Observation- and model-based estimates of particulate dry nitrogen deposition to the oceans
Observation- and model-based estimates of particulate dry nitrogen deposition to the oceans
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DOI：
10.5194/acp-17-8189-2017
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发表时间：
2017-07-05
影响因子：
6.3
通讯作者：
Prospero, Joseph M.
中科院分区：
文献类型：
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作者：
Baker, Alex R.;Kanakidou, Maria;Prospero, Joseph M.
人类向大气排放氮(N)显著增加了硝酸盐（NO3-）和铵（NH4+）在公海表层水的沉积，对海洋生产力和全球碳循环具有潜在影响。要在全球范围内了解氮沉降对海洋的影响，取决于我们是否有能力建立和验证氮排放、大气化学、运输和沉降模型。在这项工作中，类似于1995-2012年期间船上采样获得的2900次气溶胶NO3-和NH4+浓度观测，用于评估远程海洋上模拟的N浓度和沉积场的性能。选择了三个海洋区域（热带北大西洋东部、印度洋北部和西北太平洋），认为观测数据的密度和分布足以与模式产品进行有效的比较。所有这些研究区域都受到矿物粉尘的运输和沉积的影响，矿物粉尘通过吸收矿物表面的氮氧化物（NOx）而改变了氮的沉积。评估大气N沉降对海洋的影响需要大气化学输送模型来报告沉降通量；然而，这些通量无法在海洋上空测量。像大气化学和气候模式比较项目（ACCMIP）这样的模拟研究只报告了沉积通量，因此很难对干沉积进行验证。在这里，可用的观测数据在5度x 5度网格上进行了平均，并与ACCMIP氧化N （NOy）和还原N （NHx）的干沉降通量（Mod-Dep）以及环境化学过程实验室（TM4）的示踪模型4的以下参数进行了比较：NOy， NHx和颗粒NO3-和NH4+的ModDep，以及表面颗粒NO3-和NH4+浓度。作为一个模型集合，ACCMIP可以预期比TM4更鲁棒，而TM4提供了与观测参数更相关的指定参数（NO3-和NHC4+），而这些参数在ACCMIP中是不可用的。干沉积通量（CalDep）是根据观测到的浓度，利用干沉积速度的估计值计算出来的。模型-观测比（R-A,R-n）由网格单元面积和观测数加权，用于评估模型的性能。三个研究区域的对比表明，TM4高估了NO3-浓度（R-A,R-n = 1.4 ~ 2.9），低估了NH4+浓度（R-A,R-n = 0.5 ~ 0.7），模式未重现热带大西洋和北印度洋的空间分布。在印度洋NH4+的情况下，这种差异可能是由于采样的季节性偏差。在CalDep与ModDep的各种比较中也观察到类似的模式（R-A，对于NO3- R-n = 0.6-2.6，对于NH4+ R-n = 0.6-3.1）。由于TM4和ACCMIP NHx模型产品中含有大量气相NH3沉积，NHx caldeep - moddep比较的R-A、R-n值大约是NH4+ caldeep - moddep比较的相应值的两倍。由于观测数据的缺乏以及用于从浓度推导沉积通量的干沉积速度存在很大的不确定性，所有的比较都受到了影响。几十年来，这些不确定性一直是对海洋物质通量估计的主要限制。通过改变观测、建模和模式观测比较程序，提出了改进N沉积估算的建议。对模拟干沉积的验证需要与可观测到的气溶胶相物种浓度进行有效的比较，如果模式产品只报告海洋上的干沉积通量，则无法实现这一点。
Anthropogenic nitrogen (N) emissions to the atmosphere have increased significantly the deposition of nitrate (NO3-) and ammonium (NH4+) to the surface waters of the open ocean, with potential impacts on marine productivity and the global carbon cycle. Global-scale understanding of the impacts of N deposition to the oceans is reliant on our ability to produce and validate models of nitrogen emission, atmospheric chemistry, transport and deposition. In this work, similar to 2900 observations of aerosol NO3- and NH4+ concentrations, acquired from sampling aboard ships in the period 1995-2012, are used to assess the performance of modelled N concentration and deposition fields over the remote ocean. Three ocean regions (the eastern tropical North Atlantic, the northern Indian Ocean and northwest Pacific) were selected, in which the density and distribution of observational data were considered sufficient to provide effective comparison to model products. All of these study regions are affected by transport and deposition of mineral dust, which alters the deposition of N, due to uptake of nitrogen oxides (NOx) on mineral surfaces.Assessment of the impacts of atmospheric N deposition on the ocean requires atmospheric chemical transport models to report deposition fluxes; however, these fluxes cannot be measured over the ocean. Modelling studies such as the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP), which only report deposition flux, are therefore very difficult to validate for dry deposition. Here, the available observational data were averaged over a 5 degrees x 5 degrees grid and compared to ACCMIP dry deposition fluxes (Mod-Dep) of oxidised N (NOy) and reduced N (NHx) and to the following parameters from the Tracer Model 4 of the Environmental Chemical Processes Laboratory (TM4): ModDep for NOy, NHx and particulate NO3- and NH4+, and surface-level particulate NO3- and NH4+ concentrations. As a model ensemble, ACCMIP can be expected to be more robust than TM4, while TM4 gives access to speciated parameters (NO3- and NHC4+) that are more relevant to the observed parameters and which are not available in ACCMIP. Dry deposition fluxes (CalDep) were calculated from the observed concentrations using estimates of dry deposition velocities. Model-observation ratios (R-A,R-n), weighted by gridcell area and number of observations, were used to assess the performance of the models. Comparison in the three study regions suggests that TM4 overestimates NO3- concentrations (R-A,R-n = 1.4-2.9) and underestimates NH4+ concentrations (R-A,R-n = 0.5-0.7), with spatial distributions in the tropical Atlantic and northern Indian Ocean not being reproduced by the model. In the case of NH4+ in the Indian Ocean, this discrepancy was probably due to seasonal biases in the sampling. Similar patterns were observed in the various comparisons of CalDep to ModDep (R-A,R-n = 0.6-2.6 for NO3-, 0.6-3.1 for NH4+). Values of R-A,R-n for NHx CalDep-ModDep comparisons were approximately double the corresponding values for NH4+ CalDep-ModDep comparisons due to the significant fraction of gas-phase NH3 deposition incorporated in the TM4 and ACCMIP NHx model products. All of the comparisons suffered due to the scarcity of observational data and the large uncertainty in dry deposition velocities used to derive deposition fluxes from concentrations. These uncertainties have been a major limitation on estimates of the flux of material to the oceans for several decades. Recommendations are made for improvements in N deposition estimation through changes in observations, modelling and model-observation comparison procedures. Validation of modelled dry deposition requires effective comparisons to observable aerosol-phase species' concentrations, and this cannot be achieved if model products only report dry deposition flux over the ocean.