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Highly concentrated layers of atomic K have been observed in the mesosphere above Yanqing near Beijing (40 degrees N, 116 degrees E). The K density in these narrow layers exceeds 1,100cm(-3) (at least 4 times higher than reported elsewhere), and the K/Na ratio is superchondritic by a factor of 3-4. A model with detailed metal ion chemistry, supported by ancillary measurements from a nearby ionosonde and meteor radar, is used to show that these sporadic K layers can be produced from a strong sporadic E layer (critical frequency>11MHz) that descends from above 100km at a velocity of 1-2kmh(-1). This allows most of the Na+ ions to be neutralized before the remaining ions are dumped around 90km, where the higher pressures and colder temperatures facilitate the formation of K+.N-2 and K+.CO2 cluster ions. These cluster ions then undergo dissociative recombination with electrons to form K.

Plain Language Summary The source of metallic atoms in the Earth's upper atmosphere is the ablation of cosmic dust particles. These metals exist in layers between 80 and 105 km. Under most circumstances, the abundance of K is about 1/15 that of Na, the same ratio as in cosmic dust. However, in this study near Beijing, thin layers of K with a concentration more than 4 times that previously reported were observed, accompanied by a dramatic increase in the K/Na ratio to 1/5. These observations are explained by a wind shear above 100 km producing a highly concentrated layer of metal ions and electrons, which then descends over more than 5 h to around 90 km. K ions only neutralize at relatively high pressures and low temperatures, and this occurs around 90 km where a concentrated layer of neutral K atoms then forms. Because Na ions are mostly neutralized at higher altitudes, a large increase in the K/Na in the layer results.