Abstract
Abstract. The ability of aerosol particles to uptake water (hygroscopic growth) is an important determinant of aerosol optical properties and radiative effects. Aerosol hygroscopic growth is traditionally measured by humidified tandem differential mobility analyzers (HTDMA), in which size-selected dry particles are exposed to elevated relative humidity (RH), and the size distribution of humidified particles is subsequently measured using a scanning mobility particle sizer. As a scanning mobility particle sizer can measure only one particle size at a time, HTDMA measurements are time consuming, and ambient measurements are often limited to a single RH level. Pinterich et al. (2017b) showed that fast measurements of aerosol hygroscopic growth are possible using a humidity-controlled fast integrated mobility spectrometer (HFIMS). In HFIMS, the size distribution of humidified particles is rapidly captured by a water-based fast integrated mobility spectrometer (WFIMS), leading to a factor of ∼10 increase in measurement time resolution. In this study we present a prototype HFIMS that extends fast hygroscopic growth measurements to a wide range of atmospherically relevant RH values, allowing for more comprehensive characterizations of aerosol hygroscopic growth. A dual-channel humidifier consisting of two humidity conditioners in parallel is employed such that aerosol RH can be quickly stepped among different RH levels by sampling from alternating conditioners. The measurement sequence is also optimized to minimize the transition time between different particle sizes. The HFIMS is capable of measuring aerosol hygroscopic growth of six particle diameters under five RH levels ranging from 20 % to 85 % (30 separate measurements) every 25 min. The performance of this HFIMS is characterized and validated using laboratory-generated ammonium sulfate aerosol standards. Measurements of ambient aerosols are shown to demonstrate the capability of HFIMS to capture the rapid evolution of aerosol hygroscopic growth and its dependence on both size and RH.
Highlights
Hygroscopic growth is a key determinant of the liquid water content, optical properties, and radiative effects of atmospheric aerosols (Tang and Munkelwitz, 1994; Pilinis et al, 1995; Swietlicki et al, 2008)
Whereas in theory humidified tandem differential mobility analyzer (HTDMA) can measure aerosol hygroscopic growth at a variety of humidity values, field measurements are mostly limited to a single relative humidity (RH) level in practice because of (1) the time-consuming scanning mobility measurements and (2) the long transition time required for aerosol sample flow RH to stabilize after setpoint change (Santarpia et al, 2005)
We further examine the accuracy of humidity-controlled fast integrated mobility spectrometer (HFIMS) measurements by comparing hygroscopic growth of ammonium sulfate particles measured by the HFIMS with theoretical values
Summary
Hygroscopic growth is a key determinant of the liquid water content, optical properties, and radiative effects of atmospheric aerosols (Tang and Munkelwitz, 1994; Pilinis et al, 1995; Swietlicki et al, 2008). Aerosol hygroscopic growth is mostly measured using a humidified tandem differential mobility analyzer (HTDMA) (Rader and McMurry, 1986; Swietlicki et al, 2008; Duplissy et al, 2009; Massling et al, 2011; Lopez-Yglesias et al, 2014). In an HTDMA, dry and charged particles are first size-selected by a differential mobility analyzer (DMA), exposed to elevated relative humidity (RH) in a humidity conditioner. Whereas in theory HTDMA can measure aerosol hygroscopic growth at a variety of humidity values, field measurements are mostly limited to a single RH level in practice because of (1) the time-consuming scanning mobility measurements and (2) the long transition time required for aerosol sample flow RH to stabilize after setpoint change (Santarpia et al, 2005). A measurement cycle for five different particle sizes at a single RH requires a minimum of 30 min (e.g., Cerully et al, 2011)
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