Abstract
AbstractHeterogeneous 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.
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