Heterogeneous Reaction of CaCO 3 With NO 2 at Different Relative Humidities: Kinetics, Mechanisms, and Impacts on Aerosol Hygroscopicity

Heterogeneous reaction of NO 2 with CaCO 3 , an abundant and reactive component in mineral dust aerosol, was investigated in this work at different relative humidifies (RH, up to 80%), using a fixed‐bed reactor. Ion chromatograph and a vapor sorption analyzer were employed to measure changes in particulate nitrate and water with reaction time (up to 24 h). When NO 2 concentration was ∼10 ppmv (∼2.5 × 10 14 molecule cm −3 ), CaCO 3 showed very low reactivity toward NO 2 at <1% RH, and γ (NO 2 ) was estimated to be <2 × 10 −8 ; consequently, no significant change in hygroscopicity of CaCO 3 particles was observed after reaction with NO 2 for 24 h at <1% RH, as the amount of nitrate formed was very limited. Heterogeneous reactivity was significantly enhanced at elevated RH (20%–80%), and during the reaction CaCO 3 was covered with a deliquesced layer resulting from water uptake by formed nitrate; in addition, the average γ (NO 2 ) was determined to be (1.21 ± 0.45) × 10 −7 , independent of RH (20%–80%) and reaction time (3–24 h). After reaction with 10 ppmv NO 2 for 24 h at elevated RH (20%–80%), the mass of particulate water associated with reacted CaCO 3 at 90% RH was equal to ∼45% of the mass of unreacted CaCO 3 , suggesting that heterogeneous reaction of CaCO 3 with NO 2 at 20%–80% RH could substantially increase its hygroscopicity. Overall, our laboratory study suggested that heterogeneous reaction with NO 2 may significantly impact composition and hygroscopicity of CaCO 3 particles.