Pyruvic acid photolysis: Characterization of the secondary organic aerosol formed

Secondary organic aerosol (SOA) was produced from laboratory aqueous photolysis of pyruvic acid, and aerosol yield and cloud condensation nuclei (CCN) activity was examined. Pyruvic acid is ubiquitous in the atmosphere and currently used as a proxy in for similar molecules in climate models. 0.1M pyruvic acid (pH ~2) was photolyzed in a glass photoreactor for 6 hours. Aqueous samples were atomized to obtain aerosol yield (volume of aerosol produced per volume of initial pyruvic acid) of aqueous samples and the hygroscopicity parameter κ. In a room temperature (21°C) condition, unreacted pyruvic acid was atomized to yield ~ 4% aerosol. κ was found to be 0.22. Significant formation of aerosol from unreacted volatile pyruvic acid implicates the potential involvement of self-oligomerization via hemiacetal formation. After photolysis, aqueous sample yielded an increase in aerosol yield to ~ 15%, and a decrease in κ to 0.13. This decrease in κ after photochemical aging is in contrast to conventional hypotheses regarding in-cloud SOA formation, which generally expect an increase in hygroscopicity. The potential roles of variable environmental conditions in SOA formation were also investigated. Pyruvic acid photolysis was carried out in two secondary conditions: lower temperature (15°C at pH 2) and higher pH (pH 7 at 21°C after addition of ammonium hydroxide). Values for κ and aerosol yield in the 15°C conditions were similar to those obtained from room temperature photolysis after accounting for the temperature dependence of UV light intensity. At a pH of 7, photolysis resulted in a lesser increase in aerosol yield from ~ 6% to only 10%, while κ values were similar to those in the pH 2 condition. These results support the importance of further investigation of SOA formation and composition and of the effects of fluctuating environmental conditions—these effects are not currently well understood, but are likely important on a global scale.