Advancing CH₄ and N₂O retrieval strategies for NDACC/IRWG high-resolution direct-sun FTIR Observations

Abstract. Atmospheric methane (CH₄) and nitrous oxide (N₂O) are potent greenhouse gases with significant impacts on climate change. Accurate measurement of their atmospheric abundance is essential for understanding their sources, sinks, and the impact of human activities on the atmosphere. Ground-based high-resolution Fourier Transform Infrared (FTIR) observations, employed by collaborative international initiatives like the Infrared Working Group (IRWG) within the Network for the Detection of Atmospheric Composition Change (NDACC), play a vital role in retrieving the atmospheric amounts of these gases. Network wide consistent data products rely on consistent observations and retrievals. Recent developments in spectroscopy, a priori data, retrieval software and techniques underscores the necessity to revisit the retrieval strategies for all NDACC/IRWG species currently ongoing. This study investigates various retrieval strategies of CH₄ and N₂O utilizing high-resolution FTIR observations in Boulder, Colorado, and compares them with unique airborne in situ measurements. The initial focus is on characterizing retrieval differences across spectroscopy databases. While it is challenging to identify the best retrievals purely based on spectroscopy, as they produce similar outcomes, notable differences in profile shapes and magnitudes underscore the importance of independent validation. Specifically, when multi-year independent nearby AirCore and aircraft in situ profile measurements are used to evaluate vertical distributions and biases in partial columns, they reveal excellent agreement in relative differences with FTIR retrievals and thereby strengthening confidence in the assessment. The final optimized retrievals for CH₄ and N₂O are presented incorporating quantitative fitting results and comparisons of vertical profiles, partial and total columns. We find that employing a priori profiles using the latest simulations of the Whole Atmosphere Community Climate Model (WACCM) enhances accuracy relative to in situ profiles. While the HITRAN 2020 spectroscopic database is effective for N₂O, ATM 2020 provides better results for CH₄, with slight improvement observed when paired with the water vapor line list from DLR; however, this improvement may be site-dependent. Regarding regularization, both first-order Tikhonov and Optimal Estimation produce comparable outcomes, as long as the fitted profile degrees of freedom remain between 2 and 2.5. Correspondingly, profile results comparisons yield biases of -0.08 ± 0.38 % and 0.89 ± 0.28 % for tropospheric and stratospheric layers of CH₄ relative to AirCore, respectively, and 0.39 ± 0.42 % for aircraft comparisons in the troposphere. For N₂O, the bias in the troposphere using aircraft measurements is approximately 0.18 ± 0.2 %. Uncertainty budgets combining random and systematic sources are provided. Random errors, mainly stemming from temperature profile uncertainties and measurement noise dominate in the troposphere for both gases with a retrieval random error of 0.5 %. Systematic errors primarily arise from HITRAN based spectral line parameters, predominantly the line intensity and air-broadened half-width. Finally, we present long-term time series of CH₄ derived from the recommended retrieval strategies applied to observations at Boulder. To contrast these findings with the southern hemisphere, we successfully extended this analysis to the site in Lauder, New Zealand. These findings contribute to advancing our understanding of atmospheric composition and will support the improvement of a harmonized approach for all IRWG/NDACC sites.