Aerosol backscatter fraction and single scattering albedo: Measured values and uncertainties at a coastal station in the Pacific Northwest

Light scattering, hemispheric backscattering, and absorption properties of submicrometer and supermicrometer aerosol particles at low relative humidity and 550 nm wavelength are investigated as a function of air mass category during a 2-month campaign at a midlatitude Pacific coastal station at Cheeka Peak, Washington. The main source of uncertainty in single scattering albedo (omega) measurements, namely, the measurement of light absorption, is addressed by the deployment of three identical absorption photometers and by relying on a recent calibration of this device using direct optical measurements. The absorption photometer measurement is corrected for response to light scattering, and measurements of sea-salt aerosol in this campaign provide a partial validation of this correction. Scattering measurements by nephelometry are also corrected for known instrumental nonidealities. Uncertainties stemming from instrumental noise, drift, calibration, and correction factors are propagated to allow comparisons among air mass categories and with other data sets and, ultimately, to constrain the values of omega and other optical properties used in climate models. Marine aerosol over the midlatitude eastern Pacific is found to be weakly absorbing for the sub-mu m component and virtually nonabsorbing for the super-mu m component (separated at 1 mu m, low-relative humidity, aerodynamic diameter). A distinct increase in sub-mu m light extinction (especially absorption) observed during 2 days of sustained marine flow appears to be Asian pollution transported across the Pacific. Low levels of gaseous NOx during this period rule out nearby combustion sources, and low levels of particulate Fe, Al, and Si rule out a significant contribution from mineral dust. Excluding this episode, both scattering and absorption properties for marine sampling conditions are similar to those observed in the clean midlatitude Southern Hemisphere (Cape Grim, Tasmania), In general, continental influence, as indicated by trends over the air mass categories, tends to raise the backscatter ratio and lower omega. Light absorption values compared to previous marine and coastal measurements confirm the range of values found by others and the highly variable nature of this quantity.