Spatially and temporally resolved measurements of NO<sub>x</sub> fluxes by airborne eddy-covariance over Greater London

Abstract

Abstract. Flux measurements of nitrogen oxides (NOx) were made over London using airborne eddy-covariance from a low flying aircraft. Seven low altitude flights were conducted over Greater London performing multiple over-passes across the city during eight days in July 2014. NOx fluxes across the Greater London region exhibited high heterogeneity and strong diurnal variability, with central areas responsible for the highest emission rates (20–30 mg m−2 h−1). Other high emission areas included the M25 orbital motorway. The complexity of London’s emission characteristics makes it challenging to pinpoint single emission sources definitively using airborne measurements. Multiple sources, including road transport and residential, commercial and industrial combustion sources are all likely to contribute to measured fluxes. Measured flux estimates were compared to scaled National Atmospheric Emissions Inventory (NAEI) estimates, accounting for; monthly, daily and hourly variability. Significant differences were found between the flux-driven emissions and the NAEI estimates across Greater London, with measured values up to two times higher in Central London than those predicted by the inventory. To overcome the limitations of using the national inventory to contextualise measured fluxes, we used physics-guided flux data fusion to train environmental response functions (ERF) between measured flux and environmental drivers (meteorological and surface). The aim was to generate time-of-day emission surfaces using calculated ERF relationships for the entire Greater London region (GLR). 98 % spatial coverage was achieved across GLR at 400 m2 spatial resolution. All flight leg projections showed substantial heterogeneity across the domain, with high emissions emanating from Central London and major road infrastructure. The diurnal emission structure of the GLR was also investigated, through ERF, with the morning rush-hour distinguished from lower emissions during the early afternoon. Overall, the integration of airborne fluxes with an ERF-driven strategy enabled the first independent generation of surface NOx emissions, at high resolution using an eddy-covariance approach, for an entire city region.

Large Methane Emission Fluxes Observed From Tropical Wetlands in Zambia

“…Airborne eddy‐covariance (AEC) gives direct quantification of vertical eddy‐fluxes from aircraft (Hannun et al., 2020; Metzger et al., 2013; Vaughan et al., 2016, 2021). A vertical eddy‐flux is defined as the product of the fluctuating vertical wind speed and the fluctuating concentration (of CH 4 ), averaged over a defined period.…”

“…The airborne eddy‐covariance approach is an emerging technique which provides spatially resolved flux measurements that are especially useful for heterogeneous sources. However, eddy‐covariance measurements require expensive, high time‐resolution instrumentation with parallel sampling of three‐dimensional winds, and is highly dependent on meteorology (e.g., Hannun et al., 2020; Metzger et al., 2013; Vaughan et al., 2016, 2021). Finally, atmospheric inversion modeling is driven by numerical weather prediction models and thus can account for spatial and temporal variability in meteorological conditions (Ganesan et al., 2014, 2017; Rigby et al., 2019).…”

Multi-scale observations of mangrove blue carbon fluxes; the NASA Carbon Monitoring System BlueFlux field campaign

“…When combined with wavelet transforms (Wolfe et al 2018), AEC can characterize spatial gradients in fluxes at model-relevant scales (1-100 km). Flux footprint modeling allows for evaluation of fluxes within the context of surface properties and modeled fluxes (Hannun et al 2020; Vaughan et al 2021). Such data is complementary to ground-based observations, which integrate over a relatively small area but may better constrain site-specific processes and temporal variability.…”

“…This system includes a probe (mounted under the left wing) for meteorological measurements coupled with high-resolution differential GPS and inertial navigation systems. Similar systems have been utilized for airborne EC (Vaughan et al, 2021). Ambient air is sampled through a gas inlet mounted under the right wing and transferred through a teflon tube (in the wing) to two gas sensors in the cabin.…”

Novel approach to observing system simulation experiments improves information gain of surface–atmosphere field measurements

“…NS-ERF thus provides a framework to prepare and test the quantification of science objectives well ahead of the actual field measurements, thus reducing latency from field data capture to knowledge creation. More generally, NS-ERF can extend to any sort of study in which spatially and/or temporally (Hemes et al, 2021), emission inventory validation (Desjardins et al, 2018), urban air quality (Vaughan et al, 2021), industry leak detection (Kohnert et al, 2017), and multi-species applications (Vaughan et al, 2017).…”