Smog Formation Research Articles

A Relative Relationship between Low-Stack Emissions and the Quality of Fuels Used in Heating Boilers and Grills

Fuel combustion conditions in modern boilers are currently fully automated. Controlling the combustion process with preset parameters with an appropriate air distribution adapted to the type of fuel ensures maximum use of fuel properties. Combustion of heating materials in such controlled conditions allows to minimally affect the environment of emitted flue gases and solid residues (ashes). Unfortunately, the best boiler with the best regulator will not provide optimal combustion conditions without good fuel quality. Contaminated biomass (pellets, wood chips, wood) with plastic, fossil coal, colored metals not only effectively affects the low emission burden, but also the failure rate of boilers. Analogically, the use of poor-quality solid fossil fuels translates into the formation of smog and sometimes irreparable damage to CO equipment. According to the authors of the study, in which the combination of low and medium power boilers with dedicated tested fuels to a given type of heating installation will ensure maximum reduction of low emissions. In the summer period, we can observe the phenomenon of fog caused by the use of grill fuels to prepare dishes. In case of high insolation and accumulation of devices (grills), e.g., on plots, recreational areas, smog phenomenon may also occur. The more the grills are polluted, the more the combustion fumes are burdensome and harmful for users of both active and passive barbecues. Based on the analysis of flue gases from the combustion of charcoal and charcoal briquettes, it can be stated that their particulate matter emissions, which are mainly responsible for the formation of smog, range from about 110 mg/m³ to 280 mg/m³. It was found that charcoal briquettes show extremely high emission levels for both PM 2.5 and PM 10 and other compounds emitted during the combustion process. Optical analyses performed on fuels used in household furnaces revealed, in the case of the samples analyzed, a low content of additives such as plastics and metals; the only significant and unnecessary component in all the fuels analyzed was the mineral matter.

Comparison of the Parameters of the Exergoeconomic Environmental Analysis of Two Combined Cycles of Three Pressure Levels with and without Postcombustion.

Nowadays, in Mexico, most of the installed electricity generation capacity corresponds to combined cycles, representing 37.1%. For this reason, it is important to maintain these cycles in good operating conditions, with the least environmental impacts. An exergoeconomic and environmental analysis is realized to compare the operation of the combined cycle, with and without postcombustion, with the comparison of exergoeconomic and environmental indicators. With the productive structure of the energy system, the process of formation of the final products and the residues are identified, and an allocation criterion is also used to impute the formation cost of residue to the productive components related to its formation. This criterion considers the irreversibilities generated in each productive component that participates in the formation of a residue. The compositions of pollutant gases emitted are obtained, and their environmental impact is determined. The unit exergoeconomic cost of the power output in the gas turbine is lower in the combined cycle with postcombustion, indicating greater efficiency in the process of obtaining this energy stream, and the environmental indicators of global warming, smog formation and acid rain formation are higher in the combined cycle with postcombustion, these differences being 5.22%, 5.53% and 5.30%, respectively.

Photocatalytic Cementitious Materials to Reduce Air Pollution: Review

Oxides of nitrogen (NOx) are highly reactive and major air pollutants generated from the vehicular emission and burning of fossil fuels in industries. These pollutants will create tropospheric ozone, formation of urban smog and occurrence of acid rain, etc. A potential solution for this would be heterogeneous photocatalysis. Using photocatalysts such as titanium dioxide (TiO2) inorganic and organic matter present in the air, such as NOx and VOCs can be adsorbed and oxidized by the action of ultraviolet (UV) light (sunlight). This technology is promising for cities with highly congested areas having high ground-level pollutant concentrations. Self-cleaning and air-purification coating with a super-hydrophilic photocatalyst (TiO2) coated on buildings can bring down the NOx concentration in the air. In addition, the incorporation of TiO2 in the form of nano-particles into cementitious materials will enhance their mechanical properties. This study evaluates and highlights the applications of nano titanium dioxide and its effects on the fresh and hardened characteristics of the cementitious composites.

Identification of plants releasing isoprene causing smog

The accessibility of water is most important component for plant productivity and growth. Water stress is a major threat for the agriculture system of Pakistan. The change in climate causes the change in the patterns of rainfall which may cause the extreme events. The main cause of climate change is the concentration of carbon dioxide in the air and global warming. A change in environment overall takes place by a number of causes but the most important cause of all these changes is isoprene.1 Water scarcity causes high isoprene emission from plants. The dominant volatile organic compound which is released from vegetation is Isoprene (2-methyl-1,3,-butadiene). Isoprene produced by few herbaceous plant species and many woody trees. The emission of these volatiles like isoprene has an indirect effect on climate change and cause smog formation. The examination found that isoprene responds with human made nitrogen oxide to make particulate issue. In light of the presence of nitrogen oxides, it is engaged with creating the negative impacts on climate and human wellbeing. The research investigates current knowledge about the presence of isoprene emissions within the plant kingdom under drought stress conditions. Eucalyptus, Kachnar, Mulberry and Conocarpus are few species which are able to produce isoprene. The pot experiment was designed to identify isoprene emission within these plants. The plants were exposed to decrease Fractional Transpirable Soil Water FTSW. Various VOCs emission was increase under stress conditions. The isoprene emission rate is high under mild drought stress but decline under severe drought stress. Other parameters like stomatal conductance, photosynthesis, internal CO2 were also observed.

Real-Time Monitoring of Volatile Organic Compounds in Ambient Air Using Direct-Injection Mass Spectrometry

Exposure to volatile organic compounds (VOCs) at low parts per billion (ppb) concentrations (by volume, ppbV) in ambient air is increasingly being recognized as a health risk. Furthermore, VOCs contribute to the formation of photochemical smog and particulate matter, which are hazardous and negatively impact the environment. Although most urban VOCs are anthropogenic in origin, some are produced by living organisms, especially trees. Gas chromatography (GC) is conventionally applied to VOC analysis of air, but it generally has a time resolution of tens of minutes and detection at low ppbV levels, frequently requiring sample preconcentration (such as using thermal desorption tubes (TDTs) or canister sampling with cryotrapping prior to analysis). Newer direct-injection mass spectrometry (DIMS) techniques can address the challenge of time resolution (seconds to tens of seconds) and eliminate the need for sampling via TDTs or canisters while still providing sufficient specificity. In this article, the results of a DIMS air monitoring campaign in Auckland, New Zealand, are described. Real-time selected ion flow tube mass spectrometry (SIFT–MS) measurements were obtained at three sites over a period of several weeks each: (i) adjacent to a freeway, (ii) in a residential area, and (iii) at a rural location. There were 13 compounds that were targeted in the study, covering various sources: fugitive emissions and incomplete combustion (benzene, toluene, and ethylbenzene, along with the xylene isomers, styrene, and ethanol), incomplete combustion (1,3-butadiene, formaldehyde, acetaldehyde, acrolein, benzaldehyde, and acetone), and biogenic origin (isoprene and monoterpenes (total of all isomers). At each site, SIFT–MS provided effective on-site analysis to sub-ppbV levels with better than 1-min time resolution.

Life cycle assessment of behind-the-meter Bitcoin mining at US power plant.

PurposeDue to its highly energy-intensive process, Bitcoin has attracted the global attention of climate research and media. At the time of this submission, behind-the-meter Bitcoin mining has gained significant traction; however, not a single environmental impact assessment has been conducted on this type of operation. This study seeks to fill the gap, applying the established Life Cycle Assessment methodology to estimate the environmental footprint of a single case study.MethodsA life cycle assessment methodology of a natural gas power plant mining Bitcoin behind-the-meter in the state of New York following the ISO 14040 guidelines was applied. The functional unit (FU) is defined as the attributed generation capacity of 14 MW over the course of a regular full-calendar year in the attributional model. The FU is scaled to 22 MW and 104 MW in the predictive models to represent planned expansion. The TRACI 2.1 method was applied to characterize the environmental impact. The environmental impact categories considered in this study included global warming, acidification, smog formation, and particulate emissions.Results and discussionLocated in New York State, Greenidge LLC, a natural gas power plant produces an estimated 88,440 metric tons of CO2-eq per year to mine Bitcoin behind-the-meter. Annual emissions would total 656,983 metric tons of CO2-eq if the plant devotes 100% of its generation to Bitcoin mining. The primary driver of greenhouse gas emissions is the generation of electricity itself, accounting for ~ 79% of the total emissions. At full capacity, annual emissions are comparable to the annual emissions of 140,000 passenger vehicles or the emissions resulting from the burning of 600 million lb of coal. Further, additional planned cases could produce an estimated 1.9 million tons tCO2-eq per annum.ConclusionsBehind-the-meter Bitcoin mining makes the power plant a significant contributor to global warming at a time when New York State is attempting to radically reduce its greenhouse gas emissions by 85% by 2050 and to have 100% carbon-free electricity by 2040. The environmental impact of this business model is not limited to individual sites but is spread out over upstream impacts as well. In combination, we see that behind-the-meter Bitcoin mining not only goes against local climate initiatives but also poses a significant danger to national initiatives due to feasible scalability, caused by an availability of existing infrastructure and favorable financials.Supplementary informationThe online version contains supplementary material available at 10.1007/s11367-022-02025-0.

How does COVID-19 affect the life cycle environmental impacts of U.S. household energy and food consumption?

The COVID-19 pandemic has reduced travel but led to an increase in household food and energy consumption. Previous studies have explored the changes in household consumption of food and energy during the pandemic; however, the economy-wide environmental implications of these changes have not been investigated. This study addresses the knowledge gap by estimating the life cycle environmental impacts of U.S. households during the pandemic using a hybrid life cycle assessment. The results revealed that the reduction in travel outweighed the increase in household energy consumption, leading to a nationwide decrease in life cycle greenhouse gas emissions (−255 Mton CO2 eq), energy use (−4.46 EJ), smog formation (−9.17 Mton O3 eq), minerals and metal use (−16.1 Mton), commercial wastes (−8.31 Mton), and acidification (−226 kton SO2 eq). However, U.S. households had more life cycle freshwater withdrawals (+8.6 Gton) and slightly higher eutrophication (+0.2%), ozone depletion (+0.7%), and freshwater ecotoxicity (+2.1%) caused by increased household energy and food consumption. This study also demonstrated the environmental trade-offs between decreased food services and increased food consumption at home, resulting in diverse trends for food-related life cycle environmental impacts.

Sustainability Assessment of a Low-Income Building: A BIM-LCSA-FAHP-Based Analysis

The construction industry is one of the most significant consumers of environmental resources worldwide. Faced with the need to produce new buildings, but without further burdening the environment, attempts to improve social, economic, and environmental indicators have turned attention to building construction in recent decades. The objective of this research is to develop a novel framework to assess the most sustainable choice of materials applied to the construction of low-income buildings, according to the three pillars of the Triple Bottom Line (TBL). A BIM-LCSA-FAHP-based model was proposed with the creation of nine different scenarios, where the materials of the structure (precast concrete, cast-in-place concrete, and structural masonry), painting (PVA water-based and acrylic), and roofing (ceramic and fiber cement tiles) varied. The proposed procedure consists of the elaboration of a 3D Building Information Modeling (BIM) model, for which the parameters described above were evaluated according to the Life Cycle Sustainability Assessment (LCSA)-TBL-based criteria, divided into ten sub-criteria, that includes: (1) environmental (acidification, eutrophication, global warming, ozone depletion, smog formation, primary energy, non-renewable energy, and mass total), (2) economic (construction cost) and (3) socio-political issues (community impact). Finally, the Fuzzy Analytical Hierarchy Process (AHP) was used as a multi-criteria decision-making technique that helps in aggregating and classifying the impacts of each scenario in a sustainability index (SI). Regarding the best option for low-income construction, the results indicated that precast concrete when combined with acrylic paint and fiber cement tiles (scenario 3) proved to be the most advantageous and achieved first place in the sustainability index (SI) developed in this work. This methodology is replicable for different construction typologies and several categories of materials, making it a robust decision-aiding tool for engineers, architects, and decision makers.

Analyzing variability and decomposing electricity-generation emission factors for three U.S. states

Electricity generation emission factors (EGEF) quantify the relationship between amount of pollutant emitted and amount of electricity generated. Quantifying variability among calculated EGEFs is important when EGEFs are used for decision-making. Variabilities in EGEF on an annual basis due to variability in the amount of coal, natural gas, and petroleum emissions within the fuel mix are quantified for California, Texas, and New York in 2017. The results show a higher coefficient of variation for SO2 and NOx compared to CO2 EGEF. Changes in the EGEF over time are studied using decomposition analysis for California, Texas, and New York from 1990 to 2017. The results show that the main factor in reducing EGEF in California is the improving generation efficiency of power plants; in Texas, it is the increasing ratio of renewable to non-renewable electricity generation; and in New York, it is the changing mix of fossil fuels that are consumed for electricity generation. The effect of variability in EGEF on environmental impact categories is analyzed. Eutrophication of air, eutrophication of water, and smog formation are subject to high uncertainty because SO2 and NOx EGEFs are used to quantify these impacts, whereas global warming potential has less uncertainty because it only uses CO2 EGEF.

Noble-Metal-Based Catalytic Oxidation Technology Trends for Volatile Organic Compound (VOC) Removal

Volatile organic compounds (VOCs) are toxic and are considered the most important sources for the formation of photochemical smog, secondary organic aerosols (SOAs), and ozone. These can also greatly affect the environment and human health. For this reason, VOCs are removed by applying various technologies or reused after recovery. Catalytic oxidation for VOCs removal is widely applied in the industry and is regarded as an efficient and economical method compared to other VOCs removal technologies. Currently, a large amount of VOCs are generated in industries with solvent-based processes, and the ratio of aromatic compounds is high. This paper covers recent catalytic developments in VOC combustion over noble-metal-based catalysts. In addition, this report introduces recent trends in the development of the catalytic mechanisms of VOC combustion and the deactivation of catalysts, such as coke formation, poisoning, sintering, and catalyst regeneration. Since VOC oxidation by noble metal catalysts depends on the support of and mixing catalysts, an appropriate catalyst should be used according to reaction characteristics. Moreover, noble metal catalysts are used together with non-noble metals and play a role in the activity of other catalysts. Therefore, further elucidation of their function and catalytic mechanism in VOC removal is required.

Analysis of a Haze Weather Process in Eastern China from November 11th to 17th, 2020

This study mainly introduces a large-scale heavy haze weather process in northern China from November 11 to 17, 2020, and analyzes the weather process. Methods such as comparative analysis and data analysis were used in this study. The main reasons for the formation of smog are as follows. First, the relative humidity is high and the water vapor content is relatively high. Second, the atmosphere is in a static and stable state, and suspended particulate matter in the atmosphere is not easy to diffuse and dilute, so it will gradually accumulate in urban areas and suburban areas. Third, there is a temperature inversion in the vertical direction, and the suspended particles in the air are difficult to drift to high altitudes and are blocked at low altitudes and near the ground. Fourth, there must be cooling conditions. Meteorological factors are the external causes for the formation of smog. And inversion intensity has a negative correlation with visibility, which has some indicating importance in fog and haze predictions. Large-scale haze weather mainly occurs in the large-scale atmospheric circulation with weak cold air and good water vapor conditions, and wind near the ground is small. Due to the high humidity in haze weather, fog droplets provide adsorption and reaction sites, accelerating the conversion of gaseous pollutants to liquid particles. At the same time, particles are also easy to act as condensation nucleus to accelerate the generation of haze. When the two interact, the pollutants are not easy to diffuse outward, resulting in agglomeration effect, and the pollution is getting heavier.

Using an alternatives assessment framework to evaluate waterborne versus solventborne basecoats used in automotive refinishing.

We evaluated two types of automotive basecoats used in automotive refinishing (i.e., in auto body shops): waterborne and solventborne. The primary tool we used for this evaluation was the Alternatives Assessment Guide (version 1.0) developed by the Interstate Chemicals Clearinghouse (IC2). The term "solventborne," as used in this article, refers to products that contain relatively high concentrations of volatile organic compounds (referred to as "high-volatile organic compound [VOC] basecoats" in the industry). These products are distinct from the "low-VOC" solventborne products used widely in California and elsewhere. From a health and environmental hazard perspective, our evaluation found that waterborne basecoats contained fewer hazardous ingredients and at lower concentrations than their solventborne counterparts. Automotive painters who spray-painted vehicles with waterborne products experienced significantly lower exposures to several harmful solvents. Waterborne products are readily available and offer advantages with regard to social impacts. Some of these advantages include lower worker and community exposure to VOCs, reduced VOC release and smog formation, and reduced potential for generation of and exposure to hazardous waste. Based on our assessment in auto body shops, we consider waterborne basecoats to be safer alternatives from both a human health and environmental perspective. Integr Environ Assess Manag 2022;18:1101-1113. Published 2021. This article is a U.S. Government work and is in the public domain in the USA.

Green Coalescent Synthesis Based on the Design for Environment (DfE) Principles: Brazilian Experience

Coalescents are compounds with a high potential for generating tropospheric ozone, which causes adverse effects on humans and their surroundings. This study designed a coalescent for decorative paints that reached technical levels equivalent to those obtained by StC, a product on the market, but with better environmental and economic performance. The strategy adopted in creating the green coalescent (GrC) improved film formation and reduced the product’s atmospheric emission rate. Regarding the environmental performance, GrC outperformed StC in terms of water consumption, global warming potential, and human toxicity by 30%, 35%, and 91%, but had a high smog formation potential even with a reduced loss to the air. The redesign of the molecule gave rise to AGrC, which achieved a more homogeneous environmental performance. The results of an economic analysis indicated that the procedures adopted to reduce environmental impacts could also make the coalescent more competitive if the lowest market prices were practiced. On the other hand, if the products are sold at high prices, the paint industry tends to privilege the economic dimension and disregard environmental performance for decision purposes. This research succeeded in reconciling technical functions and aspects related to sustainability to design more competitive products in the Brazilian market.

Comparative Life Cycle Assessment of Gasification and Landfilling for Disposal of Municipal Solid Wastes

Disposal of municipal solid wastes (MSW) remains a challenge to minimize its impacts on the environment and human health. Landfilling, currently the most common method used for MSW disposal, occupies land space and leads to soil and air emissions. Gasification, an alternative MSW disposal method, can convert waste to energy, but can also lead to soil and air emissions and is a more extensive operation. In this study, life cycle assessments (LCA) of the two disposal methods (landfilling without energy recovery and gasification) were compared to understand impacts on environment and health. The LCA was conducted following the ISO 14040 standards with one ton of MSW as the functional unit. The life cycle inventory was obtained from published journals, technical reports, LandGEM, HELP and GREET database. The impact assessment was done using TRACI 2.1 and categorized into eight groups. The LCA revealed that landfilling is a higher contributor in global warming, acidification, smog formation, eutrophication, ecotoxicity and human health cancer and non-cancer categories. The negative environmental impacts of MSW landfilling can be primarily attributed to the fate of leachate loss and landfill gas, while those of the MSW gasification can be attributed to the disposal of its solid residues.

Vehicle Emissions and Air Quality: The Early Years (1940s–1950s)

During the 1940s, an unusual form of air pollution was experienced in the Los Angeles (LA) area of Southern California. Referred to as LA smog, this pollution differed from previously known air pollution with respect to its temporal patterns (daytime formation and nighttime dissipation), eye irritation, high oxidant levels, and plant damage. Early laboratory and field experimentation discovered the photochemical origins of LA smog. Though mechanistic understanding was incomplete, it was determined that hydrocarbon (HC) compounds in the atmosphere participate in smog formation, enabling build-up of higher ozone concentrations than would otherwise occur. It being a significant source, there was great interest in characterizing and controlling HC emissions from motor vehicles. Considerable work was done in the 1940s and 1950s to understand how emissions varied with vehicle operating conditions and deterioration of engine components. During this time, procedures were developed (and improved) to sample and quantify vehicle emissions. Besides exhaust, HC emissions from crankcase blowby, carburetor evaporation, and fuel tank losses were measured and characterized. Initial versions of both catalytic and non-catalytic exhaust after-treatment systems were developed. The knowledge gained from this pre-1960 work laid the foundation for many advancements that reduced vehicle emissions and improved air quality during subsequent decades.

Adsorption of NO2 and subsequent formation of nitrate species in the dark on TiO2 nanoparticles exhibiting different morphologies: An in-situ FTIR study

Nitrogen oxides (NOx) are among the major air pollutants, which are responsible for the photochemical smog and haze formation and can cause acid rain. The selective catalytic reduction (SCR) process is a promising method to control the NOx emission from various stationery and mobile resources. The chemisorption of NO2 is regarded as the crucial step in this heterogeneous catalytic process. Here, the adsorption and formation of nitrate species on TiO2 nanoparticles (NPs) with different morphologies, designated as TiO2 trigonal, TiO2 nanowires, TiO2 nanocubes was examined in the dark using an in-situ FTIR spectroscopy, under NO2 as the feeding gas. The N2O4 formation is the initial step as soon as TiO2 NPs are subjected to NO2 adsorption, that is simultaneously transformed into other nitrate species via intramolecular disproportionation reaction. However, TiO2 NPs with different morphologies displayed a distinguished trend towards the stability and formation of various nitrate species. In particular, formation of NO+ was noticed only in TiO2 nanowires and TiO2 nanocubes (10mL_HF/40 h). Also, NO2 was detected in TiO2 trigonal shaped NPs, TiO2 nanocubes (4mL_HF/24 h), and TiO2 nanocubes (10mL_HF/24 h). The formation of different NO3- species, including monodentate, bidentate, and bridged NO3- (m-, b-, br-NO3-) were also observed. This study provides valuable information to modify the TiO2-based supports for the selective catalytic reduction technology.

Cradle-to-Grave Life-Cycle Assessment of Cellulosic Fiberboard

According to the United Nations Environment Programme (UNEP), the construction and operation of buildings accounted for nearly 38% of total global energy-related CO₂ emissions in 2019. The construction sector has been striving to use more low-carbon footprint building products to mitigate climate change and enhance environmentally preferable purchasing. Over the last several decades, there has been substantial growth in engineered wood products for the construction industry. To assess these products used in construction for their environmental profile, lifecycle assessments (LCAs) are performed. This study performed an LCA to estimate environmental impacts (cradle-to-gate and gate-to-grave) of cellulosic fiberboard (CFB) per m³ functional unit basis. The lifecycle inventory data developed were representative of CFB production in North America. Overall, the cradle-to-grave LCA results per m3 of CFB were estimated at 305 kg CO₂ e global warming (GW), 19.3 kg O₃ e photochemical smog formation, 1.03 kg SO₂ e acidification, 0.33 kg N e eutrophication, and 415 MJ fossil-fuel depletion. Except for smog formation, most environmental impacts of CFB were from cradle-to-gate. For example, 71% and 29% of total GW impacts were from cradle-to-gate and gate-to-grave lifecycle stages, respectively. The sensitivity analysis showed that reducing transport distance, on-site electricity use, natural gas for drying, and starch additives in the manufacturing phase had the most influence. Around 353 kg CO₂ e/m³ of CFB is stored as long-term carbon during CFB’s life which is higher than the total cradle-to-grave greenhouse gases (CO₂ e) emissions. Thus, the net negative GW impact of CFB (-47 kg CO₂ e/m³ of CFB) asserted its environmental advantages as an engineered wood panel construction material. Overall, the findings of the presented study would prove useful for improving the decision-making in the construction sector.

Environmental impact assessment of biomass supported electricity generation for sustainable rural energy systems - A case study of Grenada County, Mississippi, USA

Fossil fuels have been the major source of electricity supply in the United States of America (USA) for many decades. While a significant shift has occurred from coal-dominated supply to natural gas and renewable source based supplies in recent decades, there is still large potential to improve biomass supported electricity contribution in rural communities, which depend majorly on forest-related activities. Wood waste, an underutilized renewable resource, has the potential for electricity generation in rural areas to reduce the contribution of electricity generated from fossil fuels and assist in greenhouse gas savings. In this study, a life cycle assessment approach was used to estimate the emissions of electricity generated by wood residues in a rural community (Grenada County, Mississippi (MS), USA) and compared with those emissions from the recent electricity mix (2018), a previous electricity mix (2010) supply, along with natural gas (NG) and coal (Bituminous Coal, BC) based electricity generation options. A significant reduction (85.9–94.6%) in global warming impact was observed when compared with BC, NG and grid supplied electricity for years 2010 and 2018. When compared to the current electricity mix at the grid (2018), acidification, eutrophication, respiratory effects and smog formation showed higher emissions ranging from 30.8% to 72.4%. The sensitivity analysis showed an improvement in emissions savings with increased biomass to power conversion efficiency (40%), lower moisture content of the biomass (0%, bone dry biomass), and reduced transportation distance (35 km). This study showed an improvement in the electricity mix supplied to Grenada county, MS, which implies sustainable development opportunities for promoting energy security in rural communities with forest-based industries.

Ultraviolet Radiation Environment of a Tropical Megacity in Transition: Mexico City 2000-2019.

Tropical regions experience naturally high levels of UV radiation, but urban pollution can reduce these levels substantially. We analyzed 20 years of measurements of the UV index (UVI) at several ground-level locations in the Mexico City Metropolitan Area and compared these data with the UVI values derived from the satellite observations of ozone and clouds (but not local pollution). The ground-based measurements were systematically lower than the satellite-based estimates by ca. 40% in 2000 and 25% in 2019. Calculations with a radiative transfer model using observed concentrations of air pollutants explained well the difference between satellite and ground-based UVI and showed specific contributions from aerosols, O3, NO2, and SO2 in decreasing order of importance. Such large changes in UV radiation between 2000 and 2019 have important implications ranging from human health (skin cancer and cataract induction) to air pollution control (photochemical smog formation).

A review on nitrogen transformation and conversion during coal pyrolysis and combustion based on quantum chemical calculation and experimental study

• Review of nitrogen migration and transformation based on quantum chemistry and experiment. • Summarized the mechanism and regulation of nitrogen migration and transformation. • Emphasized the importance of N 2 formation during pyrolysis. The emission of NO x during coal combustion contributes to the formation of acid rain and photochemical smog, which would seriously affect the quality of atmospheric environment. Therefore, the decrease of NO x is of great importance for improving the efficient utilization of coal. The present review comprehensively summarized the influence factors and mechanisms of migration and transformation of nitrogen during the coal pyrolysis and combustion based on experimental study and quantum chemical calculation. Firstly, in the process of pyrolysis: the occurrence state and transformation of nitrogen were concluded. The influence of temperature, atmosphere, heating rate and catalyst on formation of NO x precursor and nitrogen migration path at the molecular level were summarized; Secondly, during the process of combustion: the influence of temperature, ambient oxygen concentration, physical structure of coal char, catalyst on heterogeneous oxidation of char (N) were summarized; The effects of char surface properties, catalyst and ambient atmosphere on heterogeneous reduction of NO x were also concluded. Based on the quantum chemical calculation, the reaction path of heterogeneous oxidation of char-N and heterogeneous reduction of NO x were described in detail. Current studies focus more on the generation of HCN and NH 3 , but in order to reduce the pollution of NO x from the source, it is necessary to further improve the process conditions and the optimal formula of producing more N 2 during pyrolysis, as well as clarify the path of the generation of N 2 . Experiments study and quantum chemistry calculation should be combined to complete the research of directional nitrogen reduction during pyrolysis and denitration during combustion.