Fresh Cooking Organic Aerosol Particles Exist as Liquid in Most Urban Temperature Range

Cooking organic aerosol (COA) is an important contributor to particulate matter. COA reacts with atmospheric oxidants such as ozone, as it contains unsaturated fatty acids such as oleic acid. Laboratory studies during the last decades suggested that the atmospheric chemical lifetime of oleic acid ranges from minutes, whereas atmospheric observations demonstrated that the corresponding value is several hours to half a day. To account for the disagreement, the phase state of COA particles has been hypothesized as (semi-)solid based on experimental studies for synthetic mixtures at room temperature. However, most of these studies employed substrate, which cannot fully represent the phase state of suspending aerosol particles. The phase state of suspending COA particles has rarely been measured, especially for the whole range of urban-environment relevant temperatures. By measuring the bounce behavior of suspending particles as a function of temperature, we examined the phase state of major fatty acids in COA (oleic, linoleic, pimelic, and stearic acids), in addition to laboratory-generated COA produced by heating canola oil, lard, and hotpot soup base. All types of fresh COA particles were liquid for the temperature range above −15°C. Comparison with recent laboratory kinetic study suggested that the phase transition may partially, but not fully, account for the reduced chemical reactivity at lower temperatures. As the temperature of most cities in the world is higher than −15°C, we suggest that fresh COA particles are mostly liquid on a global scale, though observational verification would still be needed.