Accurate Determination of Aerosol Activity Coefficients at Relative Humidities up to 99% Using the Hygroscopicity Tandem Differential Mobility Analyzer Technique

Abstract

Aerosol water content plays an important role in aqueous phase reactions, in controlling visibility, and in cloud formation processes. One way to quantify aerosol water content is to measure hygroscopic growth using the hygroscopicity tandem differential mobility analyzer (HTDMA) technique. However, the HTDMA technique becomes less reliable at relative humidity (RH) >90% due to the difficulty of controlling temperature and RH inside the second DMA. For this study, we have designed and implemented a new HTDMA system with improved temperature and RH control. Temperature stability in the second DMA was achieved to ±0.02• C tolerance by implementing active control using thermoelectric heat exchangers and PID control loops. The DMA size resolution was increased by operating high-flow DMA columns at a sheath:sample flow ratio of 15:0.5. This improved size resolution allowed for improving the accuracy of the RH sensors by interspersing ammonium sulfate reference scans at high frequency. We present growth factor data for pure compounds at RH up to 99% and compare the data to theoretical values and to available bulk water activity data. With this HTDMA instrument and method, the osmotic coefficients of spherical, nonvolatile aerosols of known composition between 30 and 200 nm in diameter can be determined within ±20%. We expect that data from this instrument will lead to an improvement of aerosol water content models by contributing to the understanding of aerosol water uptake at high RH.

Accurate Measurements of Aerosol Hygroscopic Growth over a Wide Range in Relative Humidity

“…31 In addition, the largest uncertainties in diameter growth factor from this technique are of order ∼0.7% (see, for example, the points that have error bars larger than the point size in Figure 8); subsaturated growth measurements by conventional instruments have associated uncertainties of ±5% with large interinstrument variabilities. 16,26 Indeed, the hygroscopic growth curves for mixed component aerosol containing (NH 4 ) 2 SO 4 /NaCl mass ratios of 90/10 and 95/5 shown in Figure 8 are clearly resolved: the difference between growth factors for these two systems varies from 0.02 at 65% RH (at growth factors of ∼1.34, i.e., a 1.5% difference) up to 0.07 at 98% RH (at growth factors of ∼3.20, The Journal of Physical Chemistry A Article i.e., a 2% difference). Such a small effect would unlikely be resolved in conventional instruments.…”

“…23−25 To address the challenges in quantifying hygroscopicity for aerosols of complex composition, refinements in laboratory and field instrumentation, and improved frameworks for representing hygroscopicity, are required. 26,27 Hygroscopic growth measurements must be made up to water activities close to the dilute limit, ideally as high as 0.999. Such high water activities (a w ) are required for measurements to be directly relevant to CCN activation 28 and to place tighter constraints on the equilibrium solution compositions required to underpin the development of predictive models.…”

Coalescence-based assessment of aerosol phase state using dimers prepared through a dual-differential mobility analyzer technique

“…The SMPS was configured to scan across a range of voltages corresponding to a range of diameters that included the expected coagulated peak. Details on the configuration of the column and SMPS configuration are described elsewhere (Suda and Petters 2013;Nguyen et al 2014). All sheath flows were controlled using critical orifices on the outlet and dry, compressed, particle free zero air on the inlet.…”

Measuring Mass-Based Hygroscopicity of Atmospheric Particles through in Situ Imaging

“…2,3 Frequently, in field measurements, hygroscopic tandem differential mobility 58 analysis (HTDMA) is used to measure particle hygroscopicity. [4][5][6] With HTDMA, hygroscopic 59 growth is quantified from changes in particle size as a function of relative humidity. However, 60 quantifying hygroscopic growth using size is susceptible to errors due to particle asphericity and 61 particle porosity.…”