Local closure during the First Aerosol Characterization Experiment (ACE 1): Aerosol mass concentration and scattering and backscattering coefficients

The radiative effects of tropospheric aerosol particles are a complex function of the chemical, physical, and optical properties of the aerosol. Closure experiments provide a means for identifying and reducing uncertainties associated with these aerosol properties and hence with the estimation of aerosol radiative forcing. In a closure experiment, an aerosol property is measured by one or more methods and calculated from a model that is based on other independently measured properties. A comparison of the measured and calculated values can reveal inadequacies in either the measurements or the model. A goal of the Aerosol Characterization Experiments (ACE) is to reduce the uncertainty associated with estimating aerosol radiative forcing through both local and column closure experiments. The remote marine aerosol encountered during ACE I was well suited for such closure studies because of its relatively simple chemical composition. Local closure experiments were conducted on board the NOAA RV Discoverer focusing on the aerosol mass concentration and scattering and backscattering coefficients. Aerosol mass was determined by gravimetric analysis, ion chromatography, and by converting the number size distribution to a mass distribution. Scattering and backscattering coefficients were measured with an integrating nephelometer and calculated from a Mie scattering model. The different measures of mass agreed for both the submicron and supermicron aerosol indicating that within experimental uncertainty, the aerosol was composed entirely of ionic species and associated water mass. Measured and calculated values of scattering and backscattering coefficients for submicron aerosol agreed within experimental uncertainty. There was an offset, however, between the Mie-calculated true values and the measured values that is not explained by nonidealities of the nephelometer response. Closure was not obtained for scattering or backscattering in the supermicron size range due to inadequacies of the techniques used to measure the size distribution and scattering in this size range. Both the mass and the scattering closure experiments indicate a need for an improved understanding of the response of aerosol growth, mixing state, and scattering to changes in relative humidity.