Characteristics and aldehyde concentrations of diamond dust events near Barrow, Alaska

Diamond dust (DD) refers to tiny ice crystals that form frequently in the Polar troposphere under clear sky conditions. They provide surfaces for chemical reactions and scatter light. We have measured the specific surface area (SSA) of DD at Barrow in March-April 2009. We have also measured its chemical composition in mineral and organic ions, dissolved organic carbon (DOC), aldehydes, H2O2, and the absorption spectra of water-soluble chromophores. Mercury concentrations were also measured in spring 2006, when conditions were similar. The SSA of DD ranges from 79.9 to 223 m**2/kg . The calculated ice surface area in the atmosphere reaches 11000 (±70%) µm**2/cm**3, much higher than the aerosol surface area. However, the impact of DD on the downwelling and upwelling light fluxes in the UV and visible is negligible. The composition of DD is markedly different from that of snow on the surface. Its concentrations in mineral ions are much lower, and its overall composition is acidic. Its concentrations in aldehydes, DOC, H2O2 and mercury are much higher than in surface snows. Our interpretation is that DOC from the oceanic surface microlayer, coming from open leads in the ice off of Barrow, is taken up by DD. Active chemistry in the atmosphere takes place on DD crystal surfaces, explaining its high concentrations in aldehydes and mercury. After deposition, active photochemistry modifies DD composition, as seen from the modifications in its absorption spectra and aldehyde and H2O2 content. This probably leads to the emissions of reactive species to the atmosphere.

钻石尘（Diamond dust, DD）指晴朗天空条件下频繁形成于极地对流层的微小冰晶，可为化学反应提供反应界面，同时散射光线。本研究于2009年3—4月在巴罗地区测定了DD的比表面积（specific surface area, SSA）；同时还测定了其矿物与有机离子组分、溶解性有机碳（dissolved organic carbon, DOC）、醛类、过氧化氢（H₂O₂）以及水溶性生色团的吸收光谱。2006年春季（彼时大气条件与本次实验相近），研究团队还同步测定了其中的汞浓度。DD的比表面积范围为79.9~223 m²/kg，大气中推算得到的冰表面积可达11000（±70%）μm²/cm³，远高于气溶胶表面积。不过，DD对紫外（UV）与可见光波段的下行、上行光通量的影响可忽略不计。DD的组分与地表积雪存在显著差异：其矿物离子浓度远低于地表积雪，整体呈酸性；而醛类、DOC、H₂O₂及汞的浓度则显著高于地表积雪。本研究认为，源自巴罗附近冰面冰间水道的海洋表层微层溶解性有机碳，可被DD吸附捕获；大气中发生的活跃化学反应发生在DD晶体表面，这也解释了其醛类与汞浓度偏高的原因。沉降后的DD会受到活跃光化学作用的改造，这一点可从其吸收光谱、醛类及H₂O₂含量的变化中得到印证，该过程可能会向大气中释放活性物种。