Control Technology-Driven Changes to In-Use Heavy-Duty Diesel Truck Emissions of Nitrogenous Species and Related Environmental Impacts

Unraveling Reactive Nitrogen Emissions in Heavy-Duty Diesel Vehicles across Evolving Standards and Cheating Tactics

Reactive nitrogen (Nr) emissions significantly affect air quality and the nitrogen cycle in ecosystems. Heavy-duty diesel vehicles (HDDVs), as major sources of these emissions, exhibit complex emission characteristics because of the combined effects of different driving conditions and aftertreatment technologies. This study first investigated the emission factors (EFs) of Nr species, including NO, NO2, HONO, N2O, and NH3, from HDDVs under different emission standards (China IV/V/VI) and cheating strategies, with a particular focus on the impact of selective catalytic reduction (SCR) systems. Vehicles employing water injection cheating present NO, NO2, and HONO EFs that are consistent with the China III standards, significantly undermining the effectiveness of Nr emission control. The evolution of SCR technology in China IV, V, and VI standards has generally led to substantial reductions in NO, NO2, and HONO emissions, yet the integration of ammonia slip catalysts (ASC) systems in China VI vehicles presents new challenges. While ASCs have successfully reduced NH3 slip to an average of 17 ± 12 mg/km, they have also caused a 6-13-fold increase in N2O emissions compared with those of China IV and V vehicles, reaching levels of 205 ± 85 mg/km. Additionally, China VI vehicles exhibit a marked increase in the HONO/NOx ratio, which increases from 0.9% in China V to 4.6%. These increases are attributed to high-temperature oxidation of NH3 within the ASC catalyst, leading to undesirable byproducts. The temporal dynamics of Nr emissions under real-world driving conditions further reveal that the effectiveness of aftertreatment technologies and their selectivity toward byproducts vary depending on the driving mode. This variability underscores the need for further optimization of the SCR and ASC technologies to balance the control of all the reactive nitrogen species effectively.

Source apportionment of ultrafine particles in urban Europe

There is a body of evidence that ultrafine particles (UFP, those with diameters ≤ 100 nm) might have significant impacts on health. Accordingly, identifying sources of UFP is essential to develop abatement policies. This study focuses on urban Europe, and aims at identifying sources and quantifying their contributions to particle number size distribution (PNSD) using receptor modelling (Positive Matrix Factorization, PMF), and evaluating long-term trends of these source contributions using the non-parametric Theil-Sen's method. Datasets evaluated include 14 urban background (UB), 5 traffic (TR), 4 suburban background (SUB), and 1 regional background (RB) sites, covering 18 European and 1 USA cities, over the period, when available, from 2009 to 2019. Ten factors were identified (4 road traffic factors, photonucleation, urban background, domestic heating, 2 regional factors and long-distance transport), with road traffic being the primary contributor at all UB and TR sites (56-95 %), and photonucleation being also significant in many cities. The trends analyses showed a notable decrease in traffic-related UFP ambient concentrations, with statistically significant decreasing trends for the total traffic-related factors of -5.40 and -2.15 % yr-1 for the TR and UB sites, respectively. This abatement is most probably due to the implementation of European emissions standards, particularly after the introduction of diesel particle filters (DPFs) in 2011. However, DPFs do not retain nucleated particles generated during the dilution of diesel exhaust semi-volatile organic compounds (SVOCs). Trends in photonucleation were more diverse, influenced by a reduction in the condensation sink potential facilitating new particle formation (NPF) or by a decrease in the emissions of UFP precursors. The decrease of primary PM emissions and precursors of UFP also contributed to the reduction of urban and regional background sources.

Long-term plume-chasing measurements: Emission characteristics and spatial patterns of heavy-duty trucks in a megacity

Assessing the emissions of heavy-duty diesel trucks (HDDTs) is crucial for managing air quality in megacities, especially concerning nitrogen oxides (NOX) and black carbon (BC). This study employed mobile plume chasing to monitor the real-world emissions of over 7778 HDDTs in Shenzhen. The findings indicate that the real-world NOX emission factors (EF) of China IV trucks did not differ significantly from those of China III, whereas China V and VI vehicles demonstrated fleet-averaged reductions of 27% and 85%, respectively. For China V, a significant decrease in the NOX EF for HDDTs registered after 2017 was attributed to the installation of advanced aftertreatment systems, including diesel oxidation catalysts (DOC) and Diesel Particle Filters (DPF), along with selective catalytic reduction (SCR). These technologies led to an average reduction of 42% in NOX and 61% in BC emissions. Seasonal variations were pronounced, with winter (∼20 °C) NOX EF 40% higher than summer (∼35 °C) levels. Conversely, BC EF decreased by 26% in winter, indicating significant impacts of ambient temperature on emissions. Spatial analysis revealed that the average NOX EF of HDDTs on east freeways was 1.4 times higher than that on urban expressways, influenced by variations in the proportion of vehicle types segmented by usage. These findings offer a comprehensive perspective on HDDTs emissions, highlighting the importance of large-scale emission monitoring through plume chasing for precise and effective control of real-world HDDTs emissions.

PM2.5 exposure disparities persist despite strict vehicle emissions controls in California

As policymakers increasingly focus on environmental justice, a key question is whether emissions reductions aimed at addressing air quality or climate change can also ameliorate persistent air pollution exposure disparities. We examine evidence from California's aggressive vehicle emissions control policy from 2000 to 2019. We find a 65% reduction in modeled statewide average exposure to PM2.5 from on-road vehicles, yet for people of color and overburdened community residents, relative exposure disparities increased. Light-duty vehicle emissions are the main driver of the exposure and exposure disparity, although smaller contributions from heavy-duty vehicles especially affect some overburdened groups. Our findings suggest that a continued trend of emissions reductions will likely reduce concentrations and absolute disparity but may not reduce relative disparities without greater attention to the systemic factors leading to this disparity.

Regulated and unregulated emissions from Euro VI Diesel and CNG heavy-duty vehicles

This study compares emissions from Euro VI-D Diesel and CNG buses across temperatures from -7 °C to 35 °C. Pollutants including NOx, THC, CH4, CO, NH3, N2O, HCHO, Solid Particle Number larger than 23 nm (SPN23) and larger than 10 nm (SPN10) were measured. Both buses complied with Euro VI-D but exceeded European Commission's proposed Euro 7 limits, notably for NOx and SPN10. The CNG bus also surpassed NH3, CO, and CH4 limits, while the Diesel exceeded N2O limits. High NH3 emissions were observed from CNG (up to 0.320 g/kWh), with Diesel reporting lower levels (up to 0.021 g/kWh). HCHO emission from both vehicles were very low. SPN23 was under limits, but SPN10 exceeded Euro 7 limits at cold start tests. CNG's CH4 and N2O emissions constituted up to 4.6% and 3.5% of CO2 equivalent, respectively. Diesel bus showed negligible CH4 but N2O emissions represented up to 37% of CO2 equivalent.

Characterization of ammonia emissions from light-duty gasoline vehicles based on real-world driving and dynamometer measurements

Ammonia (NH3) contributes significantly to the formation of particulate matter, and vehicles represent a major source of NH3 in urban areas. However, there remains a lack of comprehensive understanding regarding the emission characteristics of NH3 from vehicles. This study conducted real-world driving emission (RDE) measurements and dynamometer measurements on 33 light-duty gasoline vehicles (LDGVs) to investigate their emission characteristics and impact factors. The tested vehicles include China 3 to China 6 emission standards. The results show that the average NH3 emission factors of LDGVs decreased by >80 % from China 3 to China 6 emission standards. The results obtained from dynamometer measurements reveal that independent from other conventional pollutants (such as HCHO and NOx), NH3 emissions do not exhibit significant emission peaks during the hot- or cold-start phase. The RDE measurement covers a more comprehensive range of the vehicle's real-world driving conditions, resulting in higher NH3 emission factors compared with dynamometer measurements. The analysis of RDE measurements revealed that NH3 emissions are influenced by vehicle speeds and accelerations. Acceleration processes contribute approximately 50 % of total NH3 emissions over a driving period. Finally, using real driving speed, acceleration, and road gradient as input parameters, an NH3 emission rate model based on vehicle specific power was developed. This emission rate model enables a more precise reflection of LDGVs' NH3 emissions of LDGVs across diverse driving conditions and provides valuable data support for high-resolution inventories of vehicle NH3 emissions.

Sensitivity of air quality to vehicle ammonia emissions in the United States

The US Environmental Protection Agency (EPA) estimates on-road vehicles emissions using the Motor Vehicle Emission Simulator (MOVES). We developed updated ammonia emission rates for MOVES based on road-side exhaust emission measurements of light-duty gasoline and heavy-duty diesel vehicles. The resulting nationwide on-road vehicle ammonia emissions are 1.8, 2.1, 1.8, and 1.6 times higher than the MOVES3 estimates for calendar years 2010, 2017, 2024, and 2035, respectively, primarily due to an increase in light-duty gasoline vehicle NH3 emission rates. We conducted an air quality simulation using the Community Multi-Scale Air Quality (CMAQv5.3.2) model to evaluate the sensitivity of modeled ammonia and fine particulate matter (PM2.5) concentrations in calendar year 2017 using the updated on-road vehicle ammonia emissions. The average monthly urban ammonia ambient concentrations increased by up to 2.3 ppbv in January and 3.0 ppbv in July. The updated on-road NH3 emission rates resulted in better agreement of modeled ammonia concentrations with 2017 annual average ambient ammonia measurements, reducing model bias by 5.8 % in the Northeast region. Modeled average winter PM2.5 concentrations increased in urban areas, including enhancements of up to 0.5 μg/m3 in the northeast United States. The updated ammonia emission rates have been incorporated in MOVES4 and will be used in future versions of the NEI and EPA's modeling platforms.

Atmospheric reduced nitrogen: Sources, transformations, effects, and management

Human activities have increased atmospheric emissions and deposition of oxidized and reduced forms of nitrogen, but emission control programs have largely focused on oxidized nitrogen. As a result, in many regions of the world emissions of oxidized nitrogen are decreasing while emissions of reduced nitrogen are increasing. Emissions of reduced nitrogen largely originate from livestock waste and fertilizer application, with contributions from transportation sources in urban areas. Observations suggest a discrepancy between trends in emissions and deposition of reduced nitrogen in the U.S., likely due to an underestimate in emissions. In the atmosphere, ammonia reacts with oxides of sulfur and nitrogen to form fine particulate matter that impairs health and visibility and affects climate forcings. Recent reductions in emissions of sulfur and nitrogen oxides have limited partitioning with ammonia, decreasing long-range transport. Continuing research is needed to improve understanding of how shifting emissions alter formation of secondary particulates and patterns of transport and deposition of reactive nitrogen. Satellite remote sensing has potential for monitoring atmospheric concentrations and emissions of ammonia, but there remains a need to maintain and strengthen ground-based measurements and continue development of chemical transport models. Elevated nitrogen deposition has decreased plant and soil microbial biodiversity and altered the biogeochemical function of terrestrial, freshwater, and coastal ecosystems. Further study is needed on differential effects of oxidized versus reduced nitrogen and pathways and timescales of ecosystem recovery from elevated nitrogen deposition. Decreases in deposition of reduced nitrogen could alleviate exceedances of critical loads for terrestrial and freshwater indicators in many U.S. areas. The U.S. Environmental Protection Agency should consider using critical loads as a basis for setting standards to protect public welfare and ecosystems. The U.S. and other countries might look to European experience for approaches to control emissions of reduced nitrogen from agricultural and transportation sectors.Implications: In this Critical Review we synthesize research on effects, air emissions, environmental transformations, and management of reduced forms of nitrogen. Emissions of reduced nitrogen affect human health, the structure and function of ecosystems, and climatic forcings. While emissions of oxidized forms of nitrogen are regulated in the U.S., controls on reduced forms are largely absent. Decreases in emissions of sulfur and nitrogen oxides coupled with increases in ammonia are shifting the gas-particle partitioning of ammonia and decreasing long-range atmospheric transport of reduced nitrogen. Effort is needed to understand, monitor, and manage emissions of reduced nitrogen in a changing environment.

Long term trends in source apportioned particle number concentrations in Rochester NY

During the past two decades, efforts have been made to further reduce particulate air pollution across New York State through various Federal and State policy implementations. Air quality has also been affected by economic drivers like the 2007-2009 recession and changing costs for different approaches to electricity generation. Prior work has focused on particulate matter with aerodynamic diameter ≤2.5 μm. However, there is also interest in the effects of ultrafine particles on health and the environment and analyses of changes in particle number concentrations (PNCs) are also of interest to assess the impacts of changing emissions. Particle number size distributions have been measured since 2005. Prior apportionments have been limited to seasonal analyses over a limited number of years because of software limitations. Thus, it has not been possible to perform trend analyses on the source-specific PNCs. Recent development have now permitted the analysis of larger data sets using Positive Matrix Factorization (PMF) including its diagnostics. Thus, this study separated and analyzed the hourly averaged size distributions from 2005 to 2019 into two data sets; October to March and April to September. Six factors were resolved for both data sets with sources identified as nucleation, traffic 1, traffic 2, fresh secondary inorganic aerosol (SIA), aged SIA, and O3-rich aerosol. The resulting source-specific PNCs were combined to provide continuous data sets and analyzed for trends. The trends were then examined with respect to the implementation of regulations and the timing of economic drivers. Nucleation was strongly reduced by the requirement of ultralow (<15 ppm) sulfur on-road diesel fuel in 2006. Secondary inorganic particles and O3-rich PNCs show strong summer peaks. Aged SIA was constant and then declined substantially in 2015 but rose in 2019. Traffic 1 and 2 have steadily declined bur rose in 2019.

Corridor-Level Impacts of Battery-Electric Heavy-Duty Trucks and the Effects of Policy in the United States

Electrifying freight trucks will be key to alleviating air pollution burdens on disadvantaged communities and mitigating climate change. The United States plans to pursue this aim by adding vehicle charging infrastructure along specific freight corridors. This study explores the coevolution of the electricity grid and freight trucking landscape using an integrated assessment framework to identify when each interstate and drayage corridor becomes advantageous to electrify from a climate and human health standpoint. Nearly all corridors achieve greenhouse gas emission reductions if electrified now. Most can reduce health impacts from air pollution if electrified by 2040 although some corridors in the Midwest, South, and Mid-Atlantic regions remain unfavorable to electrify from a human health standpoint, absent policy support. Recent policy, namely, the Inflation Reduction Act, accelerates this timeline to 2030 for most corridors and results in net human health benefits on all corridors by 2050, suggesting that near-term investments in truck electrification, particularly drayage corridors, can meaningfully reduce climate and health burdens.

Improved Spatial Resolution in Modeling of Nitrogen Oxide Concentrations in the Los Angeles Basin

The extent to which emission control technologies and policies have reduced anthropogenic NOx emissions from motor vehicles is large but uncertain. We evaluate a fuel-based emission inventory for southern California during the June 2021 period, coinciding with the Re-Evaluating the Chemistry of Air Pollutants in CAlifornia (RECAP-CA) field campaign. A modified version of the Fuel-based Inventory of Vehicle Emissions (FIVE) is presented, incorporating 1.3 km resolution gridding and a new light-/medium-duty diesel vehicle category. NOx concentrations and weekday-weekend differences were predicted using the WRF-Chem model and evaluated using satellite and aircraft observations. Model performance was similar on weekdays and weekends, indicating appropriate day-of-week scaling of NOx emissions and a reasonable distribution of emissions by sector. Large observed weekend decreases in NOx are mainly due to changes in on-road vehicle emissions. The inventory presented in this study suggests that on-road vehicles were responsible for 55-72% of the NOx emissions in the South Coast Air Basin, compared to the corresponding fraction (43%) in the planning inventory from the South Coast Air Quality Management District. This fuel-based inventory suggests on-road NOx emissions that are 1.5 ± 0.4, 2.8 ± 0.6, and 1.3 ± 0.7 times the reference EMFAC model estimates for on-road gasoline, light- and medium-duty diesel, and heavy-duty diesel, respectively.

In-use NOx and black carbon emissions from heavy-duty freight diesel vehicles and near-zero emissions natural gas vehicles in California's San Joaquin Air Basin

This study assessed the real-world nitrogen oxide (NOx) and black carbon emissions from six goods movement heavy-duty diesel and compressed natural gas (CNG) vehicles operating in California's San Joaquin Valley and Sacramento regions. The diesel vehicles were all equipped with diesel oxidation catalysts (DOCs) and diesel particulate filters (DPFs), while two diesel vehicles were also equipped with selective catalytic reduction (SCR). All CNG vehicles were equipped with three-way catalysts and fitted with stoichiometric engines meeting the optional ultra-low NOx standard of 0.02 g/bhp-hr. Emissions measurements were conducted with a portable emissions measurement systems (PEMS) during typical goods movement vehicle operation. Black carbon emissions were about 3-7 times higher for the CNG vehicles than those of the DPF-equipped diesel vehicles. NOx emissions for the CNG vehicles were found at or below the optional NOx standard and on average 35 times lower NOx than those of the diesel vehicles. Diesel vehicle NOx hotspots were identified in urban areas and intersections with frequent stop-and-go driving events, whereas the CNG vehicles showed uniform NOx emissions rates along the route. The dispersion modeling results showed elevated NOx and PM emissions exposures to receptors in close proximity to the highway. Our findings suggest that real-time emissions measurements at the tailpipe provide more accurate population exposure assessments near freight corridors compared to utilizing trip-averaged emissions rates values in dispersion models. Under the present test conditions, >70 % of black carbon and NOx were emitted within disadvantaged communities, characterized by low-income minority populations.

Trends of inorganic sulfur and nitrogen species at an urban site in western Canada (2004-2018)

A 15-year (2004-2018) record of measurement data of gaseous and particulate sulfur and nitrogen pollutants in air collected at an urban site in Burnaby in western Canada was analyzed for generating their decadal trends using three different methods, including linear regression, Mann-Kendall test and Theil-Sen trend estimator (MK-TS), and ensemble empirical mode decomposition (EEMD). Annual mean concentration of SO2 and SO42- decreased by about 59% and 42%, respectively, during the 15-year period. The slower decreases of SO42- than SO2 were mainly caused by the increased O3 concentration and temperature in spring and summer, which promoted conversion of SO2 to SO42- through gas-phase reaction, and by the increased aerosol pH value and availability of H2O2 in winter, which enhanced aqueous-phase SO42- formation. Accordingly, the sulfur oxidation ratio (SOR) increased by 23% or more in spring, summer, and winter during the 15-year period. Annual mean concentrations of NO2 and NO3- declined by 36% and 38%, respectively, during this period. On seasonal basis, NO3- decreased faster than NO2 in autumn and slower in winter. The non-linear responses of NO3- to NO2 concentration decreases were more evident in winter than the other seasons, partly due to the increased particulate NO3- fraction caused by decreased temperature, increased aerosol pH value, and enhanced NO3- formation caused by increased O3 concentrations. Annual mean concentration of NH3 showed small increases due to stable NH3 emission and reduced conversion of NH3 to NH4+. NH4+ concentration decreased by 51% during the 15-year period. These results suggest that reduced oxidants levels are likely responsible for weakened formation of secondary inorganic aerosols, besides emission reductions for SO2 and NO2.

Cold-Start NOx Mitigation by Passive Adsorption Using Pd-Exchanged Zeolites: From Material Design to Mechanism Understanding and System Integration

It remains a major challenge to abate efficiently the harmful nitrogen oxides (NO_x) in low-temperature diesel exhausts emitted during the cold-start period of engine operation. Passive NO_x adsorbers (PNA), which could temporarily capture NO_x at low temperatures (below 200 °C) and release the stored NO_x at higher temperatures (normally 250-450 °C) to downstream selective catalytic reduction unit for complete abatement, hold promise to mitigate cold-start NO_x emissions. In this review, recent advances in material design, mechanism understanding, and system integration are summarized for PNA based on palladium-exchanged zeolites. First, we discuss the choices of parent zeolite, Pd precursor, and synthetic method for the synthesis of Pd-zeolites with atomic Pd dispersions, and review the effect of hydrothermal aging on the properties and PNA performance of Pd-zeolites. Then, we show how different experimental and theoretical methodologies can be integrated to gain mechanistic insights into the nature of Pd active sites, the NO_x storage/release chemistry, as well as the interactions between Pd and typical components/poisons in engine exhausts. This review also gathers several novel designs of PNA integration into modern exhaust after-treatment systems for practical application. At the end, we discuss the major challenges, as well as important implications, for the further development and real application of Pd-zeolite-based PNA in cold-start NO_x mitigation.

The impact of hydrogenated vegetable oil (HVO) on the formation of secondary organic aerosol (SOA) from in-use heavy-duty diesel vehicles

This manuscript contains an assessment of tailpipe emissions and secondary aerosol formation from two in-use heavy-duty diesel vehicles (HDDVs) with different aftertreatment systems when operated with ultra-low sulfur diesel (ULSD) and hydrogenated vegetable oil (HVO) operated on a chassis dynamometer. Secondary aerosol formation was characterized from the HDDVs' diluted exhaust collected and photochemically aged in a 30 m³ mobile atmospheric chamber. Primary nitrogen oxide (NOx) and particulate matter (PM) emissions were reduced for both vehicles operating on HVO compared to ULSD. For the vehicles with no selective catalytic reduction (SCR) system, secondary aerosol production was ~2 times higher for ULSD compared to HVO. The composition of primary aerosol was exclusively organic for the vehicle with no SCR system regardless of fuel type. The composition of secondary aerosol with HVO was primarily organic for the vehicle equipped with diesel particulate filter (DPF)/SCR system; however, when the same vehicle was tested with ULSD, the composition was ~20% organic (80% ammonium nitrate). The results reported here revealed that the in-use vehicle with no-SCR had a non-functioning DPF leading to dramatic increases in secondary aerosol formation when compared to the DPF/SCR vehicle. The high-resolution mass spectra analysis showed that the POA of HVO combustion contained relatively lower portion of CH class compounds (or higher CHO class compounds) compared to ULSD under the similar conditions, which can be rationalized by the higher cetane number of HVO. Substantial growth of oxidized organic aerosol (such as m/z 44 peak) were observed after 5 h of photochemical oxidation, consistent with aged organic aerosols present in the atmosphere. The C₄H₉⁺ fragment at m/z 57 peak was used as a tracer to calculate evolution of secondary organic aerosol formation.

Integrated effects of SCR, velocity, and Air-fuel Ratio on gaseous pollutants and CO2 emissions from China V and VI heavy-duty diesel vehicles

Vehicle exhaust, an important source of air pollution, is affected by many factors, including driving conditions, combustion efficiencies, and the usage of emission control devices. In this study, the Portable Emission Measurement System (PEMS) was used to test the emissions from China V and China VI heavy-duty diesel vehicles to evaluate the integrated effects of Selective Catalytic Reduction (SCR), velocity, and air-fuel ratio on carbon dioxide (CO₂) and nitrogen oxide (NO_x) emissions. Our results reveal that the average distance-based CO₂ and CO emission factors at high velocities (50-90 km/h) are 25% and 61% lower than those at low velocities (less than 50 km/h). The use of SCR increases CO₂ emissions in the range of 70-90 km/h (an average increase of 10.9%). In addition, SCR leads to a 55% NO_x emission reduction at low velocities and 89% at high velocities, with an overall average reduction of 84%. We also find that SCR leads to a significant reduction in the correlation between NO_x emissions and air-fuel ratio (0.76 vs 0.47 for China V truck; 0.72 vs 0.05 for China VI truck), but it does not cause a drastic reduction in the correlation coefficients between CO₂ emissions and air-fuel ratio, which can be used to detect whether SCR is working effectively.

Assessment of In-Use NOx Emissions from Heavy-Duty Diesel Vehicles Equipped with Selective Catalytic Reduction Systems

This work evaluated the nitrogen oxide (NOx) emissions of 277 heavy-duty diesel vehicles (HDDVs) from three portable emission measurement system testing programs. HDDVs in these programs were properly maintained before emission testing, so the malfunction indicator lamp (MIL) was not illuminated. NOx emissions of some HDDVs were significantly higher than the certification standard even during hot operations where exhaust temperature was ideal for selective catalytic reduction to reduce NOx. For engines certified to the 0.20 g/bhp-hr NOx standard, hot operation NOx emissions increased with engine age at 0.081 ± 0.016 g/bhp-hr per year. The correlation between emissions and mileage was weak because six trucks showed extraordinarily high apparent emission increase rates reaching several multiples of the standard within the first 15,000 miles of operation. The overall annual increase in NOx emissions for the HDDVs in this study was two-thirds of what was observed in real-world emissions for HDDVs at the Caldecott Tunnel over the past decade. The vehicles at the Caldecott Tunnel would include those without proper maintenance, and the inclusion of these vehicles possibly explains the difference in the rate of emission increase. The results suggest that HDDVs need robust strategies to better control in-use NOx emissions.

High-Resolution Modeling and Apportionment of Diesel-Related Contributions to Black Carbon Concentrations

Exposure to diesel-related air pollution, which includes black carbon (BC) as a major component of the particulate matter emitted in engine exhaust, is a known human health hazard. The resulting health burden falls heavily on vulnerable communities located close to major sources including highways, rail yards, and ports. Determination of source contributions to the overall pollution burden is challenging due to collinearity in the exhaust composition profiles for relevant sources including heavy-duty diesel trucks, railroad locomotives, cargo-handling equipment, and marine engines. Additionally, the impact of each source depends not just on the magnitude of emissions but on its location relative to receptors as well as on meteorology. We modeled source-resolved BC concentrations in West Oakland, California, at a high (150 m) spatial resolution using the Weather Research and Forecasting model. The ability of the model to predict hourly and 24 h average BC concentrations is evaluated for a 100-day period in summer 2017 when BC was measured at 100 sites within the community. We find that a community monitoring site is representative of population-weighted average BC exposure in the community. Major contributing sources to BC in West Oakland include on-road diesel trucks (44 ± 5%) and three off-road diesel sources: ocean-going vessels (19 ± 1%), railroad locomotives (16 ± 2%), and harbor craft such as tugboats and ferries (11 ± 1%).

Inhibition Effect of Phosphorus Poisoning on the Dynamics and Redox of Cu Active Sites in a Cu-SSZ-13 NH3-SCR Catalyst for NOx Reduction

Phosphorus (P) stemming from biodiesel and/or lubricant oil additives is unavoidable in real diesel exhausts and deactivates gradually the Cu-SSZ-13 zeolite catalyst for ammonia-assisted selective catalytic NO_x reduction (NH₃-SCR). Here, the deactivation mechanism of Cu-SSZ-13 by P-poisoning was investigated by ex situ examination of the structural changes and by in situ probing the dynamics and redox of Cu active sites via a combination of impedance spectroscopy, diffuse reflection infrared Fourier transform spectroscopy, and ultraviolet-visible spectroscopy. We unveiled that strong interactions between Cu and P led to not only a loss of Cu active sites for catalytic turnovers but also a restricted dynamic motion of Cu species during low-temperature NH₃-SCR catalysis. Furthermore, the Cu^II ↔ Cu^I redox cycling of Cu sites, especially the Cu^I → Cu^II reoxidation half-cycle, was significantly inhibited, which can be attributed to the restricted Cu motion by P-poisoning disabling the formation of key dimeric Cu intermediates. As a result, the NH₃-SCR activity at low temperatures (200 °C and below) decreased slightly for the mildly poisoned Cu-SSZ-13 and considerably for the severely poisoned Cu-SSZ-13.

Variability in real-world emissions and fuel consumption by diesel construction vehicles and policy implications

Strategically reducing the emission of non-road mobile source especially diesel construction vehicle (DCV) has a large potential in improving air quality and has attracted much scientific and public attention in recent years around the world. In this study, we explored real-world fuel consumption rate and gaseous emissions factors for multiple pollutants of three typical DCVs in China. The sampling campaign considered the operation mode, cumulative operation hour, emission standard stage and engine power. Results show that the accumulated fuel consumption per hour of vehicle weight for working, load-free moving and idling modes was 0.3, 0.2 and 0.1 kg/h·tons, respectively. The fuel-based NOx emission factor exhibited a bimodal distribution at 27 and 41 g/kg. The fuel-based emission factors for volatile organic compounds (VOCs) were in the range of 0.8 to 2.6 g/kg, where alkene and alkane were the dominant components (>80%), i.e., ethylene, acetylene, propylene, and isobutane. We observed that the ratio of toluene and benzene concentration (T/B) (1.4 ± 1.3) differed from other key emission sources and may be used as the specific indicator of DCV emission exhaust. Our estimates suggest that in 2017 the fuel consumption and NOx emissions of DCV emission accounted for 22-28% of non-road mobile sources in China; NO_X emissions were 2.7 times higher than those in 2006, and it is forecasted that NOx emissions would reduce by 23% between 2017 and 2025 with the implementation of stage IV and the strict supervision policy. The comprehensive dataset on DCV emissions will either guide the government to establish precise and effective policies to regulate the non-road mobile source or significantly improve our understanding of source apportionment of atmospheric NOx and VOCs, both of which are key precursors of haze and ozone pollution.

Health and Climate Impacts from Long-Haul Truck Electrification

Long-haul truck electrification has attracted nascent policy support, but the potential health and climate impacts remain uncertain. Here, we developed an integrated assessment approach with high spatial-temporal (km and hourly) resolution to characterize the causal chain from truck operation to charging loads, electricity grid response, changes in emissions and atmospheric concentrations, and the resulting health and climate impacts across the United States. Compared to future diesel trucks, electrified trucking's net health benefits are concentrated only along the West Coast with a business-as-usual electricity grid. However, with an 80%-renewable electricity grid, most regions would experience net health benefits, and the economic value of avoided climate and health damages exceeds $5 billion annually, an 80% reduction relative to future diesel trucks. Electric trucks with larger batteries may increase health and climate impacts due to additional trips needed to compensate for the payload penalty, but a 2× improvement in the battery specific energy (to ∼320 Wh/kg) could eliminate the additional trips.

From COVID-19 to future electrification: Assessing traffic impacts on air quality by a machine-learning model

The large fluctuations in traffic during the COVID-19 pandemic provide an unparalleled opportunity to assess vehicle emission control efficacy. Here we develop a random-forest regression model, based on the large volume of real-time observational data during COVID-19, to predict surface-level NO2, O3, and fine particle concentration in the Los Angeles megacity. Our model exhibits high fidelity in reproducing pollutant concentrations in the Los Angeles Basin and identifies major factors controlling each species. During the strictest lockdown period, traffic reduction led to decreases in NO2 and particulate matter with aerodynamic diameters <2.5 μm by -30.1% and -17.5%, respectively, but a 5.7% increase in O3 Heavy-duty truck emissions contribute primarily to these variations. Future traffic-emission controls are estimated to impose similar effects as observed during the COVID-19 lockdown, but with smaller magnitude. Vehicular electrification will achieve further alleviation of NO2 levels.

Evaluation of Nitrogen Oxide Emission Inventories and Trends for On-Road Gasoline and Diesel Vehicles

On-road vehicles continue to be a major source of nitrogen oxide (NO_x) emissions in the United States and in other countries around the world. The goal of this study is to compare and evaluate emission inventories and long-term trends in vehicular NO_x emissions. Taxable fuel sales data and in-use measurements of emission factors are combined to generate fuel-based NO_x emission inventories for California and the US over the period 1990-2020. While gasoline and diesel fuel sales increased over the last three decades, total on-road NO_x emissions declined by approximately 70% since 1990, with a steeper rate of decrease after 2004 when heavy-duty diesel NO_x emission controls finally started to gain traction. In California, additional steps have been taken to accelerate the introduction of new heavy-duty engines equipped with selective catalytic reduction systems, resulting in a 48% decrease in diesel NO_x emissions in California compared to a 32% decrease nationally since 2010. California EMFAC model predictions are in good agreement with fuel-based inventory results for gasoline engines and are higher than fuel-based estimates for diesel engines prior to the mid-2010s. Similar to the findings of recent observational and modeling studies, there are discrepancies between the fuel-based inventory and national MOVES model estimates. MOVES predicts a steeper decrease in NO_x emissions and predicts higher NO_x emissions from gasoline engines over the entire period from 1990 to 2020.