Large and seasonally varying biospheric CO₂ fluxes in the Los Angeles megacity revealed by atmospheric radiocarbon

Measurements of Δ¹⁴C and CO₂ can cleanly separate biogenic and fossil contributions to CO₂ enhancements above background. Our measurements of these tracers in air around Los Angeles in 2015 reveal high values of fossil CO₂ and a significant and seasonally varying contribution of CO₂ from the urban biosphere. The biogenic CO₂ is composed of sources such as biofuel combustion and human metabolism and an urban biospheric component likely originating from urban vegetation, including turf and trees. The urban biospheric component is a source in winter and a sink in summer, with an estimated amplitude of 4.3 parts per million (ppm), equivalent to 33% of the observed annual mean fossil fuel contribution of 13 ppm. While the timing of the net carbon sink is out of phase with wintertime rainfall and the sink seasonality of Southern California Mediterranean ecosystems (which show maximum uptake in spring), it is in phase with the seasonal cycle of urban water usage, suggesting that irrigated urban vegetation drives the biospheric signal we observe. Although 2015 was very dry, the biospheric seasonality we observe is similar to the 2006–2015 mean derived from an independent Δ¹⁴C record in the Los Angeles area, indicating that 2015 biospheric exchange was not highly anomalous. The presence of a large and seasonally varying biospheric signal even in the relatively dry climate of Los Angeles implies that atmospheric estimates of fossil fuel–CO₂ emissions in other, potentially wetter, urban areas will be biased in the absence of reliable methods to separate fossil and biogenic CO₂

Large and seasonally varying biospheric CO₂ fluxes in the Los Angeles megacity revealed by atmospheric radiocarbon

Measurements of Δ¹⁴C and CO₂ can cleanly separate biogenic and fossil contributions to CO₂ enhancements above background. Our measurements of these tracers in air around Los Angeles in 2015 reveal high values of fossil CO₂ and a significant and seasonally varying contribution of CO₂ from the urban biosphere. The biogenic CO₂ is composed of sources such as biofuel combustion and human metabolism and an urban biospheric component likely originating from urban vegetation, including turf and trees. The urban biospheric component is a source in winter and a sink in summer, with an estimated amplitude of 4.3 parts per million (ppm), equivalent to 33% of the observed annual mean fossil fuel contribution of 13 ppm. While the timing of the net carbon sink is out of phase with wintertime rainfall and the sink seasonality of Southern California Mediterranean ecosystems (which show maximum uptake in spring), it is in phase with the seasonal cycle of urban water usage, suggesting that irrigated urban vegetation drives the biospheric signal we observe. Although 2015 was very dry, the biospheric seasonality we observe is similar to the 2006–2015 mean derived from an independent Δ¹⁴C record in the Los Angeles area, indicating that 2015 biospheric exchange was not highly anomalous. The presence of a large and seasonally varying biospheric signal even in the relatively dry climate of Los Angeles implies that atmospheric estimates of fossil fuel–CO₂ emissions in other, potentially wetter, urban areas will be biased in the absence of reliable methods to separate fossil and biogenic CO₂

Preindustrial atmospheric ethane levels inferred from polar ice cores: A constraint on the geologic sources of atmospheric ethane and methane

Ethane levels were measured in air extracted from Greenland and Antarctic ice cores ranging in age from 994 to 1918 Common Era (C.E.) There is good temporal overlap between the two data sets from 1600 to 1750 C.E. with ethane levels stable at 397 ± 28 parts per trillion (ppt) (±2 standard error (s.e.)) over Greenland and 103 ± 9 ppt over Antarctica. The observed north/south interpolar ratio of ethane (3.9 ± 0.1, 1σ) implies considerably more ethane emissions in the Northern Hemisphere than in the Southern Hemisphere, suggesting geologic ethane sources contribute significantly to the preindustrial ethane budget. Box model simulations based on these data constrain the global geologic emissions of ethane to 2.2-3.5 Tg yr-1 and biomass burning emissions to 1.2-2.5 Tg yr-1 during the preindustrial era. The results suggest biomass burning emissions likely increased since the preindustrial period. Biomass burning and geologic outgassing are also sources of atmospheric methane. The results place constraints on preindustrial methane emissions from these sources

Spatio-temporally Resolved Methane Fluxes From the Los Angeles Megacity

We combine sustained observations from a network of atmospheric monitoring stations with inverse modeling to uniquely obtain spatiotemporal (3‐km, 4‐day) estimates of methane emissions from the Los Angeles megacity and the broader South Coast Air Basin for 2015–2016. Our inversions use customized and validated high‐fidelity meteorological output from Weather Research Forecasting and Stochastic Time‐Inverted Lagrangian model for South Coast Air Basin and innovatively employ a model resolution matrix‐based metric to disentangle the spatiotemporal information content of observations as manifested through estimated fluxes. We partially track and constrain fluxes from the Aliso Canyon natural gas leak and detect closure of the Puente Hills landfill, with no prior information. Our annually aggregated fluxes and their uncertainty excluding the Aliso Canyon leak period lie within the uncertainty bounds of the fluxes reported by the previous studies. Spatially, major sources of CH_4 emissions in the basin were correlated with CH_4‐emitting infrastructure. Temporally, our findings show large seasonal variations in CH_4 fluxes with significantly higher fluxes in winter in comparison to summer months, which is consistent with natural gas demand and anticorrelated with air temperature. Overall, this is the first study that utilizes inversions to detect both enhancement (Aliso Canyon leak) and reduction (Puente Hills) in CH_4 fluxes due to the unintended events and policy decisions and thereby demonstrates the utility of inverse modeling for identifying variations in fluxes at fine spatiotemporal resolution

Spatio-temporally Resolved Methane Fluxes From the Los Angeles Megacity

We combine sustained observations from a network of atmospheric monitoring stations with inverse modeling to uniquely obtain spatiotemporal (3‐km, 4‐day) estimates of methane emissions from the Los Angeles megacity and the broader South Coast Air Basin for 2015–2016. Our inversions use customized and validated high‐fidelity meteorological output from Weather Research Forecasting and Stochastic Time‐Inverted Lagrangian model for South Coast Air Basin and innovatively employ a model resolution matrix‐based metric to disentangle the spatiotemporal information content of observations as manifested through estimated fluxes. We partially track and constrain fluxes from the Aliso Canyon natural gas leak and detect closure of the Puente Hills landfill, with no prior information. Our annually aggregated fluxes and their uncertainty excluding the Aliso Canyon leak period lie within the uncertainty bounds of the fluxes reported by the previous studies. Spatially, major sources of CH_4 emissions in the basin were correlated with CH_4‐emitting infrastructure. Temporally, our findings show large seasonal variations in CH_4 fluxes with significantly higher fluxes in winter in comparison to summer months, which is consistent with natural gas demand and anticorrelated with air temperature. Overall, this is the first study that utilizes inversions to detect both enhancement (Aliso Canyon leak) and reduction (Puente Hills) in CH_4 fluxes due to the unintended events and policy decisions and thereby demonstrates the utility of inverse modeling for identifying variations in fluxes at fine spatiotemporal resolution

Carbon dioxide and methane measurements from the Los Angeles Megacity Carbon Project – Part 1: calibration, urban enhancements, and uncertainty estimates

We report continuous surface observations of carbon dioxide (CO_2) and methane (CH_4) from the Los Angeles (LA) Megacity Carbon Project during 2015. We devised a calibration strategy, methods for selection of background air masses, calculation of urban enhancements, and a detailed algorithm for estimating uncertainties in urban-scale CO_2 and CH_4 measurements. These methods are essential for understanding carbon fluxes from the LA megacity and other complex urban environments globally. We estimate background mole fractions entering LA using observations from four extra-urban sites including two marine sites located south of LA in La Jolla (LJO) and offshore on San Clemente Island (SCI), one continental site located in Victorville (VIC), in the high desert northeast of LA, and one continental/mid-troposphere site located on Mount Wilson (MWO) in the San Gabriel Mountains. We find that a local marine background can be established to within  ∼  1 ppm CO_2 and  ∼  10 ppb CH_4 using these local measurement sites. Overall, atmospheric carbon dioxide and methane levels are highly variable across Los Angeles. Urban and suburban sites show moderate to large CO_2 and CH_4 enhancements relative to a marine background estimate. The USC (University of Southern California) site near downtown LA exhibits median hourly enhancements of  ∼  20 ppm CO_2 and  ∼  150 ppb CH_4 during 2015 as well as  ∼  15 ppm CO_2 and  ∼  80 ppb CH_4 during mid-afternoon hours (12:00–16:00 LT, local time), which is the typical period of focus for flux inversions. The estimated measurement uncertainty is typically better than 0.1 ppm CO_2 and 1 ppb CH_4 based on the repeated standard gas measurements from the LA sites during the last 2 years, similar to Andrews et al. (2014). The largest component of the measurement uncertainty is due to the single-point calibration method; however, the uncertainty in the background mole fraction is much larger than the measurement uncertainty. The background uncertainty for the marine background estimate is  ∼  10 and  ∼  15 % of the median mid-afternoon enhancement near downtown LA for CO_2 and CH_4, respectively. Overall, analytical and background uncertainties are small relative to the local CO_2 and CH_4 enhancements; however, our results suggest that reducing the uncertainty to less than 5 % of the median mid-afternoon enhancement will require detailed assessment of the impact of meteorology on background conditions

Recent decreases in fossil-fuel emissions of ethane and methane derived from firn air

Methane and ethane are the most abundant hydrocarbons in the atmosphere and they affect both atmospheric chemistry and climate. Both gases are emitted from fossil fuels and biomass burning, whereas methane (CH(4)) alone has large sources from wetlands, agriculture, landfills and waste water. Here we use measurements in firn (perennial snowpack) air from Greenland and Antarctica to reconstruct the atmospheric variability of ethane (C(2)H(6)) during the twentieth century. Ethane levels rose from early in the century until the 1980s, when the trend reversed, with a period of decline over the next 20 years. We find that this variability was primarily driven by changes in ethane emissions from fossil fuels; these emissions peaked in the 1960s and 1970s at 14-16 teragrams per year (1 Tg = 10(12) g) and dropped to 8-10 Tg  yr(-1) by the turn of the century. The reduction in fossil-fuel sources is probably related to changes in light hydrocarbon emissions associated with petroleum production and use. The ethane-based fossil-fuel emission history is strikingly different from bottom-up estimates of methane emissions from fossil-fuel use, and implies that the fossil-fuel source of methane started to decline in the 1980s and probably caused the late twentieth century slow-down in the growth rate of atmospheric methane

Methyl chloride variability in the Taylor Dome ice core during the Holocene

Methyl chloride (CH3Cl) is a naturally occurring, ozone-depleting trace gas and one of the most abundant chlorinated compounds in the atmosphere. CH3Cl was measured in air from the Taylor Dome ice core in East Antarctica to reconstruct an atmospheric record for the Holocene (11–0 kyr B.P.) and part of the last glacial period (50–30 kyr B.P.). CH3Cl variability throughout the Holocene is strikingly similar to that of atmospheric methane (CH4), with higher levels in the early and late Holocene, and a well-defined minimum during mid-Holocene. The sources and sinks of atmospheric CH3Cl and CH4 are located primarily in the tropics, and variations in their atmospheric levels likely reflect changes in tropical conditions. CH3Cl also appears to correlate with atmospheric CH4 during the last glacial period (50–30 kyr B.P.), although the temporal resolution of sampling is limited. The Taylor Dome data provide information about the range of natural variability of atmospheric CH3Cl and place a new constraint on the causes of past CH4variability

Methyl chloride variability in the Taylor Dome ice core during the Holocene

Methyl chloride (CH3Cl) is a naturally occurring, ozone-depleting trace gas and one of the most abundant chlorinated compounds in the atmosphere. CH3Cl was measured in air from the Taylor Dome ice core in East Antarctica to reconstruct an atmospheric record for the Holocene (11–0 kyr B.P.) and part of the last glacial period (50–30 kyr B.P.). CH3Cl variability throughout the Holocene is strikingly similar to that of atmospheric methane (CH4), with higher levels in the early and late Holocene, and a well-defined minimum during mid-Holocene. The sources and sinks of atmospheric CH3Cl and CH4 are located primarily in the tropics, and variations in their atmospheric levels likely reflect changes in tropical conditions. CH3Cl also appears to correlate with atmospheric CH4 during the last glacial period (50–30 kyr B.P.), although the temporal resolution of sampling is limited. The Taylor Dome data provide information about the range of natural variability of atmospheric CH3Cl and place a new constraint on the causes of past CH4variability

Methyl chloride in a deep ice core from Siple Dome, Antarctica

Methyl chloride (CH3Cl) is a naturally occurring ozone-depleting substance and a significant component of the atmospheric chlorine burden. In this study CH3Cl was analyzed in air bubbles from the West Antarctic Siple Dome deep ice core with gas ages ranging from about 65 kyr BP to the Late Holocene. CH3Cl levels were below the modern Antarctic atmospheric level of 530 ppt in glacial ice (456 ± 46 ppt, 33–65 kyr BP) and above it during the early Holocene (650–700 ppt, 10–11 kyr BP). For most of the Holocene, CH3Cl levels were 500–550 ppt, with good agreement between CH3Cl levels in this core and in the Dome Fuji ice core (Saito et al., 2007). Several late Holocene ice core samples (<2 kyr BP), show evidence of enrichment in CH3Cl relative to South Pole ice core samples of overlapping gas age. The Siple Dome record suggests that CH3Cl levels in the glacial Southern Hemisphere atmosphere were about 16% lower than those during the mid-late Holocene