Characterizing Anthropogenic and Biogenic Sources of CO2 and CH4 in Houston, Texas, USA

This study presents a comprehensive 2022 dataset of continuous in-situ measurements of δCO₂ and CH_4, δ¹³CO₂ and δ¹³CH₄ in Houston, Texas, USA complemented by targeted canister sampling to characterize key anthropogenic and biogenic emission sources. It also integrates ground-based in-situ measurements with satellite observations to characterize CO₂ and CH₄ emission hotspots in Houston, Texas.

Seasonal background variability reflects distinct biogeochemical processes: CO₂ declines from ~435 ppm in winter to ~410 ppm in summer due to photosynthetic uptake, while CH₄ decreases from ~2.02 to ~1.88 ppm primarily through OH oxidation. Regional contrasts are evident, with lower marine-influenced backgrounds (~410 ppm CO₂, ~1.85 ppm CH₄) compared to continental sectors (>435 ppm CO₂, >2.05 ppm CH₄).

Boundary-layer-height (BLH)-corrected enhancements reveal strong seasonal patterns in anthropogenic emissions. ΔCO₂ peaks in winter-fall (up to ~138 ppm hourly; ~20.6 ppm monthly) and drops to ~8.96 ppm in summer, while ΔCH₄ shows maxima in winter-spring (~5.45 ppm hourly; ~0.10 ppm monthly) and a summer minimum (~0.08 ppm). The 2022 mean ΔCH₄/ ΔCO₂ ratio (8.8 ppb/ppm) is ~14% higher than Philadelphia's in-situ winter value and ~35-40% above EDGAR v8.1 and EPA inventories, but broadly consistent with recent satellite-based estimates across U.S. cities.

Bivariate ΔCH₄/ ΔCO₂ mapping identifies major hotspots at McCarty, Blue Ridge, and Coastal Plains landfills (>40-60 ppb/ppm), which are underestimated or absent in inventories. Sharp enhancements at the Ship Channel and McCarty landfill align with satellite NO₂ and HCHO peaks, confirming these as multi-source hotspots. The BLH-corrected in-situ approach captures rapid emission events and diurnal variability, providing finer-scale source attribution and plume detection within the satellite sub-pixel domain that are missed by single overpasses.

Temporal analysis revealed distinct seasonal variability: δ¹³CO₂ was most depleted in winter, reflecting enhanced combustion-related CO₂, whereas δ¹³ CO₂ showed the most negative values in summer and fall, consistent with intensified microbial methanogenesis under warm, humid conditions. Background δ¹³CO₂ ranged from –11.8‰ to –13.2‰ depending on air mass origin, while δ¹³CH₄ varied between –47.2‰ and –50.7‰, reflecting marine–continental transitions. Spatially, bivariate plots and isotopic mapping identified strong CH₄ enhancements (>0.5 ppm) and highly depleted δ¹³ CH₄  (−50.5‰ to −51‰) over the McCarty landfill, indicating dominant microbial methane generation, further confirmed by canister measurements δ¹³CH₄ ≈ −60.3‰. Estimated emissions from the McCarty Landfill, based on our isotopic mass-balance analysis, were ~2,100 kg CH₄ h⁻¹ ±55%, exceeding the EPA GHGRP inventory value (~857 kg CH₄ h⁻¹) and moderately higher than the Carbon Mapper satellite-derived flux but within combined uncertainties (~1,427 kg CH₄ h⁻¹ ±91%). highlighting that bottom-up inventories underestimate methane emissions from large urban landfills such as McCarty. Overall, the isotopic evidence demonstrates that integrating δ¹³C analyses data provides critical insights into source attribution and the relative roles of combustion, industrial, and microbial processes shaping Houston’s CO₂ and CH₄ emission landscape.