Quantifying stink: An investigation of nitrogen and carbon emissions from waste, vehicles, and agriculture

Abstract

Atmospheric measurements are needed to investigate the observed uncertainty in emissions of important trace gases like methane (CH4), nitrous oxide (N2O) and ammonia (NH3) from high-emitting sectors, such as agriculture, transportation and waste. Upon release to the atmosphere, these gases can negatively impact human health and the environment, both directly and indirectly, through pollution and climate change, for instance. However, understanding of the magnitude and variability of their emissions is limited. In this work, I used multiple observational datasets in conjunction with several quantification methods to investigate the characteristics of CH4, N2O and NH3 emissions from these three important sectors. A first-ever, statistically robust sector-wide inventory of CH4, N2O and NH3 emissions from US wastewater treatment was developed using measurements captured from a mobile-based platform and a facility-level quantification technique rooted in Bayesian statistics. This measurement-informed inventory was found to be significantly greater than current national inventories by factors of 2.3 (95% CI: 1.8- 2.9) and 2.0 (95% CI: 1.4-2.8) for CH4 and N2O, respectively. Wastewater NH3 emissions were only identified from a subset of facilities, but scaled emissions are comparable to current estimates of on-road vehicle exhaust emissions. I also measured thousands of vehicle exhaust plumes and used them to characterize vehicular N2O and NH3 emissions across seasonal, diurnal, and regional scales. Measured N2O and NH3 emission ratios were scaled to the entire US using well-constrained carbon dioxide (CO2) emissions, resulting in sector-wide emissions of 123 kt N2O yr−1 and 215 kt NH3 yr−1, or roughly 3.4 (95% CI: 3.2-3.9) and 2.4 (95% CI: 2.2-2.6) times greater than current estimates, respectively. Finally, satellite-based measurements were employed as an alternative observational platform to investigate gas fluxes on larger and longer scales than in-situ measurements. Specifically, a physics-based flux calculation method was utilized to quantify NH3 fluxes across the US over the 15-year period of 2008-2022 at high spatial and temporal resolutions. NH3 fluxes (11.7 ± 3.3 Mt NH3 yr−1) were 2.1 times greater than current estimates and are increasing at a rate of 6.0 ± 0.7% per year. Comparisons between livestock- and cropland-dominated regions magnify the complexities of NH3 fluxes.