Photochemical and ozone-induced aging significantly alter the viscosity of aqueous <i>trans</i> -aconitic acid aerosol particles

Abstract. Aging processes of organic aerosols, including reactions with gas phase oxidants, such as ozone (O3), as well as photochemical reactions, can significantly alter their physicochemical properties. While previous research has examined how photochemical aging and ozonolysis affect the physicochemical properties of organic aerosols, our study investigates the combined effect of photolysis and ozonolysis. We use aqueous trans-aconitic acid as a proxy for secondary organic aerosol particles (SOA), selected for its ability to absorb UV light and for containing a C=C double bond that is susceptible to ozonolysis. We observe significant mass loss in single particles levitated in an electrodynamic balance when exposed to either O3 or UV light (375 nm), as well as to both aging processes simultaneously, resulting from fragmentation reactions followed by the volatilization of some of the products. Viscosity measurements at 17 % relative humidity revealed an increase of nearly 4 orders of magnitude after both UV exposure and combined UV and O3 exposure at 60 % mass loss. Interestingly, continued UV-aging beyond 60 % mass loss resulted in a viscosity decrease, whereas combined UV and O3 exposure led to a further viscosity increase. Hygroscopicity exhibited only a modest decline after 20 % mass loss during UV-aging and remained constant with further UV exposure; this reduction was less pronounced when UV-aging occurred in the presence of O3. Overall, our results indicate that the mixing times within accumulation mode SOA particles may increase from 4 s to 4 h after aging under dry boundary layer conditions.