Cloud Activating Properties of Aerosol Observed during CELTIC

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 Ϫ1 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 Ϫ180 cm Ϫ3 , 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.

Characterization of aerosol associated with enhanced small particle number concentrations in a suburban forested environment

“…Inorganic aerosol constituents appear to have similar concentrations and variability as those presented for the current study, likely implying that anthropogenic influences between the two summer periods of different years were similar. Organic concentrations presented by Stroud et al [2007] (maxima approaching 20 mg m −3 ) are significantly larger than those presented in the current study. Although the measurements cannot be compared directly, it is possible that this increase in OM in the study of Stroud et al [2007] results from increased BVOC emission and chemistry as a result of stronger photochemistry during July relative to September.…”

“…Organic concentrations presented by Stroud et al [2007] (maxima approaching 20 mg m −3 ) are significantly larger than those presented in the current study. Although the measurements cannot be compared directly, it is possible that this increase in OM in the study of Stroud et al [2007] results from increased BVOC emission and chemistry as a result of stronger photochemistry during July relative to September. Unfortunately, an extensive spectral analysis is not presented by Stroud et al [2007] so that comparisons of OM components, contributions of specific m/z, etc., are not possible.…”

“…[45] One previous study also presents Q-AMS measurements of particulate matter in the Duke Forest [Stroud et al, 2007]. The previous study was conducted during July 2003.…”

“…Although the measurements cannot be compared directly, it is possible that this increase in OM in the study of Stroud et al [2007] results from increased BVOC emission and chemistry as a result of stronger photochemistry during July relative to September. Unfortunately, an extensive spectral analysis is not presented by Stroud et al [2007] so that comparisons of OM components, contributions of specific m/z, etc., are not possible. However, based on a closure study focused on prediction of cloud condensation nuclei concentrations, Stroud et al [2007] inferred a mixed anthropogenic/ biogenic influence on the atmosphere above the Duke FACE site, a conclusion in agreement with that of the present study.…”

“…Unfortunately, an extensive spectral analysis is not presented by Stroud et al [2007] so that comparisons of OM components, contributions of specific m/z, etc., are not possible. However, based on a closure study focused on prediction of cloud condensation nuclei concentrations, Stroud et al [2007] inferred a mixed anthropogenic/ biogenic influence on the atmosphere above the Duke FACE site, a conclusion in agreement with that of the present study.…”