Preliminary observation of strong NOx release over Qiyi Glacier in the northeast of the Tibetan Plateau†

NO_x is released from sunlit snowpack surfaces, and this considerably influences the oxidizing capacity of the clean boundary layer atmosphere in Antarctic and Arctic regions and the potential interpretation of the historical atmospheric composition recorded in the ice core. The Tibetan Plateau is an important snow-covered region in the northern midlatitudes, with strong solar radiation and relatively high NO₃⁻ in snow/ice. Released NO_x on the glacier surface of the Tibetan Plateau should be strong. To confirm this hypothesis, field observations were performed at 4600 m above the sea level in Qiyi Glacier in late August 2004. The surface ultraviolet-B (UVB) radiation level reached >4.5 W m⁻² and was increased by the strong reflection of snow/ice and clouds against the Sun and strengthened by the topographical effect. The concentrations of NO₃⁻ and NH₄⁺ in water from melting snow were hardly detected, but the average concentration (±1σ) of NO₃⁻ in snow samples was 8.7 ± 2.7 μmol L⁻¹. Strong correlations were observed between NO_x (NO₂) mixing ratios and UVB radiation levels in the Tibetan glacier. Vertical experiments revealed a negative gradient of NO_x (NO₂) mixing ratios from the glacier snow surface to a height of 30 cm. As a result of the high levels of UV radiation and high NO₃⁻ concentrations in snow/ice, the mixing ratios of NOx released by fresh snow in Qiyi Glacier in late August reached several parts per billion (ppbv) and were approximately one order of magnitude higher than those observed in polar regions. This observation provides direct evidence to support the research hypothesis and confirms the release of high concentrations of NOx in the boundary layer of the highland glaciers and snow surfaces.