Direct atmospheric evidence for the irreversible formation of aqueous secondary organic aerosol

The reversible nature of aqueous secondary organic aerosol (aqSOA) formation was characterized for the first time through direct atmospheric measurements. Water-soluble organic carbon in the gas and particle phases (WSOCg and WSOCp) was measured simultaneously to quantify aqSOA formation in Baltimore, Maryland. During the nighttime, aqSOA formation was evident as WSOCg increasingly partitioned to the particle phase with increasing relative humidity (RH). To characterize the reversible/irreversible nature of this aqSOA, the WSOCp measurement was alternated through an unperturbed ambient channel and through a dried channel maintained at similar to 40% RH (with 7s residence time) to mimic the natural drying particles undergo throughout the day. Across the entire RH range encountered, there was no statistically significant difference in WSOCp concentrations through the dry and ambient channels, indicating that the aqSOA remained in the condensed phase upon the evaporation of aerosol water. This strongly suggests that the observed aqSOA was formed irreversibly.