Evidence for the Importance of Semivolatile Organic Ammonium Salts in Ambient Particulate Matter

The gas/particle phase partitioning behavior of NH₃/NH₄⁺ and other semivolatile constituents was measured by a custom-designed Denuder-MOUDI-Denuder integrated sampling system in Toronto, Canada. In this setup, upstream denuders were used to capture alkaline and acidic gaseous components, and particle phase components were captured by the filters on MOUDI stages. Downstream denuders captured any alkaline and acidic gases that exited the MOUDI apparatus, likely representing semivolatile constituents. In the ambient gas phase HCOOH was the most abundant acidic gas, with an average mixing ratio ∼2-3 times higher than that of SO₂ and HNO₃. It was found that the majority (49-96%) of filter-collected NH₄⁺ volatilized during collection. NO₃^- volatilization could only explain 0.9-15% of NH₄⁺ loss from the filters. Instead, a strong correlation and nearly 1:1 molar ratio between downstream HCOO^- and NH₄⁺ indicated that most of the semivolatile NH₄⁺ was originally balanced by organic acids in the ambient particle phase. The thermodynamic properties of HCOOH/HCOO^- suggest that it should not have been present at high levels in the ambient particle phase, and we interpret its detection in the downstream denuder as evidence for larger organic acids that reacted to generate HCOOH prior to our offline measurement.