Collaborative Research: A Synthesis of Existing and New Observations of Air-Snowpack Exchanges to Assess the Arctic

Chemical and biological processes occuring within, above, and below snowpacks influence tropospheric ozone. Measurements available to date and simplified modeling studies indicate that the resulting impact on tropospheric O3 is significant, but available measurements and current modeling capabilities are insufficient for a quantitative estimate of its magnitude. It is of particular importance to improve our understanding of snowpack-atmosphere O3 exchanges because of ongoing and expected future alterations in snow, sea-ice and permafrost extent resulting from climate change, which will alter snowpack O3 impacts in the future. This project provides an integrated approach to address this need, using field measurements to fill key gaps in current knowledge and synthesizing the new and existing data into a chemistry-climate model. Air-snow exchange fluxes of O3 and NOx (NO+NO2) will be measured at multiple sites with different snow/land types, each for an extended period to capture effects of changing insolation, snowpack properties and (where applicable) soil temperature and soil NOx emissions. Measurements will include O3 and NOx levels and gradients both within and above the snowpack and eddy-correlation O3 fluxes at two heights above the snowpack; ancillary measurements will characterize atmospheric turbulence, actinic flux, micrometeorological parameters and the snowpack's physical and radiative properties. Field locations include Summit, Greenland (representing glacial snowpack), Toolik Lake, Alaska (representing snowpack above permafrost soil and snowpack over frozen lakes) and the Aspen FACE research site operated by Michigan Tech (representing snowpack above biologically active soil). Additional measurements for instrument shakedown at Table Mountain, Colorado, will provide information on intermittent snowpack. New parameterizations of snowpack processes will be developed and incorporated into single column model (SCM) versions of the global chemistry-climate models ECHAM4 and ECHAM5-MESSy. These parameterizations will be designed to describe the underlying processes and to capture variations among the available and new field measurements, which will be used for model evaluation. The new model system will be used to simulate the impact of air-snow O3 and NOx exchange upon the arctic tropospheric O3 budget. This work will close existing gaps in understanding of snowpack processes affecting O3, including O3 uptake to snow and the role of biological activity below snowpacks, NOx release from snow, and boundary layer O3 production resulting from snowpack emissions of O3 precursors. It will provide the first measurements of snow-air O3 and NOx fluxes for snow over permafrost and snow over frozen lakes, and the most thorough measurements available for snow over non-permafrost soil. The new snowpack-exchange model will be a very significant advancement over current simulations of snowpack impacts in chemistry-climate models, and will allow such models to better describe the connections between changing Arctic climate and environmental systems. Its use will produce the first assessment of the impact of snowpack photochemical processes upon the arctic and subarctic tropospheric O3 budget, and will provide the basis for assessing the expected impact that climate change will exert upon the tropospheric O3 budget through changing snowcover and permafrost extent.

积雪层内部、上方及下方发生的化学与生物过程会影响对流层臭氧。迄今为止的观测数据及简化模拟研究表明，其对对流层O3的影响显著，但现有观测与当前模拟能力仍不足以对其影响量级进行定量估算。鉴于气候变化导致积雪、海冰及多年冻土范围正在并预计将进一步发生改变，这将在未来改变积雪层对O3的影响，因此提升对积雪层-大气O3交换的理解尤为重要。本项目采用综合方法应对这一需求，通过实地观测填补当前认知的关键空白，并将新数据与现有数据整合到化学-气候模型中。将在具有不同积雪/土地类型的多个站点测量O3与NOx（NO+NO2）的气-雪交换通量，每个站点观测周期较长，以捕捉日照变化、积雪层特性及（如适用）土壤温度与土壤NOx排放的影响。观测内容将包括积雪层内部及上方的O3与NOx浓度及梯度，以及积雪层上方两个高度处的涡动相关（eddy-correlation）O3通量；辅助观测将表征大气湍流、光化通量、微气象参数及积雪层的物理与辐射特性。实地观测地点包括格陵兰岛萨米特（代表冰川积雪层）、阿拉斯加图利克湖（代表多年冻土土壤上方的积雪层及冰封湖泊上方的积雪层），以及由密歇根理工大学运营的阿斯彭FACE研究站点（代表生物活性土壤上方的积雪层）。在科罗拉多州桌山进行的仪器调试附加观测将提供间歇性积雪层的相关信息。将开发积雪层过程的新参数化方案，并整合到全球化学-气候模型ECHAM4与ECHAM5-MESSy的单柱模型（SCM）版本中。这些参数化方案将旨在描述潜在过程，并捕捉现有及新实地观测中的变化，后者将用于模型评估。新模型系统将用于模拟气-雪O3与NOx交换对北极对流层O3收支的影响。本研究将填补对影响O3的积雪层过程认知的现有空白，包括积雪对O3的吸收、积雪层下方生物活动的作用、积雪释放NOx，以及积雪层排放O3前体物导致的边界层O3生成。本研究将首次提供多年冻土上方积雪及冰封湖泊上方积雪的雪-气O3与NOx通量观测数据，并提供非多年冻土土壤上方积雪的最全面观测数据。新的积雪层交换模型将是化学-气候模型中积雪层影响模拟的重大进步，将使此类模型能更好地描述北极气候变化与环境系统之间的联系。该模型的应用将首次评估积雪层光化学过程对北极及亚北极对流层O3收支的影响，并为评估气候变化通过改变积雪覆盖与多年冻土范围对对流层O3收支的预期影响提供基础。