Joint application of Doppler Lidar and differential absorption lidar to estimate the atomic mercury flux from a chlor-alkali plant

We have combined differential absorption lidar (DIAL) measurements of mercury concentrations downwind of a chlor-alkali plant (Rosignano Solvay) with measurements of wind profiles made with a Doppler Lidar based on modern fibre-optic technology. Since the flux of pollutant is equal to the cross-wind integral of the product of concentration and wind speed, this should permit us to make a more precise estimate of the fugitive emission of mercury from the plant than could be obtained by using anemometer measurements of the wind. The flux was estimated to be 54 g Hg h−1 using an anemometer on the plant building; 49 g Hg h−1 using an anemometer on a nearby 10 m mast; and 48 g Hg h−1 using wind speed corrections estimated from the Doppler Lidar measurements. Because of difficulties with the range resolution of the Doppler Lidar, the precision of this estimate was not as good as it should have been, though the difference from the rooftop anemometer remains statistically significant. Corrections of this magnitude are irrelevant to the Rosignano plant, where the emission rate varies strongly with the meteorological conditions. Where a precise estimate of a steady flux is required, however, reliable measurements of the wind profile in the wake of the source are essential. Doppler Lidar provides a possible method for acquiring such measurements.