Cloud Processing of Secondary Organic Aerosol fromIsoprene and MethacroleinPhotooxidation

摘要

Aerosol-cloud interaction contributes to the largest uncertainties in the estimation and interpretation of the Earth’s changing energy budget. The present study explores experimentally the impacts of water condensation-evaporation events, mimicking processes occurring in atmospheric clouds, on the molecular composition of secondary organic aerosol (SOA) from the photooxidation of methacrolein. A range of on- and off-line mass spectrometry techniques were used to obtain a detailed chemical characterization of SOA formed in control experiments in dry conditions, in triphasic experiments simulating gas-particle-cloud droplet interactions (starting from dry conditions and from 60% relative humidity (RH)), and in bulk aqueous-phase experiments. We observed that cloud events trigger fast SOA formation accompanied by evaporative losses. These evaporative losses decreased SOA concentration in the simulation chamber by 25–32% upon RH increase, while aqueous SOA was found to be metastable and slowly evaporated after cloud dissipation. In the simulation chamber, SOA composition measured with a high-resolutiontime-of-flight aerosol mass spectrometer, did not change during cloudevents compared with high RH conditions (RH > 80%). In all experiments,off-line mass spectrometry techniques emphasize the critical roleof 2-methylglyceric acid as a major product of isoprene chemistry,as an important contributor to the total SOA mass (15–20%)and as a key building block of oligomers found in the particulatephase. Interestingly, the comparison between the series of oligomersobtained from experiments performed under different conditions showa markedly different reactivity. In particular, long reaction timesat high RH seem to create the conditions for aqueous-phase processingto occur in a more efficient manner than during two relatively shortcloud events.