Boiling of catechol secondary organic aerosol when heated to mild temperatures (36-52°C) due to carbon dioxide formation and high viscosity

Thermal desorption measurements, including thermal desorption mass spectrometry, are often used to determine the volatility and chemical composition of secondary organic aerosol (SOA). Accurately interpreting such measurements requires an understanding of the response of SOA to heat. Using optical microscopy, we monitored catechol + O₃ SOA during heating to mild temperatures (36–52 °C). Catechol + O₃ SOA is a type of SOA formed in wildfire plumes. Surprisingly, the SOA particles appeared to boil when heated to these temperatures. We identified acetone and CO₂ as the dominant species emitted from the SOA during heating, implying decomposition of the SOA components. Using mass spectrometry techniques, we observed catechol dimers to be the major product in unheated SOA and observed the degradation of these dimers after heating. Viscosity calculations suggested the mixing time of acetone and CO₂ within the particles was 11 and 1 h at temperatures of 36 and 52 °C, respectively. The observed boiling can be explained by the production and slow mixing of CO₂ within the SOA particles when subjected to mild temperatures. Our results underscore the importance of considering decomposition, high viscosities, and slow mixing times when interpreting thermal desorption measurements of SOA, even upon heating to mild temperatures.