sampler's complete size-selective efficiency curve can be estimated with polydisperse aerosols in a single test, as opposed to the use of monodisperse aerosols which require conducting multiple tests using several different particle sizes. While this polydisperse aerosol technique is not proposed as a regulatory substitute for use of monodisperse aerosols, the use of polydisperse aerosols is advantageous during an inlet's development where variables of sampling flow rate and inlet geometry are often iteratively evaluated before a final inlet design can be successfully achieved. Complete Standard Operating Procedures for the generation, collection, and analysis of polydisperse calibration aerosols are available from EPA as downloadable files. The described experimental methods will be of value to other researchers during development of ambient sampling inlets and size-selective evaluation of the inlets in aerosol wind tunnels.

Calibration Approaches for Measurement of Aerosol Multielemental Concentration using Spark Emission Spectroscopy.

A multivariate calibration approach, using partial least squares regression, has been developed for measurement of aerosol elemental concentration. A training set consisting of 25 orthogonal aerosol samples with 9 factors (elements: Cr, Mn, Fe, Ni, Cu, Zn, Cd, Pb, Ti) and 5 levels (elemental concentrations) was designed. Spectral information was obtained for each aerosol sample using aerosol spark emission spectroscopy (ASES) at a time resolution of 1 minute. Simultaneous filter samples were collected for determination of elemental concentration using an inductively coupled plasma mass spectrometry (ICP-MS) analysis. Two regression models, PLS1 and PLS2, were developed to predict mass concentration from spectral measurements. Prediction ability of the models improved substantially when only signature wavelengths were included instead of the entire spectrum. The PLS1 model with 45 selected spectral variables (PLS1-45 model) presented the lowest relative root mean square error of cross validation (RMSECV; 16 - 35%). The detection limits using the PLS1-45 model, for the nine elements were in the range of 0.16 - 0.50 μg/m3. The performance of both multivariate and univariate regression models were tested for an unknown sample of welding fume aerosol. The multivariate model did not provide significantly better prediction compared to the univariate model. In spite of the difference in matrices of calibration aerosol and the unknown test aerosol, the results from PLS model show good agreement with those from filter measurements. The relative root mean square error of prediction (RMSEP) obtained from PLS1-45 model was 13% for Cr, 23% for Fe, 22% for Mn and 12% for Ni. The study shows that in spite of lower spectral resolution and lack of sample preparation, reliable and robust measurements can be obtained using the proposed calibration method based on PLS regression.

Dependence of CPC cut-off diameter on particle morphology and other factors

ABSTRACTIn this investigation, we summarize performance parameters of 24 TSI CPCs model 3772 and 9 TSI CPCs model 3790 determined at the World Calibration Aerosol Centre Physics hosted by the Leibniz Institute for Tropospheric Research. Model 3790 CPCs are basically identical to model 3772 laminar continuous flow type butanol-based CPCs with a modified temperature difference between saturator and condenser. The average 50% detection efficiency for silver particles for 3772 and 3790 instruments was found to be 7.52 ± 0.04 nm and 24.34 ± 0.29 nm (average mobility diameter ± standard deviation), respectively. Small changes of the temperature difference between saturator and condenser cause larger shifts of the 50% detection efficiencies of 3790 CPCs compared to 3772 CPCs. In addition to the known calibration material dependence of the 50% detection efficiencies of 3790 CPCs, we found a dependence on the morphology of the particles used for calibration. In our experiments more spherical particles shifted the ...

CCN Properties of Organic Aerosol Collected Below and within Marine Stratocumulus Clouds near Monterey, California

The composition of aerosol from cloud droplets differs from that below cloud. Its implications for the Cloud Condensation Nuclei (CCN) activity are the focus of this study. Water-soluble organic matter from below cloud, and cloud droplet residuals off the coast of Monterey, California were collected; offline chemical composition, CCN activity and surface tension measurements coupled with Köhler Theory Analysis are used to infer the molar volume and surfactant characteristics of organics in both samples. Based on the surface tension depression of the samples, it is unlikely that the aerosol contains strong surfactants. The activation kinetics for all samples examined are consistent with rapid (NH4)2SO4 calibration aerosol. This is consistent with our current understanding of droplet kinetics for ambient CCN. However, the carbonaceous material in cloud drop residuals is far more hygroscopic than in sub-cloud aerosol, suggestive of the impact of cloud chemistry on the hygroscopic properties of organic matter.

Single-particle characterization of ice-nucleating particles and ice particle residuals sampled by three different techniques

Abstract. In the present work, three different techniques to separate ice-nucleating particles (INPs) as well as ice particle residuals (IPRs) from non-ice-active particles are compared. The Ice Selective Inlet (ISI) and the Ice Counterflow Virtual Impactor (Ice-CVI) sample ice particles from mixed-phase clouds and allow after evaporation in the instrument for the analysis of the residuals. The Fast Ice Nucleus Chamber (FINCH) coupled with the Ice Nuclei Pumped Counterflow Virtual Impactor (IN-PCVI) provides ice-activating conditions to aerosol particles and extracts the activated particles for analysis. The instruments were run during a joint field campaign which took place in January and February 2013 at the High Alpine Research Station Jungfraujoch (Switzerland). INPs and IPRs were analyzed offline by scanning electron microscopy and energy-dispersive X-ray microanalysis to determine their size, chemical composition and mixing state. Online analysis of the size and chemical composition of INP activated in FINCH was performed by laser ablation mass spectrometry. With all three INP/IPR separation techniques high abundances (median 20–70%) of instrumental contamination artifacts were observed (ISI: Si-O spheres, probably calibration aerosol; Ice-CVI: Al-O particles; FINCH + IN-PCVI: steel particles). After removal of the instrumental contamination particles, silicates, Ca-rich particles, carbonaceous material and metal oxides were the major INP/IPR particle types obtained by all three techniques. In addition, considerable amounts (median abundance mostly a few percent) of soluble material (e.g., sea salt, sulfates) were observed. As these soluble particles are often not expected to act as INP/IPR, we consider them as potential measurement artifacts. Minor types of INP/IPR include soot and Pb-bearing particles. The Pb-bearing particles are mainly present as an internal mixture with other particle types. Most samples showed a maximum of the INP/IPR size distribution at 200–400 nm in geometric diameter. In a few cases, a second supermicron maximum was identified. Soot/carbonaceous material and metal oxides were present mainly in the sub-micrometer range. Silicates and Ca-rich particles were mainly found with diameters above 1 μm (using ISI and FINCH), in contrast to the Ice-CVI which also sampled many submicron particles of both groups. Due to changing meteorological conditions, the INP/IPR composition was highly variable if different samples were compared. Thus, the observed discrepancies between the different separation techniques may partly result from the non-parallel sampling. The differences of the particle group relative number abundance as well as the mixing state of INP/IPR clearly demonstrate the need of further studies to better understand the influence of the separation techniques on the INP/IPR chemical composition. Also, it must be concluded that the abundance of contamination artifacts in the separated INP and IPR is generally large and should be corrected for, emphasizing the need for the accompanying chemical measurements. Thus, further work is needed to allow for routine operation of the three separation techniques investigated.

Response Characteristics of PMP Compliant Condensation Particle Counters Toward Various Calibration Aerosols

Within the European legislation, a new limit for particle emissions of light and heavy duty engines based on the particle number (PN) was established in 2011. For PN determination, solid exhaust particles are quantified by means of a condensation particle counter (CPC). In literature, deviations in PN of up to 30% are reported for a comparison of different measurement set-ups. Among others variations in the counting efficiency (CE) of different CPCs have to be considered as possible error sources that contribute to the overall deviation in PN. Thereby the uncertainties in CE may result from variations in the calibration procedure of different manufacturers (e.g., calibration aerosol). To investigate this circumstance, devices from three different manufacturers were directly compared according to their CE for model aerosols. The subject CPCs exhibited differences of up to 17% (23 nm particles) in the counting efficiency when measuring simultaneously the same test aerosol. Depending on the PN size distribution in real exhaust, this might result in an error (∼9%) in the finally determined PN. Additionally, the CPC response for selected volatile exhaust components was investigated. In this way, we found out that the fraction of detected nucleation mode particles increases approximately by factor 3 in case particles consist of or contain volatile material (e.g., sulfuric acid).Copyright 2015 American Association for Aerosol Research

Seeded growth of monodisperse and spherical silver nanoparticles

Aiming at spherical and monodisperse silver nanoparticles with diameters up to 100nm, the potential of heterogeneous nucleation of silver particles was explored. Gold seed particles, mainly produced with a spark discharge generator, were carried by nitrogen through a three-zone tube furnace. Silver was evaporated at 1210°C in the first zone of the furnace and particle growth and shaping took place in the subsequent zones, heated to 730°C and 390°C, respectively. The generated aerosol was monitored by a scanning mobility particle sizer (SMPS), while parameters, such as furnace temperature, seed particle size and concentration and nitrogen carrier gas flow, were investigated. Off-line atomic force microscopy (AFM) and transmission electron microscopy (TEM) were used to characterize the morphology of the silver nanoparticles in addition to the SMPS scans. Spherical silver nanoparticles with a mobility diameter of more than 115nm and a geometric standard deviation of typically 1.09 or lower at concentrations as large as 5×105cm−3 could be produced. The mobility diameter of the monodisperse aerosol could be varied in the range of 50nm to 115nm by changing the furnace temperature or the gold seed particle size. Elemental analysis revealed that the gold from the seed particles formed a homogeneous alloy with the silver (≤3.5at.% of gold). The growth mechanism could not be identified unambiguously since the obtained silver particles could both originate from heterogeneous nucleation of silver vapour on the seed particles or from coagulation and coalescence of the seed particles with smaller, homogeneously nucleated silver particles. Moreover, the narrow size distribution opens the opportunity to obtain an exclusively singly charged, monodisperse calibration aerosol at sufficient concentrations after an additional mobility selection process.

Measuring soot particles from automotive exhaust emissions

The European Metrology Research Programme participating countries and the European Union jointly fund a three year project to address the need of the automotive industry for a metrological sound base for exhaust measurements. The collaborative work on particle emissions involves five European National Metrology Institutes, the Tampere University of Technology, the Joint Research Centre for Energy and Transport and the Leibniz Institute for Tropospheric Research. On one hand, a particle number and size standard for soot particles is aimed for. Eventually this will allow the partners to provide accurate and comparable calibrations of measurement instruments for the type approval of Euro 5b and Euro 6 vehicles. Calibration aerosols of combustion particles, silver and graphite proof partially suitable. Yet, a consensus choice together with instrument manufactures is pending as the aerosol choice considerably affects the number concentration measurement. Furthermore, the consortium issued consistent requirements for novel measuring instruments foreseen to replace today’s opacimeters in regulatory periodic emission controls of soot and compared them with European legislative requirements. Four partners are conducting a metrological validation of prototype measurement instruments. The novel instruments base on light scattering, electrical, ionisation chamber and diffusion charging sensors and will be tested at low and high particle concentrations. Results shall allow manufacturers to further improve their instruments to comply with legal requirements.

Determining the unique refractive index properties of solid polystyrene aerosol using broadband Mie scattering from optically trapped beads

A method is described to measure the refractive index dispersion with wavelength of optically trapped solid particles in air. Knowledge of the refraction properties of solid particles is critical for the study of aerosol; both in the laboratory and in the atmosphere for climate studies. Single micron-sized polystyrene beads were optically trapped in air using a vertically aligned counter-propagating configuration of focussed laser beams. Each bead was illuminated using white light from a broadband light emitting diode (LED) and elastic scattering within the bead was collected onto a spectrograph. The resulting Mie spectra were analysed to accurately determine polystyrene bead radii to ±0.4 nm and values of the refractive index to ±0.0005 over a wavelength range of 480-700 nm. We demonstrate that optical trapping combined with elastic scattering can be used to both accurately size polystyrene beads suspended in air and determine their wavelength dependent refractive index. The refractive index dispersions are in close agreement with reported values for polystyrene beads in aqueous dispersion. Our results also demonstrate a variation in the refractive index of polystyrene, from bead to bead, in a commercial sample. The measured variation highlights that care must be taken when using polystyrene beads as a calibration aerosol.

Experimental and Theoretical Investigations of the Effect of the Calibration Aerosol Material on the Counting Efficiencies of TSI 3790 Condensation Particle Counters

The counting efficiencies of two TSI 3790 Condensation Particle Counters (CPCs) were investigated experimentally for graphite, poly-(alpha)-olefin (PAO), tetradecane (C14), and hexadecane (C16) particles at saturator-to-condenser temperature differences spanning from 5.6°C to 11.3°C. The efficiencies determined with PAO, C14, and C16 particles were broadly similar, while tests with graphite particles resulted in systematically lower counting efficiencies. The differences between PAO and graphite particles were reduced at elevated temperature differences, i.e., as the saturation ratios inside the condenser increased. The possibility to predict measured counting efficiencies by heterogeneous nucleation theory was also assessed. The results for PAO, C14, and C16 were representative of perfectly wettable particles, while graphite data could only be reproduced with a contact angle of 6–12°, for all temperatures examined. Line tension fits revealed a linear correlation with the graphite mobility diameter for all operating temperatures (5 × 10−11 N at 15 nm to 4 × 10−10 N at 70 nm). This could actually indicate that the mobility diameter underestimates the contact line for these complex geometry graphite aggregates. An examination of the calculated activation regions inside the condenser indicated that the upper part of the counting efficiency curve (>50%) is very sensitive to flow and temperature nonidealities. This observation is in quantitative agreement with systematic deviations observed between theoretical predictions and experimental data. Numerical calculations for a range of working fluids suggested that for a given affinity of the calibration particle to the examined vapors (i.e., for a finite contact angle), the benefit from shifting to a fluid alternative to butanol is limited. Further investigations on the reduction of the material dependence should focus on the identification of working fluids exhibiting greater affinity for different particle materials (e.g., lower contact angle). Copyright 2012 American Association for Aerosol Research

Mixing state and compositional effects on CCN activity and droplet growth kinetics of size-resolved CCN in an urban environment

Abstract. Aerosol composition and mixing state near anthropogenic sources can be highly variable and can challenge predictions of cloud condensation nuclei (CCN). The impacts of chemical composition on CCN activation kinetics is also an important, but largely unknown, aspect of cloud droplet formation. Towards this, we present in-situ size-resolved CCN measurements carried out during the 2008 summertime August Mini Intensive Gas and Aerosol Study (AMIGAS) campaign in Atlanta, GA. Aerosol chemical composition was measured by two particle-into-liquid samplers measuring water-soluble inorganic ions and total water-soluble organic carbon. Size-resolved CCN data were collected using the Scanning Mobility CCN Analysis (SMCA) method and were used to obtain characteristic aerosol hygroscopicity distributions, whose breadth reflects the aerosol compositional variability and mixing state. Knowledge of aerosol mixing state is important for accurate predictions of CCN concentrations and that the influence of an externally-mixed, CCN-active aerosol fraction varies with size from 31% for particle diameters less than 40 nm to 93% for accumulation mode aerosol during the day. Assuming size-dependent aerosol mixing state and size-invariant chemical composition decreases the average CCN concentration overprediction (for all but one mixing state and chemical composition scenario considered) from over 190–240% to less than 20%. CCN activity is parameterized using a single hygroscopicity parameter, κ, which averages to 0.16 ± 0.07 for 80 nm particles and exhibits considerable variability (from 0.03 to 0.48) throughout the study period. Particles in the 60–100 nm range exhibited similar hygroscopicity, with a κ range for 60 nm between 0.06–0.076 (mean of 0.18 ± 0.09). Smaller particles (40 nm) had on average greater κ, with a range of 0.20–0.92 (mean of 0.3 ± 0.12). Analysis of the droplet activation kinetics of the aerosol sampled suggests that most of the CCN activate as rapidly as calibration aerosol, suggesting that aerosol composition exhibits a minor (if any) impact on CCN activation kinetics.

A coupled observation – modeling approach for studying activation kinetics from measurements of CCN activity

Abstract. This paper presents an approach to study droplet activation kinetics from measurements of CCN activity by the Continuous Flow Streamwise Thermal Gradient CCN Chamber (CFSTGC) and a comprehensive model of the instrument and droplet growth. The model, which can be downloaded from http://nenes.eas.gatech.edu/Experiments/CFSTGC.html , is evaluated against a series of experiments with ammonium sulfate calibration aerosol. Observed and modeled droplet sizes are in excellent agreement for a water vapor uptake coefficient ~0.2, which is consistent with theoretical expectations. The model calculations can be considerably accelerated without significant loss of accuracy by assuming simplified instrument geometry and constant parabolic flow velocity profiles. With these assumptions, the model can be applied to large experimental data sets to infer kinetic growth parameters while fully accounting for water vapor depletion effects and changes in instrument operation parameters such as the column temperature, flow rates, sheath and sample flow relative humidities, and pressure. When the effects of instrument operation parameters, water vapor depletion and equilibrium dry particle properties on droplet size are accounted for, the remaining variations in droplet size are most likely due to non-equilibrium processes such as those caused by organic surface films, slow solute dissociation and glassy or highly viscous particle states. As an example of model application, data collected during a research flight in the ARCTAS 2008 campaign are analyzed. The model shows that water vapor depletion effects can explain changes in the observed average droplet size.

Aerosol hygroscopicity and CCN activation kinetics in a boreal forest environment during the 2007 EUCAARI campaign

Abstract. Measurements of size-resolved cloud condensation nuclei (CCN) concentrations, subsaturated hygroscopic growth, size distribution, and chemical composition were collected from March through May, 2007, in the remote Boreal forests of Hyytiälä, Finland, as part of the European Integrated project on Aerosol Cloud Climate and Air Quality Interactions (EUCAARI) campaign. Hygroscopicity parameter, κ, distributions were derived independently from Continuous Flow-Streamwise Thermal Gradient CCN Chamber (CFSTGC) and Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA) measurements. CFSTGC-derived κ values for 40, 60, and 80 nm particles range mostly between 0.10 and 0.40 with an average characteristic of highly oxidized organics of 0.20 ± 0.10, indicating that organics play a dominant role for this environment. HTDMA-derived κ were generally 30% lower. Diurnal trends of κ show a minimum at sunrise and a maximum in the late afternoon; this trend covaries with inorganic mass fraction and the m/z 44 organic mass fraction given by a quadrupole aerosol mass spectrometer, further illustrating the importance of organics in aerosol hygroscopicity. The chemical dispersion inferred from the observed κ distributions indicates that while 60 and 80 nm dispersion increases around midday, 40 nm dispersion remains constant. Additionally, 80 nm particles show a markedly higher level of chemical dispersion than both 40 and 60 nm particles. An analysis of droplet activation kinetics for the sizes considered indicates that most of the CCN activate as rapidly as (NH4)2SO4 calibration aerosol.

Cloud condensation nuclei activity and droplet activation kinetics of wet processed regional dust samples and minerals

Abstract. This study reports laboratory measurements of particle size distributions, cloud condensation nuclei (CCN) activity, and droplet activation kinetics of wet generated aerosols from clays, calcite, quartz, and desert soil samples from Northern Africa, East Asia/China, and Northern America. The dependence of critical supersaturation, sc, on particle dry diameter, Ddry, is used to characterize particle-water interactions and assess the ability of Frenkel-Halsey-Hill adsorption activation theory (FHH-AT) and Köhler theory (KT) to describe the CCN activity of the considered samples. Wet generated regional dust samples produce unimodal size distributions with particle sizes as small as 40 nm, CCN activation consistent with KT, and exhibit hygroscopicity similar to inorganic salts. Wet generated clays and minerals produce a bimodal size distribution; the CCN activity of the smaller mode is consistent with KT, while the larger mode is less hydrophilic, follows activation by FHH-AT, and displays almost identical CCN activity to dry generated dust. Ion Chromatography (IC) analysis performed on regional dust samples indicates a soluble fraction that cannot explain the CCN activity of dry or wet generated dust. A mass balance and hygroscopicity closure suggests that the small amount of ions (from low solubility compounds like calcite) present in the dry dust dissolve in the aqueous suspension during the wet generation process and give rise to the observed small hygroscopic mode. Overall these results identify an artifact that may question the atmospheric relevance of dust CCN activity studies using the wet generation method. Based on the method of threshold droplet growth analysis, wet generated mineral aerosols display similar activation kinetics compared to ammonium sulfate calibration aerosol. Finally, a unified CCN activity framework that accounts for concurrent effects of solute and adsorption is developed to describe the CCN activity of aged or hygroscopic dusts.

Water Vapor Depletion in the DMT Continuous-Flow CCN Chamber: Effects on Supersaturation and Droplet Growth

The continuous-flow streamwise thermal-gradient cloud condensation nuclei counter (CFSTGC) is a commercially available instrument that is widely used for laboratory and field measurements of cloud condensation nuclei (CCN). All studies to date assume that the supersaturation profile generated in its growth chamber is not influenced by the condensation of water vapor upon the growing CCN. The validity of this assumption, however, has never been systematically explored. This work examines when water vapor depletion from CCN can have an important impact on supersaturation, measured CCN concentration, and droplet growth. A fully coupled numerical flow model of the instrument is used to simulate the water vapor supersaturation, temperature, velocity profiles, and CCN growth in the CFSTGC for a wide range of operation and CCN concentrations. Laboratory CCN activation experiments of polydisperse calibration aerosol (with a DMT CFSTGC operated in constant flow mode) are used to evaluate the simulations. The simul...