Observing Anthropogenic and Biogenic CO2 Emissions in Los Angeles Using a Dense Sensor Network

Urban areas are major contributors to greenhouse gas emissions, necessitating effective monitoring systems to evaluate mitigation strategies. A dense sensor network, such as the Berkeley Environmental Air-quality & CO2 Observation Network (BEACO2N), offers a unique opportunity to monitor urban emissions at high spatial resolution. Here, we describe a simple approach to quantifying urban emissions with sufficient precision to constrain seasonal and annual trends. Measurements from 12 BEACO2N sites in Los Angeles (called the USC Carbon Census) are analyzed within a box model framework. By combining CO2 and CO observations, we partition total CO2 emissions into fossil fuel and biogenic emissions. We infer temporal changes in biogenic emissions that correspond to the MODIS enhanced vegetation index (EVI) and show that net biogenic exchange can consume up to 60% of fossil fuel emissions in the growing season during daytime hours. While we use the first year of observations to describe seasonal variation, we demonstrate the feasibility of this approach to constrain annual and longer trends.

Impact of urban trees on carbon dioxide exchange: Mechanistic pathways, environmental controls, and feedback

The increase of carbon dioxide (CO2) concentration in the atmosphere is held responsible for global climate changes. To meet the objective of achieving carbon neutrality and keeping global warming in check, many cities, as hotspots of CO2 emissions, have been promoting the use of urban greenery, urban trees in particular, to mitigate carbon emissions from the built environment. However, there remain large uncertainty and divergence of the potential of urban trees for carbon mitigation, with the underlying mechanisms poorly understood. In this study, we conducted a comprehensive survey of the biophysical functions, their environmental controls, and possible heat-carbon-water feedback that mechanistically govern the CO2 exchange processes of trees in the built environment. This review helps to clarify some disparities and enables us to gain clearer insights into the participatory role of urban trees in the dynamics of CO2 exchange. In addition, we proposed a few guidelines for urban planning and management strategies of using trees to promote the sustainability of urban ecosystems.

Contrasting Summertime Trends in Vehicle Combustion Efficiency in Los Angeles, CA and Salt Lake City, UT

Policy interventions and technological advances are mitigating emissions of air pollutants from motor vehicles. As a result, vehicle fleets are expected to progressively combust fuel more efficiently, with a declining ratio of carbon monoxide to carbon dioxide (CO/CO2) in their emissions. We assess trends in traffic combustion efficiency in Los Angeles (LA) and Salt Lake City (SLC) by measuring changes in summertime on-road CO/CO2 between 2013 and 2021 using mobile observations. Our data show a reduction in CO/CO2 in LA, indicating an improvement in combustion efficiency that likely resulted from stringent regulation of CO emissions. In contrast, we observed an increase in CO/CO2 values in SLC. While slower progress in SLC compared to LA may be partially due to a later adoption of vehicle emission regulations in Utah compared to California, differing driving conditions and fleet composition may also be playing a role. This is evidenced by increased CO/CO2 in LA during the COVID-19 pandemic, which led to faster driving speeds and changes to the fleet composition. Our results demonstrate the success of California's CO-reducing policy interventions and illustrate the impacts of traffic characteristics on vehicle combustion efficiency and air pollutant emissions.

Stable isotope analysis of atmospheric CO2 using a Gasbench II-Cold Trap-IRMS setting

RATIONALE

The measurement of the stable carbon and oxygen isotope ratio of (atmospheric) carbon dioxide (CO2 ) is a useful technique for the investigation and identification of the sources and sinks of the most abundant greenhouse gases by far. For this reason, we are presenting a measuring system here that enables a wide range of users to carry out stable isotope analysis of atmospheric CO2 using off-the-bench hardware and software.

METHODS

The fully automated system uses cryogenic and gas chromatographic separation to analyse CO2 from 12-mL whole air samples and consists of an autosampler, a Gasbench II (GB), a downstream cryo trap and a continuous flow gas interface feeding into a sector field mass spectrometer (GC Pal/GB/Cold Trap/ConFlo IV/DeltaV Plus). The evaluation of the system performance was based on the analysis of samples prepared from eight CO2 sources (four CO2 reference gases and four artificial air tanks).

RESULTS

The overall measurement uncertainty (averaged single standard deviation (1σ) of measurement replicates from each CO2 source) in the determination of the carbon and oxygen isotope ratio was 0.04‰ and 0.09‰ (n = 24). Furthermore, we were able to show that the measurement data also allowed for the quantification of the CO2 mole fraction, with a precision of 1.2 μmol mol-1 in the analysis range of 400-500 μmol mol-1 .

CONCLUSIONS

Our protocol provides a detailed description of the measurement set-up and the analysis procedure, how raw data should be evaluated and gives recommendations for sample preparation and sampling to enable a fully automated whole air sample analysis. The quantification limit of CO2 mole fractions and measurement precision for carbon and oxygen isotope ratios of CO2 should meet the requirements of a wide range of users.

Circular Carbon Economy (CCE): A Way to Invest CO2 and Protect the Environment, a Review

Increased levels of carbon dioxide have revolutionised the Earth; higher temperatures, melting icecaps, and flooding are now more prevalent. Fortunately, renewable energy mitigates this problem by making up 20% of human energy needs. However, from a “green environment” perspective, can carbon dioxide emissions in the atmosphere be reduced and eliminated? The carbon economic circle is an ideal solution to this problem, as it enables us to store, use, and remove carbon dioxide. This research introduces the circular carbon economy (CCE) and addresses its economic importance. Additionally, the paper discusses carbon capture and storage (CCS), and the utilisation of CO2. Furthermore, it explains current technologies and their future applications on environmental impact, CO2 capture, utilisation, and storage (CCUS). Various opinions on the best way to achieve zero carbon emissions and on CO2 applications and their economic impact are also discussed. The circular carbon economy can be achieved through a highly transparent global administration that is supportive of advanced technologies that contribute to the efficient utilisation of energy sources. This global administration must also provide facilities to modernise and develop factories and power stations, based on emission-reducing technologies. Monitoring emissions in countries through a global monitoring network system, based on actual field measurements, linked to a worldwide database allows all stakeholders to track the change in greenhouse gas emissions. The process of sequestering carbon dioxide in the ocean is affected by the support for technologies and industries that adopt the principle of carbon recycling in order to maintain the balance. This includes supporting initiatives that contribute to increasing vegetation cover and preserving oceans from pollutants, especially chemicals and radioactive pollutants, which will undoubtedly affect the process of sequestering carbon dioxide in the oceans, and this will contribute significantly to maintaining carbon dioxide at acceptable levels.

Fossil fuel CO2 traced by radiocarbon in fifteen Chinese cities

China is an important fossil fuel CO₂ (CO_2ff) emitter and the international community is thus concerned with quantifying reductions in Chinese carbon emissions in the recent past. Compared to traditional statistical method, radiocarbon (¹⁴C) offers a different approach to quantify atmospheric CO₂ derived from fossil fuel emissions. Here, we carry out a multi-year (2011-2016) CO_2ff tracing by ¹⁴C in Xi'an, and a three-year (2014-2016) CO_2ff tracing in 15 Chinese cities. The Xi'an results show that average CO_2ff concentrations fell 35.9 ± 6.6% from 2014- 2016, compared to 2011-2013, and the timing of this decrease coincides with the implementation of nationwide carbon reduction measures in China, known as the Action Plan on Prevention and Control of Air Pollution. A WRF-Chem forward modeling simulation reveals that the CO_2ff in Xi'an is mainly derived from local sources, and a source apportionment combined stable-carbon isotope showed that the CO_2ff in this city is dominated by coal combustion (72.6 ± 10.4%). Strong CO_2ff differences are found between January and July in most Chinese cities. High CO_2ff concentrations often correspond to severe haze episodes and there are generally positive correlations between CO_2ff and fine particulate (PM_2.5) concentrations. Our study provides scientific data to understand the effects of CO_2ff reduction strategies in China that can be applied to other countries as well.

Synthesis of Urban CO2 Emission Estimates from Multiple Methods from the Indianapolis Flux Project (INFLUX)

Urban areas contribute approximately three-quarters of fossil fuel derived CO₂ emissions, and many cities have enacted emissions mitigation plans. Evaluation of the effectiveness of mitigation efforts will require measurement of both the emission rate and its change over space and time. The relative performance of different emission estimation methods is a critical requirement to support mitigation efforts. Here we compare results of CO₂ emissions estimation methods including an inventory-based method and two different top-down atmospheric measurement approaches implemented for the Indianapolis, Indiana, U.S.A. urban area in winter. By accounting for differences in spatial and temporal coverage, as well as trace gas species measured, we find agreement among the wintertime whole-city fossil fuel CO₂ emission rate estimates to within 7%. This finding represents a major improvement over previous comparisons of urban-scale emissions, making urban CO₂ flux estimates from this study consistent with local and global emission mitigation strategy needs. The complementary application of multiple scientifically driven emissions quantification methods enables and establishes this high level of confidence and demonstrates the strength of the joint implementation of rigorous inventory and atmospheric emissions monitoring approaches.

Atmospheric fossil fuel CO2 traced by 14CO2 and air quality index pollutant observations in Beijing and Xiamen, China

Radiocarbon (¹⁴C) is the most accurate tracer available for quantifying atmospheric CO₂ derived from fossil fuel (CO_2ff), but it is expensive and time-consuming to measure. Here, we used common hourly Air Quality Index (AQI) pollutants (AQI, PM_2.5, PM₁₀, and CO) to indirectly trace diurnal CO_2ff variations during certain days at the urban sites in Beijing and Xiamen, China, based on linear relationships between AQI pollutants and CO_2ff traced by ¹⁴C ([Formula: see text]) for semimonthly samples obtained in 2014. We validated these indirectly traced CO_2ff (CO_2ff-in) concentrations against [Formula: see text] concentrations traced by simultaneous diurnal ¹⁴CO₂ observations. Significant (p < 0.05) strong correlations were observed between each of the separate AQI pollutants and [Formula: see text] for the semimonthly samples. Diurnal variations in CO_2ff traced by each of the AQI pollutants generally showed similar trends to those of [Formula: see text], with high agreement at the sampling site in Beijing and relatively poor agreement at the sampling site in Xiamen. AQI pollutant tracers showed high normalized root-mean-square (NRMS) errors for the summer diurnal samples due to low [Formula: see text] concentrations. After the removal of these summer samples, the NRMS errors for AQI pollutant tracers were in the range of 31.6-64.2%. CO generally showed a high agreement and low NRMS errors among these indirect tracers. Based on these linear relationships, monthly CO_2ff averages at the sampling sites in Beijing and Xiamen were traced using CO concentration as a tracer. The monthly CO_2ff averages at the Beijing site showed a shallow U-type variation. These results indicate that CO can be used to trace CO_2ff variations in Chinese cities with CO_2ff concentrations above 5 ppm.

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 (CO₂) and methane (CH₄) 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 CO₂ and CH₄ 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 CO₂ and ~10 ppb CH₄ 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 CO₂ and CH₄ enhancements relative to a marine background estimate. The USC (University of Southern California) site near downtown LA exhibits median hourly enhancements of ~20 ppm CO₂ and ~150 ppb CH₄ during 2015 as well as ~15 ppm CO₂ and ~80 ppb CH₄ 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 CO₂ and 1 ppb CH₄ 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 CO₂ and CH₄, respectively. Overall, analytical and background uncertainties are small relative to the local CO₂ and CH₄ 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.

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 (CO2) and methane (CH4) 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 CO2 and CH4 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 CO2 and ~10 ppb CH4 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 CO2 and CH4 enhancements relative to a marine background estimate. The USC (University of Southern California) site near downtown LA exhibits median hourly enhancements of ~20 ppm CO2 and ~150 ppb CH4 during 2015 as well as ~15 ppm CO2 and ~80 ppb CH4 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 CO2 and 1 ppb CH4 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 CO2 and CH4, respectively. Overall, analytical and background uncertainties are small relative to the local CO2 and CH4 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.

High-resolution atmospheric inversion of urban CO2 emissions during the dormant season of the Indianapolis Flux Experiment (INFLUX)

Based on a uniquely dense network of surface towers measuring continuously the atmospheric concentrations of greenhouse gases (GHGs), we developed the first comprehensive monitoring systems of CO₂ emissions at high resolution over the city of Indianapolis. The urban inversion evaluated over the 2012-2013 dormant season showed a statistically significant increase of about 20% (from 4.5 to 5.7 MtC ± 0.23 MtC) compared to the Hestia CO₂ emission estimate, a state-of-the-art building-level emission product. Spatial structures in prior emission errors, mostly undetermined, appeared to affect the spatial pattern in the inverse solution and the total carbon budget over the entire area by up to 15%, while the inverse solution remains fairly insensitive to the CO₂ boundary inflow and to the different prior emissions (i.e., ODIAC). Preceding the surface emission optimization, we improved the atmospheric simulations using a meteorological data assimilation system also informing our Bayesian inversion system through updated observations error variances. Finally, we estimated the uncertainties associated with undetermined parameters using an ensemble of inversions. The total CO₂ emissions based on the ensemble mean and quartiles (5.26-5.91 MtC) were statistically different compared to the prior total emissions (4.1 to 4.5 MtC). Considering the relatively small sensitivity to the different parameters, we conclude that atmospheric inversions are potentially able to constrain the carbon budget of the city, assuming sufficient data to measure the inflow of GHG over the city, but additional information on prior emission error structures are required to determine the spatial structures of urban emissions at high resolution.

Impact of direct greenhouse gas emissions on the carbon footprint of water reclamation processes employing nitrification-denitrification

Water reclamation has the potential to reduce water supply demands from aquifers and more energy-intensive water production methods (e.g., seawater desalination). However, water reclamation via biological nitrification-denitrification is also associated with the direct emission of the greenhouse gases (GHGs) CO₂, N₂O, and CH₄. We quantified these direct emissions from the nitrification-denitrification reactors of a water reclamation plant in Southern California, and measured the (14)C content of the CO₂ to distinguish between short- and long-lived carbon. The total emissions were 1.5 (±0.2) g-fossil CO₂ m(-3) of wastewater treated, 0.5 (±0.1) g-CO₂-eq of CH₄ m(-3), and 1.8 (±0.5) g-CO₂-eq of N₂O m(-3), for a total of 3.9 (±0.5) g-CO₂-eqm(-3). This demonstrated that water reclamation can be a source of GHGs from long lived carbon, and thus a candidate for GHG reduction credit. From the (14)C measurements, we found that between 11.4% and 15.1% of the CO₂ directly emitted was derived from fossil sources, which challenges past assumptions that the direct CO₂ emissions from water reclamation contain only modern carbon. A comparison of our direct emission measurements with estimates of indirect emissions from several water production methods, however, showed that the direct emissions from water reclamation constitute only a small fraction of the plant's total GHG footprint.