Nevertheless, even at a time of 3000 s the total ion signal and the acetone signals were still above background levels recorded prior to introducing the catechol SOA sample into the heated cell. Acetone has been observed previously from the photodegradation of various types of SOA using near-UV radiation (300 nm). Acetone has also been observed as a minor product from the pyrolysis of carboxylic acids, alcohols, and peroxides, although typical at higher temperatures than used in our experiments. Below, we show that CO2 is produced in the heating experiments, in addition to acetone. Both acetone and CO2 can form from the thermal decomposition of acetoacetic acid (Fig. ). Acetoacetic acid is a beta-keto acid, i.e., an acid where the ketone group is located at the second carbon from the carboxylic acid functional group. This keto group at the beta position allows them to readily undergo thermal decomposition, such as decarboxylation. The thermal decomposition of this beta-keto acid occurs at 37 °C with a half-life of 140 min, which is roughly consistent with our experimental conditions. Acetoacetic acid was not observed in the condensed phase in our experiments using UHPLC-HESI-HRMS, but this species may have decomposed in the heated electrospray source. Additional studies are needed to determine the mechanism of acetone production from catechol SOA.