Influence of High Relative Humidity on Secondary Organic Carbon: Observations at a Background Site in East China

To investigate the impacts of relative humidity (RH) on secondary organic aerosol (SOA) concentrations and chemical reactions, the carbonaceous aerosol components [i.e., organic carbon (OC) and element carbon (EC)] were quantified in daily PM2.5 samples collected at a background site in East China during summer 2015. Based on the method of EC-tracer, the concentration of secondary organic carbon (SOC) demonstrated an obvious negative relationship with RH higher than 60%. Moreover, the ratio of SOC/EC also exhibited obvious decreasing trends with increasing RH, indicating negative effects for chemical production of SOA under high RH conditions. Due to high RH, photochemistry was weakened, gaseous oxidant concentrations was lowered (e.g., significantly decreased O-3 levels), and the production rates of SOA were relatively low. On the other hand, because of more water uptake under higher RH conditions, the aerosol droplet acidity was reduced and enhancement of SOA formation by acidity was accordingly absent. In addition, high RH also plays an important role in changing viscosity of pre-existing aerosol coatings, which can affect reactive uptake yield of SOA. Overall, the results from this study imply that SOA production may be more associated with photochemical processes, while aqueous-phase chemistry is not very important for some SOA formation in a moist ambient environment. In the ambient atmosphere, oxidant concentrations, reaction rates, airborne species, etc., are highly variable. How do these factors affect SOA yields under given ambient environment warrants further detailed investigations.