From California dreaming to California data: Challenging historic models for landfill CH4 emissions

Abstract

Improved quantifi cation of diverse CH 4 sources at the urban scale is needed to guide local GHG mitigation strategies in the Anthropocene. Herein, we focus on landfi ll CH 4 emissions in California, challenging the current IPCC methodology which focuses on a climate dependency for landfi ll CH 4 generation (methanogen-esis), but does not explicitly consider climate or soil dependencies for emissions. Relying on a comprehensive California landfi ll database, a fi eld-validated process-based model for landfi ll CH 4 emissions (CALMIM), and select fi eld measurements at 10 California sites with a variety of methods, we support the contrary position: Limited climate dependency for methanogenesis, but strong climate dependency for landfi ll CH 4 emissions. Contrary to the historic IPCC empirical model for methanogenesis with kinetic constants related to climate, we demonstrate a simpler and more robust linear empirical relationship (r 2 = 0.85; n=128) between waste mass and landfi ll biogas recovery [126 × 10-6 Nm 3 CH 4 hr-1 Mg waste-1 ]. More interestingly, there are no statistically signifi cant relationships with climate, site age, or status (open/closed) for landfi ll biogas recovery. Th e current IPCC methodology does not consider soil or climate drivers for gaseous transport or seasonal methanotrophy in diff erent cover soils. On the other hand, we illustrate strong climate and soil dependencies for landfi ll emissions-e.g., average intermediate cover emissions below 20 g CH 4 m-2 d-1 when the site's mean annual precipitation is >500 mm y-1. Th ereby, for the California landfi ll CH 4 inventory, the highest-emitting sites shift from landfi lls containing the largest mass of waste to sites dominated by intermediate cover types having a reduced rate of soil CH 4 oxidation during the annual cycle. Th ese diff erences have profound implications for developing more realistic, science-based urban and regional scale GHG inventories for landfi ll CH 4 while reducing uncertainties for this important anthropogenic source.

Spatio‐temporally Resolved Methane Fluxes From the Los Angeles Megacity

“…We first evaluate the nonleak periods in 2015 and 2016. In this study (Wong et al, 2016), a larger peak in winter that cannot be attributed to landfill emissions (see section 4.5 in Wong et al, 2016, andK. This agrees with the prior work using independent top-down (tracertracer) flux estimation methods by Wong et al (2016), which showed significant variability in monthly top-down CH 4 emissions estimates.…”

Assessing the optimized precision of the aircraft mass balance method for measurement of urban greenhouse gas emission rates through averaging

“…If we assume that the CH 4 average we obtained (67 mol/s) for the city-wide total is representative, the Lamb et al (2016) value for the SSLF represents 46% of the city-wide total. Variability of CH 4 emissions from landfills is partially dependent on the local climate, which can influence the seasonal oxidation of landfill covers (Xu et al, 2014;Spokas et al, 2015). To better understand the variability of the SSLF emissions, we considered several factors known to influence landfill emissions, such as change in barometric pressure over time, air temperature and precipitation (Xu et al, 2014;Spokas et al, 2015).…”

“…Variability of CH 4 emissions from landfills is partially dependent on the local climate, which can influence the seasonal oxidation of landfill covers (Xu et al, 2014;Spokas et al, 2015). To better understand the variability of the SSLF emissions, we considered several factors known to influence landfill emissions, such as change in barometric pressure over time, air temperature and precipitation (Xu et al, 2014;Spokas et al, 2015). However, no particular correlations were found between aircraft-determined CH 4 emission rates (the part attributed to the landfill) and average atmospheric temperature recorded at Indianapolis International airport during the experiments (data downloaded from http://www.ncdc.noaa.gov/qclcd/QCLCD, accessed on 10/05/2015), as well as with relative humidity and barometric pressure.…”

Field measurements and modeling to resolve m2 to km2 CH4 emissions for a complex urban source: An Indiana landfill study

“…Neither the theoretical generation nor the % recovery efficiency can be routinely validated by field data and, indeed, has been rarely quantified at field scale (Spokas et al, 2006). Recently, as discussed in Spokas et al (2015), 2010 field data for 129 well-managed California landfills indicated a robust linear relationship (r 2 = 0.85) for measured landfill CH 4 recovery compared to measured wastein-place (approx. 125 Nm 3 CH 4 per million Mg landfilled waste).…”

“…125 Nm 3 CH 4 per million Mg landfilled waste). As the California dataset included open/closed sites, small/large sites, old/new sites in all climatic regions of the state, this simple relationship contradicts the HH-1 first order kinetic equation where biogas generation peaks in the year of disposal with an exponential decline thereafter (See additional discussion in Spokas et al, 2015;Bogner et al, 2016). Moreover, under current regulations, U.S. landfills are required to perform routine quarterly monitoring for CH 4 concentrations near the ground surface followed by remediation and re-testing where elevated concentrations are observed, thus periodically checking for CH 4 leakages.…”