Abstract
Ice crystals occurring in mixed-phase clouds play a vital role in global precipitation and energy balance because of the unstable equilibrium between co-existent liquid droplets and ice crystals, which affects cloud lifetime and radiative properties, as well as precipitation formation. Satellite observations proved that immersion freezing, i.e., ice formation on particles immersed within aqueous droplets, is the dominant ice nucleation (IN) pathway in mixed-phase clouds. However, the impact of anthropogenic emission on atmospheric IN in the urban environment remains ambiguous. In this study, we present in situ observations of ambient ice nucleating particle number concentration (NINP) measured at mixed-phase cloud conditions (−30 °C, relative humidity with respect to liquid water RHw = 104 %) and the physicochemical properties of ambient aerosol, including chemical composition and size distribution, at an urban site in Beijing during the traditional Chinese Spring Festival. The impact of multiple aerosol sources such as firework emissions, local traffic emissions, mineral dust and urban secondary aerosols on NINP is investigated. The results show that NINP during the dust event reaches up to 160 # L−1, with an activation fraction (AF) of 0.0036 % ± 0.0011 %. During the rest of the observation, NINP is on the order of 10−1 to 10 # L−1, with an average AF between 0.0001 to 0.0002 %. No obvious dependence of NINP on the number concentration of particles larger than 500 nm (N500) or black carbon (BC) mass concentration (mBC) is found throughout the field observation. The results indicate that mineral dust dominates NINP, although the observation took place at an urban site with high background aerosol concentration. Meanwhile, the presence of atmospheric BC from firework and traffic emissions, along with urban aerosols formed via secondary transformation during heavily polluted periods do not influence the observed INP concentration. Our study corroborates previous laboratory and field findings that anthropogenic BC emission has a negligible effect on NINP, and that NINP is unaffected by heavy pollution in the urban environment under mixed-phase cloud conditions.
Highlights
Mixed-phase clouds occur where super-cooled liquid water droplets co‐exist with ice crystals and are normally sustained 35 between -38 and 0 °C in the atmosphere, with ice melting rapidly at warmer temperature and droplets freezing homogeneously at colder temperature (Boucher et al, 2013; Korolev et al, 2017)
This study reports positive NINP only, because the negative values indicate that the signal of optical particle counter (OPC) during the measurement is undistinguishable from background noise
335 Continuous in situ observation of ice nucleating particles (INPs) number concentration (NINP) and physiochemical properties, including chemical composition and size distribution, of ambient particles at an urban site in Beijing during the traditional Chinese Spring Festival has been performed at mixed-phase cloud condition (-30 °C, RHw = 104%) for 18 days
Summary
Mixed-phase clouds occur where super-cooled liquid water droplets co‐exist with ice crystals and are normally sustained 35 between -38 and 0 °C in the atmosphere, with ice melting rapidly at warmer temperature and droplets freezing homogeneously at colder temperature (Boucher et al, 2013; Korolev et al, 2017). Satellite observations demonstrate that the predominant ice formation pathway in mixed-phase clouds is immersion freezing (e.g., Ansmann et al, 2008; de Boer et al, 2011; Silber et al, 2021). In this mode, ice nucleating particles (INPs) immersed within super-cooled aqueous droplets provide an interface that decreases the liquid-solid phase transition energy barrier and aids droplet freezing by so called heterogeneous ice nucleation (IN, Pruppacher and Klett, 2010; Vali et al, 2015; Kanji et al, 2017). Previous studies have confirmed that several kinds of atmospheric particles, including mineral dusts, carbonaceous particles, and biogenic species, can act as immersion INP and catalyze ice crystal formation below 0 °C (Murray et al, 2012; Kanji et al, 2017 and references therein)
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