The Influence of Near-field Fluxes on Seasonal Carbon Dioxide Enhancements: Results From the Indianapolis Flux Experiment (INFLUX)

Abstract

BackgroundNetworks of tower-based CO2 mole fraction sensors have been deployed in and around cities across the world to quantify anthropogenic CO2 emissions from metropolitan areas. A critical aspect in these approaches is the separation of atmospheric signatures from distant sources and sinks (i.e., the background) from local emissions and biogenic fluxes. We examined CO2 enhancements compared to forested and agricultural background towers in Indianapolis, Indiana, USA, as a function of season and compared them to modeled results, as a part of the Indianapolis Flux (INFLUX) project.ResultsAt the INFLUX urban tower sites, daytime growing season enhancement on a monthly timescale was up to 4.3 – 6.5 ppm, 2.6 times as large as those in the dormant season, on average. The enhancement differed significantly depending on choice of background and time of year, being 2.8 ppm higher in June and 1.8 ppm lower in August using a forested background tower compared to an agricultural background tower. A prediction based on land cover and observed CO2 fluxes showed that differences in phenology and drawdown intensities drove measured differences in enhancements. Forward modelled CO2 enhancements using fossil fuel and biogenic fluxes indicated growing season model-data mismatch of 1.1 ± 1.7 ppm for the agricultural background and 2.1 ± 0.5 ppm for the forested background, corresponding to 25 – 29 % of the modelled CO2 enhancements. The model-data total CO2 mismatch during the dormant season was low, – 0.1 ± 0.5 ppm. ConclusionsBecause growing season biogenic fluxes at the background towers are large, the urban enhancements must be disentangled from the biogenic signal, and growing season increases in CO2 enhancement could be misinterpreted as increased anthropogenic fluxes if the background ecosystem CO2 drawdown is not considered. The magnitude and timing of enhancements depend on the land cover type and net fluxes surrounding each background tower, so a simple box model is not appropriate for interpretation of these data. Quantification of the seasonality and magnitude of the biological fluxes in the study region using high-resolution and detailed biogenic models is necessary for the interpretation of tower-based urban CO2 networks for cities with significant vegetation.

Quantification of Urban Forest and Grassland Carbon Fluxes Using Field Measurements and a Satellite‐Based Model in Washington DC/Baltimore Area

“…Our study adds to a growing body of literature illustrating the importance of including biogenic sources of CO 2 when generating estimates of CO 2 fluxes in urban areas at fine spatial and temporal resolutions (Miles et al., 2021; Sargent et al., 2018). Similar to the top‐down study by Sargent et al.…”

High‐Resolution Lagrangian Inverse Modeling of CO2 Emissions Over the Paris Region During the First 2020 Lockdown Period

“…(2015) pointed out that for the city‐scale inversion, current regional models would fail to provide sufficient accuracy while using spatially resolved modeled values. Several other studies have discussed the impact of background mixing ratios on inversion (e.g., Lauvaux et al., 2016; Miles et al., 2021; Mueller et al., 2018; Pisso et al., 2019; Turnbull et al., 2015). Here, we present the results from different strategies used to define the background values.…”

“…Therefore, the estimation of CO 2 fluxes in the city needs to consider observations from within and around the city (McKain et al., 2012). Fossil fuel emissions are the dominant source of CO 2 in an urban area, whereas, in rural remote locations, the biogenic influence (net ecosystem exchange) dominates the observed variations (Miles et al., 2021). Biogenic sources can partially offset the emissions during the growing season, even at the urban scale, depending on the green vegetation fraction and the regional ecosystems (Miller et al., 2020; Staufer et al., 2016).…”

Assimilating Morning, Evening, and Nighttime Greenhouse Gas Observations in Atmospheric Inversions

“…This network is equipped with Cavity Ring-Down Spectrometers (CRDS; Picarro, Inc.) and regular discrete flask measurements. As many as 12 telecommunication towers have been instrumented at any one time since 2010 (Miles et al, 2017a;Richardson et al, 2017;Lauvaux et al, 2016), recording high-frequency continuous measurements of CO 2 (all sites), CH 4 , and CO (some sites). At some sites, there are measurements at multiple heights.…”

“…At some sites, there are measurements at multiple heights. Hourly outputs of these measurements are publicly available (Miles et al, 2017a).…”