Abstract
Abstract. The humidified tandem differential mobility analyzer (HTDMA) is widely used to measure the hygroscopic properties of submicron particles. The size-resolved aerosol hygroscopicity κ measured by a HTDMA will be influenced by the contribution of multiply charged aerosols, but this effect on field measurements has seldom been discussed for previous field measurements. Our calculations demonstrate that the number ratio of multiply charged particles is quite considerable for some specific sizes between 100 and 300 nm, especially during a pollution episode. The presence of multiple charges will lead to a compression effect on the aerosol hygroscopicity in HTDMA measurements. Therefore, we propose a new algorithm that performs multi-charge correction of the size-resolved hygroscopicity κ, taking both the compression effect and the multi-charge number contribution into consideration. Application of the algorithm to field measurements showed that the relatively high hygroscopicity in the accumulation size range leads to the overestimation of the hygroscopicity of particles smaller than 200 nm. The low hygroscopicity of coarse-mode particles leads to the underestimation of the hygroscopicity of accumulation particles between 200 and 500 nm in size. The difference between the corrected and measured κ values can be as large as 0.05, highlighting that special attention must be paid to the multi-charge effect when a HTDMA is used for aerosol hygroscopicity measurements.
Highlights
Atmospheric particles can scatter solar radiation and absorb longwave radiation, which directly affects the Earth’s radiation balance (Haywood and Boucher, 2000; Bond et al, 2013)
When the scan diameter in the differential mobility analyzers (DMAs) is set to Dp∗, the number fraction of less hygroscopic (LH) particles observed by the humidified tandem differential mobility analyzer (HTDMA) can be expressed as
HTDMAs have been extensively used in numerous field measurements to obtain the hygroscopic properties of submicron particles
Summary
Atmospheric particles can scatter solar radiation and absorb longwave radiation, which directly affects the Earth’s radiation balance (Haywood and Boucher, 2000; Bond et al, 2013). Shen et al.: Multi-charge effects dem differential mobility analyzer (HTDMA) is one of the most widely used (Swietlicki et al, 2008; Tang et al, 2019) As it can directly provide the particle size distribution after water uptake, a HTDMA can be employed to obtain both the mixing state and the bulk mean hygroscopic properties of ambient aerosol particles. Gysel et al (2009) obtained the kernel functions for multiply charged particles, and Duplissy et al (2008) applied them in the data inversion to retrieve the correct growth factors (GFs) The dry sizes he selected are dominated by doubly or triply charged particles. The effects of multi-charge correction using the algorithm on the particle hygroscopicity are discussed
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