A novel atmospheric pressure glow microdischarge system coupled with chemical vapor generation was applied to the optical emission spectrometry determination of Hg in samples of mosses (Pleurozium schreberi) from parks and surrounding forests of Wroclaw (Poland). The design of experiment (DOE) was used to optimize operating parameters of the microdischarge combined with a reaction/separation unit used for the chemical vapor generation. Seven experimental factors were examined, i.e., concentrations of NaBH4 and HCl, the discharge current and flow rates of reagents, the jet-supporting gas, the shielding gas and the liquid cathode solution, on the intensity of the Hg I 253.7 nm emission line and the standard deviation of background in its vicinity. Optimized operating conditions allowed to obtain the detection limit for Hg of 0.066 µg L-1 that was close to the predicted value with a fitted model. To separate Hg species from moss samples, a 2-step extraction procedure with a 5.0 mol L-1 HCl solution was carried out instead of the complete wet digestion with concentrated HNO3. To validate the reliability of results, the spike-and-recovery experiment was performed. Obtained results were close to 100% proving the good accuracy of the methodology proposed. The average concentration of Hg in mosses in the urban area of Wroclaw was high (0.37 µg g-1) but no specific source of contamination was found.