Rainfall-induced changes in vertical O3 and SO2 within and above a boreal-temperate forest

This study investigates the impact of rainfall on the vertical distribution of ozone (O3) and sulfur dioxide (SO2) in a boreal-temperate forest, using multi-year observations (2009-2013) from a 42-m tower. Based on 194 persistent rainfall events, we compare the vertical O3 and SO2 concentrations between rain and non-rain conditions at the same height, hour, and month to minimize biases introduced by the background variations in time and space. In contrast to the observed O3 increases during rainfall over tropical forests, O3 during rainfall predominantly declines 2-6 ppb (10-20 %) from the surface throughout the canopy and above, except at lower layers during nighttime rainfall. SO2 exhibits a much larger reduction of 0.3-0.5 ppb (40-50 %) throughout the vertical profile. We further assess the role of four key processes, including washout, photochemistry, deposition, and vertical transport, in modulating O3 and SO2 vertical profiles during rainfall. Our analysis suggests that the substantial reduction in SO2 is mainly attributable to its high solubility, while O3 reductions during the day are largely due to declined photochemical production. Both O3 and SO2 depositions in nighttime could be moderately enhanced during rainfall with wet non-stomatal depositions, while the rainfall-induced change in daytime O3 deposition is small because rainfall-induced changes in stomatal and non-stomatal deposition offset each other. Furthermore, upward transport prevails in the rainfall periods, contributing to O3 reduction, whereas downward transport could enhance O3. These findings provide new insights into how rainfall impacts air pollutants and underscore the role of the four processes in shaping O3 and SO2 vertical distributions in boreal-temperate forests.