Heterogeneous Reaction of CaCO₃ With NO₂ at Different Relative Humidities: Kinetics, Mechanisms, and Impacts on Aerosol Hygroscopicity

Abstract

Heterogeneous reaction of NO2 with CaCO3, 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 NO2 concentration was ∼10 ppmv (∼2.5 × 1014 molecule cm−3), CaCO3 showed very low reactivity toward NO2 at <1% RH, and γ(NO2) was estimated to be <2 × 10−8; consequently, no significant change in hygroscopicity of CaCO3 particles was observed after reaction with NO2 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 CaCO3 was covered with a deliquesced layer resulting from water uptake by formed nitrate; in addition, the average γ(NO2) 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 NO2 for 24 h at elevated RH (20%–80%), the mass of particulate water associated with reacted CaCO3 at 90% RH was equal to ∼45% of the mass of unreacted CaCO3, suggesting that heterogeneous reaction of CaCO3 with NO2 at 20%–80% RH could substantially increase its hygroscopicity. Overall, our laboratory study suggested that heterogeneous reaction with NO2 may significantly impact composition and hygroscopicity of CaCO3 particles.