Wintertime vertical stratification of air pollutants and meteorological associations within an urban forest canopy in Shenyang, China.

Urban forests modify near-surface air quality; however, the vertical distribution of air pollutants within winter canopies remains poorly understood. This is especially true in cold-climate cities, where heating emissions and stable atmospheric conditions intensify pollution. To address this, we established a multi-level monitoring system in an urban forest in Shenyang, Northeast China, to measure the concentrations of PM2.5, PM10, O3, NO2, SO2, CO, and meteorological variables at 1.5, 10, and 20 m. The observations showed a pronounced height-dependent stratification during winter. PM2.5 and PM10 concentrations were generally higher at 10 and 20 m than at 1.5 m, whereas O3 and CO concentrations were relatively higher at the pedestrian level. NO2 and SO2 exhibited mid-canopy concentration maxima. Statistical and machine learning analyses revealed that meteorological factors, particularly relative humidity and wind speed, were associated with pollutant variability, although these associations were highly dependent on canopy height and pollutant type. These findings provide field evidence of the distinct vertical distributions of air pollutants within a northern urban forest during winter and demonstrate the importance of considering vertical heterogeneity when assessing air quality within urban forest canopies.