A Differential Absorption Lidar (DIAL) system based on a dye laser pumped by a 20 Hz Nd:YAG laser was constructed and used for range-resolved atomic mercury and sulfur dioxide monitoring. These gases are both severe pollutants in China, and there is a considerable interest to map such gases. Measurements were performed from a fixed laboratory, with the eye- safe laser beam pointing upwards at a 15◦ angle, to assess the vertical distribution of the gases. Long term quasi-continuous monitoring of background mercury and sulfur dioxide concentrations is presented. In particular, the influence of weather conditions was studied during prolonged measurement sequences. We have now studied prominent wash-out effects on mercury due to rainfall. For sulfur dioxide, a pollutant that causes acid rain quite high concentrations measured in Guangzhou were observed to fall drastically at rainfall due to wash-out.