Primary NO2 Emissions Research Articles

Tropospheric ozone (O3) pollution in Johannesburg, South Africa: Exceedances, diurnal cycles, seasonality, Ox chemistry and O3 production rates

Ground-level ozone (O3) is an air pollutant of major health and environmental concern. The Johannesburg-Pretoria megacity in South Africa is the industrial and economical capital of the country with more than 10 million inhabitants experiencing poor air quality. In 2004, the City of Johannesburg (CoJ) began monitoring trace gases to assess ground-level O3 pollution. Here, we use CoJ’s publicly available air quality data, and present the first long-term data analysis of O3, nitric oxide (NO), nitrogen dioxide (NO2), NOx and carbon monoxide (CO) in the City from 2004 to 2011 at three air quality monitoring sites: Buccleuch, Delta Park and Newtown. We quantified CoJ’s South African National Ambient Air Quality Standards (NAAQS) exceedances for O3 and NO2, and demonstrate the City’s substantial O3 and NO2 air pollution problem. O3 mixing ratios peak in the early afternoon as expected due to photochemical production. To estimate O3 production rates, we summed O3 and NO2 diurnal profiles to obtain Ox mixing ratios at each site. This analysis provided insight into missing volatile organic compound (VOC) reactivity as well as primary NO2 emissions information necessary for developing tropospheric O3 pollution mitigation strategies. Furthermore, CoJ experiences high O3 mixing ratios on weekends due to lower NOx traffic emissions titrating the O3, thereby providing evidence of a VOC-limited regime for O3 production. Seasonal peak O3 occurs in the austral spring, a maximum that we link to increases in water (H2O) concentrations which in turn increases radical chemistry leading to O3. In addition, wintertime VOC and aerosol emissions from biomass burning over the winter add important precursors for O3 formation once radical chemistry is initiated during the first rain events in early spring. In all, this study will help inform air quality modelling and policy work on air pollutants in the City of Johannesburg, South Africa.

High urban NOx triggers a substantial chemical downward flux of ozone.

Nitrogen oxides (NOx) play a central role in catalyzing tropospheric ozone formation. Nitrogen dioxide (NO2) has recently reemerged as a key target for air pollution control measures, and observational evidence points toward a limited understanding of ozone in high-NOx environments. A complete understanding of the mechanisms controlling the rapid atmospheric cycling between ozone (O3)-nitric oxide (NO)-NO2 in high-NOx regimes at the surface is therefore paramount but remains challenging because of competing dynamical and chemical effects. Here, we present long-term eddy covariance measurements of O3, NO, and NO2, over an urban area, that allow disentangling important physical and chemical processes. When generalized, our findings suggest that the depositional O3 flux near the surface in urban environments is negligible compared to the flux caused by chemical conversion of O3. This leads to an underestimation of the Leighton ratio and is a key process for modulating urban NO2 mixing ratios. As a consequence, primary NO2 emissions have been significantly overestimated.

Roadside NO2/NOx and primary NO2 from individual vehicles

Traffic-derived nitrogen oxides (NOx) is a major component of kerbside pollution in large cities and represent an important source of exposure and health risk to urban populations, especially during the use of roadside pavements. The ratio of ambient NO2/NOx (xNO2) varies widely in urban environments, influenced by relative source strengths, but also oxidation. Primary NO2 emissions have changed over the years, with changes in engine and exhaust controls. Over the years 1998/2021 the NO2 fraction in exhaust (fNO2) in Hong Kong has changed from 0.05 in the late 1990s to values ∼0.2 in the years 2010/2015, and has fallen to ∼0.08 in recent years. To better understand fNO2 from individual on-road vehicles, this study monitored NO and NO2 at the kerb in a busy street canyon in Hong Kong, capturing the rapid concentration changes in response to vehicles passing by. The kerbside xNO2 was much lower (∼0.2) than that at the roadside air quality monitoring station (∼0.5) whose inlet was elevated some 3 m above the road nearby. Output from pollution sensors was deconvoluted to explore rapidly changing concentrations in the plume segments from individually identifiable vehicles as they passed and enable to estimate of the NO2 fraction in the exhaust for individual vehicle. Each vehicle was assigned to a type, Euro emission class and year of manufacture. The fNO2 from individual vehicles depends on emission class, increasing from Euro 4 to Euro 6, while fNO2 decreases as vehicles get older. The estimates of fNO2 made at the kerb were similar to those derived for the local air quality monitoring station, but not necessarily that of the city as a whole, where fNO2 varies by a factor of two.

Performance evaluation of the VITROS® hs Troponin I assay⁎ on the VITROS® ECi/ECiQ and 3600 immunodiagnostic and VITROS® 5600/XT 7600 integrated systems

A large part of the European population is still exposed to ambient nitrogen dioxide (NO2) levels exceeding the European Union (EU) air quality standards, being a key challenge to reduce NO2 concentrations across many European urban areas, particularly close to roads. In this work, a trend analysis of pollutants involved in NO2 processes was done for the period 2003–2014 in traffic sites from three Spanish cities (Barcelona, Madrid and Granada) that still exceed the European NO2 air quality standard limits. We also estimated the contributions of primary NO2 emissions and photo-chemically formed NO2 to the observed ambient NO2 concentrations in order to explore their possible role in the observed NO2 concentration trends. The NOx and NO concentrations at these traffic sites showed significant decreasing trends during the period 2003–2014, especially at Barcelona (BARTR) and Madrid (MADTR) traffic stations. The NO2 concentrations showed statistically significant downward trends at BARTR and MADTR and remained unchanged at Granada traffic station (GRATR) during the study period. Despite the significant decrease in NO2 concentrations in BCNTR and MADTR during the analysed period, the NO2 concentrations observed over these sites still above the annual NO2 standard limit of 40 μg m−3 and, therefore, more efficient measures are still needed. Primary NO2 emissions significantly influence NO2 concentrations at the three analysed sites. However, as no drastic changes are expected in the after-exhaust treatment technology that can reduce primary NO2 emissions to zero in the near future, only a substantial reduction in NOx emissions will help to comply with the NO2 European air quality standards. Reduction of 78%, 56% and 16% on NOx emissions in Barcelona, Madrid and Granada were estimated to be necessary to comply with the NO2 annual limit of 40 μg m−3.

Investigation of vehicle cold start primary NO2 emissions inferred from ambient monitoring data in the UK and their implications for urban air quality

Nitrogen oxides (NO and NO2, collectively NOX) derived from vehicle exhausts are critical pollutants with significant implications for urban air quality and human and environmental health. In this study, we investigate trends in measured ambient nitrogen dioxide (NO2) and NOx mixing ratios at urban traffic - dominated monitoring sites in the UK for the period 2009–2016. We apply an oxidant analysis approach alongside a number of assumptions to the ambient data to determine trends in the inferred primary NO2/NOx ratio, and examine evidence for enhanced vehicle “cold start” effects upon these inferred emissions. Ambient NO2 and NOX mixing ratios have experienced an overall decrease of 17.2% and 11.3% respectively for the locations considered over this time period. The inferred primary NO2/NOx ratio for the majority of the study locations is found to have fallen by 29% (from 0.175 to 0.125) as a monthly mean from 2009 to 2016, with a statistically significant median decrease of 0.32 percentage points per year. However, during cold weather (temperatures less than or equal to 5 OC), the inferred primary NO2/NOx ratio averaged across all locations, when compared with normal conditions (temperatures higher than 5 OC) increased from 0.062 (±0.004) to 0.102 (±0.001) (64.5% higher) and from 0.056 (±0.004) to 0.098 (±0.001) (75% higher) for cold morning and evening rush hours, with substantially greater increases at some sites. This “cold start” result suggests that the combination of recent vehicle driving history and ambient weather conditions, in conjunction with technological constraints on the operating temperature range of emission control systems in some vehicles, affects NOx emissions and hence has a detrimental impact upon air quality in urban environments. Increased cold start emissions imply an increased NO2 - derived health burden from air pollution, under certain meteorological conditions, assessment of which should consider changes in vehicle use as a result of weather, and hence altered personal exposure.

Using meteorological normalisation to detect interventions in air quality time series

Interventions used to improve air quality are often difficult to detect in air quality time series due to the complex nature of the atmosphere. Meteorological normalisation is a technique which controls for meteorology/weather over time in an air quality time series so intervention exploration (and trend analysis) can be assessed in a robust way. A meteorological normalisation technique, based on the random forest machine learning algorithm was applied to routinely collected observations from two locations where known interventions were imposed on transportation activities which were expected to change ambient pollutant concentrations. The application of progressively stringent limits on the content of sulfur in marine fuels was very clearly represented in ambient sulfur dioxide (SO2) monitoring data in Dover, a port city in the South East of England. When the technique was applied to the oxides of nitrogen (NOx and NO2) time series at London Marylebone Road (a Central London monitoring site located in a complex urban environment), the normalised time series highlighted clear changes in NO2 and NOx which were linked to changes in primary (directly emitted) NO2 emissions at the location. The clear features in the time series were illuminated by the meteorological normalisation procedure and were not observable in the raw concentration data alone. The lack of a need for specialised inputs, and the efficient handling of collinearity and interaction effects makes the technique flexible and suitable for a range of potential applications for air quality intervention exploration.

Impact of primary NO2 emissions at different urban sites exceeding the European NO2 standard limit

A large part of the European population is still exposed to ambient nitrogen dioxide (NO2) levels exceeding the European Union (EU) air quality standards, being a key challenge to reduce NO2 concentrations across many European urban areas, particularly close to roads. In this work, a trend analysis of pollutants involved in NO2 processes was done for the period 2003–2014 in traffic sites from three Spanish cities (Barcelona, Madrid and Granada) that still exceed the European NO2 air quality standard limits. We also estimated the contributions of primary NO2 emissions and photo-chemically formed NO2 to the observed ambient NO2 concentrations in order to explore their possible role in the observed NO2 concentration trends. The NOx and NO concentrations at these traffic sites showed significant decreasing trends during the period 2003–2014, especially at Barcelona (BARTR) and Madrid (MADTR) traffic stations. The NO2 concentrations showed statistically significant downward trends at BARTR and MADTR and remained unchanged at Granada traffic station (GRATR) during the study period. Despite the significant decrease in NO2 concentrations in BCNTR and MADTR during the analysed period, the NO2 concentrations observed over these sites still above the annual NO2 standard limit of 40 μg m−3 and, therefore, more efficient measures are still needed. Primary NO2 emissions significantly influence NO2 concentrations at the three analysed sites. However, as no drastic changes are expected in the after-exhaust treatment technology that can reduce primary NO2 emissions to zero in the near future, only a substantial reduction in NOx emissions will help to comply with the NO2 European air quality standards. Reduction of 78%, 56% and 16% on NOx emissions in Barcelona, Madrid and Granada were estimated to be necessary to comply with the NO2 annual limit of 40 μg m−3.

A Portable Emissions Measurement System (PEMS) study of NOx and primary NO2 emissions from Euro 6 diesel passenger cars and comparison with COPERT emission factors

Real world emissions of oxides of nitrogen (NOx) often greatly exceed those achieved in the laboratory based type approval process. In this paper the real world emissions from a substantial sample of the latest Euro 6 diesel passenger cars are presented with a focus on NOx and primary NO2. Portable Emissions Measurement System (PEMS) data is analysed from 39 Euro 6 diesel passenger cars over a test route comprised of urban and motorway sections. The sample includes vehicles installed with exhaust gas recirculation (EGR), lean NOx traps (LNT), or selective catalytic reduction (SCR). The results show wide variability in NOx emissions from 1 to 22 times the type approval limit. The average NOx emission, 0.36 (sd. 0.36) g km−1, is 4.5 times the Euro 6 limit. The average fraction primary NO2 (fNO2) is 44 (sd. 20) %. Higher emissions during the urban section of the route are attributed to an increased number of acceleration events. Comparisons between PEMS measurements and COPERT speed dependent emissions factors show PEMS measurements to be on average 1.6 times higher than COPERT estimates for NOx and 2.5 times for NO2. However, by removing the 5 most polluting vehicles average emissions were reduced considerably.

Synergistic use of OMI NO2 tropospheric columns and LOTOS–EUROS to evaluate the NOx emission trends across Europe

In this study trends in the tropospheric NO2 concentrations during 2005–2010 across Europe were derived from the synergistic use of OMI NO2 tropospheric columns and the chemistry transport model LOTOS–EUROS and were compared to reported NOx emissions. The chemistry transport model captures a large fraction of the variability in NO2 columns at a synoptic timescale, although a seasonal signal in the bias between the modelled and retrieved column data remains. Using a simulation with constant emissions in time, trends were derived on the basis of the systematically changing bias between the modelled and retrieved columns. Significant negative trends of 5–6%a−1 were found in highly industrialized areas across Western Europe. Strongest decreases in NO2 concentrations are observed over a region with many power plants in Northern Spain (10–20%a−1) and over the Po Valley (11%a−1). A source apportionment simulation was performed to evaluate the sensitivity of the Ozone Monitoring Instrument (OMI) to NOx emission sources across Europe, identifying the importance of changes in the energy sector in Northern Spain. A comparative study on in-situ NO2 measurements shows that annual reductions at the surface are of 2–3%a−1. This trend increases from rural polluted sites to remote areas (~4.5%a−1). The observed variability with station type may be explained by the increase in primary NO2 emissions combined with the representativeness of the measurement sites. Comparing country average trends in NO2 columns with national NOx emission totals shows that these are generally within a factor 2 of each other. A better agreement was found for western European countries than for eastern European countries. The method described here is a promising methodology to complement and evaluate trends in NO2 columns and indirectly emission strengths. A strong advantage is the fact that the methodology using satellite data is in principle consistent throughout the entire domain.

Primary NO2 emissions and their impact on air quality in traffic environments in Germany

Abstract Background The decreasing NOX concentrations at urban measurement stations in Germany are in agreement with the reduction of NOX emissions from vehicular traffic. However, the measured NO2 concentrations are stagnating nationwide. In 2010, at more than the half of the urban measurement stations in Germany, annual mean values for NO2 exceeded the new Europe-wide limit value of 40 μg/m3 (20 ppbv) NO2. Similar findings are reported from many other member states of the European Union. Results The observed trend of the airborne NO2 concentrations has different reasons. Firstly, the NO2/NOx emission ratio has increased significantly during the last two decades. Furthermore, secondary NO2, caused by the titration reactions of NO with ozone (O3) and peroxy radicals (RO2), is responsible for the major fraction (approximately 70%) of the measured NO2. However, secondary NO2 shows a highly nonlinear dependency on NOx and thus, is decreasing much more slowly than expected from the decreasing NOx levels. Based on the results from the present study, the increased NO2/NOX emission ratio can only explain a minor fraction of the observed high airborne NO2 concentration in the city center. Conclusions A further reduction of primary NO2 emissions, due to improved exhaust gas treatment, will not have a strong influence on urban NO2 levels, and a further significant reduction of the NOX emissions, in particular from vehicular traffic, is necessary in order to meet the annual mean limit value for NO2 of about 20 ppb in the future.

Trends in primary NO2 and exhaust PM emissions from road traffic for the period 2000–2020 and implications for air quality and health in the Netherlands

Abstract Application of an oxidation catalyst mainly by diesel-fuelled passenger cars reduces harmful exhaust emissions of particulate matter (PM). As a side effect, the primary NO 2 /NO x emission ratio by these vehicles increased from 10% in 2000 (before the introduction of the oxidation catalyst) to between 55% and 70% in 2010. The impact of this evolution in traffic emissions was studied from both a health and a regulatory perspective. Primary NO 2 emissions from road traffic in the Netherlands is expected to increase from 8 kt in 2000 to 15 kt by 2015 and subsequently to decrease to 9 kt by 2020. Meanwhile, exhaust PM emissions from road traffic in the Netherlands will decrease from 7 kt in 2000 to 3 kt by 2020. The impact of exhaust PM on air quality and health was assessed according to the mass concentrations of elemental carbon (EC) in ambient air, as EC is a more sensitive indicator than PM. Monitoring data on the NO 2 /EC concentration ratios near road traffic between 2000 and 2010 indicate no significant change in ambient air quality. This indicates that health effects in epidemiological studies associated with long-term exposure to NO 2 concentrations are still valid. The health impact from the introduction of the oxidation catalyst was assessed by comparing the relatively higher NO 2 (“cost”) and lower EC (“benefit”) concentrations at street locations. “Relative” refers to traffic emissions in situations “with” and “without” the oxidation catalyst being introduced. The cost–benefit ratio in 2010 was in balance, but benefits are expected to outweigh costs by 2015 and 2020. It is concluded that the application of oxidation catalysts is beneficial from a health perspective, but from a regulatory perspective it complicates compliance with the average annual limit value of NO 2 . This indicates that additional local measures may be required in order to meet air quality standards at locations with high traffic intensities.

Long-term trends of primary and secondary NO2 production in the Athens area. Variation of the NO2/NOx ratio

Abstract This paper examines the contribution of primary and secondary NO2 production in NOx concentrations and offers a comprehensive analysis of the long-term trends of NOx, NO2 and O3 concentrations, as well as of the NO2/NOx ratio, in the Athens urban conurbation. Long-term pollutant concentration time series show that NO2 concentrations in Athens have decreased since 1987 but at a slower rate than those of NOx, resulting to an increasing NO2/NOx concentration ratio. However, this increasing trend is much smaller than those observed in urban areas of other European countries. The possible causes of this trend are examined and especially the interaction with ozone and the amount of direct NO2 traffic emissions. The results indicate that the increasing NO2/NOx ratio in the Athens area is mainly attributable to an increased secondary formation of NO2 through photochemical reactions in the atmosphere. More specifically, two different empirical methodologies were applied to examine the primary NO2 concentration fraction in Athens, using ambient monitoring data from a kerbside station. Both methods indicate that the primary NO2 concentration share has not altered significantly between 1998 and 2006. This is mainly attributed to the fact that in the Athens area diesel passenger cars are not allowed and after-treatment technologies such as particle filters and oxidation catalysts are not yet applied in Greece. A probable future penetration of diesel passenger cars in Athens should be combined with inventories of primary NO2 emissions and with the development of appropriate policies to reduce ambient NO2 concentrations below the EU limit values.

Primary NO2 emissions and their role in the development of NO2 concentrations in a traffic environment

Abstract An assessment of the formation of NO2 concentrations in heavily traffic-influenced environments in Helsinki, Finland was carried out. The proportion of primary NO2 emissions from road traffic was estimated using a statistical model for the relationship between the mixing ratios of nitrogen oxides (NO + NO2) and total oxidant (O3 + NO2) measured in 1994–2009. Based on this analysis, a quantitative estimate was derived for the relative importance of the primary NO2 emissions, ambient NO–NO2–O3 equilibrium and background concentrations in the observed NO2 concentrations. The proportion of primary NO2 in the vehicular NOx emissions increased from below 10% in the 1990s to about 20% in 2009, with a more distinctive increase during the most recent years. This development was related to the changes in the proportion of diesel-powered passenger cars in Finland. Between 1994 and 2004, the photochemical NO-to-NO2 conversion comprised on average 51% of the mean NO2 concentration, while the primary NO2 emissions contributed 31%. The role of the primary NO2 emissions was limited by the steeply-decreasing total NOx emissions. More recent data (2005–2009) yielded higher primary NO2 emission fractions (15 –21%), with a clearly increasing trend. As a result, the contribution of chemical conversion steadily decreased from 54% in 2005 to 43% in 2009, while that of the primary NO2 emissions increased from 32 to 44%. In order not to exceed in future the annual limit of NO2 concentration, set by the European Union, in the busiest street canyons in downtown Helsinki, the primary NO2 emissions need to be addressed alongside the total NOx emissions.

Effects of Retrofitting Emission Control Systems on In-Use Heavy Diesel Vehicles

Diesel engines are now the largest source of nitrogen oxides (NO(x)) and fine particulate black carbon (soot) emissions in California. The California Air Resources Board recently adopted a rule requiring that by 2014 all in-use heavy trucks and buses meet current (2007) exhaust particulate matter (PM) emission standards. Also by 2023 all in-use heavy-duty vehicles will have to meet current NO(x) emission standards, with significant progress in achieving the requirements for NO(x) control expected by 2014. This will require retrofit or replacement of older in-use engines. Diesel particle filters (DPF) reduce PM emissions but may increase the NO(2)/NO(x) emission ratio to approximately 35%, compared to approximately 5% typical of diesel engines without particle filters. Additionally, DPF with high oxidative capacity reduce CO and hydrocarbon emissions. We evaluate the effects of retrofitting trucks with DPF on air quality in southern California, using an Eulerian photochemical air quality model. Compared to a 2014 reference scenario without the retrofit program, black carbon concentrations decreased by 12 +/- 2% and 14 +/- 2% during summer and fall, respectively, with corresponding increases in ambient ozone concentrations of 3 +/- 2% and 7 +/- 3%. NO(2) concentrations decreased by 2-4% overall despite the increase in primary NO(2) emissions because total NO(x) emissions were reduced as part of the program to retrofit NO(x) control systems on in-use engines. However, in some cases NO(2) concentrations may increase at locations with high diesel truck traffic.

Analysis of sources and partitioning of oxidant in the UK—Part 2: contributions of nitrogen dioxide emissions and background ozone at a kerbside location in London

Abstract Hourly mean concentration data for nitric oxide (NO), nitrogen dioxide (NO2) and ozone (O3) at Marylebone Rd (an urban kerbside site in London) are used to investigate the diurnal and seasonal dependence of sources of oxidant, and their origins. The observed concentrations of oxidant ([OX], defined as [NO2]+[O3]) are interpreted in terms of a the sum of a NOX-independent ‘regional’ contribution, and a linearly NOX-dependent ‘local’ contribution. The former is believed to equate to the background O3 concentration, whereas the latter is likely to be dominated by emissions of NO2 from road transport at the studied location. The derived regional OX concentrations display a significant seasonal variation, with a springtime maximum value of ca. 43 ppb in April, the results being broadly similar to those reported for background ozone at low altitude sites in northwest Europe. A strong diurnal variation in the local OX contribution is observed throughout the year, with the highest values (typically ca. 0.11 ppb ppb−1 NOX) during the daytime. Diurnal profiles averaged over the periods of the year when the UK operates under GMT and BST demonstrate a clear temporal shift, consistent with the local OX contribution being due to primary NO2 emissions from road transport. In conjunction with traffic flow statistics and associated NOX emissions estimates, the results suggest that primary NO2 from diesel-fuelled vehicles accounts for 0.996 v−0.6 of diesel NOX emissions, by volume, where v is the mean vehicle speed in km h−1 (valid range, 30–60 km h−1). This corresponds to (11.8±1.2)% of NOX emissions integrated over the average diurnal cycle for the conditions at Marylebone Rd. The results also suggest that primary NO2 emissions from petrol-fuelled vehicles are far less important, with an upper limit NO2/NOX emission ratio of

Investigating the potential importance of primary NO2 emissions in a street canyon

Abstract Monitoring data from a busy street canyon location in central London and a background site have been used to estimate the potential importance of primary NO2 emissions from road vehicles travelling along the street canyon. By considering the difference in the total ‘oxidant’, OX (NO2+O3) between the two sites insight has been gained into the potential magnitude of primary NO2 emissions from road vehicles. It is shown that the day of the week and hour of the day variation in OX closely matches some attributes of the variation in road traffic flows and vehicle composition. In particular, the variation in OX compares well the variation in estimated NOX emissions from diesel vehicles by hour of the day and day of the week. A multiple regression is used to estimate the fraction of NO2 in petrol and diesel exhausts that is directly emitted. These results suggest that petrol and diesel vehicles in the street canyon emit 0.6±0.2 and 12.7±0.1 vol% NO2, respectively. Primary emissions of this magnitude appear to explain much of the variation observed from the measurements. These results have implications for modelling studies in terms of source apportionment and how NO2 concentrations respond to NOX control. It is recommended that detailed primary NO2 emissions inventories are developed and used by dispersion modellers for concentration predictions and comparisons with UK national and international limits for NO2.

Investigating the potential importance of primary NO2 emissions in a street canyon

Investigating the potential importance of primary NO2 emissions in a street canyon