Abstract CaCO
3 in mineral dusts can be converted to highly hygroscopic substances, which affect the formation of clouds, by reactions with gaseous oxidative species in the atmosphere. It is important to investigate CaCO
3 reactions in the presence of various kinds of gaseous oxidative species in order to improve the understanding of climate change. In this study, heterogeneous chemical reaction of CaCO
3 particles in the presence of HCl, NO
2 and O
3 was evaluated using the Raman microscope coupled with aerosol reaction chamber. The reaction of CaCO
3 particles with HCl and NO
2 resulted in the formation of internal mixtures of Ca(NO
3)
2 and CaCl
2, both of which were very hygroscopic and undergoes deliquescence at low relative humidity. Ca(NO
3)
2 did not change into CaCl
2 by the subsequent reactions with HCl, whereas the produced CaCl
2 reacted partially with NO
2 and O
3 to form Ca(NO
3)
2. Mixed particles including large amounts of Ca(NO
3)
2 were most frequently seen in microscope observation and existed in the state of supersaturated droplets under a low humidity condition. On the other hand, the particles containing large amount of CaCl
2 were observed as transparent particles with wrinkled surfaces, which included CaCl
2・2H
2O hydrate under low humidity condition.
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