Tall tower measurements of methane, carbon monoxide and carbon dioxide emissions in London, UK

London, with a population of 8.2 million, is the largest city in Europe. It is heavily built-up (typically 8% vegetation cover within the central boroughs) and boasts some of the busiest arteries in Europe despite efforts to reduce traffic in the city centre with the introduction of a congestion charging scheme in 2007. We report on over two years of continuous measurements atop a tall tower in the heart of London between October 2011 and present. Fluxes of methane (CH4), carbon monoxide (CO) and carbon dioxide (CO2) are measured by eddy-covariance from the top of the British Telecom (BT) tower in central London (51° 31’ 17.4” N, 0° 8’ 20.04” W). The eddy-covariance system consists of a Gill R3-50 ultrasonic anemometer located 192 m above street level , a Picarro G2301-f cavity ring-down spectrometer for the measurement of CH4, CO2 and water, and an Aero-Laser AL5002 carbon monoxide analyser. Air is sampled 0.3 m below the sensor head of the ultrasonic anemometer and pulled down 45 m of 12.7 mm OD Teflon tubing. CO2 emissions were found to be mainly controlled by fossil fuel combustion (e.g. traffic, commercial and domestic heating) and diurnal averages of CO2 fluxes are highly correlated to traffic. However changes in heating-related natural gas consumption and, to a lesser extent, photosynthetic activity in two large city centre green spaces (Hyde Park and Regent’s Park) explained the seasonal variability. Annual estimates of net exchange of CO2 (41 ktons m-2) obtained by eddy-covariance agreed well with up-scaled data from the UK National Atmospheric Emissions Inventory (NAEI). CO fluxes were correlated to both CO2 and CH4; the estimated net emissions of CO for 2013 were 156 ± 40 tons km-2 which is in reasonable agreement with the 2012 London Atmospheric Emissions Inventory (LAEI) value of 90 tons km-2 and with independent measurement-based estimates which report a range of 105 to 220 tons km-2 (Harrison et al., 2012; O’Shea et al., 2014). Methane emissions from central London exhibit diurnal trends both for concentrations and fluxes. Fluxes are strongly correlated to those of carbon dioxide and although flux ratios exhibit diurnal cycles they are relatively constant on an annual basis. The baseline for methane fluxes is thought to result from leaks in the natural gas distribution network at a rate of 30 tons km-2 yr-1. However, a two- to three-fold difference was found between inventory and measured total fluxes, which could indicate an underestimation of CH4 emissions from combustion sources by the inventory (e.g. road traffic, domestic and commercial heating). Central London methane emissions are estimated at 70 tons km-2 yr-1 and the global warming effect of CO2 was found to be 25 times greater than that of CH4 (100-year horizon). References: Harrison et al., 2012, Atmospheric Chemistry and Physics, 12(6), 3065-3114. O’Shea et al., 2014, Journal of Geophysical Research: Atmospheres, 119(8), 4940–4952

Phenology is the dominant control of methane emissions in a tropical non-forested wetland

Tropical wetlands are a significant source of atmospheric methane (CH4), but their importance to the global CH4 budget is uncertain due to a paucity of direct observations. Net wetland emissions result from complex interactions and co-variation between microbial production and oxidation in the soil, and transport to the atmosphere. Here we show that phenology is the overarching control of net CH4 emissions to the atmosphere from a permanent, vegetated tropical swamp in the Okavango Delta, Botswana, and we find that vegetative processes modulate net CH4 emissions at sub-daily to inter-annual timescales. Without considering the role played by papyrus on regulating the efflux of CH4 to the atmosphere, the annual budget for the entire Okavango Delta, would be under- or over-estimated by a factor of two. Our measurements demonstrate the importance of including vegetative processes such as phenological cycles into wetlands emission budgets of CH4

From sink to source: high inter-annual variability in the carbon budget of a southern African wetland

We report on three years of continuous monitoring of carbon dioxide (CO2) and methane (CH4) emissions in two contrasting wetland areas of the Okavango Delta, Botswana: a perennial swamp and a seasonal floodplain. The hydrographic zones of the Okavango Delta possess distinct attributes (e.g. vegetation zonation, hydrology) which dictate their respective greenhouse gas (GHG) temporal emission patterns and magnitude. The perennial swamp was a net source of carbon (expressed in CO2-eq units), while the seasonal swamp was a sink in 2018. Despite differences in vegetation types and lifecycles, the net CO2 uptake was comparable at the two sites studied in 2018/2020 (−894.2 ± 127.4 g m−2 yr−1 at the perennial swamp, average of the 2018 and 2020 budgets, and −1024.5 ± 134.7 g m−2 yr−1 at the seasonal floodplain). The annual budgets of CH4 were however a factor of three larger at the permanent swamp in 2018 compared to the seasonal floodplain. Both ecosystems were sensitive to drought, which switched these sinks of atmospheric CO2 into sources in 2019. This phenomenon was particularly strong at the seasonal floodplain (net annual loss of CO2 of 1572.4 ± 158.1 g m−2), due to a sharp decrease in gross primary productivity. Similarly, drought caused CH4 emissions at the seasonal floodplain to decrease by a factor of 4 in 2019 compared to the previous year, but emissions from the perennial swamp were unaffected. Our study demonstrates that complex and divergent processes can coexist within the same landscape, and that meteorological anomalies can significantly perturb the balance of the individual terms of the GHG budget. Seasonal floodplains are particularly sensitive to drought, which exacerbate carbon losses to the atmosphere, and it is crucial to improve our understanding of the role played by such wetlands in order to better forecast how their emissions might evolve in a changing climate. Studying such hydro-ecosystems, particularly in the data-poor tropics, and how natural stressors such as drought affect them, can also inform on the potential impacts of man-made perturbations (e.g. construction of hydro-electric dams) and how these might be mitigated. Given the contrasting effects of drought on the CO2 and CH4 flux terms, it is crucial to evaluate an ecosystem's complete carbon budget instead of treating these GHGs in isolation

Illumination Geometry and Flying Height Influence Surface Reflectance and NDVI Derived from Multispectral UAS Imagery

Small unmanned aerial systems (UAS) have allowed the mapping of vegetation at very high spatial resolution, but a lack of standardisation has led to uncertainties regarding data quality. For reflectance measurements and vegetation indices (Vis) to be comparable between sites and over time, careful flight planning and robust radiometric calibration procedures are required. Two sources of uncertainty that have received little attention until recently are illumination geometry and the effect of flying height. This study developed methods to quantify and visualise these effects in imagery from the Parrot Sequoia, a UAV-mounted multispectral sensor. Change in illumination geometry over one day (14 May 2018) had visible effects on both individual images and orthomosaics. Average near-infrared (NIR) reflectance and NDVI in regions of interest were slightly lower around solar noon, and the contrast between shadowed and well-illuminated areas increased over the day in all multispectral bands. Per-pixel differences in NDVI maps were spatially variable, and much larger than average differences in some areas. Results relating to flying height were inconclusive, though small increases in NIR reflectance with height were observed over a black sailcloth tarp. These results underline the need to consider illumination geometry when carrying out UAS vegetation survey

UKCEH at the Edinburgh Climate Festival, 14th Aug 2021, Leith Links, Edinburgh

In total over 200 people engaged with the UKCEH Team between noon and 6 pm on the 14th Aug 2021 at the Edinburgh Climate Festival. The stand fulfilled its aim to raise awareness of publicly funded research conducted at UKCEH Edinburgh, including the national capability project UK-SCAPE, as evidenced by the number of people attracted into the stand, the remarks made in conversations with the team members and written in the answers to the poster quiz. Three activities were offered to target different age groups: • Carbon Game – target children- duration typically 2-4 min, estimated 100 children and adults participated • Intergenerational trend in CO2 concentration – target all age ranges – duration typically 1 to 5 min, estimated 80 children and adults participated • Poster quiz – target adults - duration typically 5-20 min, 42 primarily adults participated. A wide range of conversations were noted by the UKCEH team members primarily focused on: • the role of carbon in the environment and link to climate change • the steep rise in CO2 concentration in the lifetime of the people present • the range of science conducted at a local institution • the variety of options that people could make to their life choices that could improve the environment • routes for a career in STEM subjects

From sink to source: long-term (2002-2019) trends and anomalies in net ecosystem exchange of CO2 from a Scottish temperate peatland

A 'display' at the EGU General Assembly 2020. Peatlands North of 45˚ represent one of the largest terrestrial carbon (C) stores. They play an important role in the global C-cycle, and their ability to sequester carbon is controlled by multiple, often competing, factors including precipitation, temperature and phenology. Land-atmosphere exchange of carbon dioxide (CO2) is dynamic, and exhibits marked seasonal and inter-annual variations which can effect the overall carbon sink strength in both the short- and long-term. Due to increased incidences of climate anomalies in recent years, long-term datasets are essential to disambiguate natural variability in Net Ecosystem Exchange (NEE) from shorter-term fluctuations. This is particularly important at high latitudes (>45˚N) where the majority of global peatlands are found. With increasing pressure from stressors such as climate and land-use change, it has been predicted that with a ca. 3oC global temperature rise by 2100, UK peatlands could become a net source of C. NEE of CO2 has been measured using the eddy-covariance (EC) method at Auchencorth Moss (55°47’32 N, 3°14’35 W, 267 m a.s.l.), a temperate, lowland, ombrotrophic peatland in central Scotland, continuously since 2002. Alongside EC data, we present a range of meteorological parameters measured at site including soil temperature, total solar and photosynthetically active radiation (PAR), rainfall, and, since April 2007, half-hourly water table depth readings. The length of record and range of measurements make this dataset an important resource as one of the longest term records of CO2 fluxes from a temperate peatland. Although seasonal cycles of gross primary productivity (GPP) were highly variable between years, the site was a consistent CO2 sink for the period 2002-2012. However, net annual losses of CO2 have been recorded on several occasions since 2013. Whilst NEE tends to be positively correlated with the length of growing season, anomalies in winter weather also explain some of the variability in CO2 sink strength the following summer. Additionally, water table depth (WTD) plays a crucial role, affecting both GPP and ecosystem respiration (Reco). Relatively dry summers in recent years have contributed to shifting the balance between Reco and GPP: prolonged periods of low WTD were typically accompanied by an increase in Reco, and a decrease in GPP, hence weakening the overall CO2 sink strength. Extreme events such as drought periods and cold winter temperatures can have significant and complex effects on NEE, particularly when such meteorological anomalies co-occur. For example, a positive annual NEE occurred in 2003 when Europe experienced heatwave and summer drought. More recently, an unusually long spell of snow lasting until the end of March delayed the onset of the 2018 growing season by up to 1.5 months compared to previous years. This was followed by a prolonged dry spell in summer 2018, which weakened GPP, increased Reco and led to a net annual loss of 47.4 ton CO2-C km-2. It is clear that the role of Northern peatlands within the carbon cycle is being modified, driven by changes in climate at both local and global scales

Spatial and temporal variability of urban fluxes of methane, carbon monoxide and carbon dioxide above London, UK

We report on more than 3 years of measurements of fluxes of methane (CH4), carbon monoxide (CO) and carbon dioxide (CO2) taken by eddy-covariance in central London, UK. Mean annual emissions of CO2 in the period 2012–2014 (39.1 ± 2.4 ktons km−2 yr−1) and CO (89 ± 16 tons km−2 yr−1 ) were consistent (within 1 and 5% respectively) with values from the London Atmospheric Emissions Inventory, but measured CH4 emissions (72 ± 3 tons km−2 yr−1) were over two-fold larger than the inventory value. Seasonal variability was large for CO with a winter to summer reduction of 69 %, and monthly fluxes were strongly anti-correlated with mean air temperature. The winter increment in CO emissions was attributed mainly to vehicle cold starts and reduced fuel combustion efficiency. CO2 fluxes were 33 % higher in winter than in summer and anti-correlated with mean air temperature, albeit to a lesser extent than for CO. This was attributed to an increased demand for natural gas for heating during the winter. CH4 fluxes exhibited moderate seasonality (21 % larger in winter), and a spatially variable linear anti-correlation with air temperature. Differences in resident population within the flux footprint explained up to 90 % of the spatial variability of the annual CO2 fluxes and up to 99 % for CH4. Furthermore, we suggest that biogenic sources of CH4, such as wastewater, which is unaccounted for by the atmospheric emissions inventories, make a substantial contribution to the overall bud- get and that commuting dynamics in and out of central business districts could explain some of the spatial and temporal variability of CO2 and CH4 emissions. To our knowledge,this study is unique given the length of the data sets presented, especially for CO and CH4 fluxes. This study offers an independent assessment of “bottom-up” emissions inventories and demonstrates that the urban sources of CO and CO2 are well characterized in London. This is however not the case for CH4 emissions which are heavily underestimated by the inventory approach. Our results and others point to opportunities in the UK and abroad to identify and quantify the “missing” sources of urban methane, revise the methodologies of the emission inventories and devise emission reduction strategies for this potent greenhouse gas