NO2 Trends Research Articles

Four Decades of United States Mobile Source Pollutants: Spatial-Temporal Trends Assessed by Ground-Based Monitors, Air Quality Models, and Satellites.

On-road emissions sources degrade air quality, and these sources have been highly regulated. Epidemiological and environmental justice studies often use road proximity as a proxy for traffic-related air pollution (TRAP) exposure, and other studies employ air quality models or satellite observations. To assess these metrics' abilities to reproduce observed near-road concentration gradients and changes over time, we apply a hierarchical linear regression to ground-based observations, long-term air quality model simulations using Community Multiscale Air Quality (CMAQ), and satellite products. Across 1980-2019, observed TRAP concentrations decreased, and road proximity was positively correlated with TRAP. For all pollutants, concentrations decreased fastest at locations with higher road proximity, resulting in "flatter" concentration fields in recent years. This flattening unfolded at a relatively constant rate for NOx, whereas the flattening of CO concentration fields has slowed. CMAQ largely captures observed spatial-temporal NO2 trends across 2002-2010 but overstates the relationships between CO and elemental carbon fine particulate matter (EC) road proximity. Satellite NOx measures overstate concentration reductions near roads. We show how this perspective provides evidence that California's on-road vehicle regulations led to substantial decreases in NO2, NOx, and EC in California, with other states that adopted California's light-duty automobile standards showing mixed benefits over states that did not adopt these standards.

Reductions in nitrogen oxides over the Netherlands between 2005 and 2018 observed from space and on the ground: Decreasing emissions and increasing O3 indicate changing NOx chemistry

Satellite measurements of tropospheric NO2 columns are valuable for monitoring long-term changes in air quality. However, direct linkage of satellite-derived NO2 trends with changes in underlying NOx emissions and NOx surface concentrations is complicated by the contribution of background NO2 to the column, by changes in the chemical regime wherein emissions take place, and by data sampling differences. Here we study the 2005–2018 changes in nitrogen oxides concentrations over the polluted Netherlands. We use the QA4ECV OMI NO2 retrievals, RIVM surface measurements of NO, NO2, and O3, wet deposition fluxes of nitrate, and NOx emissions reported in two European inventories (EMEP and TNO-MACC-III). We interpret the observed changes in concentrations with simulations by CLASS, a box model accounting for boundary layer dynamics and chemistry. Nationally averaged, OMI column NO2 and RIVM surface NO2 concentrations are reduced by 30% and 32% respectively between 2005 and 2018. This is in line with Dutch national NOx emissions from the TNO-MACC-III and EMEP inventories showing decreases of 32%–39%, respectively, between 2005 and 2018. There is no indication that the decrease in NO2 concentrations slows down after 2010. The observed reductions in nitrogen oxides differ between winter and summer and turn out to be modulated by ozone chemistry. The RIVM surface measurements show a stronger reduction in NOx than in NO2, accompanied by an increase in O3 of 4–6 ppbv, which is due to diminished NO-titration following the lower NOx emissions, especially in winter. CLASS simulations confirm that daytime O3 increases have shifted the NO–NO2 equilibrium more towards NO2, explaining the weaker reductions in NO2 (−30%) than in NOx (−40%) concentrations over time. The O3 increases occur both during day and during night, and have likely shortened the NOx lifetimes both in summer (via faster OH + NO2+M during daytime) and in winter (via faster nighttime N2O5 formation and subsequent hydrolysis). Our findings suggest a reduction in anthropogenic NOx emissions of approximately 30% in the southern part of the Netherlands, and propose that interpreting satellite NO2 trends as a proxy for trends in NOx emissions is well-possible over a high-NOx region, but requires careful analysis in terms of changes in the chemical regimes and NOx lifetime.

Analysis of Atmospheric Pollution Status over Nanjing Based on Multi-Source Detection

Nanjing, as one of the core cities in the Yangtze River Delta, has developed industries and serious air pollution. In order to further study the pollution status in Nanjing, MODIS and CALIPSO's aerosol products were used to analyze the aerosol characteristics of Nanjing from 2011 to 2019. Then, four kinds of common reactive gases (SO2, NO, NO2 and O3) in the atmosphere of Nanjing in winter were analyzed based on the data of ground-based reactive gas analyzer. The results shows that: (1) The AOD over Nanjing showed a downward trend in the whole nine years, and the AOD could reach 0.7 in winter. The pollution degree was the highest in the four seasons. The aerosol was mainly composed of coarse particles and absorbent particles in winter. (2) The near surface atmosphere (within 2 km) of Nanjing was dominated by large particles and irregular particles. Mainly, the proportion of fine particles and regular particles gradually increased within 2-6 km, and the volume of particles changed little within 2-6 km. (3) Among the four common reactive gases in winter, the φ(NO) in the atmosphere was the most significant, and the maximum value was 160×10-9, and the change trend of NO2 and O3 was opposite. Due to the emission limitation and the promotion of desulfurization measures, and the influence of photochemical reaction, the φ(SO2) was the lowest, only about 3×10-9. Combined with OMI data, the φ(NOx) over Nanjing remained at a low level in early 2020 due to the shutdown of factories because of COVID-19. Studies have shown that the particulate pollution in Nanjing has improved in recent years, but it is still necessary to pay attention to the emission of coarse particles, and it is necessary to strictly control the emission of gaseous pollutants from factories, especially NOx. © 2021, Editorial Board, Research of Environmental Sciences. All right reserved.

Long-term MAX-DOAS measurements of NO<sub>2</sub>, HCHO, and aerosols and evaluation of corresponding satellite data products over Mohali in the Indo-Gangetic Plain

Abstract. We present comprehensive long-term ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements of aerosols, nitrogen dioxide (NO2), and formaldehyde (HCHO) from Mohali (30.667∘ N, 76.739∘ E; ∼310 m above mean sea level), located in the densely populated Indo-Gangetic Plain (IGP) of India. We investigate the temporal variation in tropospheric columns, surface volume mixing ratio (VMR), and vertical profiles of aerosols, NO2, and HCHO and identify factors driving their ambient levels and distributions for the period from January 2013 to June 2017. We observed mean aerosol optical depth (AOD) at 360 nm, tropospheric NO2 vertical column density (VCD), and tropospheric HCHO VCD for the measurement period to be 0.63 ± 0.51, (6.7 ± 4.1) × 1015, and (12.1 ± 7.5) × 1015 molecules cm−2, respectively. Concerning the tropospheric NO2 VCDs, Mohali was found to be less polluted than urban and suburban locations of China and western countries, but comparable HCHO VCDs were observed. For the more than 4 years of measurements during which the region around the measurement location underwent significant urban development, we did not observe obvious annual trends in AOD, NO2, and HCHO. High tropospheric NO2 VCDs were observed in periods with enhanced biomass and biofuel combustion (e.g. agricultural residue burning and domestic burning for heating). Highest tropospheric HCHO VCDs were observed in agricultural residue burning periods with favourable meteorological conditions for photochemical formation, which in previous studies have shown an implication for high ambient ozone also over the IGP. Highest AOD is observed in the monsoon season, indicating possible hygroscopic growth of the aerosol particles. Most of the NO2 is located close to the surface, whereas significant HCHO is present at higher altitudes up to 600 m during summer indicating active photochemistry at high altitudes. The vertical distribution of aerosol, NO2, and HCHO follows the change in boundary layer height (BLH), from the ERA5 dataset of European Centre for Medium-Range Weather Forecasts, between summer and winter. However, deep convection during the monsoon transports the pollutants at high altitudes similar to summer despite a shallow ERA5 BLH. Strong gradients in the vertical profiles of HCHO are observed during the months when primary anthropogenic sources dominate the formaldehyde production. High-resolution MODIS AOD measurements correlate well but were systematically higher than MAX-DOAS AODs. The ground-based MAX-DOAS measurements were used to evaluate three NO2 data products and two HCHO data products of the Ozone Monitoring Instrument (OMI) for the first time over India and the IGP. NO2 VCDs from OMI correlate reasonably with MAX-DOAS VCDs but are lower by ∼30 %–50 % due to the difference in vertical sensitivities and the rather large OMI footprint. OMI HCHO VCDs exceed the MAX-DOAS VCDs by up to 30 %. We show that there is significant scope for improvement in the a priori vertical profiles of trace gases, which are used in OMI retrievals. The difference in vertical representativeness was found to be crucial for the observed biases in NO2 and HCHO surface VMR intercomparisons. Using the ratio of NO2 and HCHO VCDs measured from MAX-DOAS, we have found that the peak daytime ozone production regime is sensitive to both NOx and VOCs in winter but strongly sensitive to NOx in other seasons.

Impacts of global NO<sub><i>x</i></sub> inversions on NO<sub>2</sub> and ozone simulations

Abstract. Tropospheric NO2 and ozone simulations have large uncertainties, but their biases, seasonality, and trends can be improved with NO2 assimilations. We perform global top-down estimates of monthly NOx emissions using two Ozone Monitoring Instrument (OMI) NO2 retrievals (NASAv3 and DOMINOv2) from 2005 to 2016 through a hybrid 4D-Var/mass balance inversion. Discrepancy in NO2 retrieval products is a major source of uncertainties in the top-down NOx emission estimates. The different vertical sensitivities in the two NO2 retrievals affect both magnitude and seasonal variations of top-down NOx emissions. The 12-year averages of regional NOx budgets from the NASA posterior emissions are 37 % to 53 % smaller than the DOMINO posterior emissions. Consequently, the DOMINO posterior surface NO2 simulations greatly reduced the negative biases in China (by 15 %) and the US (by 22 %) compared to surface NO2 measurements. Posterior NOx emissions show consistent trends over China, the US, India, and Mexico constrained by the two retrievals. Emission trends are less robust over South America, Australia, western Europe, and Africa, where the two retrievals show less consistency. NO2 trends have more consistent decreases (by 26 %) with the measurements (by 32 %) in the US from 2006 to 2016 when using the NASA posterior emissions. The performance of posterior ozone simulations has spatial heterogeneities from region to region. On a global scale, ozone simulations using NASA-based emissions alleviate the double peak in the prior simulation of global ozone seasonality. The higher abundances of NO2 from the DOMINO posterior simulations increase the global background ozone concentrations and therefore reduce the negative biases more than the NASA posterior simulations using GEOS-Chem v12 at remote sites. Compared to surface ozone measurements, posterior simulations have more consistent magnitude and interannual variations than the prior estimates, but the performance from the NASA-based and DOMINO-based emissions varies across ozone metrics. The limited availability of remote-sensing data and the use of prior NOx diurnal variations hinder improvement of ozone diurnal variations from the assimilation, and therefore have mixed performance on improving different ozone metrics. Additional improvements in posterior NO2 and ozone simulations require more precise and consistent NO2 retrieval products, more accurate diurnal variations of NOx and VOC emissions, and improved simulations of ozone chemistry and depositions.

Trends in ambient air pollution levels and PM2.5 chemical compositions in four Chinese cities from 1995 to 2017.

An in-depth analysis of the specific evolution of air pollution in a given city can provide a better understanding of the chronic effects of air pollution on human health. In this study, we reported trends in ambient concentrations of particulate matter (PM) and gaseous pollutants [sulfur dioxide (SO2), nitrogen dioxide (NO2), and ozone (O3)] from 1995 to 2017 and PM2.5 composition for the period of 2000–2017 in Guangzhou, Wuhan, Chongqing, and Lanzhou. We provided socio-economic indicators to help explain the pollution trends. SO2 and PM (including PM10 and PM2.5) concentrations showed a downward trend in recent years with the most notable reduction in SO2 in Chongqing and PM2.5 in Guangzhou. There was an overall flat trend for NO2, while O3 showed an upward trend in recent years except in Lanzhou. The majority of PM2.5 mass was SO42− (6.0–30 µg/m3) and organic carbon (6.0–38 µg/m3), followed by NO3− (2.0–12 µg/m3), elemental carbon (2.1–12 µg/m3), NH4+ (1.0–10 µg/m3), K+ (0.2–2.0 µg/m3), and Cl− (0.2–1.9 µg/m3). Except for secondary inorganic aerosols in Wuhan, annual average concentrations of all PM2.5 constituents showed a declining trend after 2013, corresponding to the trend of PM2.5. The secondary sources in PM2.5 were found to be most prominent in Wuhan, while the most abundant EC and Cl− in Lanzhou was attributed to the use of coal. Despite temporal and spatial variabilities across the four cities, coal combustion, traffic emissions, and secondary pollution have been the major sources of PM2.5 pollution. These trends in ambient air pollution levels and PM2.5 composition may help understand changes in health outcomes measured at different times within the time period of 1995–2017 in the four cities.

Minimizing Soil Nitrogen Leaching by Changing Furrow Irrigation into Sprinkler Fertigation in Potato Fields in the Northwestern China Plain

Irrigation water is limiting for crop production in arid areas and application rates of fertilizers often exceed crop requirements, resulting in high accumulation of nitrate nitrogen (NO3−-N) in the soil. Management practices play a significant role in the leaching of NO3−-N. This experiment compares the effects of traditional furrow irrigation and sprinkler fertigation on the soil NO3−-N concentration trend throughout the cropping season in potato fields in China. Two irrigation systems that were fertilized, namely by furrow (NF-FI) and sprinkler fertigation (NF-SI), and two controlling without any fertilizer (C-FI and C-SI) were tested in the same experimental site for three consecutive years. Both the NF-FI soils and NF-SI soils with three replications and fertilizer applications of 273 kg N ha−1 exhibited a different trend of NO3−-N accumulation at different depths of soil profile. However, the magnitude of NO3−-N accumulation was low in the NF-SI soil profile. In NF-SI treatments, higher NO3−-N was observed at 20–40 cm soil layer. In the NF-FI, the concentration of the highest nitrate was observed at the 40–120 cm soil layer. The concentrations of NO3−-N in the fertilized soil were higher than those of the control soil for each irrigation system. Residual levels of NO3−-N in the soil depth of 40–120 cm from NF-FI were 1.54, 3.45 and 5.28 times higher than NF-SI after harvesting potatoes from 2015 to 2017. In NF-FI treatments, apparent nitrogen loss was 234.7, 237.5 and 276.7 kg ha−1 after harvesting potatoes in 2015, 2016 and 2017. Meanwhile, apparent nitrogen loss from NF-SI treatments was only 161.9, 132.1 and 148.9 kg ha−1, which was 31.0%, 44.4% and 46.2% lower than that of NF-FI in 2015, 2016 and 2017, respectively. The risk of NO3−-N leaching below the root zone from NF-FI was higher than that from NF-SI. It has been demonstrated that sprinkler fertigation can also be used as a tool for mitigating NO3−-N accumulation and apparent nitrogen loss.

The influence of different concentrations of flavanol chocolate bars under acute supplement conditions on exercise and performance.

The purpose of this study was to assess the effects and acute dosage of different flavanol concentrations in a dark chocolate bar on physiological parameters during steady state (SS) and incremental exercise. In a double-blind, randomised, crossover study, 15 healthy participants with a mean ± SD age of 30 ± 7years; stature 176.8 ± 8.6cm and body mass 80.3 ± 8.4kg supplemented with high flavanol (HF) (1060mg), moderate flavanol (MF) (746mg), low flavanol (LF) (406mg), or a control (CON) (88mg) chocolate bar (~ 34g), 2h prior to 40min of SS cycling (80% gas-exchange threshold) followed by an incremental test to volitional fatigue. During the SS cycle oxygen consumption ([Formula: see text]), respiratory exchange ratio (RER) and heart rate (HR) were continuously monitored. Plasma samples were collected prior to commencing exercise to determine nitrate (NO3-) and nitrite (NO2-) levels under each condition. There was no observed effect between flavanol concentrations on [Formula: see text], RER, and HR during SS cycling (P > 0.05). [Formula: see text], peak power, HR peak, and RER peak also did not significantly differ between conditions (P > 0.05). There was a small trend for higher plasma NO2- levels following higher flavanol concentration; however, this did not reach statistical significance (P > 0.05). Acute supplementation with cocoa of differing flavanol concentrations does not appear to have any effect on exercise and performance. It is plausible that longer flavanol supplementation periods might have greater accumulative effects and thus may potentially elicit a larger effect.

Analysis of Ambient Air Quality Based On Exceedance Factor and Air Quality Index for Siliguri City, West Bengal

Bad air is one of the key concerns for most of the urban centres today, and Siliguri is no exceptions to this. In order to assess the air quality of Siliguri, Exceedance factor (EF) method was applied based on the average annual concentration of the pollutants named as; NO2, SO2, PM2.5 and PM10 and it is found that PM2.5 and PM10 are the major pollutants that pose a severe threat for the city. After applying the EF method, it is found that the values of PM2.5 was between moderate to high pollution level and for PM10 it falls under high to critical pollution level. On the other hand, the concentration of NO2 and SO2 falls under moderate to low pollution level. Through trend analysis of the various pollutants, it is found that their concentration was varying in nature. In case of PM10, the trend shows high concentration which exceeds national standard; whereas PM2.5 shows its concentration near towards violating the national standard soon if not checked. In contrast, trends of NO2 and SO2 were recorded lower than the national standard. The present situation of ambient air of Siliguri was analyzed based on Air Quality Index which reveals that air quality of the city can be classified into two seasons, i.e. clean air period (from April to October) and polluted period (from November to March). Lastly, the annual trends of PM2.5 and PM10 were constructed as they are the major pollutants, and it shows their skewed nature during winter months which results in smog episodes. It unveils how critical the situation of air quality of Siliguri became especially during winter months which seek immediate attention. Thus the study tries to present a vivid scenario about the present air quality of Siliguri, which concludes with some of the suggestions to restrain the air quality.

Spatio-temporal variations and trends of major air pollutants in China during 2015-2018.

The Chinese government, as a policy response, has continued to invest efforts and resources to implement cost-effective air pollution control technologies and stringent regulation to reduce emissions from the most contributing sectors to protect the environment and public health. The higher density of monitoring stations (> 1600) distributed across China provides a timely opportunity to use them to study in detail the national pollution trends in light of more stringent air pollution control policies. In the present study, air quality datasets comprising hourly concentrations of PM2.5, O3, NO2, and SO2 collected across 1309, 1341, 1289, and 1347 monitoring stations respectively were obtained from the National Environmental Monitoring Centre over 4years (2015-2018) and trend analysis was performed. Results indicate that the overall annual trends for PM2.5 and SO2 were - 2.9 ± 2.7 and - 3.2 ± 3.2μg/m3/year, while the winter trends were - 4.8 ± 5.8 and - 6.9 ± 8.4μg/m3/year respectively across China. The daily maximum 8-h average (DMA8) ozone concentration showed a significant positive trend of 2.4 ± 4.6μg/m3/year, which was comparatively higher in summer at 4.4 ± 9.0μg/m3/year. On the other side, NO2 trend is not great in number (- 0.45 ± 2.0μg/m3/year). Overall, 62.2%, 61.8%, and 20.9% of PM2.5, SO2, and NO2 monitoring stations were associated with a negative trend of ≥ - 2μg/m3/year. For O3 DMA8 concentrations, 50.7% of the monitoring stations showed a significant positive trend of ≥ 2μg/m3/year. In light of the Chinese government's increasing impetus on combating air pollution and climate change via new policy regulations, it is important to understand the spatio-temporal distributions and relative contributions of the spectrum of gaseous pollutants to the pollution loads as well as identify changing emission loads across sectors. The results of this study will facilitate the formulation of evidence-based air pollution reduction strategies and policies.

Evaluating in-use vehicle emissions using air quality monitoring stations and on-road remote sensing systems

This study investigated real world in-use vehicle emissions using two regulatory techniques simultaneously, namely on-road remote sensing (RS) systems and air quality (AQ) monitoring stations, aiming to provide a full pollution profile from tailpipe to roadside and atmosphere. Two large AQ and RS datasets collected during 2012–2018 were analyzed. The effects of various emission control programmes on the trends of tailpipe emissions and air quality were evaluated. Correlations between tailpipe emissions and roadside and ambient air quality were also explored. The results showed a decreasing trend of NO2 at both roadside and ambient AQ stations from 2013 to 2016, which was attributed to the intensive implementation of a series of vehicle emissions control programmes. Although NO2 was decreasing, O3 was generally increasing for all AQ stations. AQ data showed that O3 had little correlation with either NO2 or NOx, but was mainly determined by NO2/NOx ratio. Roadside NO2/NOx ratio increased first and then decreased or stabilized after 2014, while ambient NO2/NOx ratio increased steadily. RS data showed that the overall NO decreased quickly during 2012–2015 and then decreased moderately after 2015. The decrease was mainly attributed to the effective NO reduction from LPG vehicles. However, diesel NO remained high and reduced relatively slowly during the study period. Gasoline vehicles were relatively clean compared with LPG and diesel vehicles. Finally, good correlations were demonstrated between NO measured by RS sites and NOx measured by roadside AQ stations, indicating that vehicle emissions were the major contributor to roadside NOx pollution. Ambient NOx emissions could be affected by various sources, leading to different correlation levels between RS and ambient AQ results.

Trend analysis and first time observations of sulphur dioxide and nitrogen dioxide in South Africa using TROPOMI/Sentinel-5 P data

The 17 Sustainable Development Goals (SDGs) aim to end extreme poverty and create a healthy, sustainable world by the year 2030. Goal 7 is of interest to this study as it targets access to clean and affordable energy. However, in this study we show that the energy created in South Africa is not necessary clean. South Africa has numerous coal-fired power station located in the Mpumalanga (MP), Gauteng (GP) and Limpopo (LP) provinces. These power station produce tons of toxic pollutants including sulphur dioxide (SO2), nitrogen dioxide (NO2) and sulphates (SO4). These pollutants are known to have a negative impact on human health, climate and the environment. In this study we use the sequential Mann-Kendall test to investigate the 39 year (1980–2019) trends of SO2, NO2 and SO4 from these source areas. We also report for the first time on the observations of SO2 and NO2 from the Sentinel-5 P sensor over South Africa. Increasing trends of SO2 were observed in the MP, LP and GP regions. The increase was mostly due to the emissions from coal-fired power stations. Moreover, the increase of SO2 over the years could be due to the increasing demand in electricity, aging power stations and the low quality of coal used. Sentinel-5 P observations of SO2 and NO2 over South Africa were observed in the MP, GP and LP regions as a result of coal-fired power stations. Dispersion of SO2 and NO2 over South Africa were observed in the winter months, while confined SO2 and NO2 in the source region were observed in the summer months.

Monthly physicochemical variation of tropical island groundwater of Pulau Bidong, south China sea

The groundwater of Pulau Bidong, Terengganu, Malaysia was investigated to monitor the quality of freshwater source. Groundwater samples were collected from five sampling stations from June 2016 to October 2016. Physical parameters such as temperature, specific conductivity, dissolved oxygen (DO), pH, salinity, and DO saturation were measured in-situ by using handheld device. Meanwhile, total suspended solid (TSS), total dissolved solid (TDS), nitrate (NO3−), nitrite (NO2−), ammonium (NH4+) and phosphate (PO43−) were also analysed. The inorganic nutrients of NO3−, NO2−, NH4+, and PO43− were ranged from 0.000 to 4.310 mg/L, 0.000–0.190 mg/L, 0.000–0.807 mg/L and 0.003–0.028 mg/L, respectively. The monthly trends of specific conductivity, DO, salinity, DO saturation, NH4, NO3 and NO2 demonstrated significant variation in June (the lowest rainfall) compared to other months. Correlation matrix revealed that temperature was associated with the specific conductivity, and NH4+ strongly correlated with DO, NO3− and NO2−. Nevertheless, there is a strong negative correlation between physicochemical parameters and monthly rainfall distribution. The physicochemical parameters were within the recommended safe limits suggested by the World Health Organization (WHO) and National Drinking Water Quality Standard (NDWQS) except for pH. Nevertheless, pH of water does not directly impact on the human health. Environmetrics analysis demonstrated that NO3− and PO43− significantly influenced the groundwater quality spatially. Furthermore, the temporal variation of groundwater quality was affected by pH and salinity. Overall, the groundwater sources from Pulau Bidong are considered safe to be used as drinking water. Notably, future studies are required for long-term monitoring to ensure the good quality of groundwaters from Pulau Bidong.

Spatio-temporal variations in NO2 and SO2 over Shanghai and Chongming Eco-Island measured by Ozone Monitoring Instrument (OMI) during 2008–2017

High resolution satellite maps are useful for the identification of pollution hotspots. In this work, nitrogen dioxide (NO2) and sulfur dioxide (SO2) observations from the Ozone Monitoring Instrument (OMI) have been averaged and gridded for Shanghai and surrounding areas with a spatial resolution of 0.01° × 0.01°. The pollution maps have been used to investigate the spatio-temporal variations in NO2 and SO2 from 2008 to 2017. The averaged tropospheric NO2 and planetary boundary layer (PBL) SO2 columns in Shanghai are on average 1.40 × 1016 molec·cm−2 and 1.21 × 1016 molec·cm−2, respectively. As the government has put a lot of effort to improve the air quality, both the NO2 and SO2 columns reveal a significant decline in Shanghai during the past decade, with a reduction of ∼24% and ∼56%, respectively, which is also confirmed by the good agreement between satellite observations and ground-based in-situ measurements. The NO2 and SO2 time series also show distinct seasonal characteristics with higher pollution level during winter and lower in the summer. Due to the impact of ship and industrial emissions, the hotspots of NO2 and SO2 are concentrated in the northern part of Shanghai, while the pollution level is lower in the southern and eastern regions. Decreasing trends in NO2 and SO2 are found for most areas of Shanghai during the past decade, though the strongest decrease in the northern part of the city (1.4 × 1015 molec·cm−2·year−1 for NO2 and 3.0 × 1015 molec·cm−2·year−1 for SO2). Comparisons with the China Multi-Resolution Emissions Inventory (MEIC) indicate that the driving factors for the decline of NO2 and SO2 are mainly denitration and desulfurization in the power sector, and the reduction of emissions in the industry sector. We also use aerosol optical depth (AOD) measurements from MODerate-resolution Imaging Spectroradiometer (MODIS) to investigate the relationship between gas phase pollutants and formation of secondary aerosol. Compared to SO2, the contribution of NO2 to secondary aerosol formation increased in the central part of Shanghai. Although Chongming Eco-Island has developed rapidly in recent years, both the SO2 and NO2 levels decreased significantly. This is mainly related to the strict emission control policy and improved urban planning, and indicates that ecological development is achievable under the guidance of policies.

Effect of changing NO<sub><i>x</i></sub> lifetime on the seasonality and long-term trends of satellite-observed tropospheric NO<sub>2</sub> columns over China

Abstract. Satellite observations of tropospheric NO2 columns are extensively used to infer trends in anthropogenic emissions of nitrogen oxides (NOx≡NO+NO2), but this may be complicated by trends in NOx lifetime. Here we use 2004–2018 observations from the Ozone Monitoring Instrument (OMI) satellite-based instrument (QA4ECV and POMINO v2 retrievals) to examine the seasonality and trends of tropospheric NO2 columns over central–eastern China, and we interpret the results with the GEOS-Chem chemical transport model. The observations show a factor of 3 increase in NO2 columns from summer to winter, which we explain in GEOS-Chem as reflecting a longer NOx lifetime in winter than in summer (21 h versus 5.9 h in 2017). The 2005–2018 summer trends of OMI NO2 closely follow the trends in the Multi-resolution Emission Inventory for China (MEIC), with a rise over the 2005–2011 period and a 25 % decrease since. We find in GEOS-Chem no significant trend of the NOx lifetime in summer, supporting the emission trend reported by the MEIC. The winter trend of OMI NO2 is steeper than in summer over the entire period, which we attribute to a decrease in NOx lifetime at lower NOx emissions. Half of the NOx sink in winter is from N2O5 hydrolysis, which counterintuitively becomes more efficient as NOx emissions decrease due to less titration of ozone at night. The formation of organic nitrates also becomes an increasing sink of NOx as NOx emissions decrease but emissions of volatile organic compounds (VOCs) do not.

Measurement and Correlation of the Solubility of Theobromine and Theophylline in Seven Imidazolium-Based Ionic Liquids at Various Temperatures

The solubility of theobromine and theophylline in seven pure imidazolium-based ionic liquids, including 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim]BF4), 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim]PF6), 1-butyl-3-methylimidazolium nitrate ([C4mim]NO3), 1-hexyl-3-methylimidazolium tetrafluoroborate ([C6mim]BF4), 1-hexyl-3-methylimidazolium hexafluorophosphate ([C6mim]PF6), 1-hexyl-3-methylimidazolium nitrate ([C6mim]NO3), and 1-octyl-3-methylimidazolium tetrafluoroborate ([C8mim]BF4), were measured via a static method combined with high-performance liquid chromatography between T = 303.15–353.15 K at atmospheric pressure. The solubility of theobromine and theophylline in the selected ionic liquids increased as the temperature increased. Both cations and anions in the ionic liquids affected the solubility, but anions had a more pronounced effect than the cations with a trend of NO3– > BF4– > PF6–. The simplified dual-parameter equation, the Apelblat empirical equation, and the λh equa...

Air pollution associated respiratory mortality risk alleviated by residential greenness in the Chinese Elderly Health Service Cohort

BackgroundAlthough residing in lower surrounding greenness and transient exposure to air pollution are independently associated with higher risk of adverse health outcomes, little is known about their interactions. ObjectivesWe examine whether residential neighborhood greenness modifies the short-term association between air pollution and respiratory mortality among the participants of Chinese Elderly Health Service Cohort in Hong Kong. MethodsWe estimated residential surrounding greenness by measuring satellite-derived normalized difference vegetation index (NDVI) from Landsat within catchments of residential addresses of participants who died of respiratory diseases between 1998 and 2011. We first dichotomized NDVI into low and high greenness and used a time-stratified case-crossover approach to estimate the percent excess risk of respiratory mortality associated with fine particulate matter (PM2.5), respirable particulate matter (PM10), nitrogen dioxide (NO2), and ozone (O3). We further classified NDVI into greenness quartiles and introduced an interaction term between air pollution and the assigned median values of the NDVI quartiles into the models to assess the trend of greenness modification on the air pollution and respiratory mortality associations. ResultsAmong 3159 respiratory deaths during the follow-up, 2058 were from pneumonia and 947 from chronic obstructive pulmonary disease. Elders living in the low greenness areas were associated with a higher risk of pneumonia mortality attributed to NO2 (p = 0.049) and O3 (p = 0.025). The mortality risk of pneumonia showed a decreasing trend for NO2 (p for trend = 0.041), O3 (p for trend = 0.006), and PM2.5 (p for trend = 0.034) with greenness quartiles increasing from Quartile 1 (lowest) to Quartile 4 (highest). ConclusionsOur findings suggest that elders living in higher greenness areas are less susceptible to pneumonia mortality associated with air pollution, which provides evidence for optimizing allocation, siting, and quality of urban green space to minimize detrimental health effects of air pollution.

Pollution Trends in China from 2000 to 2017: A Multi-Sensor View from Space

Satellite sensors can provide unique views of global pollution information from space. In particular, a series of aerosol and trace gas monitoring instruments have been operating for more than a decade, providing the opportunity to analyze temporal trends of major pollutants on a large scale. In this study, we integrate aerosol products from MODIS (MODIS Resolution Imaging Spectroradiometer, all abbreviations and their definitions are listed alphabetically in Abbreviations) and MISR (Multi-angle Imaging Spectroradiometer), the AAI (Absorbing Aerosol Index) product from OMI (Ozone Monitoring Instrument), column SO2 and NO2 concentrations from OMI, and tropospheric column ozone concentration from OMI/MLS (Microwave Limb Sounder) to study temporal changes in major pollutants over China. MODIS and MISR consistently revealed that column AOD (Aerosol Optical Depth) increased from 2000, peaked around 2007, and started to decline afterward, except for northwest and northeast China, where a continuous upward trend was found. Extensive negative trends in both SO2 and NO2 have also been found over major pollution source regions since ~2005. On the other hand, the OMI AAI exhibited significant increases over north China, especially the northeast and northwest regions. These places also have a decreased Angstrom exponent as revealed by MISR, indicating an increased fraction of large particles. In general, summer had the largest AOD, SO2, and NO2 trends, whereas AAI trends were strongest for autumn and winter. A multi-regression analysis showed that much of the AOD variance over major pollution source regions could be explained by SO2, NO2, and AAI combined, and that the SO2 and NO2 reduction was likely to be responsible for the negative AOD trends, while the AOD increase over NE and NW China may be associated with an increase of coarse particles revealed by increased AAI and decreased AE. In contrast to aerosols, tropospheric ozone exhibited a steady increase from 2005 throughout China. This indicates that although the recent emission control effectively reduced aerosol pollutants, ozone remains a challenging issue and may dominate future air pollution.

Impact of natural gas production on nitrogen dioxide and sulphur dioxide over Northeast British Columbia, Canada

Northeast British Columbia (BC) Canada, is a region in which natural gas production has undergone rapid development since 2007. We used nitrogen dioxide (NO2) and sulphur dioxide (SO2) data products of the Ozone Monitoring Instrument (OMI) to assess the impact of natural gas development activity on air quality in this region from 2005 to 2018. We noticed that values of both pollutants were elevated in the immediate vicinity of large emission sources within the Montney formation and Horn River Basin – regions, which have experienced an increase in unconventional natural gas activities. Places with elevated NO2 Vertical Column Densities (VCDs) are Fort St. John, Taylor, and Dawson Creek located in the Montney formation, and higher SO2 VCDs are found near the Fort Nelson gas plant situated in the Horn River and Liard Basin areas. Although all the OMI data products consistently reported relatively high NO2 VCDs in the same areas, VCD values vary substantially with data products largely due to differences in Air Mass Factor (AMF) calculations. The rate of increase in NO2 VCDs and mass between 2005 and 2018 in the Dawson Creek area was assessed at 2.34% yr−1 and 4.32% yr−1, respectively, and these rates of change are statistically significant. Although we obtained an overall increasing trend for NO2 in Northeast BC, we also noticed a decreasing trend in the period of 2011–2013, which may be attributed, in part, to compliance and enforcement of regulations concerning flaring activities from oil and gas activities in Northeast BC, or due to less development activities. From our analysis, we suggest that the current air quality monitoring network in Northeast BC should be expanded to capture the spatial distribution of SO2 by deploying one additional station near Fort Nelson equipped with meteorology and SO2 monitoring systems.

Tropospheric SO2 and NO2 in 2012–2018: Contrasting views of two sensors (OMI and OMPS) from space

The global long-term Climate Data Records (CDRs) of atmospheric SO2 and NO2 have been obtained from multiple satellite sensors since 1990s, and all these CDRs show consistently decreasing trends in developed countries and increasing trends in developing countries prior to 2010. However, much less clear is the quantitative differences among these CDRs and how such differences affect the inferences for atmospheric SO2 and NO2 climatology in terms of their annual means as well as their frequency distributions. Here, we compare and contrast the CDRs from the aged OMI sensor (the flagship for measuring NO2 and SO2 since 2005) and the young OMPS sensor series (that started measuring NO2 and SO2 in 2012 and will continue in next 2-3 decades). We show that after 2012, the difference of average SO2 between OMPS and OMI is 0.12 DU and it only decreases to 0.04 DU after bias correction, despite their consistence in spatial pattern. NO2 CDRs from OMPS and OMI overall exhibit general agreement in both magnitude and spatial pattern. Furthermore, the CDR differences can lead to the opposite trend signs in developed countries and the difficulty to reconcile trend magnitude in developing countries. Notable consistence in trend signs does exist, regardless of radiative cloud fraction, mainly showing decline of SO2 and NO2 in China and increasing in India; much inconsistence is, however, found in many parts of developed countries. No SO2 trends and inconsistent NO2 trends are found over Europe, and notable differences are found over U.S. where OMI SO2 and NO2's declining trends are consistent with surface observations, but OMPS SO2, albeit its better spatial agreement with surface data, shows increasing trend. This study calls the importance to assess CDRs from different satellite sensors with the account of frequency distributions for extreme events. This importance is emergent as the atmospheric SO2 and NO2 amounts are closer to the uncertainties of satellite-based retrievals in developed countries and are or will be declining in developing countries in the coming decades, all of which make the detection of signs, magnitudes, and spatiotemporal dichotomy a challenge from space.