Abstract
Abstract Measurements of aerosol size distribution, chemical composition, and cloud condensation nuclei (CCN) concentration were performed during the Chemical Emission, Loss, Transformation, and Interactions with Canopies (CELTIC) field program at Duke Forest in North Carolina. A kinetic model of the cloud activation of ambient aerosol in the chamber of the CCN instrument was used to perform an aerosol–CCN closure study. This study advances prior investigations by employing a novel fitting algorithm that was used to integrate scanning mobility particle sizer (SMPS) measurements of aerosol number size distribution and aerosol mass spectrometer (AMS) measurements of the mass size distribution for sulfate, nitrate, ammonium, and organics into a single, coherent description of the ambient aerosol in the size range critical to aerosol activation (around 100-nm diameter). Three lognormal aerosol size modes, each with a unique internally mixed composition, were used as input into the kinetic model. For the two smaller size modes, which control CCN number concentration, organic aerosol mass fractions for the defined cases were between 58% and 77%. This study is also unique in that the water vapor accommodation coefficient was estimated based on comparing the initial timing for CCN activation in the instrument chamber with the activation predicted by the kinetic model. The kinetic model overestimated measured CCN concentrations, especially under polluted conditions. Prior studies have attributed a positive model bias to an incomplete understanding of the aerosol composition, especially the role of organics in the activation process. This study shows that including measured organic mass fractions with an assumed organic aerosol speciation profile (pinic acid, fulvic acid, and levoglucosan) and an assumed organic aerosol solubility of 0.02 kg kg−1 still resulted in a significant model positive bias for polluted case study periods. The slope and y intercept for the CCN predicted versus CCN observed regression was found to be 1.9 and −180 cm−3, respectively. The overprediction generally does not exceed uncertainty limits but is indicative that a bias exists in the measurements or application of model. From this study, uncertainties in the particle number and mass size distributions as the cause for the model bias can be ruled out. The authors are also confident that the model is including the effects of growth kinetics on predicted activated number. However, one cannot rule out uncertainties associated with poorly characterized CCN measurement biases, uncertainties in assumed organic solubility, and uncertainties in aerosol mixing state. Sensitivity simulations suggest that assuming either an insoluble organic fraction or external aerosol mixing were both sufficient to reconcile the model bias.
Highlights
Predictions for climate change vary, in large part, because of differences in the way the various feedback processes involving water vapor, aerosols, and clouds are represented in models (Nenes and Seinfeld 2003)
Surface-active organic compounds can form a film on the cloud condensation nuclei (CCN) and reduce the ability for water vapor to condense onto the CCN (Nenes et al 2002; Feingold and Chuang 2002; Chuang 2003)
One final factor that could impact the CCN model– measurement bias is the degree of aerosol external mixing
Summary
Predictions for climate change vary, in large part, because of differences in the way the various feedback processes involving water vapor, aerosols, and clouds are represented in models (Nenes and Seinfeld 2003). Quantifying the impact of the indirect aerosol effect is a challenge because it requires that global models predict the aerosol size and composition as well as the interactions between gases, aerosols, water vapor, and clouds, most of which operate at the subgrid scale (Houghton et al 2001). Organic compounds can provide solute to the particle aqueous phase, affecting the water activity (Shulman et al 1996; Laaksonen et al 1998) and can directly affect particle surface tension (Facchini et al 2000). Both of these effects alter the thermodynamic equilibrium of CCN with water vapor (so-called Köhler curve). Surface-active organic compounds can form a film on the CCN and reduce the ability for water vapor to condense onto the CCN (Nenes et al 2002; Feingold and Chuang 2002; Chuang 2003)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
