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Sources of methane (CH4) and nitrous oxide (N2O) were investigatedusing measurements from a site in southeast Bakersfield as part of the CalNex(California at the Nexus of Air Quality and Climate Change) experiment frommid-May to the end of June 2010. Typical daily minimum mixing ratios of CH4and N2O were higher than daily minima that were simultaneously observedat a mid-oceanic background station (NOAA, Mauna Loa) by approximately70 ppb and 0.5 ppb, respectively. Substantial enhancements of CH4 andN2O (hourly averages > 500 and > 7 ppb,respectively) were routinely observed, suggesting the presence of largeregional sources. Collocated measurements of carbon monoxide (CO) and a rangeof volatile organic compounds (VOCs) (e.g., straight-chain and branchedalkanes, 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 sourcesto observed enhancements of CH4 and N2O. The PMF technique provided a "top-down" deconstruction of ambientgas-phase observations into broad source categories, yielding a seven-factorsolution. We identified these emission source factors as follows: evaporativeand fugitive; motor vehicles; livestock and dairy; agricultural and soilmanagement; daytime light and temperature driven; non-vehicular urban; andnighttime terpene biogenics and anthropogenics. The dairy and livestockfactor accounted for the majority of the CH4 (70–90 %) enhancementsduring the duration of experiments. The dairy and livestock factor was alsoa principal contributor to the daily enhancements of N2O (60–70 %).Agriculture and soil management accounted for ~ 20–25 % ofN2O enhancements over a 24 h cycle, which is not surprising giventhat organic and synthetic fertilizers are known to be a major source ofN2O. The N2O attribution to the agriculture and soil managementfactor had a high uncertainty in the conducted bootstrapping analysis. Thisis most likely due to an asynchronous pattern of soil-mediated N2Oemissions from fertilizer usage and collocated biogenic emissions from cropsfrom the surrounding agricultural operations that is difficult to apportionstatistically when using PMF. The evaporative/fugitive source profile,which resembled a mix of petroleum operation and non-tailpipe evaporativegasoline sources, did not include a PMF resolved-CH4 contribution thatwas significant (< 2 %) compared to the uncertainty in thelivestock-associated CH4 emissions. The uncertainty of the CH4estimates in this source factor, derived from the bootstrapping analysis, isconsistent with the ~ 3 % contribution of fugitive oil andgas emissions to the statewide CH4 inventory. The vehicle emissionsource factor broadly matched VOC profiles of on-road exhaust sources. Thissource factor had no statistically significant detected contribution to theN2O signals (confidence interval of 3 % of livestock N2Oenhancements) and negligible CH4 (confidence interval of 4 % oflivestock CH4 enhancements) in the presence of a dominant dairy andlivestock factor. The CalNex PMF study provides a measurement-basedassessment of the state CH4 and N2O inventories for the southernSan Joaquin Valley (SJV). The state inventory attributes ~ 18 % oftotal N2O emissions to the transportation sector. Our PMF analysisdirectly contradicts the state inventory and demonstrates there were nodiscernible N2O emissions from the transportation sector in thesouthern SJV region

Source apportionment of methane and nitrous oxide in California's San Joaquin Valley at CalNex 2010 via positive matrix factorization