Modification of a Commercial Integrating Nephelometer for Outdoor Measurements

Abstract

One of the major challenges in atmospheric aerosol research is the quantification of aerosol optical properties at ambient relative humidities (RHs) and their attribution to natural and anthropogenic aerosol sources. As an essential tool for this task, a commercial single wavelength integrating nephelometer built for dry indoor aerosol measurements was modified and complemented for outdoor operating conditions. The temperature difference ?T between air inlet and outlet is the crucial parameter indicating the effect of the instrument on RH of the sampled aerosol. By removing all heat sources from the optics compartment and by doubling the length of the light pipe between lamp and sensing chamber, ?T became smaller in the laboratory than the uncertainty of the temperature sensors. After mounting the instrument in a protective housing an average ?T = 0.55°C (0.22°C standard deviation) was maintained under nonoptimized ventilation conditions for one week of unattended outdoor operation parallel with an unmodified unit indoors. Data evaluation of the field test provided promising results concerning the temporal variability of the humidity dependence of aerosol optical properties.