The large amount of carbon stored in trees and soils of the Amazon rain forest is under pressure from land use as well as climate change. Therefore, various efforts to monitor greenhouse gas exchange between the Amazon forest and the atmosphere are now ongoing, including regular vertical profile (surface to 4.5 km) greenhouse gas measurements across the Amazon. These profile measurements can be used to calculate fluxes to and from the rain forest to the atmosphere at large spatial scales by considering the enhancement or depletion relative to the mole fraction of air entering the Amazon basin from the Atlantic, providing an important diagnostic of the state, changes and sensitivities of the forests. Previous studies have estimated greenhouse gas mole fractions of incoming air (‘background’) as a weighted mean of mole fractions measured at two background sites, Barbados (Northern Hemisphere) and Ascension (Southern hemisphere) in the Tropical Atlantic, where the weights were based on sulphur hexafluoride (SF₆) measured locally (in the Amazon vertical profiles) and at the two background sites. However, this method requires the accuracy and precision of SF₆ measurements to be significantly better than 0.1 parts per trillion (picomole mole⁻¹), which is near the limit for the best SF₆ measurements and assumes that there are no SF₆ sources in the Amazon basin. We therefore present here an alternative method. Instead of using SF₆, we use the geographical position of each air-mass back-trajectory when it intersects the limit connecting these two sites to estimate contributions from Barbados versus Ascension. We furthermore extend the approach to include an observation site further south, Cape Point, South Africa. We evaluate our method using CO₂ vertical profile measurements at a coastal site in Brazil comparing with values obtained using this method where we find a high correlation (r² = 0.77). Similarly, we obtain good agreement for CO₂ background when comparing our results with those based on SF₆, for the period 2010–2011 when the SF₆ measurements had excellent precision and accuracy. We also found high correspondence between the methods for background values of CO, N₂O and CH₄. Finally, flux estimates based on our new method agree well with the CO₂ flux estimates for 2010 and 2011 estimated using the SF₆-based method. Together, our findings suggest that our trajectory-based method is a robust new way to derive background air concentrations for the purpose of greenhouse gas flux estimation using vertical profile data.

中文翻译：

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基于亚马逊后向轨迹的温室气体通量计算的新背景方法

亚马逊雨林的树木和土壤中储存的大量碳正受到土地利用和气候变化的压力。因此，目前正在进行各种监测亚马逊森林与大气之间的温室气体交换的努力，包括在整个亚马逊地区进行定期的垂直剖面（地表至4.5公里）温室气体测量。通过考虑相对于从大西洋进入亚马逊盆地的空气的摩尔分数的增加或减少，这些廓线测量值可用于计算在大空间尺度上往返于雨林和大气的通量，为该州提供了重要的诊断信息，森林的变化和敏感性。₆）在本地（在Amazon垂直配置文件中）和两个背景站点进行测量。但是，此方法要求SF ₆测量的准确性和精密度要显着优于每万亿分之0.1（picomole mole ^-1），这接近于最佳SF ₆测量的极限，并且假设在该状态下没有SF ₆来源。亚马逊盆地。因此，我们在这里提出一种替代方法。代替使用SF ₆，我们使用每个空气质量回溯轨迹与连接这两个站点的界限相交时的地理位置来估算巴巴多斯与提升之间的贡献。我们进一步将方法扩展到包括南非开普角更南端的观察点。我们将巴西沿海地区的CO ₂垂直剖面测量结果与使用该方法获得的相关性较高（r ² = 0.77）的值进行比较，以评估我们的方法。同样，我们得到的CO吻合₂比较我们基于SF的结果时背景₆，2010-2011年期间，当SF ₆测量具有极好的精度和准确性。我们还发现CO，N ₂ O和CH _4的背景值方法之间的高度对应性。最后，基于我们的新方法的通量估算值与使用基于SF ₆的方法估算的2010年和2011年的CO ₂通量估算值非常吻合。总之，我们的发现表明，基于轨迹的方法是一种强大的新方法，可以使用垂直剖面数据估算背景空气浓度，用于估算温室气体通量。