Atmospheric Factors Affecting a Decrease in the Night-Time Concentrations of Tropospheric Ozone in a Low-Polluted Urban Area

Ozone (O₃) decomposition in the troposphere is a very important process which prevents excessive O₃ accumulation in the air. It is particularly significant on warm summer days which are marked by a high risk of photochemical smog. We used Spearman's rank correlation test to determine relationships between the drop in O₃ concentrations over time (-ΔO₃), nitrogen oxide (NO), nitrogen dioxide (NO₂), and total nitrogen oxide (NO_x) concentrations and meteorological factors (1-h average) in low-polluted urban area in Olsztyn (north-eastern Poland). Nitrogen oxide concentrations were measured continuously by the chemiluminescence method, and O₃ concentrations were determined by the UV photometric method. The obtained results suggest that the rate of decomposition of tropospheric O₃ is affected mostly by the presence of NO_x, high temperature, and air humidity (positive correlation) as well as by wind speed (negative correlation). Maximum correlation coefficient values were reported between –ΔO₃ and air temperature, –ΔO₃ and absolute air humidity when NO_x concentrations were low (below 1.0 microgram per cubic meter), reaching 0.271 and 0.243, respectively. These results indicate that O₃ also reacted with air components other than NO and NO₂. Precipitation at average temperature of < 0 °C did not significantly contribute to a drop in O₃ concentrations at night-time. In the warm season, precipitation slowed down the rate of O₃ decomposition, mostly because NO_x were scrubbed by rain. An analysis of seasonal and daily –ΔO₃ fluctuations revealed that –ΔO₃ values were highest in the summer and shortly after sunset in the diurnal cycle.