Performance Characteristics of Integrating Nephelometers in the Australian Outback

Abstract

Radiance Research M903 nephelometers have been operated at remote Australian Outback sites since April 1998. This paper describes the calibration procedures applied to these instruments and reports on the noise performance and other operational issues. It is found that instrument noise leads to a detection limit of ?0.2 Mm?1 in scattering coefficient at the 95% confidence interval for a 5-min integration. Changes in ambient temperature cause drift with a coefficient of ?0.06 Mm?1 K?1, leading to a typical diurnal drift of amplitude ?0.9 Mm?1. Over the 10-yr deployment at an Outback station, the accuracy of the derived scattering coefficient is compromised by drifts in sensitivity and offset, in part related to gross changes in bandpass filter characteristics resulting from environmental degradation. A method is developed to track these changes. An uncertainty analysis suggests that the typical background scattering coefficient of ?10 Mm?1 can be measured to within 15% at the 95% confidence level. For events where the scattering coefficient is >100 Mm?1, the uncertainty falls to ?5%. Correction factors are derived for angular truncation error and inlet efficiency for the particular inlet configuration adopted and illustrated via a case study using size distributions guided by collocated NASA Aerosol Robotic Network (AERONET) data.