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

Ozone (O(3)) decomposition in the troposphere is a very important process which prevents excessive O(3) 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 O(3) concentrations over time (-ΔO(3)), nitrogen oxide (NO), nitrogen dioxide (NO(2)), 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 O(3) concentrations were determined by the UV photometric method. The obtained results suggest that the rate of decomposition of tropospheric O(3) 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 –ΔO(3) and air temperature, –ΔO(3) 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 O(3) also reacted with air components other than NO and NO(2). Precipitation at average temperature of < 0 °C did not significantly contribute to a drop in O(3) concentrations at night-time. In the warm season, precipitation slowed down the rate of O(3) decomposition, mostly because NO(x) were scrubbed by rain. An analysis of seasonal and daily –ΔO(3) fluctuations revealed that –ΔO(3) values were highest in the summer and shortly after sunset in the diurnal cycle.