Abstract
Abstract. Size-resolved observations of aerosol particles and cloud droplet residuals were studied at a marine boundary layer site (251 m a.m.s.l.) in La Jolla, San Diego, California, during 2012. A counterflow virtual impactor (CVI) was used as the inlet to sample cloud residuals while a total inlet was used to sample both cloud residuals and interstitial particles. Two cloud events totaling 10 h of in-cloud sampling were analyzed. Based on bulk aerosol particle concentrations, mass concentrations of refractory black carbon (rBC), and back trajectories, the two air masses sampled were classified as polluted marine air. Since the fraction of cloud droplets sampled by the CVI was less than 100%, the measured activated fractions of rBC should be considered as lower limits to the total fraction of rBC activated during the two cloud events. Size distributions of rBC and a coating analysis showed that sub-100 nm rBC cores with relatively thick coatings were incorporated into the cloud droplets (i.e., 95 nm rBC cores with median coating thicknesses of at least 65 nm were incorporated into the cloud droplets). Measurements also show that the coating volume fraction of rBC cores is relatively large for sub-100 nm rBC cores. For example, the median coating volume fraction of 95 nm rBC cores incorporated into cloud droplets was at least 0.9, a result that is consistent with κ-Köhler theory. Measurements of the total diameter of the rBC-containing particles (rBC core and coating) suggest that the total diameter of rBC-containing particles needed to be at least 165 nm to be incorporated into cloud droplets when the core rBC diameter is ≥ 85 nm. This result is consistent with previous work that has shown that particle diameter is important for activation of non-rBC particles. The activated fractions of rBC determined from the measurements ranged from 0.01 to 0.1 for core rBC diameters ranging from 70 to 220 nm. This type of data is useful for constraining models used for predicting rBC concentrations in the atmosphere.
Highlights
Black carbon (BC) is a subset of the aerosol population that is emitted as a result of incomplete combustion
Specific questions to be addressed include the following: (1) what is the activated fraction of refractory black carbon (rBC) as a function of particle size in the marine stratocumulus clouds studied? (2) Do small rBC cores get incorporated into the cloud droplets? (3) What is the thickness of the coating on the rBC cores that are incorporated into the cloud droplets? (4) Is the rBC coating volume fraction and the total diameter important for activation of rBC into cloud droplets? (5) Are the results consistent with κ-Köhler theory, which is used in advanced modeling studies to describe the activation of rBC particles into cloud droplets (e.g., Fierce et al, 2013; Riemer et al, 2010)?
Based on measured bulk aerosol concentrations the two air masses sampled were classified as polluted marine air, a classification consistent with the back trajectories and measured concentrations of black carbon
Summary
Black carbon (BC) is a subset of the aerosol population that is emitted as a result of incomplete combustion. There, a statistically significant difference in scavenging was observed; the scavenged fraction of sulfate at this site was 0.52 while the scavenged fraction of BC was 0.15 (Hallberg et al, 1994) In this latter case, observations showed that BC particles found as cloud droplet residuals were of mixed composition, often having a water-soluble component that varied as a function of size. The single particle soot photometer (SP2) is an instrument recently developed to determine the refractory black carbon (rBC) mass of individual particles (Moteki and Kondo, 2007; Schwarz et al, 2006; Stephens et al, 2003) With this instrument, size distributions of rBC can be obtained in real time. Specific questions to be addressed include the following: (1) what is the activated fraction of rBC as a function of particle size in the marine stratocumulus clouds studied? (2) Do small (sub-100 nm) rBC cores get incorporated into the cloud droplets? (3) What is the thickness of the coating on the rBC cores that are incorporated into the cloud droplets? (4) Is the rBC coating volume fraction and the total diameter (rBC core and coating thickness) important for activation of rBC into cloud droplets? (5) Are the results consistent with κ-Köhler theory, which is used in advanced modeling studies to describe the activation of rBC particles into cloud droplets (e.g., Fierce et al, 2013; Riemer et al, 2010)?
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