Abstract. Asian dust is an important source of atmospheric ice nucleating particles (INPs). However, the freezing activity of airborne Asian dust, especially its sensitivity to particle size, is poorly understood. In this study we report the first INP measurement of size-resolved airborne mineral dust collected during East Asian dust events. The measured total INP concentrations in the immersion mode ranged from 10−2 to 102 L−1 in dust events at temperatures between −25 and −5 °C. The average contributions of heat-sensitive INPs at three temperatures, −10, −15, and −20 °C, were 81 ± 12 %, 70 ± 15 %, and 38 ± 21 %, respectively, suggesting that proteinaceous biological materials have a substantial effect on the ice nucleation properties of Asian atmospheric mineral dust at warm temperatures. The dust particles which originated from China's northwest deserts are more efficient INPs compared to those from northern regions. There was no significant difference in the ice nucleation properties between East Asian dust particles and other regions in the world. An explicit size dependence of both INP concentration and surface ice active density was observed. The nucleation efficiency of dust particles increased with increasing particle size, while the INP concentration first increased rapidly and then levelled, due to the significant decrease in the number concentration of larger particles. A new set of parameterizations for INP activity based on size-resolved nucleation properties of Asian mineral dust particles were developed over an extended temperature range (−35 ~ −6 °C). These size-dependent parameterizations require only particle size distributions as input, and can be easily applied in models.

No abstract

<p>Mineral dust is an important type of ice nucleating particles in the troposphere; however, the effects of heterogeneous reactions on ice nucleation (IN) activities of mineral dust remain to be elucidated. A droplet-freezing apparatus (Guangzhou Institute of Geochemistry Ice Nucleation Apparatus, GIGINA) was developed in this work to measure IN activities of atmospheric particles in the immersion freezing mode, and its performance was validated by a series of experimental characterizations. This apparatus was then employed to measure IN activities of feldspar and Arizona Test Dust (ATD) particles before and after heterogeneous reaction with NO<sub>2</sub> (10±0.5 ppmv) at 40% relative humidity. The fractional surface coverage of nitrate, <em>θ</em>(NO<sub>3</sub><sup>-</sup>), increased to 3.1±0.2 for feldspar after reaction with NO<sub>2</sub> for 6 h, and meanwhile the active site density per unit surface area (<em>n</em><sub>s</sub>) at -20 <sup>o</sup>C was reduced from 92±5 to <1.0<sup></sup>cm<sup>-2</sup> by about two orders of magnitude; however, no changes in nitrate content or IN activities were observed for further increase in reaction time (up to 24 h). Both nitrate content and IN activities changed continuously with reaction time (up to 24 h) for ATD particles; after reaction with NO<sub>2</sub> for 24 h, <em>θ</em>(NO<sub>3</sub><sup>-</sup>) increased to 1.4±0.1 and <em>n</em><sub>s</sub> at -20 <sup>o</sup>C was reduced from 20±4 to 9.7±1.9 cm<sup>-2</sup> by a factor of ~2. Our work suggests that heterogeneous reaction with NO<sub>2</sub> may significantly reduce IN activities of mineral dust in the immersion freezing mode.</p>

No abstract