Spatiotemporal characterization of PM 2.5 , O 3 , and trace gases associated with East Asian continental outflows via drone sounding.

East Asian continental outflows with PM _2.5 , O ₃ , 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 , O ₃ , CO ₂ , 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 O ₃ concentrations (70-100 ppbv) but relatively low concentrations of PM _2.5 (10-20 μg/m ³ ), CO ₂ (420-425 ppmv), and VOCs in some air masses. The low levels of PM _2.5 , CO ₂ , and VOCs ruled out photochemistry as the cause of the formation of high-level O ₃ . Further coordination of spatiotemporal data with air mass trajectories and O ₃ cross sections provided by MERRA-2 suggested that the high O ₃ concentrations could be attributed to stratospheric intrusion and advection via continental outflows. High-level O ₃ concentrations persisted in the lower troposphere, even reaching the surface, suggesting that stratospheric intrusion O ₃ may be involved in the rising trend in O ₃ concentrations in parts of East Asia in recent years in addition to surface photochemical factors.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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