Abstract
Abstract. Sources of methane (CH4) and nitrous oxide (N2O) were investigated using measurements from a site in southeast Bakersfield as part of the CalNex (California at the Nexus of Air Quality and Climate Change) experiment from mid-May to the end of June 2010. Typical daily minimum mixing ratios of CH4 and N2O were higher than daily minima that were simultaneously observed at a mid-oceanic background station (NOAA, Mauna Loa) by approximately 70 ppb and 0.5 ppb, respectively. Substantial enhancements of CH4 and N2O (hourly averages > 500 and > 7 ppb, respectively) were routinely observed, suggesting the presence of large regional sources. Collocated measurements of carbon monoxide (CO) and a range of volatile organic compounds (VOCs) (e.g., straight-chain and branched alkanes, cycloalkanes, chlorinated alkanes, aromatics, alcohols, isoprene, terpenes and ketones) were used with a positive matrix factorization (PMF) source apportionment method to estimate the contribution of regional sources to observed enhancements of CH4 and N2O. The PMF technique provided a "top-down" deconstruction of ambient gas-phase observations into broad source categories, yielding a seven-factor solution. We identified these emission source factors as follows: evaporative and fugitive; motor vehicles; livestock and dairy; agricultural and soil management; daytime light and temperature driven; non-vehicular urban; and nighttime terpene biogenics and anthropogenics. The dairy and livestock factor accounted for the majority of the CH4 (70–90 %) enhancements during the duration of experiments. The dairy and livestock factor was also a principal contributor to the daily enhancements of N2O (60–70 %). Agriculture and soil management accounted for ~ 20–25 % of N2O enhancements over a 24 h cycle, which is not surprising given that organic and synthetic fertilizers are known to be a major source of N2O. The N2O attribution to the agriculture and soil management factor had a high uncertainty in the conducted bootstrapping analysis. This is most likely due to an asynchronous pattern of soil-mediated N2O emissions from fertilizer usage and collocated biogenic emissions from crops from the surrounding agricultural operations that is difficult to apportion statistically when using PMF. The evaporative/fugitive source profile, which resembled a mix of petroleum operation and non-tailpipe evaporative gasoline sources, did not include a PMF resolved-CH4 contribution that was significant (< 2 %) compared to the uncertainty in the livestock-associated CH4 emissions. The uncertainty of the CH4 estimates in this source factor, derived from the bootstrapping analysis, is consistent with the ~ 3 % contribution of fugitive oil and gas emissions to the statewide CH4 inventory. The vehicle emission source factor broadly matched VOC profiles of on-road exhaust sources. This source factor had no statistically significant detected contribution to the N2O signals (confidence interval of 3 % of livestock N2O enhancements) and negligible CH4 (confidence interval of 4 % of livestock CH4 enhancements) in the presence of a dominant dairy and livestock factor. The CalNex PMF study provides a measurement-based assessment of the state CH4 and N2O inventories for the southern San Joaquin Valley (SJV). The state inventory attributes ~ 18 % of total N2O emissions to the transportation sector. Our PMF analysis directly contradicts the state inventory and demonstrates there were no discernible N2O emissions from the transportation sector in the southern SJV region.
Highlights
Methane (CH4) and nitrous oxide (N2O) are the two most significant non-CO2 greenhouse gases (GHGs), contributing about 50 % and 17 % of the total direct non-CO2 GHG radiative forcing (∼ 1 W m−2), respectively (Fig. SPM.5; IPCC, 2013)
This study provides CH4 and N2O source attribution during a 6-week study involving a complete suite of continuous GHG and volatile organic compounds (VOCs) tracer measurements during the CalNex 2010 campaign in Bakersfield, which is located in the southern part of the Central Valley (May to June 2010)
4 Results and discussion In Bakersfield, there are a multitude of pollutant sources, ranging from local to regional, from biogenic to anthropogenic, and from primary to secondary
Summary
Methane (CH4) and nitrous oxide (N2O) are the two most significant non-CO2 greenhouse gases (GHGs), contributing about 50 % and 17 % of the total direct non-CO2 GHG radiative forcing (∼ 1 W m−2), respectively (Fig. SPM.; IPCC, 2013). CH4, with a lifetime of ∼ 10 years and global warming potential (GWP) of 34 on a 100-year basis, accounting for climate–carbon feedbacks (Table 8.7, Myhre et al, 2013; Montzka et al, 2011), is emitted by both anthropogenic and natural sources (e.g., wetlands, oceans, termites, etc.). Anthropogenic global CH4 emissions are due to agricultural activities (enteric fermentation in livestock, manure management and rice; McMillan et al, 2007; Owen and Silver, 2014), the energy sector (oil and gas operations and coal mining), waste management (landfills and wastewater treatment), and biomass burning (some of which is natural) (Smith et al, 2007; NRC, 2010). Management of livestock and animal waste is another important biological source of N2O, while industrial processes including fossil fuel combustion have been estimated to account for ∼ 10 % of total global anthropogenic N2O emissions (Denman et al, 2007)
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