Point Visibility Meter: a forward scatter instrument for the measurement of aerosol extinction coefficient.

A new instrument for the measurement of atmospheric extinction coefficient-the Point Visibility Meter (PVM)-is introduced and its principle of operation discussed. The instrument measures the amount of light scattered at a particular forward scattering angle by haze or fog; the extinction coefficient sigma is then found from the empirical law relating sigma to the scatter at this angle. Detailed calculations of aerosol scattering properties using Mie theory are given to support this empirical law. Error estimates based on these calculations indicate rms errors of less than +/-12% for a range of sigma values 2-60 km(-1). Field trials of the instrument against standard transmissometers confirm these theoretical findings and show a linearity of instrumental output with extinction coefficient, with an rms error of +/-11.7%.