Importance of water-soluble organic acid on the hygroscopicity of nitrate

Nitrates are important components of atmospheric aerosols. Organic components are widely present in the atmosphere, while their influence on the hygroscopicity of nitrates has not been fully explored. The hygroscopic growth of nitrate aerosols internally mixed with water-soluble organic acids (succinic acid, glutaric acid and citric acid) was measured using a hygroscopic tandem differential mobility analyzer (HTDMA). The hygroscopic behaviors of ammonium nitrate/succinic acid mixed particles exhibited the effect of organic acid content on the phase state of ammonium nitrate, indicating that the minor succinic acid content (less than 50% by mass) does not efficiently initiate the crystallization of NH4NO3 in submicron mixed particles. The obvious particle shrinkage with increasing RH under low humidity conditions was observed for sodium nitrate or calcium nitrate particles mixed with citric acid, suggesting the influence of citric acid on the nitrate particle structure. Our results indicate that due to the complexation of carboxylic groups of polycarboxylic acid some water-soluble carboxylic acids have a potential role in transformation of the dry submicron calcium nitrate particle structure. Our findings highlight the importance of water-soluble organic acids in water uptake behaviors of nitrates, further improving the understanding of phase behavior and water uptake of nitrates in atmospheric aerosols.