Greater fuel efficiency is potentially preferable to reducing NOx emissions for aviation's climate impacts

Trends and drivers of anthropogenic NOx emissions in China since 2020

Nitrogen oxides (NOx), significant contributors to air pollution and climate change, form aerosols and ozone in the atmosphere. Accurate, timely, and transparent information on NOx emissions is essential for decision-making to mitigate both haze and ozone pollution. However, a comprehensive understanding of the trends and drivers behind anthropogenic NOx emissions from China-the world's largest emitter-has been lacking since 2020 due to delays in emissions reporting. Here we show a consistent decline in China's NOx emissions from 2020 to 2022, despite increased fossil fuel consumption, utilizing satellite observations as constraints for NOx emission estimates through atmospheric inversion. This reduction is corroborated by data from two independent spaceborne instruments: the TROPOspheric Monitoring Instrument (TROPOMI) and the Ozone Monitoring Instrument (OMI). Notably, a reduction in transport emissions, largely due to the COVID-19 lockdowns, slightly decreased China's NOx emissions in 2020. In subsequent years, 2021 and 2022, reductions in NOx emissions were driven by the industry and transport sectors, influenced by stringent air pollution controls. The satellite-based inversion system developed in this study represents a significant advancement in the real-time monitoring of regional air pollution emissions from space.

Net-zero aviation: Transition barriers and radical climate policy design implications

While air transport decarbonization is theoretically feasible, less attention has been paid to the complexity incurred in various 'transition barriers' that act as roadblocks to net-zero goals. A total of 40 barriers related to mitigation, management, technology and fuel transition, finance, and governance are identified. As these make decarbonization uncertain, the paper analyzes air transport system's growth, revenue, and profitability. Over the period 1978-2022, global aviation has generated marginal profits of US$20200.94 per passenger, or US$202082 billion in total. Low profitability makes it unlikely that the sector can finance the fuel transition cost, at US$0.5-2.1 trillion (Dray et al. 2022). Four radical policy scenarios for air transport futures are developed. All are characterized by "limitations", such as CO2 taxes, a carbon budget, alternative fuel obligations, or available capacity. Scenario runs suggest that all policy scenarios will more reliably lead to net-zero than the continued volume growth model pursued by airlines.

Impact of Legislated and Best Available Emission Control Measures on UK Particulate Matter Pollution, Premature Mortality, and Nitrogen-Sensitive Habitats

Past emission controls in the UK have substantially reduced precursor emissions of health-hazardous fine particles (PM2.5) and nitrogen pollution detrimental to ecosystems. Still, 79% of the UK exceeds the World Health Organization (WHO) guideline for annual mean PM2.5 of 5 μg m-3 and there is no enforcement of controls on agricultural sources of ammonia (NH3). NH3 is a phytotoxin and an increasingly large contributor to PM2.5 and nitrogen deposited to sensitive habitats. Here we use emissions projections, the GEOS-Chem model, high-resolution data sets, and contemporary exposure-risk relationships to assess potential human and ecosystem health co-benefits in 2030 relative to the present day of adopting legislated or best available emission control measures. We estimate that present-day annual adult premature mortality attributable to exposure to PM2.5 is 48,625 (95% confidence interval: 45,188-52,595), that harmful amounts of reactive nitrogen deposit to almost all (95%) sensitive habitat areas, and that 75% of ambient NH3 exceeds levels safe for bryophytes and lichens. Legal measures decrease the extent of the UK above the WHO guideline to 58% and avoid 6,800 premature deaths by 2030. This improves with best available measures to 36% of the UK and 13,300 avoided deaths. Both legal and best available measures are insufficient at reducing the extent of damage of nitrogen pollution to sensitive habitats. Far more ambitious reductions in nitrogen emissions (>80%) than is achievable with best available measures (34%) are required to halve the amount of excess nitrogen deposited to sensitive habitats.

Technological solutions for NOx, SOx, and VOC abatement: recent breakthroughs and future directions

NOx, SOx, and carbonaceous volatile organic compounds (VOCs) are extremely harmful to the environment, and their concentrations must be within the limits prescribed by the region-specific pollution control boards. Thus, NOx, SOx, and VOC abatement is essential to safeguard the environment. Considering the importance of NOx, SOx, and VOC abatement, the discussion on selective catalytic reduction, oxidation, redox methods, and adsorption using noble metal and non-noble metal-based catalytic approaches were elaborated. This article covers different thermal treatment techniques, category of materials as catalysts, and its structure-property insights along with the advanced oxidation processes and adsorption. The defect engineered catalysts with lattice oxygen vacancies, bi- and tri-metallic noble metal catalysts and non-noble metal catalysts, modified metal organic frameworks, mixed-metal oxide supports, and their mechanisms have been thoroughly reviewed. The main hurdles and potential achievements in developing novel simultaneous NOx, SOx, and VOC removal technologies are critically discussed to envisage the future directions. This review highlights the removal of NOx, SOx, and VOC through material selection, properties, and mechanisms to further improve the existing abatement methods in an efficient way.

Nitrogen deposition from aviation emissions

Excess nitrogen deposition from anthropogenic sources of atmospheric emissions, such as agriculture and transportation, can have negative effects on natural environments. Designing effective conservation efforts requires knowledge of the contribution of individual sectors. This study utilizes a global atmospheric chemistry-transport model to quantify, for the first time, the contribution of global aviation NO_x emissions to nitrogen deposition for 2005 and 2019. We find that aviation led to an additional 1.39 Tg of nitrogen deposited globally in 2019, up 72 % from 2005, with 67 % of each year's total occurring through wet deposition. In 2019, aviation was responsible for an average of 0.66 %, 1.13 %, and 1.61 % of modeled nitrogen deposition from all sources over Asia, Europe, and North America, respectively. These impacts are spatially widespread, with 56 % of deposition occurring over water. Emissions during the landing, taxi and takeoff (LTO) phases of flight are responsible for 8 % of aviation's nitrogen deposition impacts on average globally, and between 16 and 32 % over most land in regions with high aviation activity. Despite currently representing less than 1.2 % of nitrogen deposition globally, further growth of aviation emissions would result in increases in aviation's contribution to nitrogen deposition and associated critical loads.

An Evolutionary Field Theorem: Evolutionary Field Optimization in Training of Power-Weighted Multiplicative Neurons for Nitrogen Oxides-Sensitive Electronic Nose Applications

Neuroevolutionary machine learning is an emerging topic in the evolutionary computation field and enables practical modeling solutions for data-driven engineering applications. Contributions of this study to the neuroevolutionary machine learning area are twofold: firstly, this study presents an evolutionary field theorem of search agents and suggests an algorithm for Evolutionary Field Optimization with Geometric Strategies (EFO-GS) on the basis of the evolutionary field theorem. The proposed EFO-GS algorithm benefits from a field-adapted differential crossover mechanism, a field-aware metamutation process to improve the evolutionary search quality. Secondly, the multiplicative neuron model is modified to develop Power-Weighted Multiplicative (PWM) neural models. The modified PWM neuron model involves the power-weighted multiplicative units similar to dendritic branches of biological neurons, and this neuron model can better represent polynomial nonlinearity and they can operate in the real-valued neuron mode, complex-valued neuron mode, and the mixed-mode. In this study, the EFO-GS algorithm is used for the training of the PWM neuron models to perform an efficient neuroevolutionary computation. Authors implement the proposed PWM neural processing with the EFO-GS in an electronic nose application to accurately estimate Nitrogen Oxides (NO_x) pollutant concentrations from low-cost multi-sensor array measurements and demonstrate improvements in estimation performance.

Stabilized Ultra-High-Temperature Sensors Based on Inert Gas-Sealed Sapphire Fiber Bragg Gratings

In situ measurement of high temperature is critical in aerospace, petrochemical, metallurgical, and power industries. The single-crystal sapphire fiber is a promising material for high-temperature measurement owing to its high melting point of ∼2045 °C. Sapphire fiber Bragg gratings (SFBGs), which could be inscribed in sapphire fibers with a femtosecond laser, are widely used as high-temperature sensors. However, conventional SFBGs typically exhibit a significant deterioration in their spectra after long-term operation at ultra-high temperatures, resulting from the formation of some unwanted microstructural features, that is, lossy spots and micro-etched lines, on the surface of the sapphire fiber. Here, we report for the first time, to the best of our knowledge, a thermally stabilized ultra-high-temperature sensor based on an SFBG created by femtosecond laser inscription, inert gas-sealed packaging, and gradient temperature-elevated annealing. The results indicate that the lossy spots are essentially aluminum hydroxide induced by high-temperature oxidation, and the inert gas-sealed packaging process can effectively insulate the sapphire fiber from the ambient air. Moreover, the formation of micro-etched lines was suppressed successfully by using the gradient temperature-elevated annealing process. As a result, the surface topography of the SFBG after operating at high temperatures was improved obviously. The long-term thermal stability of such an SFBG was greatly enhanced, showing a stable operation at 1600 °C for up to 20 h. In addition, it could withstand an even higher temperature of 1800 °C with a sensitivity of 41.2 pm/°C. The aforementioned results make it promising for high-temperature sensing in chemical, aviation, smelting, and power industries.

Accounting for the carbon emissions from domestic air routes in China

This study seeks to build a database of China's 413 main routes in 2018, which contains the carbon emission intensity of the leading aircraft types, the Climb/Cruise/Descent (CCD) carbon emissions of each airline and each route, and the probable emissions when the routes are straight. First, the Modified Fuel Percentage Method (MFPM) is applied to calculate the carbon emission intensity of main aircraft types at various distances. Next, the carbon emissions and the Revenue Passenger Kilometers of 413 routes and 40 airlines are calculated. Then the carbon emissions of the actual route and straight route are analyzed. Dali-Kunming and Joy Air have the most significant emission intensity among the routes and airlines. However, the excess emissions from the non-straight routes account for about 10.15% of the actual emissions.

Climate Change Mitigation Pathways for the Aviation Sector

Even though the contribution of the aviation sector to the global economy is very notable, it also has an adverse impact on climate change. Improvements have been made in different areas (i.e., technology, sustainable aviation fuel, and design) to mitigate these adverse effects. However, the rate of improvement is small compared to the increase in the demand for air transportation. Hence, greenhouse gas emissions in the aviation sector are steadily increasing and this trend is expected to continue unless adequately addressed. In this context, this study examined the following: (i) the factors that affect the growth of aviation, (ii) trends in greenhouse gas emissions in the sector, (iii) trends in energy demand, (iv) mitigation pathways of emissions, (v) mitigation challenges for the International Civil Aviation Organization, (vi) achievements in mitigating emissions, (vii) barriers against mitigating emissions, and (viii) approaches of overcoming barriers against emissions mitigation. This study finds that continued research and development efforts targeting aircraft fuel burn efficiency are crucial in reducing greenhouse gas emissions. Although biofuels are promising for the reduction of aviation emissions, techniques to reduce NOx emissions could enhance large-scale deployment. Pragmatic market-based mechanisms, such as the Emissions Trading Scheme (ETS) and/or carbon tax must be enforced on a global scale to capitalize on a collective stakeholder effort to curb CO2 emissions. The findings of this study will help in understanding the emissions and energy consumption scenarios, which will provide a comprehensive package of mitigation pathways to overcome future emissions reduction challenges in the aviation sector.