An Estimate of Natural Gas Methane Emissions from California Homes

We
estimate postmeter
methane (CH4) emissions from California’s
residential natural gas (NG) system using measurements and analysis
from a sample of homes and appliances. Quiescent whole-house emissions
(i.e., pipe leaks and pilot lights) were measured using a mass balance
method in 75 California homes, while CH4 to CO2 emission ratios were measured for steady operation of individual
combustion appliances and, separately, for transient operation of
three tankless water heaters. Measured quiescent whole-house emissions
are typically <1 g CH4/day, though they exhibit long-tailed
gamma distributions containing values >10 g CH4/day.
Most
operating appliances yield undetectable CH4 to CO2 enhancements in steady operation (<0.01% of gas consumed), though
storage water heaters and stovetops exhibit long-tailed gamma distributions
containing high values (∼1–3% of gas consumed), and
transients are observed for the tankless heaters. Extrapolating results
to the state-level using Bayesian Markov chain Monte Carlo sampling
combined with California housing statistics and gas use information
suggests quiescent house leakage of 23.4 (13.7–45.6, at 95%
confidence) Gg CH4, with pilot lights contributing ∼30%.
Emissions from steady operation of appliances and their pilots are
13.3 (6.6–37.1) Gg CH4/yr, an order of magnitude
larger than current inventory estimates, with transients likely increasing
appliance emissions further. Together, emissions from residential
NG are 35.7 (21.7–64.0) Gg CH4/yr, equivalent to
∼15% of California’s NG CH4 emissions, suggesting
leak repair, improvement of combustion appliances, and adoption of
nonfossil energy heating sources can help California meet its 2050
climate goals.