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

Abstract

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.