Global partitioning of NOx sources using satellite observations: Relative roles of fossil fuel combustion, biomass burning and soil emissions

Identification of NOx emissions and source characteristics by TROPOMI observations - A case study in north-central Henan, China

With the development of industries, air pollution in north-central Henan is becoming increasingly severe. The TROPOspheric Monitoring Instrument (TROPOMI) provides nitrogen dioxide (NO2) column densities with high spatial resolution. Based on TROPOMI, in this study, the nitrogen oxides (NOx) emissions in north-central Henan are derived and the emission hotspots are identified with the flux divergence method (FDM) from May to September 2021. The results indicate that Zhengzhou has the highest NOx emissions in north-central Henan. The most prominent hotspots are in Guancheng Huizu District (Zhengzhou) and Yindu District (Anyang), with emissions of 448.4 g/s and 300.3 g/s, respectively. The Gaussian Mixture Model (GMM) is applied to quantify the characteristics of emission hotspots, including the diameter, eccentricity, and tilt angle, among which the tilt angle provides a novel metric for identifying the spatial distribution of pollution sources. Furthermore, the results are compared with the CAMS global anthropogenic emissions (CAMS-GLOB-ANT) and Multi-resolution Emission Inventory model for Climate and air pollution research (MEIC), and they are generally in good agreement. However, some point sources, such as power plants, may be missed by both inventories. It is also found that for emission hotspots near transportation hubs, CAMS-GLOB-ANT may not have fully considered the actual traffic flow, leading to an underestimation of transportation emissions. These findings provide key information for the accurate implementation of pollution prevention and control measures, as well as references for future optimization of emission inventories. Consequently, deriving NOx emissions from space, quantifying the characteristics of emission hotspots, and combining them with bottom-up inventories can provide valuable insights for targeted emission control.

Nitrogen Isotopes Reveal High NOx Emissions from Arid Agricultural Soils in the Salton Sea Air Basin

Air quality management commonly aims to mitigate emissions of oxides of nitrogen (NO x ) from combustion, reducing ozone and particulate matter pollution. Despite such efforts, regulations have recently proven ineffective in rural areas like the Salton Sea Air Basin of Southern California, which routinely violates air quality standards. With $2 billion in annual agricultural sales and low population density, air quality in the region is likely influenced by year-round farming. We conducted NO x source apportionment using nitrogen stable isotopes of ambient NO 2 , which indicate a substantial contribution of soil-emitted NO x . The soil source strength was estimated based on the mean δ 15 N-NO x from each emission category in the California Air Resources Board's NO x inventory. Our annual average soil emission estimate for the air basin was 11.4 ± 4 tons/d, representing ~ 30% of the extant NO x inventory, 10× larger than the state's inventory. Therefore, the impact of soil NO x in agricultural regions must be re-evaluated.

OMI-based emission source classification in East China and its spatial redistribution in view of pollution control measures

This study aims to generate a satellite-based qualitative emission source characterization for the heavily polluted eastern part of China in the 2010-2016 time period. The applied source identification technique relies on satellite-based NOx and SO2 emission estimates by OMI, their SO2:NOx ratio, and the MIX anthropogenic emission inventory to distinguish emissions from different emission categories (urban, industrial, natural) and characterize the dominant source per 0.25° × 0.25° grid cell in East China. Overall, we find good agreement between the satellite- and emission inventory-based spatiotemporal distribution and characterization of the dominant emission sources in East China in 2010-2016. In 2010, the satellite measurements suggest an emission distribution less dominated by industrial areas, a somewhat larger role for urban/transportation areas and agricultural activities, and more natural emissions in the southern part compared to the bottom-up emission categorization. In 2016, more than half of the classified emission categories over East China have remained the same. At the same time, there is a notable increase of agricultural lands and decrease of areas dominated by industry/transportation in 2016, suggestive of an overall decrease in heavy air pollution in East China over the course of 7 years. This is likely attributed to the sustained efforts of the Chinese government to drastically improve the air quality, especially since 2013 when the National Air Pollution Prevention and Control Action Plan was enacted. However, signs of urban expansion (urbanization) and rural-urban migration ("Go West" motion) stemmed from China's rapid economic growth and labour demand are evident; escalating industrialization (even with cleaner means) and the urban population growth in East China resulted in stronger emissions from sources representing consumption and transportation which are strongly related to NO2 and PM10 pollution (rather than SO2) and are directly influenced by the population size. This resulted to a shift of the emissions from the east mainly to the north and northwest of East China. Overall, although the effectiveness of the Chinese environmental control policies has been successful, the air pollution problem remains an important concern.

In Situ Sampling of NOx Emissions from United States Natural Gas Flares Reveals Heavy-Tail Emission Characteristic

Natural gas flaring is a common practice employed in many United States (U.S.) oil and gas regions to dispose of gas associated with oil production. Combustion of predominantly hydrocarbon gas results in the production of nitrogen oxides (NOx). Here, we present a large field data set of in situ sampling of real world flares, quantifying flaring NOx production in major U.S. oil production regions: the Bakken, Eagle Ford, and Permian. We find that a single emission factor does not capture the range of the observed NOx emission factors within these regions. For all three regions, the median emission factors fall within the range of four emission factors used by the Texas Commission for Environmental Quality. In the Bakken and Permian, the distribution of emission factors exhibits a heavy tail such that basin-average emission factors are 2-3 times larger than the value employed by the U.S. Environmental Protection Agency. Extrapolation to basin scale emissions using auxiliary satellite assessments of flare volumes indicates that NOx emissions from flares are skewed, with 20%-30% of the flares responsible for 80% of basin-wide flaring NOx emissions. Efforts to reduce flaring volume through alternative gas capture methods would have a larger impact on the NOx oil and gas budget than current inventories indicate.

Air pollution mortality benefits of sustained COVID-19 mobility restrictions in Australian cities

OBJECTIVES

Emissions from road traffic, power generation and industry were substantially reduced during pandemic lockdown periods globally. Thus, we analysed reductions in traffic-related air pollution in Australian capital cities during March-April 2020 and then modelled the mortality benefits that could be realised if similar reductions were sustained by structural policy interventions.

STUDY DESIGN

Satellite, air pollution monitor and land use observations were used to estimate ground-level nitrogen dioxide (NO2) concentrations in all Australian capital cities during: (a) a typical year with no prolonged air pollution events; (b) a hypothetical sustained reduction in NO2 equivalent to the COVID-19 lockdowns.

METHODS

We use the WHO recommended NO2 exposure-response coefficient for mortality (1.023, 95 % CI: 1.008-1.037, per 10 μg/m3 annual average) to assess gains in life expectancy and population-wide years of life from reduced exposure to traffic-related air pollution.

RESULTS

We attribute 1.1 % of deaths to anthropogenic NO2 exposures in Australian cities, corresponding to a total of 13,340 years of life lost annually. Although COVID-19-related reductions in NO2 varied widely between Australian cities during April 2020, equivalent and sustained reductions in NO2 emissions could reduce NO2-attributable deaths by 27 %, resulting in 3348 years of life gained annually.

CONCLUSIONS

COVID-19 mobility restrictions reduced NO2 emissions and population-wide exposures in Australian cities. When sustained to the same extent by policy interventions that reduce fossil fuel consumption by favouring the uptake of electric vehicles, active travel and public transport, the health, mortality and economic benefits will be measurable in Australian cities.

Increasing Nonfossil Fuel Contributions to Atmospheric Nitrate in Urban China from Observation to Prediction

China's nitrogen oxide (NOx) emissions have undergone significant changes over the past few decades. However, nonfossil fuel NOx emissions are not yet well constrained in urban environments, resulting in a substantial underestimation of their importance relative to the known fossil fuel NOx emissions. We developed an approach using machine learning that is accurate enough to generate a long time series of the nitrogen isotopic composition (δ15N) of atmospheric nitrate using high-level accuracies of air pollutants and meteorology data. Air temperature was found to be the critical driver of the variation of nitrate δ15N at daily resolution based on this approach, while significant reductions of aerosol and its precursor emissions played a key role in the change of nitrate δ15N on the yearly scale. Predictions from this model found a significant decrease in nitrate δ15N in Chinese megacities (Beijing and Guangzhou as representative cities in the north and south, respectively) since 2013, implying an enhanced contribution of nonfossil fuel NOx emissions to nitrate aerosols (up to 22%-26% in 2021 from 18%-22% in 2013 quantified by an isotope mixing model), as confirmed by the Weather Research and Forecasting model coupled with online chemistry (WRF-Chem) simulation. Meanwhile, the declining contribution in coal combustion (34%-39% in 2013 to 31%-34% in 2021) and increasing contribution of natural gas combustion (11%-14% in 2013 to 14%-17% in 2021) demonstrated the transformation of China's energy structure from coal to natural gas. This approach provides missing records for exploring long-term variability in the nitrogen isotope system and may contribute to the study of the global reactive nitrogen biogeochemical cycle.

Large Contribution of Nitrous Acid to Soil-Emitted Reactive Oxidized Nitrogen and Its Effect on Air Quality

Soil emissions have long been recognized as an important source of nitric oxide (NO), which regulates atmospheric oxidative capacity and the production of air pollutants. Recent research has also indicated that nitrous acid (HONO) can be emitted in significant quantities from soil microbial activities. However, only a few studies have quantified emissions of HONO along with NO from a wide range of soil types. In this study, we measured emissions of HONO and NO from soil samples collected from 48 sites across China and found much higher emissions of HONO than of NO, especially for samples from northern China. We performed a meta-analysis of 52 field studies in China, which revealed that long-term fertilization increased the abundance of nitrite-producing genes much more than the abundance of NO-producing genes. This promotion effect was greater in northern China than in southern China. In simulations using a chemistry transport model with laboratory-derived parametrization, we found that HONO emissions had a greater effect than NO emissions on air quality. Moreover, we determined that with projected continuous reductions in anthropogenic emissions, the contribution from soils to maximum 1 h concentrations of hydroxyl radicals and ozone and daily average concentrations of particulate nitrate in the Northeast Plain will increase to 17%, 4.6%, and 14%, respectively. Our findings highlight the need to consider HONO in the assessment of the loss of reactive oxidized nitrogen from soils to the atmosphere and its effect on air quality.

A High-Precision Mid-Infrared Spectrometer for Ambient HNO3 Measurements

Precise and accurate measurements of ambient HNO3 are crucial for understanding various atmospheric processes, but its ultra-low trace amounts and the high polarity of HNO3 have strongly hindered routine, widespread, direct measurements of HNO3 and restricted field studies to mostly short-term, localized measurement campaigns. Here, we present a custom field-deployable direct absorption laser spectrometer and demonstrate its analytical capabilities for in situ atmospheric HNO3 measurements. Detailed laboratory characterizations with a particular focus on the instrument response under representative conditions for tropospheric measurements, i.e., the humidity, spectral interference, changing HNO3 amount fractions, and air-sampling-related artifacts, revealed the key aspects of our method: (i) a good linear response (R2 > 0.98) between 0 and 25 nmol·mol−1 in both dry and humid conditions with a limit of detection of 95 pmol·mol−1; (ii) a discrepancy of 20% between the spectroscopically derived amount fractions and indirect measurements using liquid trapping and ion chromatography; (iii) a systematic spectral bias due to water vapor. The spectrometer was deployed in a three-week field measurement campaign to continuously monitor the HNO3 amount fraction in ambient air. The measured values varied between 0.1 ppb and 0.8 ppb and correlated well with the daily total nitrates measured using a filter trapping method.

Local and regional air pollution characteristics in Cyprus: A long-term trace gases observations analysis

Observations of key gaseous trace pollutants, namely NO, NO_y, CO, SO₂ and O_3, performed at several curb, residential, industrial, background and free-troposphere sites were analyzed to assess the temporal and spatial variability of pollution in Cyprus. Notably, the analysis utilized one of the longest datasets of 17 years of measurements (2003-2019) in the East Mediterranean and the Middle East (EMME). This region is considered a regional hotspot of ozone and aerosol pollution. A trend analysis revealed that at several stations, a statistically significant decrease in primary pollutant concentration is recorded, most likely due to pollution control strategies. In contrast, at four stations, a statistically significant increase in ozone levels, ranging between 0.36 ppb_v y^-1 and 0.82 ppb_v y^-1_, has been observed, attributed to the above strategies targeting the reduction of nitrogen oxides (NO_x) but not that of Volatile Organic Compounds (VOCs). The NO and NO_y, and CO levels at the Agia Marina regional background station were two orders of magnitude and four times lower, respectively, than the ones of the urban centers. The latter denotes that local emissions are not negligible and control a large fraction of the observed interannual and diurnal variability. Speciation analysis showed that traffic and other local emissions are the sources of urban NO and NO_y. At the same time, 46 % of SO₂ and 40 % of CO, on average, originate from long-range regional transport. Lastly, a one-year analysis of tropospheric NO₂ vertical columns from the TROPOMI satellite instrument revealed a west-east low-to-high gradient over the island, with all major hotspots, including cities and powerplants, being visible from space. With the help of an unsupervised machine learning approach, it was found that these specific hotspots contribute overall around 10 % to the total NO₂ tropospheric columns.

Synergistic effects of biogenic volatile organic compounds and soil nitric oxide emissions on summertime ozone formation in China

Natural emissions play a key role in modulating the formation of ground-level ozone (O₃), especially emissions of biogenic volatile organic compounds (BVOCs) and soil nitric oxide (SNO), and their individual effects on O₃ formation have been previously quantified and evaluated. However, their synergistic effects remain unclear and have not yet been well assessed. By applying the Weather Research and Forecasting (WRF) model coupled with the Chemistry-Model of Emissions of Gases and Aerosols from Nature (WRF/Chem-MEGAN) model, this study reveals that in the presence of sufficient BVOC emissions, which act as a fuel, SNO emissions act as a fuel additive and promote the chemical reactions of BVOCs and the subsequent production of O₃. Consequently, the synergistic effects of BVOC and SNO emissions on summertime O₃ production surpassed the sum of their individual effects by as much as 10-20 μg m^-3 in eastern China in 2014. In order to reduce O₃ concentration to a level corresponding to no natural emissions of BVOC or SNO (i.e., the BASE scenario), the anthropogenic volatile organic compound (AVOC) emissions in the scenario considers BVOC and SNO emissions must be reduced by 1.76 times that of the BASE scenario. This study demonstrates that the synergistic effects of BVOC and SNO emissions can impede ground-level O₃ regulation and can subsequently impose stricter requirements on anthropogenic precursor emission control in China. The results of this study can also inform efforts in other regions that are still combating ground-level O₃ pollution.

Exploring the environmental impact of crop production in China using a comprehensive footprint approach

The carbon-nutrient-water cycles of farmland ecosystem not only provides support for crop production, but also has an impact on the environment. Comprehensively quantifying the impact of crop production on the environment can provide a basis for crop sustainable production. A series of environmental footprint approaches, including carbon footprint (CF), nitrogen footprint (NF) and water footprint (WF), were optimized to evaluate greenhouse gas (GHG) emissions, reactive nitrogen (Nr) loss and water resource consumption in crop production, and a comprehensive footprint method based on Endpoint modeling was proposed to evaluate the overall environmental impact of crop production in China. The CF, NF and WF of 28 forms of crop production varied from 1206.29 kg CO₂ equivalent (CO₂-eq) ha^-1 of oil crops to 7326.37 kg CO₂-eq ha^-1 of fiber crops, 16.07 kg Nr-eq ha^-1 of oil crops to 60.70 kg Nr-eq ha^-1 of sugar crops, and 4032.04 m³ ha^-1 oil crops to 12,476.28 m³ ha^-1 of sugar crops, respectively. The contribution of each component to footprints varied greatly among different crops, and fertilizer manufacture, NH₃ volatilization and green WF were generally the main contributors of CF, NF and WF, respectively. The total GHG emissions, Nr loss and water consumption were estimated to be 670.11 Tg CO₂-eq, 5.50 Tg Nr-eq and 837.06 G m³ for all crop production of China. The greenhouse vegetable with the highest area-scaled comprehensive footprint was 4.5 times that of the oil crops which had the lowest one. The contribution of crop production to the corresponding environmental impact in China was as low as 3.7%, of which NH₃ volatilization contributed 48% and grain production contributed 72%. Mineral N fertilization was the main driver of the variation of comprehensive footprint between provinces, with reduction of N fertilizer application as an important way to reduce the environmental impact of crop production.

Isotopic constraints confirm the significant role of microbial nitrogen oxides emissions from the land and ocean environment

Nitrogen oxides (NO_x, the sum of nitric oxide (NO) and N dioxide (NO₂)) emissions and deposition have increased markedly over the past several decades, resulting in many adverse outcomes in both terrestrial and oceanic environments. However, because the microbial NO_x emissions have been substantially underestimated on the land and unconstrained in the ocean, the global microbial NO_x emissions and their importance relative to the known fossil-fuel NO_x emissions remain unclear. Here we complied data on stable N isotopes of nitrate in atmospheric particulates over the land and ocean to ground-truth estimates of NO_x emissions worldwide. By considering the N isotope effect of NO_x transformations to particulate nitrate combined with dominant NO_x emissions in the land (coal combustion, oil combustion, biomass burning and microbial N cycle) and ocean (oil combustion, microbial N cycle), we demonstrated that microbial NO_x emissions account for 24 ± 4%, 58 ± 3% and 31 ± 12% in the land, ocean and global environment, respectively. Corresponding amounts of microbial NO_x emissions in the land (13.6 ± 4.7 Tg N yr⁻¹), ocean (8.8 ± 1.5 Tg N yr⁻¹) and globe (22.5 ± 4.7 Tg N yr⁻¹) are about 0.5, 1.4 and 0.6 times on average those of fossil-fuel NO_x emissions in these sectors. Our findings provide empirical constraints on model predictions, revealing significant contributions of the microbial N cycle to regional NO_x emissions into the atmospheric system, which is critical information for mitigating strategies, budgeting N deposition and evaluating the effects of atmospheric NO_x loading on the world.

Wet nitrogen (N) deposition to urban Latin America: filling in the gaps with GEOS-Chem

In Latin America, atmospheric deposition is a major vector of nitrogen (N) input to urban systems. Yet, measurements of N deposition are sparse, precluding analysis of spatial patterns, temporal trends, and ecosystem impacts. Chemical transport models can be used to fill these gaps in the absence of dense measurements. Here, we evaluate the performance of a global 3-D chemical transport model in simulating spatial and interannual variation in wet inorganic N (NH₄-N + NO₃-N) deposition across urban areas in Latin America. Monthly wet and dry inorganic N deposition to Latin America were simulated for the period 2006-2010 using the GEOS-Chem Chemical Transport Model. Published estimates of observed wet or bulk inorganic N deposition measured between 2006-2010 were compiled for 16 urban areas and then compared with model output from GEOS-Chem. Observed mean annual inorganic N deposition to the urban study sites ranged from 5.7-14.2 kg ha^-1 yr^-1, with NH₄-N comprising 48-90% of the total. Results show that simulated N deposition was highly correlated with observed N deposition across sites (R² = 0.83, NMB = -50%). However, GEOS-Chem generally underestimated N deposition to urban areas in Latin America compared to observations. Underestimation due to bulk sampler dry deposition artifacts was considered and improved bias without improving correlation. In contrast to spatial variation, the model did not capture year-to-year variation well. Discrepancies between modeled and observed values exist, in part, because of uncertainties in Latin American N emissions inventories. Our findings indicate that even at coarse spatial resolution, GEOS-Chem can be used to simulate N deposition to urban Latin America, improving understanding of regional deposition patterns and potential ecological effects.

Analyzing the role of in situ coal fire in greenhouse gases emission in a coalfield using remote sensing data and their dispersion and source apportionment study

In situ coal fires significantly pollute the environment in many countries of the world. Monitoring these pollutants is challenging due to extensive area coverage and spatial variations. Thus, the present study demonstrates the method of deriving the spatial and temporal profiles of columnar density of three major greenhouse gases (carbon monoxide (CO), sulfur dioxide (SO₂), and nitrogen dioxide (NO₂)) in an in situ coal fire region (Jharia coalfield (JCF), India) using high-resolution satellite data (TROPOMI) of the European Space Agency (ESA). The study also demonstrates a new methodology for estimating greenhouse gas emissions from in situ coal burning. JCF is one of the significant polluted mining regions with multiple in situ coal fire pockets. The columnar density of the gaseous pollutants in the mining region was compared with the same in the rural, urban, and forest regions to identify the major emission inventories. The study results indicated that coal fire is the major source of CO emission in the region, as the CO was high in the fire regions compared to that of the non-fire regions. But, the major source of NO₂ is the traffic, as the NO₂ was high in the city area as compared to other regions. The spatial profile of SO₂ does not reveal the specific emission sources. The study results indicated that TROPOMI onboard satellite sensors could be effectively used for deriving the spatial profiles of greenhouse gaseous in coal fire regions, which further assist in identifying the emission inventories. Furthermore, the satellite-based Earth observations offer information to understand and manage the greenhouse gas emissions over a large area.

Trend reversal from source region to remote tropospheric NO2 columns

Global tropospheric nitrogen dioxide (NO₂) changes have different or even opposite impacts on the photochemical formation of ozone in different regions under different weather and emission condition. However, the changes over regions affected by different levels of human activities are not well known. By using the Ozone Monitoring Instrument (OMI) measurements, we analyzed spatial and temporal variability of tropospheric NO₂ vertical column densities (VCDs) from the megacity to the background regions during 2005-2019. Consistent with previous research, our results show a rapid decline of tropospheric NO₂ column density over regions strongly affected by human activities, especially for source regions. The decline rates of annual mean NO₂ VCDs are up to - 2.44% year^-1, - 2.37% year^-1, and - 1.43% year^-1 over megacities of the USA, Europe, and China, respectively. However, the decreasing rate has slowed, and even reversed to an increasing trend, of tropospheric NO₂ from megacities to developing and remote regions, especially over ocean and background areas less affected by anthropogenic activity. From 2005 to 2019, the NO₂ VCDs over the ocean and background areas increased for all seasons, with the statistically significant (p < 0.05) trends of 1.15%/0.74% year^-1 (MAM), 1.20%/1.06% year^-1 (JJA), 1.16%/0.82% year^-1 (SON), and 0.68%/0.65% year^-1 (DJF), respectively, for ocean/background region. Such decreasing/increasing trends of tropospheric NO₂ over sources/remote regions may prevent the ozone air pollution to be effectively resolved to achieve air quality goals worldwide.

High atmospheric wet nitrogen deposition and major sources in two cities of Yangtze River Delta: Combustion-related NH3 and non-fossil fuel NOx

High ammonia (NH₃) and nitrogen oxide (NO_x) emissions are related to serious air pollution in urban areas and the negative impacts of excessive reactive nitrogen (N) deposition on many ecosystems. However, whether there is a relationship between N deposition rates and their sources with urbanization or not remains unclear in many areas. Here, we investigated the deposition rates of ammonium (NH₄⁺), nitrate (NO₃^-), dissolved organic N, and water-insoluble particular N from July 2017 to June 2018 at two urban and two suburban sites in the Yangtze River Delta (YRD). The δ¹⁵N values of precipitation NH₄⁺ and NO₃^- were measured, and major sources were analyzed using a Bayesian isotope mixing model. Wet N deposition rates were higher in Yangzhou (developing city, 20.3-22.7 kg N ha^-1 yr^-1) than those in Nanjing (developed city, 19.4-20.5 kg N ha^-1 yr^-1), and were higher at urban sites (20.4-22.5 kg N ha^-1 yr^-1) than those at suburban sites (18.7-20.3 kg N ha^-1 yr^-1). δ¹⁵N values of precipitation NH₄⁺ increased with an increase in precipitation pH because ambient acidity affects the equilibrium isotope fractionation between NH₃ and NH₄⁺ and wet scavenging coefficients of NH₃ and particulate NH₄⁺. For NH₄⁺, combustion-related NH₃ sources (62%-65% with 5.5-6.4 kg N ha^-1 yr^-1, including coal combustion, vehicle exhaust, and biomass burning) contributed more than volatilization NH₃ sources (35%-38% with 2.9-3.9 kg N ha^-1 yr^-1, including fertilizer application and waste volatilization). For NO₃^-, non-fossil fuel NO_x sources (50%-63% with 3.4-4.1 kg N ha^-1 yr^-1, including biomass burning and microbial N cycle) were comparable to fossil fuel NO_x sources (37%-50% with 2.4-3.4 kg N ha^-1 yr^-1, including coal combustion and vehicle exhaust). This study evidenced high N deposition rates and the importance of combustion-related NH₃ emissions and non-fossil fuel NO_x emissions in city areas of the YRD.

Sector-Based Top-Down Estimates of NO x , SO2, and CO Emissions in East Asia

Top-down estimates using satellite data provide important information on the sources of air pollutants. We develop a sector-based 4D-Var framework based on the GEOS-Chem adjoint model to address the impacts of co-emissions and chemical interactions on top-down emission estimates. We apply OMI NO₂, OMI SO₂, and MOPITT CO observations to estimate NO _x , SO₂, and CO emissions in East Asia during 2005-2012. Posterior evaluations with surface measurements show reduced normalized mean bias (NMB) by 7% (NO₂)-15% (SO₂) and normalized mean square error (NMSE) by 8% (SO₂)-9% (NO₂) compared to a species-based inversion. This new inversion captures the peak years of Chinese SO₂ (2007) and NO _x (2011) emissions and attributes their drivers to industry and energy activities. The CO peak in 2007 in China is driven by residential and industry emissions. In India, the inversion attributes NO _x and SO₂ trends mostly to energy and CO trend to residential emissions.

Stable isotopic characterization of nitrate wet deposition in the tropical urban atmosphere of Costa Rica

Increasing energy consumption and food production worldwide results in anthropogenic emissions of reactive nitrogen into the atmosphere. To date, however, little information is available on tropical urban environments where inorganic nitrogen is vastly transported and deposited through precipitation on terrestrial and aquatic ecosystems. To fill this gap, we present compositions of water stable isotopes in precipitation and atmospheric nitrate (δ¹⁸O-H₂O, δ²H-H₂O, δ¹⁵N-NO₃^-, and δ¹⁸O-NO₃^-) collected daily between August 2018 and November 2019 in a tropical urban atmosphere of central Costa Rica. Rainfall generation processes (convective and stratiform rainfall fractions) were identified using stable isotopes in precipitation coupled with air mass back trajectory analysis. A Bayesian isotope mixing model using δ¹⁵N-NO₃^- compositions and corrected for potential ¹⁵N fractionation effects revealed the contribution of lightning (25.9 ± 7.1%), biomass burning (21.8 ± 6.6%), gasoline (19.1 ± 6.4%), diesel (18.4 ± 6.0%), and soil biogenic emissions (15.0 ± 2.6%) to nitrate wet deposition. δ¹⁸O-NO₃^- values reflect the oxidation of NO_x sources via the ·OH + RO₂ pathways. These findings provide necessary baseline information about the combination of water and nitrogen stable isotopes with atmospheric chemistry and hydrometeorological techniques to better understand wet deposition processes and to characterize the origin and magnitude of inorganic nitrogen loadings in tropical regions.

Variations in the tropospheric concentration of NO2 in the central west of Brazil, MS, and their relationship with the COVID-19

COVID-19 (coronavirus disease 2019) started in late 2019 in Wuhan, China. Subsequently, the disease was disseminated in several cities around the world, where measures were taken to control the spread of the virus through the adoption of quarantine (social isolation and closure of commercial sectors). This article analyzed the environmental impact of the COVID-19 outbreak in the state of Mato Grosso do Sul, Brazil, regarding the variations of nitrogen dioxide (NO₂) in the atmosphere. NO₂ data from the AURA satellite, in the period before the beginning of the epidemic (2005-2019) and during the adoption of the preventive and control measures of COVID-19 in 2020, were acquired and compared. The results obtained from the analysis showed that the blockade from COVID-19, beginning in March 2020, improved air quality in the short term, but as soon as coal consumption in power plants and refineries returned to normal levels (since June 2020), due to the resumption of works, the pollution levels returned to the level of the previous years of 2020. NO₂ levels showed a significant decrease, since they were mainly associated with the decrease in economic growth and transport restrictions that led to a change in energy consumption and a reduction in emissions. This study can complement the scientific community and policy makers for environmental protection and public management, not only to assess the impact of the outbreak on air quality, but also for its effectiveness as a simple alternative program of action to improve air quality.

Multi isotope systematics of precipitation to trace the sources of air pollutants in Seoul, Korea

Multiple sulfur (δ³⁴S_sulfate, Δ³³S_sulfate, & Δ³⁶S_sulfate), nitrogen and oxygen (δ¹⁵N_nitrate & δ¹⁸O_nitrate) and strontium (⁸⁷Sr/⁸⁶Sr) isotope compositions of precipitation collected from Seoul, South Korea were analyzed to study the sources, transportation and deposition of air pollutants in East Asia. The δ³⁴S_sulfate values (from 1.9 to 14.6‰ with a median of 4.7‰) and the δ¹⁵N_nitrate values (from -2.0 to 13.3‰ with a median of 1.0‰) suggest that fossil fuel use (emission from coal combustion and vehicle exhaust) is a predominant source for sulfur and nitrogen, but other natural sources including biogenic contributions of DMS also add to their total budget. The seasonal variations are observed in δ³⁴S_sulfate and δ¹⁵N_nitrate values (both higher in winter season), which is likely to result from the increase of coal use for domestic heating in China. The δ¹⁸O_nitrate values also varied seasonally depending on the NO_x oxidation pathway, being about 20‰ higher in winter than in summer season. The Δ³³S_sulfate and Δ³⁶S_sulfate values are not anomalous, showing the absence of MIF signals in precipitation of Seoul. The ⁸⁷Sr/⁸⁶Sr ratio of the precipitation samples range from 0.70988 to 0.71487 with a median of 0.71073, indicating the influence of at least three end-member (silicate dust, carbonate dust and anthropogenic emission). Ionic ratios such as (K⁺+NH₄⁺)/(Ca²⁺+Mg²⁺) and Cl^-/Na⁺ suggest the overwhelming effect of anthropogenic input rather than carbonate dust on the end-member with lower ⁸⁷Sr/⁸⁶Sr ratios.

Ultrathin Silicon Nanowires for Optical and Electrical Nitrogen Dioxide Detection

The ever-stronger attention paid to enhancing safety in the workplace has led to novel sensor development and improvement. Despite the technological progress, nanostructured sensors are not being commercially transferred due to expensive and non-microelectronic compatible materials and processing approaches. In this paper, the realization of a cost-effective sensor based on ultrathin silicon nanowires (Si NWs) for the detection of nitrogen dioxide (NO2) is reported. A modification of the metal-assisted chemical etching method allows light-emitting silicon nanowires to be obtained through a fast, low-cost, and industrially compatible approach. NO2 is a well-known dangerous gas that, even with a small concentration of 3 ppm, represents a serious hazard for human health. We exploit the particular optical and electrical properties of these Si NWs to reveal low NO2 concentrations through their photoluminescence (PL) and resistance variations reaching 2 ppm of NO2. Indeed, these Si NWs offer a fast response and reversibility with both electrical and optical transductions. Despite the macro contacts affecting the electrical transduction, the sensing performances are of high interest for further developments. These promising performances coupled with the scalable Si NW synthesis could unfold opportunities for smaller sized and better performing sensors reaching the market for environmental monitoring.

Impacts of Soil NOx Emission on O3 Air Quality in Rural California

Nitrogen oxides (NO_x) are a key precursor in O₃ formation. Although stringent anthropogenic NO_x emission controls have been implemented since the early 2000s in the United States, several rural regions of California still suffer from O₃ pollution. Previous findings suggest that soils are a dominant source of NO_x emissions in California; however, a statewide assessment of the impacts of soil NO_x emission (SNO_x) on air quality is still lacking. Here we quantified the contribution of SNO_x to the NO_x budget and the effects of SNO_x on surface O₃ in California during summer by using WRF-Chem with an updated SNO_x scheme, the Berkeley Dalhousie Iowa Soil NO Parameterization (BDISNP). The model with BDISNP shows a better agreement with TROPOMI NO₂ columns, giving confidence in the SNO_x estimates. We estimate that 40.1% of the state's total NO_x emissions in July 2018 are from soils, and SNO_x could exceed anthropogenic sources over croplands, which accounts for 50.7% of NO_x emissions. Such considerable amounts of SNO_x enhance the monthly mean NO₂ columns by 34.7% (53.3%) and surface NO₂ concentrations by 176.5% (114.0%), leading to an additional 23.0% (23.2%) of surface O₃ concentration in California (cropland). Our results highlight the cobenefits of limiting SNO_x to help improve air quality and human health in rural California.

Urban Emissions of Nitrogen Oxides, Carbon Monoxide, and Methane Determined from Ground-Based Measurements in Philadelphia

Nitrogen oxides (NO_X) and methane impact air quality through the promotion of ozone formation, and methane is also a strong greenhouse gas. Despite the importance of these pollutants, emissions in urban areas are poorly quantified. We present measurements of NO_X, CH₄, CO, and CO₂ made at Drexel University in Philadelphia along with NO_X and CO observations at two roadside monitors. Because CO₂ concentrations in the winter result almost entirely from combustion with negligible influence from photosynthesis and respiration, we are able to infer fleet-averaged fuel-based emission factors (EFs) for NO_X and CO, similar in some ways to how EFs are determined from tunnel studies. Comparison of the inferred NO_X and CO fuel-based EF to the National Emissions Inventory (NEI) suggests errors in NEI emissions of either NO_X, CO, or both. From the measurements of CH₄ and CO₂, which are not emitted by the same sources, we infer the ratio of CH₄ emissions (from leaks in the natural gas infrastructure) to CO₂ emissions (from fossil fuel combustion) in Philadelphia. Comparison of the CH₄/CO₂ emission ratios to emission inventories from the Environmental Protection Agency suggests underestimates in CH₄ emissions by almost a factor of 4. These results demonstrate the need for the addition of long-term observations of CH₄ and CO₂ to existing monitoring networks in urban areas to better constrain emissions and complement existing measurements of NO_X and CO.

Evaluating Drought Responses of Surface Ozone Precursor Proxies: Variations With Land Cover Type, Precipitation, and Temperature

Prior work suggests drought exacerbates US air quality by increasing surface ozone concentrations. We analyze 2005-2015 tropospheric column concentrations of two trace gases that serve as proxies for surface ozone precursors retrieved from the OMI/Aura satellite: Nitrogen dioxide (ΩNO_2; NO_x proxy) and formaldehyde (ΩHCHO; VOC proxy). We find 3.5% and 7.7% summer drought enhancements (classified by SPEI) for ΩNO₂ and ΩHCHO, respectively, corroborating signals previously extracted from ground-level observations. When we subset by land cover type, the strongest ΩHCHO drought enhancement (10%) occurs in the woody savannas of the Southeast US. By isolating the influences of precipitation and temperature, we infer that enhanced biogenic VOC emissions in this region increase ΩHCHO independently with both high temperature and low precipitation during drought. The strongest ΩNO₂ drought enhancement (6.0%) occurs over Midwest US croplands and grasslands, which we infer to reflect the sensitivity of soil NO_x emissions to temperature.

Could Air Quality Get Better during Epidemic Prevention and Control in China? An Analysis Based on Regression Discontinuity Design

Though many scholars and practitioners are paying more attention to the health and life of the public after the COVID-19 outbreak, extant literature has so far failed to explore the variation of ambient air quality during this pandemic. The current study attempts to fill the gap by disentangling the causal effects of epidemic prevention on air quality in China, measured by the individual pollutant dimensionless index, from other confounding factors. Using the fixed effects model, this article finds that five air indicators, PM2.5, PM10, CO, NO2, and SO2, significantly improved during the shutdown period, with NO2 showing the most improvement. On the contrary, O3 shows an inverse pattern, that is, O3 gets worse unexpectedly. The positive impact of epidemic prevention on air quality, especially in terms of PM2.5, PM10, and NO2, become manifest five days after the resumption of labor, indicated by the result of a regression discontinuity design. These findings are still robust and consistent after the dataset of 2019 as a counterfactual sample is utilized. The findings of this paper make contributions to both environmental governance and pandemic prevention, with relevant guidelines regarding the health and life of the public and governmental behavioral management strategies discussed.

Investigating the background and local contribution of the oxidants in London and Bangkok

The contribution of NO_x emissions and background O₃ to the sources and partitioning of the oxidants [OX (= O₃ + NO₂)] at the Marylebone Road site in London during the 2000s and 2010s has been investigated to see the impact of the control measures or technology changes inline with the London Mayor's Air Quality Strategy. The abatement of the pollution emissions has an impact on the trends of local and background oxidants, [OX]_L and [OX]_B, decreasing by 1.4% per year and 0.4% per year, respectively from 2000 to 2019. We also extend our study to three roadside sites (Din Daeng, Thonburi and Chokchai) in another megacity, Bangkok, to compare [OX]_L and [OX]_B and their behavioural changes with respect to the Marylebone Road site. [OX]_L and [OX]_B at the Marylebone Road site (0.21[NO_x] and 32 ppbv) are comparable with the roadside sites of Thailand (0.12[NO_x] to 0.26[NO_x] and 29 to 32 ppbv). The seasonal variation of [OX]_B levels displays a spring maximum for London, which is due to the higher northern hemispheric ozone baseline, but a maximum during the dry season is found for Bangkok which is likely due to regional-scale long-range transport from the Asian continent. The diurnal variations of [OX]_L for both London and Bangkok roadside sites confirm the dominance of the oxidants from road transport emissions, which are found to be higher throughout the daytime. WRF-Chem-CRI model simulations of the distribution of [OX] showed that the model performed well for London background sites when predicting [OX] levels compared with the measured [OX] levels suggesting that the model is treating the chemistry of the oxidants correctly. However, there are large discrepancies for the model-measurement [OX] levels at the traffic site because of the difficulties in the modelling of [OX] at large road networks in megacities for the complex sub grid-scale dynamics that are taking place, both in terms of atmospheric processes and time-varying sources, such as traffic volumes. For roadside sites in Bangkok, the trend in changes of [OX] is predicted by the model correctly but overestimated in absolute magnitude. We suggest that this large deviation is likely to be due to discrepancies in the EDGAR emission inventory (emission overestimates) beyond the resolution of the model.

Reductions in NO2 burden over north equatorial Africa from decline in biomass burning in spite of growing fossil fuel use, 2005 to 2017

Socioeconomic development in low- and middle-income countries has been accompanied by increased emissions of air pollutants, such as nitrogen oxides [NO_x: nitrogen dioxide (NO₂) + nitric oxide (NO)], which affect human health. In sub-Saharan Africa, fossil fuel combustion has nearly doubled since 2000. At the same time, landscape biomass burning-another important NO_x source-has declined in north equatorial Africa, attributed to changes in climate and anthropogenic fire management. Here, we use satellite observations of tropospheric NO₂ vertical column densities (VCDs) and burned area to identify NO₂ trends and drivers over Africa. Across the northern ecosystems where biomass burning occurs-home to hundreds of millions of people-mean annual tropospheric NO₂ VCDs decreased by 4.5% from 2005 through 2017 during the dry season of November through February. Reductions in burned area explained the majority of variation in NO₂ VCDs, though changes in fossil fuel emissions also explained some variation. Over Africa's biomass burning regions, raising mean GDP density (USD⋅km^-2) above its lowest levels is associated with lower NO₂ VCDs during the dry season, suggesting that economic development mitigates net NO₂ emissions during these highly polluted months. In contrast to the traditional notion that socioeconomic development increases air pollutant concentrations in low- and middle-income nations, our results suggest that countries in Africa's northern biomass-burning region are following a different pathway during the fire season, resulting in potential air quality benefits. However, these benefits may be lost with increasing fossil fuel use and are absent during the rainy season.

Nitrogen dioxide and asthma emergency department visits in California, USA during cold season (November to February) of 2005 to 2015: A time-stratified case-crossover analysis

Nitrogen dioxide (NO₂) is responsible for aggravating respiratory diseases, particularly asthma. The aim of this study is to investigate the association between NO₂ exposure and asthma emergency department (ED) visits during the cold season (November-February) in five populated locations (Sacramento, San Francisco, Fresno, Los Angeles, and San Diego) of California from 2005 to 2015 (1320 Days). Conditional logistic regression models were used to obtain the odds ratio (OR) and 95% confidence interval (CI) associated with a 5 ppb increase in NO₂ concentration for the 19,735 ED visits identified. An increase in NO₂ exposure increased the odds of having asthma ED visits for the studied population. The potential effect modification by sex (female and male), race (White, Black, Hispanic, and Asian), and age (2-5, 6-18, 19-40, 41-64, and ≥65) was explored. A 5 ppb increase in the concentration of NO₂ during lag 0-30 was associated with a 56% increase in the odds of having an asthma ED visit (OR = 1.560, CI: 1.428-1.703). Sex was not found to be a modifier. Asthma ED visits among all the races/ethnicities (except Asians) were associated with NO₂ exposure. Whites had the highest OR 75% (OR = 1.750, CI: 1.417-2.160) at lag 0-30 in response to NO₂ exposure. The association between NO₂ exposure and asthma ED visits was positive among all age groups except for 19 to 40 years old; the OR was higher among 2 to 18 year old (at lag 0-30: age group 2-5 (OR = 1.699, CI: 1.399-2.062), and age group 6-18 (OR = 1.568, CI 1.348-1.825)). For stratification by location, San Diego and Fresno were found to have the highest OR, compared to the other studied locations.

Important contributions of non-fossil fuel nitrogen oxides emissions

Since the industrial revolution, it has been assumed that fossil-fuel combustions dominate increasing nitrogen oxide (NO_x) emissions. However, it remains uncertain to the actual contribution of the non-fossil fuel NO_x to total NO_x emissions. Natural N isotopes of NO₃⁻ in precipitation (δ¹⁵N_w-NO3−) have been widely employed for tracing atmospheric NO_x sources. Here, we compiled global δ¹⁵N_w-NO3− observations to evaluate the relative importance of fossil and non-fossil fuel NO_x emissions. We found that regional differences in human activities directly influenced spatial-temporal patterns of δ¹⁵N_w-NO3− variations. Further, isotope mass-balance and bottom-up calculations suggest that the non-fossil fuel NO_x accounts for 55 ± 7% of total NO_x emissions, reaching up to 21.6 ± 16.6Mt yr⁻¹ in East Asia, 7.4 ± 5.5Mt yr⁻¹ in Europe, and 21.8 ± 18.5Mt yr⁻¹ in North America, respectively. These results reveal the importance of non-fossil fuel NO_x emissions and provide direct evidence for making strategies on mitigating atmospheric NO_x pollution.

This study investigates in the importance of non-fossil fuel NO_x emissions in the surface-earth-nitrogen cycle. The study shows how changes of regional human activities directly influence δ¹⁵N signatures of deposited NO_x to terrestrial environments and that emissions have largely been underestimated.

Impacts of the COVID-19 event on the NOx emissions of key polluting enterprises in China

The unprecedented cessation of human activities during the COVID-19 pandemic has affected China's industrial production and NOx emissions. Quantifying the changes in NOx emissions resulting from COVID-19 and associated governmental control measures is crucial to understanding its impacts on the environment. Here, we divided the research timeframe into three periods: the normal operation period (P1), the Spring Festival period (P2), and the epidemic period following the Spring Festival (P3). We then calculated the NOx operating vent numbers and emission concentrations of key polluting enterprises in 29 provinces and 20 industrial sectors and compared the data for the same periods in 2020 and 2019 to obtain the impacts of COVID-19 on industrial NOx emissions. We found that spatially, from P1 to P2 in 2020, the operating NOx vent numbers in North China changed the most, with a relative change rate of -33.84%. Comparing the operating vent numbers in P1 and P3, East China experienced the largest decrease, approximately -32.72%. Among all industrial sectors, the mining industry, manufacturing industry, power, heat, gas, and water production and supply industry, and the wholesale and retail industry, were the most heavily influenced. In general, the operating vent numbers of key polluting enterprises in China decreased by 24.68%, and the standardized NOx (w)_5-day decreased by an average of -9.54 ± -6.00 due to the COVID-19 pandemic. The results suggest that COVID-19 significantly reduced the NOx emission levels of the key polluting enterprises in China.

Reactive Molecular Dynamics Simulations and Quantum Chemistry Calculations To Investigate Soot-Relevant Reaction Pathways for Hexylamine Isomers

Sooting tendencies of a series of nitrogen-containing hydrocarbons (NHCs) have been recently characterized experimentally using the yield sooting index (YSI) methodology. This work aims to identify soot-relevant reaction pathways for three selected C₆H₁₅N amines, namely, dipropylamine (DPA), diisopropylamine (DIPA), and 3,3-dimethylbutylamine (DMBA) using ReaxFF molecular dynamics (MD) simulations and quantum mechanical (QM) calculations and to interpret the experimentally observed trends. ReaxFF MD simulations are performed to determine the important intermediate species and radicals involved in the fuel decomposition and soot formation processes. QM calculations are employed to extensively search for chemical reactions involving these species and radicals based on the ReaxFF MD results and also to quantitatively characterize the potential energy surfaces. Specifically, ReaxFF simulations are carried out in the NVT ensemble at 1400, 1600, and 1800 K, where soot has been identified to form in the YSI experiment. These simulations account for the interactions among test fuel molecules and pre-existing radicals and intermediate species generated from rich methane combustion, using a recently proposed simulation framework. ReaxFF simulations predict that the reactivity of the amines decrease in the order DIPA > DPA > DMBA, independent of temperature. Both QM calculations and ReaxFF simulations predict that C₂H₄, C₃H₆, and C₄H₈ are the main nonaromatic soot precursors formed during the decomposition of DPA, DIPA, and DMBA, respectively, and the associated reaction pathways are identified for each amine. Both theoretical methods predict that sooting tendency increases in the order DPA, DIPA, and DMBA, consistent with the experimentally measured trend in YSI. This work demonstrates that sooting tendencies and soot-relevant reaction pathways of fuels with unknown chemical kinetics can be identified efficiently through combined ReaxFF and QM simulations. Overall, predictions from ReaxFF simulations and QM calculations are consistent, in terms of fuel reactivity, major intermediates, and major nonaromatic soot precursors.

Global high-resolution emissions of soil NOx, sea salt aerosols, and biogenic volatile organic compounds

Natural emissions of air pollutants from the surface play major roles in air quality and climate change. In particular, nitrogen oxides (NOx) emitted from soils contribute ~15% of global NOx emissions, sea salt aerosols are a major player in the climate and chemistry of the marine atmosphere, and biogenic emissions are the dominant source of non-methane volatile organic compounds at the global scale. These natural emissions are often estimated using nonlinear parameterizations, which are sensitive to the horizontal resolutions of inputted meteorological and ancillary data. Here we use the HEMCO model to compute these emissions worldwide at horizontal resolutions of 0.5° lat. × 0.625° lon. for 1980-2017 and 0.25° lat. × 0.3125° lon. for 2014-2017. We further offer the respective emissions at lower resolutions, which can be used to evaluate the impacts of resolution on estimated global and regional emissions. Our long-term high-resolution emission datasets offer useful information to study natural pollution sources and their impacts on air quality, climate, and the carbon cycle.

Assessment of NO2 observations during DISCOVER-AQ and KORUS-AQ field campaigns

NASA's Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER-AQ, conducted in 2011-2014) campaign in the United States and the joint NASA and National Institute of Environmental Research (NIER) Korea-United States Air Quality Study (KORUS-AQ, conducted in 2016) in South Korea were two field study programs that provided comprehensive, integrated datasets of airborne and surface observations of atmospheric constituents, including nitrogen dioxide (NO2), with the goal of improving the interpretation of spaceborne remote sensing data. Various types of NO2 measurements were made, including in situ concentrations and column amounts of NO2 using ground- and aircraft-based instruments, while NO2 column amounts were being derived from the Ozone Monitoring Instrument (OMI) on the Aura satellite. This study takes advantage of these unique datasets by first evaluating in situ data taken from two different instruments on the same aircraft platform, comparing coincidently sampled profile-integrated columns from aircraft spirals with remotely sensed column observations from ground-based Pandora spectrometers, intercomparing column observations from the ground (Pandora), aircraft (in situ vertical spirals), and space (OMI), and evaluating NO2 simulations from coarse Global Modeling Initiative (GMI) and high-resolution regional models. We then use these data to interpret observed discrepancies due to differences in sampling and deficiencies in the data reduction process. Finally, we assess satellite retrieval sensitivity to observed and modeled a priori NO2 profiles. Contemporaneous measurements from two aircraft instruments that likely sample similar air masses generally agree very well but are also found to differ in integrated columns by up to 31.9 %. These show even larger differences with Pandora, reaching up to 53.9 %, potentially due to a combination of strong gradients in NO2 fields that could be missed by aircraft spirals and errors in the Pandora retrievals. OMI NO2 values are about a factor of 2 lower in these highly polluted environments due in part to inaccurate retrieval assumptions (e.g., a priori profiles) but mostly to OMI's large footprint (> 312 km2).

The observation of isotopic compositions of atmospheric nitrate in Shanghai China and its implication for reactive nitrogen chemistry

The occurrence of PM_2.5 pollution in China is usually associated with the formation of atmospheric nitrate, the oxidation product of nitrogen oxides (NO_X = NO + NO₂). The oxygen-17 excess of nitrate (Δ¹⁷O(NO₃^-)) can be used to reveal the relative importance of nitrate formation pathways and get more insight into reactive nitrogen chemistry. Here we present the observation of isotopic composition of atmospheric nitrate (Δ¹⁷O and δ¹⁵N) collected from January to June 2016 in Shanghai China. Concentrations of atmospheric nitrate ranged from 1.4 to 24.1 μg m^-3 with the mean values being (7.6 ± 4.4 (1SD)), (10.2 ± 5.8) and (4.1 ± 2.4) μg m^-3 in winter, spring and summer respectively. Δ¹⁷O(NO₃^-) varied from 20.5‰ to 31.9‰ with the mean value being (26.9 ± 2.8) ‰ in winter, followed by (26.6 ± 1.7) ‰ in spring and the lowest (23.2 ± 1.6) ‰ in summer. Δ¹⁷O(NO₃^-)-constrained estimates suggest that the conversion of NO_X to nitrate is dominated by NO₂ + OH and/or NO₂ + H₂O, with the mean possible contribution of 55-77% in total and even higher (84-92%) in summer. A diurnal variation of Δ¹⁷O(NO₃^-) featured by high values at daytime (28.6 ± 1.2‰) and low values (25.4 ± 2.8‰) at nighttime was observed during our diurnal sampling period. This trend is related to the atmospheric life of nitrate (τ) and calculations indicate τ is around 15 h during the diurnal sampling period. In terms of δ¹⁵N(NO₃^-), it changed largely in our observation, from -2.9‰ to 18.1‰ with a mean of (6.4 ± 4.4) ‰. Correlation analysis implies that the combined effect of NO_X emission sources and isotopic fractionation processes are responsible for δ¹⁵N(NO₃^-) variations. Our observations with the aid of model simulation in future study will further improve the understanding of reactive nitrogen chemistry in urban regions.

NO2 Sensing Properties of Cr2WO6 Gas Sensor in Air and N2 Atmospheres

Gas sensors were fabricated from Cr₂WO₆ nanoparticles for NO₂ detection. Low dimensional materials Cr₂WO₆ were prepared by a wet chemistry method followed by hydrothermal treatment. The morphology of the nanoparticles and their sensing properties to NO₂ were investigated in both dry and humid conditions. Additionally, the sensing response was also characterized in a non-oxygen condition. It was concluded that the sensor responses in N₂ conditions were higher than that in air conditions at 200°C. Moreover, the sensing characteristics were inhibited by water vapor at 200°C. The oxygen adsorption behavior was also investigated to verify the basic sensing mechanism of Cr₂WO₆ in the absence and presence of NO₂ and water vapor separately. Based on the power law response, it was indicated that both NO₂ and water vapor have a strong adsorption ability than oxygen ions of Cr₂WO₆ sensors.

Present knowledge and controversies, deficiencies, and misconceptions on nitric oxide synthesis, sensing, and signaling in plants

After 30 years of intensive work, nitric oxide (NO) has just started to be characterized as a relevant regulatory molecule on plant development and responses to stress. Its reactivity as a free radical determines its mode of action as an inducer of posttranslational modifications of key target proteins through cysteine S-nitrosylation and tyrosine nitration. Many of the NO-triggered regulatory actions are exerted in tight coordination with phytohormone signaling. This review not only summarizes and updates the information accumulated on how NO is synthesized, sensed, and transduced in plants but also makes emphasis on controversies, deficiencies, and misconceptions that are hampering our present knowledge on the biology of NO in plants. The development of noninvasive accurate tools for the endogenous NO quantitation as well as the implementation of genetic approaches that overcome misleading pharmacological experiments will be critical for getting significant advances in better knowledge of NO homeostasis and regulatory actions in plants.

Temporal Analysis of OMI-Observed Tropospheric NO2 Columns over East Asia during 2006–2015

The study analyzed temporal variations of Ozone Monitoring Instrument (OMI)-observed NO2 columns, interregional correlation, and comparison between NO2 columns and NOx emissions during the period from 2006 to 2015. Regarding the trend of the NO2 columns, the linear lines were classified into four groups: (1) ‘upward and downward’ over six defined geographic regions in central-east Asia; (2) ‘downward’ over Guangzhou, Japan, and Taiwan; (3) ‘stagnant’ over South Korea; and (4) ‘upward’ over North Korea, Mongolia, Qinghai, and Northwestern Pacific ocean. In particular, the levels of NO2 columns in 2015 returned to those in 2006 over most of the polluted regions in China. Quantitatively, their relative changes in 2015 compared to 2006 were approximately 10%. From the interregional correlation analysis, it was found that unlike positive relationships between the polluted areas, the different variations of monthly NO2 columns led to negative relationships in Mongolia and Qinghai. Regarding the comparison between NO2 columns and NOx emission, the NOx emissions from the Copernicus Atmosphere Monitoring Service (CAMS) and Clean Air Policy Support System (CAPSS) inventories did not follow the year-to-year variations of NO2 columns over the polluted regions. In addition, the weekly effect was only clearly shown in South Korea, Japan, and Taiwan, indicating that the amounts of NOx emissions are significantly contributed to by the transportation sector.

Tropospheric NO2 vertical column densities retrieved from ground-based MAX-DOAS measurements at Shangdianzi regional atmospheric background station in China

Ground-basedMulti-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements were performed at Shangdianzi (SDZ) regional atmospheric background station in northern China from March 2009 to February 2011. The tropospheric NO₂ vertical column densities (VCDs) were retrieved to investigate the background condition of the Beijing-Tianjin-Hebei developed economic circle in China. The seasonal variation of mean NO₂ tropospheric VCDs (VCD_Trop) at SDZ is apparent, with the maximum (1.3 × 10¹⁶ molec/cm²) in February and the minimum (3.5 × 10¹⁵ molec/cm²) in August, much lower than those observed at the Beijing city center. The average daytime diurnal variations of NO₂ VCD_Trop are rather consistent for all four seasons, presenting the minimum at noon and the higher values in the morning and evening. The largest and lowest amplitudes of NO₂ VCD_Trop diurnal variation appear in winter and in summer, respectively. The diurnal pattern at SDZ station is similar to those at other less polluted stations, but distinct from the ones at the urban or polluted stations. Tropospheric NO₂ VCDs at SDZ are strongly dependent on the wind, with the higher values being associated with the pollution plumes from Beijing city. Tropospheric NO₂ VCDs derived from ground-based MAX-DOAS at SDZ show to be well correlated with corresponding OMI (Ozone Monitoring Instrument) satellite products with a correlation coefficient R = 0.88. However, the OMI observations are on average higher than MAX-DOAS NO₂ VCDs by a factor of 28%, probably due to the OMI grid cell partly covering the south of SDZ which is influenced more by the pollution plumes from the urban areas.

Mutagenic atmospheres resulting from the photooxidation of aromatic hydrocarbon and NOx mixtures

Although many volatile organic compounds (VOCs) are regulated to limit air pollution and the consequent health effects, the photooxidation products generally are not. Thus, we examined the mutagenicity in Salmonella TA100 of photochemical atmospheres generated in a steady-state atmospheric simulation chamber by irradiating mixtures of single aromatic VOCs, NOx, and ammonium sulfate seed aerosol in air. The 10 VOCs examined were benzene; toluene; ethylbenzene; o-, m-, and p-xylene; 1,2,4- and 1,3,5-trimethylbenzene; m-cresol; and naphthalene. Salmonella were exposed at the air-agar interface to the generated atmospheres for 1, 2, 4, 8, or 16 h. Dark-control exposures produced non-mutagenic atmospheres, illustrating that the gas-phase precursor VOCs were not mutagenic at the concentrations tested. Under irradiation, all but m-cresol and naphthalene produced mutagenic atmospheres, with potencies ranging from 2.0 (p-xylene) to 10.4 (ethylbenzene) revertants m3 mgC-1 h-1. The mutagenicity was due exclusively to direct-acting late-generation products of the photooxidation reactions. Gas-phase chemical analysis showed that a number of oxidized organic chemical species enhanced during the irradiated exposure experiments correlated (r ≥ 0.81) with the mutagenic potencies of the atmospheres. Molecular formulas assigned to these species indicated that they likely contained peroxy acid, aldehyde, alcohol, and other functionalities.

Molecular response to nitrogen starvation by Frankia alni ACN14a revealed by transcriptomics and functional analysis with a fosmid library in Escherichia coli

The transcriptome of Frankia alni strain ACN14a was compared between in vitro ammonium-replete (N-replete) and ammonium-free dinitrogen-fixing (N-fixing) conditions using DNA arrays. A Welch-test (p < 0.05) revealed significant upregulation of 252 genes under N-fixing vs. N-replete (fold-change (FC) ≥ 2), as well as significant downregulation of 48 other genes (FC ≤ 0.5). Interestingly, there were 104 Frankia genes upregulated in vitro that were also significantly upregulated in symbiosis with Alnus glutinosa, while the other 148 genes were not, showing that the physiology of in vitro fixation is markedly different from that under symbiotic conditions. In particular,in vitro fixing cells were seen to upregulate genes identified as coding for a nitrite reductase, and amidases that were not upregulated in symbiosis. Confirmatory assays for nitrite reductase showed that Frankia indeed reduced nitrite and used it as a nitrogen source. An Escherichia coli fosmid clone carrying the nirB region was able to grow better in the presence of 5 mM nitrite than without it, confirming the function of the genome region. The physiological pattern that emerges shows that Frankia undergoes nitrogen starvation that induces a molecular response different from that seen in symbiosis.

Agriculture is a major source of NO x pollution in California

Nitrogen oxides (NO _x = NO + NO₂) are a primary component of air pollution-a leading cause of premature death in humans and biodiversity declines worldwide. Although regulatory policies in California have successfully limited transportation sources of NO _x pollution, several of the United States' worst-air quality districts remain in rural regions of the state. Site-based findings suggest that NO _x emissions from California's agricultural soils could contribute to air quality issues; however, a statewide estimate is hitherto lacking. We show that agricultural soils are a dominant source of NO _x pollution in California, with especially high soil NO _x emissions from the state's Central Valley region. We base our conclusion on two independent approaches: (i) a bottom-up spatial model of soil NO _x emissions and (ii) top-down airborne observations of atmospheric NO _x concentrations over the San Joaquin Valley. These approaches point to a large, overlooked NO _x source from cropland soil, which is estimated to increase the NO _x budget by 20 to 51%. These estimates are consistent with previous studies of point-scale measurements of NO _x emissions from the soil. Our results highlight opportunities to limit NO _x emissions from agriculture by investing in management practices that will bring co-benefits to the economy, ecosystems, and human health in rural areas of California.

Scientific assessment of background ozone over the U.S.: Implications for air quality management

Ozone (O3) is a key air pollutant that is produced from precursor emissions and has adverse impacts on human health and ecosystems. In the U.S., the Clean Air Act (CAA) regulates O3 levels to protect public health and welfare, but unraveling the origins of surface O3 is complicated by the presence of contributions from multiple sources including background sources like stratospheric transport, wildfies, biogenic precursors, and international anthropogenic pollution, in addition to U.S. anthropogenic sources. In this report, we consider more than 100 published studies and assess current knowledge on the spatial and temporal distribution, trends, and sources of background O3 over the continental U.S., and evaluate how it inflattainment of the air quality standards. We conclude that spring and summer seasonal mean U.S. background O3 (USB O3), or O3 formed from natural sources plus anthropogenic sources in countries outside the U.S., is greatest at high elevation locations in the western U.S., with monthly mean maximum daily 8-hour average (MDA8) mole fractions approaching 50 parts per billion (ppb) and annual 4th highest MDA8s exceeding 60 ppb, at some locations. At lower elevation sites, e.g., along the West and East Coasts, seasonal mean MDA8 USB O3 is in the range of 20-40 ppb, with generally smaller contributions on the highest O3 days. The uncertainty in U.S. background O3 is around ±10 ppb for seasonal mean values and higher for individual days. Noncontrollable O3 sources, such as stratospheric intrusions or precursors from wildfires, can make significant contributions to O3 on some days, but it is challenging to quantify accurately these contributions. We recommend enhanced routine observations, focused fi studies, process-oriented modeling studies, and greater emphasis on the complex photochemistry in smoke plumes as key steps to reduce the uncertainty associated with background O3 in the U.S.

The Dawn of Geostationary Air Quality Monitoring: Case Studies from Seoul and Los Angeles

With the near-future launch of geostationary pollution monitoring satellite instruments over North America, East Asia, and Europe, the air quality community is preparing for an integrated global atmospheric composition observing system at unprecedented spatial and temporal resolutions. One of the ways that NASA has supported this community preparation is through demonstration of future space-borne capabilities using the Geostationary Trace gas and Aerosol Sensor Optimization (GeoTASO) airborne instrument. This paper integrates repeated high-resolution maps from GeoTASO, ground-based Pandora spectrometers, and low Earth orbit measurements from the Ozone Mapping and Profiler Suite (OMPS), for case studies over two metropolitan areas: Seoul, South Korea on June 9th, 2016 and Los Angeles, California on June 27th, 2017. This dataset provides a unique opportunity to illustrate how geostationary air quality monitoring platforms and ground-based remote sensing networks will close the current spatiotemporal observation gap. GeoTASO observes large differences in diurnal behavior between these urban areas, with NO2 accumulating within the Seoul Metropolitan Area through the day but NO2 peaking in the morning and decreasing throughout the afternoon in the Los Angeles Basin. In both areas, the earliest morning maps exhibit spatial patterns similar to emission source areas (e.g., urbanized valleys, roadways, major airports). These spatial patterns change later in the day due to boundary layer dynamics, horizontal transport, and chemistry. The nominal resolution of GeoTASO is finer than will be obtained from geostationary platforms, but when NO2 data over Los Angeles are up-scaled to the expected resolution of TEMPO, spatial features discussed are conserved. Pandora instruments installed in both metropolitan areas capture the diurnal patterns observed by GeoTASO, continuously and over longer time periods, and will play a critical role in validation of the next generation of satellite measurement.. These case studies demonstrate that different regions can have diverse diurnal patterns and that day-to-day variability due to meteorology or anthropogenic patterns such as weekday/weekend variations in emissions is large. Low Earth orbit measurements, despite their inability to capture the diurnal patterns at fine spatial resolution, will be essential for intercalibrating the geostationary radiances and cross-validating the geostationary retrievals in an integrated global observing system.

Efficacy of passive sampler collection for atmospheric NO2 isotopes under simulated environmental conditions

RATIONALE

Nitrogen oxides or NO_x (NO_x = NO + NO₂ ) play an important role in air quality, atmospheric chemistry, and climate. The isotopic compositions of anthropogenic and natural NO₂ sources are wide-ranging, and they can be used to constrain sources of ambient NO₂ and associated atmospheric deposition of nitrogen compounds. While passive sample collection of NO₂ isotopes has been used in field studies to determine NO_x source influences on atmospheric deposition, this approach has not been evaluated for accuracy or precision under different environmental conditions.

METHODS

The efficacy of NO₂ passive sampler collection for NO₂ isotopes was evaluated under varied temperature and relative humidity (RH) conditions in a dynamic flux chamber. The precision and accuracy of the filter NO₂ collection as nitrite (NO₂^- ) for isotopic analysis were determined using a reference NO₂ gas tank and through inter-calibration with a modified EPA Method 7. The bacterial denitrifer method was used to convert 20 μM of collected NO₂^- or nitrate (NO₃^- ) into N₂ O and was carried out on an Isoprime continuous flow isotope ratio mass spectrometer.

RESULTS

δ¹⁵ N-NO₂ values determined from passive NO₂ collection, in conditions of 11-34 °C, 1-78% RH, have an overall accuracy and precision of ±2.1 ‰, and individual run precision of ±0.6 ‰. δ¹⁸ O-NO₂ values obtained from passive NO₂ sampler collection, under the same conditions, have an overall precision of ± 1.3 ‰.

CONCLUSIONS

Suitable conditions for passive sampler collection of NO₂ isotopes are in environments ranging from 11 to 34 °C and 1 to 78% RH. The passive NO₂ isotope measurement technique provides an accurate method to determine variations in atmospheric δ¹⁵ N-NO₂ values and a precise method for determining atmospheric δ¹⁸ O-NO₂ values. The ability to measure NO₂ isotopes over spatial gradients at the same temporal resolution provides a unique perspective on the extent and seasonality of fluctuations in atmospheric NO₂ isotopic compositions. Copyright © 2017 John Wiley & Sons, Ltd.

Spatial and Temporal Trends in Global Emissions of Nitrogen Oxides from 1960 to 2014

The quantification of nitrogen oxide (NO_x) emissions is critical for air quality modeling. Based on updated fuel consumption and emission factor databases, a global emission inventory was compiled with high spatial (0.1° × 0.1°), temporal (monthly), and source (87 sources) resolutions for the period 1960 to 2014. The monthly emission data have been uploaded online ( http://inventory.pku.edu.cn ), along with a number of other air pollutant and greenhouse gas data for free download. Differences in source profiles, not global total quantities, between our results and those reported previously were found. There were significant differences in total and per capita emissions and emission intensities among countries, especially between the developing and developed countries. Globally, the total annual NO_x emissions finally stopped increasing in 2013 after continuously increasing over several decades, largely due to strict control measures taken in China in recent years. Nevertheless, the peak year of NO_x emissions was later than for many other major air pollutants. Per capita emissions, either among countries or over years, follow typical inverted U-shaped environmental Kuznets curves, indicating that the emissions increased during the early stage of development and were restrained when socioeconomic development reached certain points. Although the trends are similar among countries, the turning points of developing countries appeared sooner than those of developed countries in terms of development status, confirming late-move advantages.

Novel Method for Nitrogen Isotopic Analysis of Soil-Emitted Nitric Oxide

The global inventory of NO_x (NO_x = NO + NO₂) emissions is poorly constrained, with a large portion of the uncertainty attributed to soil NO emissions that result from soil abiotic and microbial processes. While natural abundance stable N isotopes (δ¹⁵N) in various soil N-containing compounds have proven to be a robust tracer of soil N cycling, soil δ¹⁵N-NO is rarely quantified due to the measurement difficulties. Here, we present a new method that collects soil-emitted NO through NO conversion to NO₂ in excess ozone (O₃) and subsequent NO₂ collection in a 20% triethanolamine (TEA) solution as nitrite and nitrate for δ¹⁵N analysis using the denitrifier method. The precision and accuracy of the method were quantified through repeated collection of an analytical NO tank and intercalibration with a modified EPA NO_x collection method. The results show that the efficiency of NO conversion to NO₂ and subsequent NO₂ collection in the TEA solution is >98% under a variety of controlled conditions. The method precision (1σ) and accuracy across the entire analytical procedure are ±1.1‰. We report the first analyses of soil δ¹⁵N-NO (-59.8‰ to -23.4‰) from wetting-induced NO pulses at both laboratory and field scales that have important implications for understanding soil NO dynamics.

Modified Method for Trapping and Analyzing 15N in NO Released from Soils

¹⁵N isotope tracing is an effective and direct approach to investigate sources of nitric oxide (NO) formed in soils. However, NO is highly reactive and rapidly converted to nitrogen dioxide (NO₂) in the presence of ozone, making it impossible to directly measure ¹⁵N in NO. Various wet-chemical methods for conversion of NO to nitrite (NO₂^-) and nitrate (NO₃^-) have been proposed for ¹⁵N analysis in high-concentration NO sources, such as combustion processes. In contrast, NO concentrations in the soil surface-near air are usually small (ppbv-range), posing major challenges to conversion efficiency and blank correction. Here, we present a modified method in which NO is oxidized quantitatively to NO₂ by chromium trioxide (CrO₃), before conversion to NO₂^- and NO₃^- in an alkaline hydrogen peroxide (H₂O₂) solution. A denitrifier method was used to reduce NO₂^- and NO₃^- in the trapping solution quantitatively to nitrous oxide (N₂O) for subsequent ¹⁵N analysis. NO trapping efficiencies of >85% were obtained with 50 ppb NO in a 0.5 L min^-1 air stream bubbling through a solution of 1.2 M H₂O₂ and 0.5 M NaOH. In a laboratory test with distinct ¹⁵NO abundances, the overall precision was 0.29‰ (δ-values) for natural abundance NO and 0.13 atom % for labeled NO, suggesting that our method can be used for both natural abundance studies and ¹⁵N labeling experiments. In a soil incubation experiment with ¹⁵NH₄NO₃, NH₄¹⁵NO₃, or Na¹⁵NO₂ amendments, we found distinct ¹⁵N abundances in NO, indicating that our method is well suited to investigate NO sources in soils.

Tropospheric Emissions: Monitoring of Pollution (TEMPO)

TEMPO was selected in 2012 by NASA as the first Earth Venture Instrument, for launch between 2018 and 2021. It will measure atmospheric pollution for greater North America from space using ultraviolet and visible spectroscopy. TEMPO observes from Mexico City, Cuba, and the Bahamas to the Canadian oil sands, and from the Atlantic to the Pacific, hourly and at high spatial resolution (~2.1 km N/S×4.4 km E/W at 36.5°N, 100°W). TEMPO provides a tropospheric measurement suite that includes the key elements of tropospheric air pollution chemistry, as well as contributing to carbon cycle knowledge. Measurements are made hourly from geostationary (GEO) orbit, to capture the high variability present in the diurnal cycle of emissions and chemistry that are unobservable from current low-Earth orbit (LEO) satellites that measure once per day. The small product spatial footprint resolves pollution sources at sub-urban scale. Together, this temporal and spatial resolution improves emission inventories, monitors population exposure, and enables effective emission-control strategies. TEMPO takes advantage of a commercial GEO host spacecraft to provide a modest cost mission that measures the spectra required to retrieve ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2), formaldehyde (H2CO), glyoxal (C2H2O2), bromine monoxide (BrO), IO (iodine monoxide),water vapor, aerosols, cloud parameters, ultraviolet radiation, and foliage properties. TEMPO thus measures the major elements, directly or by proxy, in the tropospheric O3 chemistry cycle. Multi-spectral observations provide sensitivity to O3 in the lowermost troposphere, substantially reducing uncertainty in air quality predictions. TEMPO quantifies and tracks the evolution of aerosol loading. It provides these near-real-time air quality products that will be made publicly available. TEMPO will launch at a prime time to be the North American component of the global geostationary constellation of pollution monitoring together with the European Sentinel-4 (S4) and Korean Geostationary Environment Monitoring Spectrometer (GEMS) instruments.

Nitric oxide triggers a transient metabolic reprogramming in Arabidopsis

Nitric oxide (NO) regulates plant growth and development as well as responses to stress that enhanced its endogenous production. Arabidopsis plants exposed to a pulse of exogenous NO gas were used for untargeted global metabolomic analyses thus allowing the identification of metabolic processes affected by NO. At early time points after treatment, NO scavenged superoxide anion and induced the nitration and the S-nitrosylation of proteins. These events preceded an extensive though transient metabolic reprogramming at 6 h after NO treatment, which included enhanced levels of polyamines, lipid catabolism and accumulation of phospholipids, chlorophyll breakdown, protein and nucleic acid turnover and increased content of sugars. Accordingly, lipid-related structures such as root cell membranes and leaf cuticle altered their permeability upon NO treatment. Besides, NO-treated plants displayed degradation of starch granules, which is consistent with the increased sugar content observed in the metabolomic survey. The metabolic profile was restored to baseline levels at 24 h post-treatment, thus pointing up the plasticity of plant metabolism in response to nitroxidative stress conditions.