Long-Range Transport of Air Pollutants

Atmospheric deposition of pollutants at three altitudes on Mount Emei, Sichuan Basin, southwestern China

Comparing the atmospheric deposition chemistry between above and below the planetary boundary layer (PBL) may help understand the impacts from inter-regional air pollutant transport and local emissions to air pollutant deposition. In this study, we monitored ions, soluble and insoluble potentially toxic elements (PTEs), and dissolved organic nitrogen and carbon at the base (M-base: 551 m asl), middle (M-middle: 2400 m asl), and summit (M-summit: 3077 m asl) on Mount Emei. The annual volume-weighted mean (VWM) concentrations of all measured components were substantially higher at M-base than at M-summit, except for Na+, Cl-, and NO2- as Na+ and Cl- at M-summit may be largely from ocean and NO2- to NO3- transformation may be faster at M-base. Not all components had their highest annual deposition fluxes at M-base mainly due to the differences in precipitation. Organic nitrogen and organic carbon accounted for 19 %-26 % and 63 %-71 % of the annual total dissolved nitrogen and carbon at the three sites, respectively. The contributions of soluble and insoluble fractions showed a large variability among the nine PTEs measured (3 %-75 % and 97 %-25 %, respectively) but they were similar among the three sites, as the PETs' solubility largely depended on pH. From low to high elevations, the contributions to the air pollutant deposition fluxes within and outside the SCB decreased and increased, respectively. Even south and southeast Asia were important source regions for some pollutants at M-summit. In sum, this study revealed the basin's large effects on air pollutant accumulation and deposition and the importance of non-SCB emissions above the SCB's PBL; However, either above or within the PBL, the inorganic, organic, soluble, and insoluble portions of air pollutants (particularly nitrogen and PTEs) should be considered together for a better understanding on air pollutants' transport, deposition, and ecological risks.