Hydrogen soil deposition at an urban site in Finland

Abstract. Hydrogen deposition velocities (v_d) were estimated by field chamber measurements and model simulations. A closed-chamber method was used for soil deposition studies in Helsinki, Finland, at an urban park inhabited by broad-leaved trees. Radon tracer method was used to estimate the v_d in nighttime when photochemical reactions were minimal and radon gas was concentrated in the shallow boundary layer due to exhalation from soil. A two-dimensional atmospheric model was used for the calculation of respective v_d values and radon exhalation rates. The v_d and radon exhalation rates were lower in winter than in summer according to all methods. The radon tracer method and the two-dimensional model results for hydrogen deposition velocity were in the range of 0.13 mm s⁻¹ to 0.93 mm s⁻¹ (radon tracer) and 0.12 mm s⁻¹ to 0.61 mm s⁻¹ (two-dimensional). The soil chamber results for v_d were 0.00 mm s⁻¹ to 0.70 mm s⁻¹. Both models and chamber measurements revealed a relation between one week cumulative rain sum and deposition velocity. When precipitation events occurred a few days before the chamber measurements, lower v_d values were observed. A snow cover also lowered v_d.