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. We measured the global distribution of tropospheric N2Omixing ratios during the NASA airborne Atmospheric Tomography (ATom)mission. ATom measured concentrations of ∼ 300 gas species andaerosol properties in 647 vertical profiles spanning the Pacific, Atlantic,Arctic, and much of the Southern Ocean basins, nearly from pole to pole,over four seasons (2016–2018). We measured N2O concentrations at 1 Hzusing a quantum cascade laser spectrometer (QCLS). We introduced a new spectralretrieval method to account for the pressure and temperature sensitivity ofthe instrument when deployed on aircraft. This retrieval strategy improvedthe precision of our ATom QCLS N2O measurements by a factor of three (basedon the standard deviation of calibration measurements). Our measurements show that mostof the variance of N2O mixing ratios in the troposphere is driven bythe influence of N2O-depleted stratospheric air, especially at mid- andhigh latitudes. We observe the downward propagation of lower N2O mixingratios (compared to surface stations) that tracks the influence ofstratosphere–troposphere exchange through the tropospheric column down tothe surface. The highest N2O mixing ratios occur close to the Equator,extending through the boundary layer and free troposphere. We observedinfluences from a complex and diverse mixture of N2O sources, withemission source types identified using the rich suite of chemical speciesmeasured on ATom and the geographical origin calculated using anatmospheric transport model. Although ATom flights were mostly over theoceans, the most prominent N2O enhancements were associated withanthropogenic emissions, including from industry (e.g., oil and gas), urban sources, and biomassburning, especially in the tropical Atlantic outflow from Africa. EnhancedN2O mixing ratios are mostly associated with pollution-related tracersarriving from the coastal area of Nigeria. Peaks of N2O are oftenassociated with indicators of photochemical processing, suggesting possibleunexpected source processes. In most cases, the results show howdifficult it is to separate the mixture of different sources in the atmosphere,which may contribute to uncertainties in the N2O global budget. Theextensive data set from ATom will help improve the understanding of N2Oemission processes and their representation in global models.

Impact of stratospheric air and surface emissions on tropospheric nitrous oxide during ATom