A new method for estimating megacity NOx emissions and lifetimes from satellite observations
Abstract. We present a new method for estimating NOx emissions and effective lifetimes from large cities. As in previous studies, the estimate is based on the downwind plume evolution for different wind directions separately. The novelty of the presented approach lies in the simultaneous fit of downwind patterns for opposing wind directions, which makes the method far more robust (i.e. less prone to local minima with nonphysically high or low lifetimes) than a single exponential decay fit. In addition, the new method does not require the assumption of a city being a "point source", but derives also the spatial distribution of emissions.
The method was successfully applied to 100 cities worldwide on seasonal scale. Fitted emissions generally agree reasonably with EDGAR v6 (R=0.76) and are on average 16 % lower, while estimated uncertainties are still rather large (≈ 30–50 %). Lifetimes were found to be rather short (2.44±0.68 h) and show no distinct dependency on season or latitude, which might be a consequence of discarding observations at high solar zenith angles (>65°).
Main limitations of this (and similar) methods are the underlying assumptions of steady state (meaning constant emissions, wind fields and chemical conditions) within about 100 km downwind from a city, which is probably a too strong simplification in order to reach higher accuracies.
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