Propagated Uncertainty in Scattering in Humidified Nephelometers

Atmospheric aerosols exert a cooling effect at the surface by directly scattering and absorbing incident sunlight and indirectly by serving as seeds for cloud droplets. They are highly variable liquid or solid particles suspended in gas phase whose climate impact is associated with their chemical composition and microphysical properties. One such aerosol property is the hygroscopic growth or increase in aerosol size and scattering with the uptake of water with increasing relative humidity (RH). Particle size is strongly linked to the wavelength of light scattered and absorbed. Defined as the parameter which characterizes the dispersion of the values about the measured quantity1, uncertainty can effectively place a measured value into perspective. Small uncertainties in instrument sensors can propagate to large errors in the measured hygroscopic growth of aerosols. The uncertainties in the aerosol scattering coefficients and hygroscopic growth fit parameter were calculated. Among the propagated uncertainties stems a considerable contribution from imprecise RH sensors. RH dependent uncertainty of the aerosol hygroscopic growth has never been reported in the literature; however, an increased uncertainty was calculated in aerosols with lower hygroscopic growth, particularly those in clean and wet conditions. 1. Cook, R. R., ASSESSMENT OF UNCERTAINTIES OF MEASUREMENT for calibration & testing laboratories. In National Association of Testing Authorities, Australia, 2002.