Ice-nucleating particles from multiple aerosol sources in the urban environment of Beijing under mixed-phase cloud conditions

Ice crystals occurring in mixed-phase clouds play a vital role in global precipitation and energy balance because of the unstable equilibrium between coexistent 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 emissions on atmospheric IN in the urban environment remains ambiguous. In this study, we present in situ observations of ambient ice-nucleating particle number concentration (N-INP) measured at mixed-phase cloud conditions (-30 degrees 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 N-INP is investigated. The results show that N(INP )during the dust event reaches up to 160 # L-1 (where "#" represents number of particles), with an activation fraction (AF) of 0.0036 % +/- 0.0011 %. During the rest of the observation, N(INP )is on the order of 10(-1) to 10#L-1, with an average AF between 0.0001 % and 0.0002 %. No obvious dependence of N-INP on the number concentration of particles larger than 500 nm (N-500) or black carbon (BC) mass concentration (mBc) is found throughout the field observation. The results indicate a substantial N-INP increase during the dust event, 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, does not influence the observed INP concentration. Our study corroborates previous laboratory and field findings that anthropogenic BC emission has a negligible effect on N(INP )and that N(INP )is unaffected by heavy pollution in the urban environment under mixed-phase cloud conditions.