Abstract
Abstract. We measured the global distribution of tropospheric N2O mixing ratios during the NASA airborne Atmospheric Tomography (ATom) mission. ATom measured concentrations of ∼ 300 gas species and aerosol 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 Hz using a quantum cascade laser spectrometer (QCLS). We introduced a new spectral retrieval method to account for the pressure and temperature sensitivity of the instrument when deployed on aircraft. This retrieval strategy improved the precision of our ATom QCLS N2O measurements by a factor of three (based on the standard deviation of calibration measurements). Our measurements show that most of the variance of N2O mixing ratios in the troposphere is driven by the influence of N2O-depleted stratospheric air, especially at mid- and high latitudes. We observe the downward propagation of lower N2O mixing ratios (compared to surface stations) that tracks the influence of stratosphere–troposphere exchange through the tropospheric column down to the surface. The highest N2O mixing ratios occur close to the Equator, extending through the boundary layer and free troposphere. We observed influences from a complex and diverse mixture of N2O sources, with emission source types identified using the rich suite of chemical species measured on ATom and the geographical origin calculated using an atmospheric transport model. Although ATom flights were mostly over the oceans, the most prominent N2O enhancements were associated with anthropogenic emissions, including from industry (e.g., oil and gas), urban sources, and biomass burning, especially in the tropical Atlantic outflow from Africa. Enhanced N2O mixing ratios are mostly associated with pollution-related tracers arriving from the coastal area of Nigeria. Peaks of N2O are often associated with indicators of photochemical processing, suggesting possible unexpected source processes. In most cases, the results show how difficult it is to separate the mixture of different sources in the atmosphere, which may contribute to uncertainties in the N2O global budget. The extensive data set from ATom will help improve the understanding of N2O emission processes and their representation in global models.
Highlights
Nitrous oxide (N2O) is a powerful greenhouse gas and, due to its oxidation to NOx, a major contributor to both stratospheric ozone loss and to the passivation of stratospheric oxy-halogen radicals (Forster et al, 2007; Ravishankara et al, 2009)
We present atmospheric N2O altitude profiles at high temporal resolution collected during the NASA Atmospheric Tomography (ATom) mission
The final official archive data file includes a new column where these corrections have been applied (N2O_QCLS_ad). These results show the very close comparability of the ATom airborne N2O instruments relative to the surface stations and demonstrate the feasibility of using ATom N2O measurements to evaluate the impact of stratospheric air and meridional transport of N2O emissions on N2O tropospheric column measurements over the ocean basins
Summary
Nitrous oxide (N2O) is a powerful greenhouse gas and, due to its oxidation to NOx, a major contributor to both stratospheric ozone loss and to the passivation of stratospheric oxy-halogen radicals (Forster et al, 2007; Ravishankara et al, 2009). The most recent estimates of the global ocean emissions of N2O range between 2.5 and 4.3 Tg N yr−1 (∼ 20 % of total emissions), with the tropics, upwelling coastal areas, and subpolar regions the major contributors to these fluxes (Yang et al, 2020; Tian et al, 2020). Recent estimates of N2O emissions from fertilized tropical and subtropical agricultural systems are 3 ± 5 kg N ha−1 yr−1 (Albanito et al, 2017). Most of these estimates are derived from short-term local-scale in-situ measurements and are difficult to extrapolate with confidence to large regions or to the globe. We report on the global distribution of N2O from the surface to 13 km and examine the processes contributing to the variability of tropospheric N2O based on the vertical profiles of N2O and a broad variety of covariate chemical species and aerosol properties
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