Effects of heterogeneous reaction with NO2 on ice nucleation activities of feldspar and Arizona Test Dust

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 NO₂ (10±0.5 ppmv) at 40% relative humidity. The fractional surface coverage of nitrate, θ(NO₃^-), increased to 3.1±0.2 for feldspar after reaction with NO₂ for 6 h, and meanwhile the active site density per unit surface area (n_s) at -20 ^oC was reduced from 92±5 to <1.0cm^-2 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 NO₂ for 24 h, θ(NO₃^-) increased to 1.4±0.1 and n_s at -20 ^oC was reduced from 20±4 to 9.7±1.9 cm^-2 by a factor of ~2. Our work suggests that heterogeneous reaction with NO₂ may significantly reduce IN activities of mineral dust in the immersion freezing mode.