Abstract
Abstract. Measurements of cloud condensation nuclei (CCN), aerosol size distributions, and submicron aerosol composition were made as part of the Intercontinental Chemical Transport Experiment Phase B (INTEX-B) campaign during spring 2006. Measurements were conducted from an aircraft platform over the northeastern Pacific and western North America with a focus on how the transport and evolution of Asian pollution across the Pacific Ocean affected CCN properties. A broad range of air masses were sampled and here we focus on three distinct air mass types defined geographically: the Pacific free troposphere (FT), the marine boundary layer (MBL), and the polluted continental boundary layer in the California Central Valley (CCV). These observations add to the few observations of CCN in the FT. CCN concentrations showed a large range of concentrations between air masses, however CCN activity was similar for the MBL and CCV (κ~0.2–0.25). FT air masses showed evidence of long-range transport from Asia and CCN activity was consistently higher than for the boundary layer air masses. Bulk chemical measurements predicted CCN activity reasonably well for the CCV and FT air masses. Decreasing trends in κ with organic mass fraction were observed for the combination of the FT and CCV air masses and can be explained by the measured soluble inorganic chemical components. Changes in hygroscopicity associated with differences in the non-refractory organic composition were too small to be distinguished from the simultaneous changes in inorganic ion composition in the FT and MBL, although measurements for the large organic fractions (0.6–0.8) found in the CCV showed values of the organic fraction hygroscopicity consistent with other polluted regions (κorg~0.1–0.2). A comparison of CCN-derived κ (for particles at the critical diameter) to H-TDMA-derived κ (for particles at 100 nm diameter) showed similar trends, however the CCN-derived κ values were significantly higher.
Highlights
Single variable parameterizations simplify estimates of cloud condensation nuclei (CCN) concentrations in climate models; yet their application to a range of aerosol types is still not well characterized
We discuss CCN results grouped by the three major categories of air masses discussed above: free troposphere (FT), marine boundary layer (MBL), and the polluted continental boundary layer
Air mass categories are similar to those discussed in Dunlea et al (2009), except that our criteria for FT is less strict than theirs, and included flight legs slightly inland (2300 m altitude) The analysis presented here does not differentiate between FT and Asian-influenced FT by the same definition described in Dunlea et al (2009), as the two air mass categories were separated by sulfate loading greater or less than 1 μg SO4 m−3, respectively
Summary
Single variable parameterizations simplify estimates of cloud condensation nuclei (CCN) concentrations in climate models; yet their application to a range of aerosol types is still not well characterized. Roberts et al.: Characterization of particle cloud droplet activity and composition the wet season in the Amazon Basin and show that, within the limits of expected solubility of organics, predicted and observed CCN were in agreement Their results suggested that CCN activity can be predicted with a simplified binary aerosol composition of a water soluble and insoluble species. A similar comparison using AMS measurements was reported for a recent study conducted in the Amazon Basin (Gunthe et al, 2009) Their results show that the best predictions of CCN concentrations are achieved with highly time-resolved calculations of κ (a single variable hygroscopicity parameter, κ; Petters and Kreidenweis, 2007) based on individual observations of size-resolved chemistry and aerosol size distributions. These results and other recent studies (Andreae, 2009; Jimenez et al, 2009; Sullivan et al, 2009) suggest that aerosols from various sources undergo atmospheric processes that tend to homogenize their chemical and physical properties with respect to CCN activity
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