Rapid Size-Resolved Aerosol Hygroscopic Growth Measurements: Differential Aerosol Sizing and
3.2 Introduction
Uptake of water by atmospheric aerosols is important in a variety of phenomena:
light scattering, ability to act as cloud condensation nuclei, and deposition within the human respiratory system. Aerosol hygroscopicity measurements have been made with a variety of systems (Table 3.1). The parameter normally quantified is the diameter growth factor (GF = Dp,wet/Dp,dry). Laboratory growth factor measurements are commonly performed by levitating a particle in an electrodynamic balance (Cohen et al., 1987a, 1987b; Tang and Munkelwitz, 1993, 1994a, 1994b; Chan et al., 1997; Peng et al., 2001;
Peng and Chan, 2001). This method probes single, laboratory-generated particles, directly measuring the mass change in response to humidity variation. The need to directly measure the growth factor of ambient aerosols led to the development of the hygroscopicity tandem differential mobility analyzer (HTDMA) (Liu et al., 1978;
Sekigawa, 1983; Rader and McMurry, 1986), in which dry particles of a selected size are exposed to a specific relative humidity (RH), after which the wet size is measured by classifying the grown particles with a second DMA, and counting with a condensation particle counter (CPC). Owing to the long duration of time required to measure the size distribution of the grown particles using a DMA, the HTDMA technique is relatively slow. However, both techniques above enable high-precision measurements of the growth factor.
While these methods have produced important insights into the hygroscopic behavior of atmospheric aerosols, the long time required to determine growth factors for different particle sizes and RHs makes them impractical in a number of important measurement scenarios in which the properties of sampled particles vary rapidly,
especially during airborne measurements. A number of alternate methods, which are described in detail in Table 3.1, have been employed to probe the hygroscopic properties of the atmospheric aerosol more rapidly than is possible with the HTDMA. Several of these methods probe the entire aerosol as measured at different humidities, using nephelometers to probe ensemble light scattering (Rood et al., 1985; Dougle et al., 1998;
Carrico et al., 1998, 2000; Sheridan et al., 2002; Magi and Hobbs, 2003; Kim et al., 2006), or measuring aerosol size distributions with optical particle sizing instruments in parallel (Kotchenruther and Hobbs, 1998; Hegg et al., 2006, 2007; Snider and Petters, 2007), or multiple DMAs operated in parallel (Wang et al., 2003). The use of an optical particle counter (OPC) in place of a DMA detector is not new, as several investigators (Covert et al. 1990; Hering and McMurry 1991; Brand et al. 1992) used OPC detection of mobility-classified particles to gain insight into the mixing state of atmospheric aerosol and to determine their optical properties. One approach that attempts to provide the resolution of the HTDMA without the response time limitations is to classify particles with a DMA operated at one RH, equilibrate the classified particles at a second RH, and then measure the resulting size distribution using a fast particle sizer such as an OPC.
Kreisberg et al. (2001) classified particles at a high RH, dried the classified particles, and measured the dry particle size using an OPC in an instrument they called the relative humidity-moderated differential mobility optical particle size spectrometer (RH- DMOPSS). The RH-DMOPSS, which is an extension of the DMOPSS design of Stolzenburg et al. (1998), measures shrinkage by drying rather than growth upon humidification like most other hygroscopic growth instruments. The RH-DMOPSS is an improvement upon the HTDMA in that it is faster and can measure a larger range of
particle sizes (0.1 – 1.1 μm). However, this instrument provides a growth factor at only one RH at any given time.
In this paper, we describe a new instrument that enables simultaneous determination of growth factors at several RHs, and that overcomes the challenges of determining the size of dry particles with unknown composition. The instrument has been designed especially for aircraft-based measurements. The differential aerosol sizing and hygroscopicity spectrometer probe (DASH-SP), developed by Brechtel Manufacturing Inc. (http://www.brechtel.com/), employs DMA classification of dry aerosol particles, equilibrates the classified particles to a new RH, and then measures the sizes of the grown particles using an OPC. An iterative data processing algorithm quantifies growth factors and “effective” refractive indices (n) for wet particles based on a calculated “effective” dry particle refractive index; iterations are performed on a three- dimensional surface (pulse height – n – Dp) based on dry particle calibration data from several salts with known refractive indices. By operating four controlled-RH channels in parallel and equipping each of the humidified channels with a separate OPC, hygroscopic growth factors can rapidly be determined at a number of particle sizes without having to incur the time delays required to stabilize a humidification column to a new RH.
Depending upon the concentration and size distribution of the aerosol sampled, the growth factors of particles at any selected size within the size resolution of the instrument can be determined in as little as a few seconds.
We present first an overview of the design of the DASH-SP instrument. Results from laboratory characterization tests illustrate the performance of the instrument, including size detection limits, time resolution, stability, accuracy, and inherent
uncertainties. The data processing algorithm is subsequently described, which is used to convert pulse height data to growth factors. Hygroscopicity measurements for various inorganic salts and organic acids are compared to theoretical predictions for growth factor dependence on RH. Finally, airborne field measurements establish that the DASH-SP is capable of measuring growth factors at multiple sizes and RHs with a time resolution as much as two orders of magnitude shorter than that of the HTDMA.