Spatiotemporal characterization of PM_2.5, O₃, and trace gases associated with East Asian continental outflows via...

East Asian continental outflows with PM_2.5, O₃, and other species may determine the baseline conditions and affect the air quality in downwind areas via long-range transport (LRT). To gain insight into the impact and spatiotemporal characteristics of airborne pollutants in East Asian continental outflows, a versatile multicopter drone sounding platform was used to simultaneously observe PM_2.5, O₃, CO₂, and meteorological variables (temperature, specific humidity, pressure, and wind vector) above the northern tip of Taiwan, Cape Fuiguei, which often encounters continental outflows during winter monsoon periods. By coordinating hourly high-spatial-resolution profiles provided by drone soundings, WRF/CMAQ model air quality predictions, HYSPLIT-simulated backward trajectories, and MERRA-2 reanalysis data, we analyzed two prominent phenomena of airborne pollutants in continental outflows to better understand their physical/chemical characteristics. First, we found that pollutants were well mixed within a sounding height of 500 m when continental outflows passed through and completely enveloped Cape Fuiguei. Eddies induced by significant fluctuations in wind speeds coupled with minimal temperature inversion and LRT facilitated vertical mixing, possibly resulting in high homogeneity of pollutants within the outflow layer. Second, the drone soundings indicated exceptionally high O₃ concentrations (70-100 ppbv) but relatively low concentrations of PM_2.5 (10-20 μg/m³), CO₂ (420-425 ppmv), and VOCs in some air masses. The low levels of PM_2.5, CO₂, and VOCs ruled out photochemistry as the cause of the formation of high-level O₃. Further coordination of spatiotemporal data with air mass trajectories and O₃ cross sections provided by MERRA-2 suggested that the high O₃ concentrations could be attributed to stratospheric intrusion and advection via continental outflows. High-level O₃ concentrations persisted in the lower troposphere, even reaching the surface, suggesting that stratospheric intrusion O₃ may be involved in the rising trend in O₃ concentrations in parts of East Asia in recent years in addition to surface photochemical factors.