Formation of highly hygroscopic soot aerosols upon internal mixing with sulfuric acid vapor

The hygroscopic properties of submicron soot particles during internal mixing with gaseous sulfuric acid have been investigated using a combined tandem differential mobility analyzer (TDMA) and differential mobility analyzer–aerosol particle mass analyzer (DMA‐APM) technique. Fresh particles exhibit no change in mobility size and mass at subsaturated conditions, whereas particles exposed to gaseous sulfuric acid (10 9 –10 10 molecule cm −3 , 12 s contact time) experience significant mobility size and mass changes with increasing relative humidity (RH). The DMA‐APM measurements reveal that particles of all sizes exposed to H 2 SO 4 vapor gain mass with increasing RH because of absorption of water by sulfuric acid coating. However, on the basis of mobility size measurements using TDMA, upon humidification H 2 SO 4 ‐coated soot agglomerates display distinct hygroscopic growth patterns depending on their initial size and the mass fraction of condensed sulfuric acid. While small particles experience an increase in their mobility sizes, larger particles exhibit a marked shrinkage due to compaction. We suggest that determination of the hygroscopic properties of soot particles using a TDMA alone can be inconclusive. Restructuring of the soot agglomerates and filling of the voids that accompany the condensation of water‐soluble materials and subsequent water absorption lead to little or no observable changes in particle mobility size at subsaturated RH even for particles that contain aqueous coatings. Extrapolation of our experimental results to the urban atmosphere indicates that initially hydrophobic soot particles acquire sufficient sulfate coating to become efficient CCN (cloud condensation nuclei) within a time period ranging from a few hours to a few days, dependent on the ambient H 2 SO 4 level. The results imply that internal mixing with sulfuric acid through H 2 SO 4 vapor condensation likely represents a common aging process for a variety of atmospheric aerosols. The variations in the size and hygroscopicity of soot particles during atmospheric processing influence their optical properties, cloud‐forming potential, and human health effects.