Dry Deposition of Particulate Matter and Ions in Forest at Different Heights

Dry deposition of particulate matter (PM) indicates the capacity of atmosphere self-depuration. To investigate seasonal dry deposition of PM in an urban forest, the concentration and ionic composition of PM2.5 (particulate matter with aerodynamic equivalent diameters less than 2.5m) and the deposition of PM and ions in PM at different layer were measured at three canopy heights in the Beijing Olympic Forest Park. Results showed that diurnal trends in PM concentration varied with height and that concentrations tended to be highest in the understory. During winter, when the absorption ability of plants, which withered, was reduced, and anthropogenic emissions, primarily from coal combustion, ion concentrations tended to be highest in the forest canopy. The anions SO42-, NO3-, and Cl- were the dominant ionic species, accounting for 82% of the total mass of water-soluble ions in PM2.5. Results showed a lower mass ratio of NO3-/SO42- (mean=0.29, SD=0.24), relative to previous studies, implying that vehicle emissions may have reduced in Beijing. The chemical forms of sulfate and nitrate were likely NH4NO3 and (NH4)(2)SO4; however, Ca(NO3)(2) was the only chemical form detected based on the correlation coefficient between Ca2+ and NO3-. Concentrations of PM were negatively related to temperature and wind speed, and positively related to relative humidity. Within the forest canopy, the deposition velocity was low (high) during summer (winter). Furthermore, ion deposition fluxes were higher in the forest understory (1.5-6m) during summer and in the canopy (6-10m) during winter. This was attributed to seasonal trends in PM concentration. Article HighlightsThe objective of this study was to compare the diurnal variations of the particulate matter (PM) concentrations in an urban forest, and quantify dry deposition flux of PM and ions at three heights during the four seasons, also determine the chemical composition and possible origin of water-soluble ions in PM2.5, and analyze the influence of meteorological factors on the PM concentration.Few studies have investigated PM deposition at different heights within the forest canopy and understory, and previous researches have seldom studied the deposition of ions in PM2.5 in different layer. To address this limitation, this study aims to quantify the PM and ions in PM2.5 deposition and explore the dry deposition fluxes in different layers of forest and seasons to achieve the following objectives: (1) to quantify and compare seasonal dry deposition of PM and ions at three heights in an urban forest, and (2) to determine the chemical composition and possible origins of water-soluble ions in PM2.5. Our analysis will provide information to facilitate effective forestation initiatives to mitigate particle pollution, improving air quality and regional microclimates.