Contributions of Organic Factors to Particle Hygroscopicity and Cloud Condensation Nuclei during the Eastern Pacific Cloud Aerosol Precipitation Experiment (EPCAPE)
VERONICA BERTA, Sanghee Han, Abigail Williams, Jeramy Dedrick, Christian Pelayo, Lynn M. Russell, Elavarasi Ravichandran, Markus Petters, Ryan Farley, Allison Aiken, Manvendra Dubey, Scripps/UCSD
Abstract Number: 479
Working Group: Coast to Coast Campaigns on Aerosols, Clouds, Chemistry, and Air Quality
Abstract
Uncertainties in cloud properties associated with cloud condensation nuclei (CCN) sources remain an important issue for climate modeling of coastal areas due to the mixture of polluted urban and relatively pristine marine air masses that result in diverse aerosol compositions. During the Eastern Pacific Cloud Aerosol Precipitation Experiment (EPCAPE) in La Jolla, California, ambient High Resolution Time-of-Flight Aerosol Mass Spectrometry (HR-ToF-AMS) measurements at Mt. Soledad provided insight into the chemical composition of non-refractory components. Organic mass constituted 55% of the total measured mass on average, illustrating the importance of organic hygroscopicity for CCN activity. Positive matrix factorization (PMF) analysis was used to apportion different sources of these organics, resolving five independent factors that were characterized by (i) several hydrocarbon peaks, (ii) organic sulfate peaks, (iii) organic nitrogen peaks, (iv) high m/z 44 contribution, and (v) a lower m/z 44 contribution. Their ratios of atomic oxygen to atomic carbon (O/C), a metric of their degree of oxidation, ranged from 0.32-1.46. Hygroscopicity was estimated for each of the five factors using different methods; volume-weighted mass concentrations for each factor were then averaged to estimate organic hygroscopicity (κorg). Compared to using a constant κorg of 0.1, varying κorg based on O/C to determine hygroscopicity derived from AMS composition (κAMS) resulted in stronger correlations between κAMS and hygroscopicity measured by size-resolved CCN measurements (κCCN) at all supersaturations (0.2-1.0%). κAMS estimated from O/C derived from all factors resulted in the highest correlation of R = 0.69 at 0.2% supersaturation with κCCN. These correlations were weaker at higher supersaturations but were improved by estimating κorg from O/C of PMF factors associated with Aitken mode aerosol mass concentrations. Among the five resolved PMF factors, only the factor containing organic sulfate peaks correlated with elevated κorg (R = 0.27), κAMS (R = 0.41), and with κCCN at 0.2% supersaturation (R = 0.47). This factor’s mass concentrations were highest during late spring and early summer, peaking in May when it comprised nearly a third of organic mass. These results provide characterization of organic aerosols and their impact on aerosol-cloud interactions in coastal environments.