Hygroscopicity and composition of California CCN during summer 2010

Abstract

We present an overview and analysis of cloud condensation nuclei (CCN) sampled in California by a NOAA WP‐3D aircraft during the 2010 CalNex project. Four distinct geographical regions are characterized, including the Los Angeles basin, the San Joaquin and Sacramento Valleys, and the eastern Pacific Ocean west of southern California. Median size distributions in the Central Valley were unimodal (Dg ∼ 25 nm) with a larger fraction of organic species and smaller fraction of nitrate species in the Sacramento Valley aerosol than in the San Joaquin Valley aerosol. Size distributions in the Los Angeles basin and marine outflow were bimodal (geometric mean diameter, Dg ∼ 30, 90–100 nm) with similar organic fractions and some replacement of nitrate with sulfate in the marine outflow. Both fine particle and CCN concentrations were found to decrease rapidly above the planetary boundary layer (∼2 km altitude), with CCN concentrations in the boundary layer ranging from ∼102–104 cm−3 STP, while fine particle concentrations (0.004–1 μm diameters) ranged from ∼103–105 cm−3STP. The CCN‐active number fraction varied between 0–100% in the Los Angeles Basin and Marine Outflow, but was substantially lower (0–40%) in the San Joaquin and Sacramento Valleys. Values of the hygroscopicity parameter,κ, inferred from the CCN measurements varied from 0.1–0.25, with the highest values in the marine outflow and the lowest values in the Sacramento Valley. The κvalues agreed well with the predictions based on size‐resolved aerosol composition, but were overpredicted by almost twofold when size‐averaged composition was used. CCN closure was assessed for simplified compositional and mixing state assumptions, and it was found that assuming the aerosol to be internally mixed overpredicted CCN concentrations by 30–75% for all air mass types except within the Sacramento Valley, where good closure (overprediction < 10%) was achieved by assuming insoluble organics. Assuming an externally‐mixed aerosol fraction or incorporating size‐resolved composition data improved closure in the other three regions, consistent with the bimodal nature of the aerosol size distribution.