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[1] Measurements of aerosol and cloud properties in the Eastern Pacific Ocean were taken during an airborne experiment on the University of Wyoming's King Air during April 2004 as part of the Cloud Indirect Forcing Experiment (CIFEX). We observed a wide variety of aerosols, including those of long-range transport from Asia, clean marine boundary layer, and North American emissions. These aerosols, classified by their size distribution and history, were found in stratified layers between 500 to 7500 m above sea level and thicknesses from 100 to 3000 m. A comparison of the aerosol size distributions to measurements of cloud condensation nuclei (CCN) provides insight to the CCN activity of the different aerosol types. The overall ratio of measured to predicted CCN concentration (N CCN ) is 0.56 ± 0.41 with a relationship of N CCN,measured = N CCN,predicted 0.846±0.002 for 23 research flights and 1884 comparisons. Such a relationship does not accurately describe a CCN closure; however, it is consistent with our measurements that high CCN concentrations are more influenced by anthropogenic sources, which are less CCN active. While other CCN closures have obtained results closer to the expected 1:1 relationship, the different aerosol types (and presumably differences in aerosol chemistry) are responsible for the discrepancy. The measured N CCN at 0.3% supersaturation (S c ) ranged from 20 cm À3 (pristine) to 350 cm À3 (anthropogenic) with an average of 106 ± 54 cm À3 over the experiment. The inferred supersaturation in the clouds sampled during this experiment is $0.3%. CCN concentrations of cloud-processed aerosol were well predicted using an ammonium sulfate approximation for S c 0.4%. Predicted N CCN for other aerosol types (i.e., Asian and North American aerosols) were high compared to measured values indicating a less CCN active aerosol. This study highlights the importance of chemical effects on CCN measurements and introduces a CCN activation index as a method of classifying the efficiency of an aerosol to serve as CCN relative to an ammonium sulfate particle. This index ranged from close to unity for cloud processed aerosols to as low as 0.31 for aged aerosols transported from Asia. We also compare the performance of two CCN instruments (static thermal diffusion chamber and streamwise continuous flow chamber) on a 45 minute level leg where we observe an aged layer and a nucleation event. More than 50% of the aged aerosol served as CCN at 0.2% S c , primarily owing to their large size, while CCN concentrations during the nucleation event were close to 0 cm À3 . CCN concentrations from both instruments agreed within instrument errors; however, the continuous flow chamber effectively captured the rapid transition in aerosol properties.

[1] This study presents estimates of long-range transport of black carbon (BC) and aerosol fine mass (diameter less than 2.5 mm) across the Pacific Ocean into North America during April 2004. These transport estimates are based on simulations by the Chemical Weather Forecast System (CFORS) model and evaluated across 130°W, (30°N-60°N) from 26 March through 25 April 2004. CFORS calculates BC transport into North America at 25-32 Gg of which over 75% originates from Asia. Modeled fine aerosol mass transport is between 900 and 1100 Gg. The BC transport amounts to about 77% of the published estimates of North American BC emissions. Approximately 78% of the BC and 82% of the fine aerosol mass transport occur in the midtroposphere above 2 km. Given the relatively large magnitude of the estimated BC transport, we undertake a detailed validation of the model simulations of fine aerosol mass and BC over the west coast of North America. In situ aircraft data were available for the month of April 2004 to assess the accuracy of model simulations of aerosols in the lower troposphere. Aircraft data for aerosol mass collected in the eastern Pacific Ocean during April 2004 as part of the Cloud Indirect Forcing Experiment, as well as surface measurements of fine mass and BC at 30 west coast locations, are compared to CFORS predictions. These surface sites are part of the Interagency Monitoring of Protected Visual Environments (IMPROVE) network. Both the aircraft and the IMPROVE data sets reveal similar patterns of good agreement near and above the boundary layer accompanied by large overprediction within the boundary layer. The observational data validate the CFORS simulations of BC and fine aerosol mass above the boundary layer. The near-surface overprediction does not impair the major conclusions of this study regarding long-range aerosol and BC transport, as most of the long-range transport occurs above 2 km. From this we conclude that the transport of BC from Asia and other regions west is a major source of BC at high elevations over North America. The simulated concentrations of BC between 1 and 3 km, as well as the measured BC concentrations over the elevated IMPROVE sites, range from 0.1 to 0.3 mg/m 3 . Direct radiative forcing over North America due to the modeled BC concentration between 1 and 15 km is estimated at an additional 2.04-2.55 W/m 2 absorbed in the atmosphere and a dimming of À1.45 to À1.47 W/m 2 at the surface. The impact of transported BC on the regional radiation budget through direct and indirect effects of the transported BC and other aerosols warrants further study.

Measuring wet deposition of organic carbon (OC) and black carbon (BC) is crucial for the complete understanding of the global circulation, lifetime, and radiative forcing of these aerosols. There is currently no accepted standard analytical method for measuring OC and BC concentration in precipitation. Different analytical methods have been employed for this purpose, but their feasibility has yet to be assessed. This manuscript evaluates the use of thermaloptical analysis (TOA), single-particle soot photometry (SP2), and ultraviolet-visible (UV/VIS) spectrophotometry for measuring BC in precipitation. In addition, total organic carbon (TOC) analysis was evaluated for the measurement of dissolved organic carbon (DOC) in precipitation. Potential interferences and sources of bias were assessed for each method. Precipitation samples and reference materials containing carbon particles generated from wood combustion and a natural gas diffusion flame were used in this study. The UV/VIS spectrophotometer, despite showing linearity with BC concentration, had inadequate sensitivity (±18 μg/L) to measure the low concentrations expected in precipitation. The SP2 analysis was adequate to measure refractory BC in precipitation in terms of precision and detection limit; however, systematic loss was estimated to be 34% (±3%). Sample filtration followed by TOA was inefficient for measuring particulate carbon in rainwater, as the quartz fiber filter captured less than 38% of the BC mass. Filtration was improved by adding salts and acids into the water samples, and ammonium dihydrogen phosphate, (NH 4 )H 2 PO 4 , was determined to be the best additive by increasing the collection efficiency of quartz fiber filters up to 95% (±5%). The TOC analyzer proved to be precise in the expected concentration range (200-5000 μg-C/L) for measuring DOC and total carbon (TC), including particulate OC and 94% (±2%) of the refractory BC in solution.