Secondary NO2 Research Articles

How have surface NO2 concentrations changed as a result of the UK’s COVID-19 travel restrictions?

Restrictions as a result of the COVID-19 pandemic have led to fewer vehicles on UK roads. Since fuel combustion is responsible for a large fraction of UK emissions it is expected that surface NO2 concentrations would reduce as a result. However, over parts of the UK, surface NO2 concentrations have increased following the implementation of travel restrictions. NO2 measurements from 142 Automatic Urban and Rural Network sites are combined with meteorological data from the Met Office high-resolution weather prediction model to build site specific models. These models predict NO2 concentrations given no change in emissions. It is found that both meteorological and emission changes contribute to the observed changes in NO2 concentrations. Given no change in emissions, changes in meteorology between pre- and post-lockdown periods would have led to a mean increase in NO2 concentrations of +6%. Conversely, changes in emissions would have led to a mean reduction in NO2 concentrations of -18%, resulting in the observed total change in NO2 concentrations of −12%. However at some sites the reduction in emissions is smaller than the increase in NO2 concentrations due to meteorology. The largest increases associated with changes in the meteorology are seen at rural sites (+20%) where NO2 measurements are representative of large areas and thus dominated by the regional advection of secondary NO2 from Europe. Conversely, the largest decreases associated with reduced emissions are found at urban traffic and urban background sites (−27% and −14% respectively) where NO2 concentrations are representative of local areas and thus dominated by local reduction in emissions from vehicles. As lockdown measures are relaxed, NO2 concentrations are likely to return to pre-COVID levels, but these results demonstrate that changes in our behaviour can result in positive impacts on air quality and illustrate the effectiveness of travel-reducing strategies in urban areas.

Estimate of Secondary NO2 Levels at Two Urban Traffic Sites Using Observations and Modelling

Assessing secondary and primary NO2 in urban areas is important to support carefully designed environmental policies, particularly in areas with recurrent exceedance of NO2 regulatory limits. The share of secondary NO2 was preliminary estimated in intense traffic areas of Modena and Reggio Emilia (Northern Italy) by the combined analysis of regulatory air quality observations at urban traffic and urban background conditions. In addition simulations performed by the Lagrangian particle dispersion models Micro SWIFT SPRAY and the chemical transport model WRF-Chem were performed. The former was applied on the urban area representative of traffic conditions for both cities, in winter. The latter was applied twice in Modena, both with and without urban traffic emissions. Results suggest a large amount of secondary NO2 mainly at the Modena traffic site, and a better representativity of background conditions of the corresponding urban station in Reggio Emilia. NOx levels simulated by WRF-Chem show good results at Modena urban background and performance in line with reference benchmark values in reproducing observed NO2 and NOx concentrations at rural background sites, although a non-negligible bias in simulated urban NO2 remained. Overall the simulation models suggest that contribution to atmospheric NOx by domestic heating or industrial combustion emissions are not as relevant compared to traffic, consistently with the local emission inventory.

Diverse nitrogen cycling pathways across a marine oxygen gradient indicate nitrogen loss coupled to chemoautotrophic activity.

Genetic markers and geochemical assays of microbial nitrogen cycling processes, including autotrophic and heterotrophic denitrification, anammox, ammonia oxidation, and nitrite oxidation, were examined across the oxycline, suboxic, and anoxic zones of the Cariaco Basin, Venezuela. Ammonia and nitrite oxidation genes were expressed through the entire gradient. Transcripts associated with autotrophic and heterotrophic denitrifiers were mostly confined to the suboxic zone and below but were also present in particles in the oxycline. Anammox genes and transcripts were detected over a narrow depth range near the bottom of the suboxic zone and coincided with secondary NO2 - maxima and available NH4 + . Dissolved inorganic nitrogen (DIN) amendment incubations and comparisons between our sampling campaigns suggested that denitrifier activity may be closely coupled with NO3 - availability. Expression of denitrification genes at depths of high rates of chemoautotrophic carbon fixation and phylogenetic analyses of nitrogen cycling genes and transcripts indicated a diverse array of denitrifiers, including chemoautotrophs capable of using NO3 - to oxidize reduced sulfur species. Thus, results suggest that the Cariaco Basin nitrogen cycle is influenced by autotrophic carbon cycling in addition to organic matter oxidation and anammox.

Paired N and O isotopic analysis of nitrate and nitrite in the Arabian Sea oxygen deficient zone

The Arabian Sea is home to one of the three main oceanic oxygen deficient zones (ODZs). We present paired nitrogen (N) and oxygen (O) isotope measurements of nitrate (NO3-) and nitrite (NO2-) from the central Arabian Sea in order to understand the effects of N biogeochemistry on the distribution of these species in the low oxygen waters. Within the ODZ, NO2- accumulated in a secondary NO2- maximum (SNM), though the shape and magnitude of the SNM, along with the isotopic composition of NO3- and NO2-, were highly dependent on the location within the ODZ. We also explored water mass mixing within the Arabian Sea as an explanatory factor in the distribution of NO2- in the SNM. The intrusion of Persian Gulf Water at depth may influence the shape of the NO2- peak by introducing small amounts of dissolved oxygen (O2), favoring NO2- oxidation. There was also evidence that vertical mixing may play a role in shaping the top of the SNM peak. Finally, we present evidence for NO2- oxidation and NO2- reduction co-occurring within the ODZ, as has been previously suggested in the Arabian Sea, as well as in other ODZs. The decoupling of the N and O isotopes of NO3-, deviating from the expected 1:1 ratio for dissimilatory NO3- reduction, indicates that NO2- oxidation has a significant influence on the isotopic composition of NO3-. Additionally, the N isotopes of NO2- were generally fit well by Rayleigh curves for NO2- oxidation. However, the removal of dissolved inorganic nitrogen (DIN) within the domain reflects the importance of NO2- reduction to N2.

A new approach for estimation of the effect of NOx emission reduction on roadside NO2 concentration in Tokyo

The mean NO2/NOx ratios of vehicle emission, the mean duration time of NO reaction with O3 and background concentrations at the five most polluted roadside stations in Tokyo are estimated statistically using monitoring data at both roadside sites and urban background sites. These statistics are then used to categorize roadside NO2 into three components; NO2 emission from vehicles (primary NO2), oxidation of NO by O3 (secondary NO2) and background NO2 (BG NO2). Secondary NO2 and BG NO2 make a greater contribution to the high NO2 concentrations in spring and summer. On the other hand, BG NO2 contributes the dominant portion to high NO2 concentrations in winter. The effects of vehicle NOx emission reduction on the concentration of primary, secondary and BG NO2 are evaluated using an NO2 emission reduction scenario, an oxidation model proposed in this study and an air quality simulation, respectively. It is predicted that the shift to vehicles meeting the stringent emission regulation beginning in 2009 will lower the roadside NO2 concentration due mainly to the large decrease in secondary NO2. On the other hand, the reduction of NOx in vehicle emissions results in a smaller decrease in roadside NO2 in winter.

Following the N<sub>2</sub>O consumption in the oxygen minimum zone of the eastern South Pacific

Abstract. Oxygen minimum zones (OMZs), such as those found in the eastern South Pacific (ESP), are the most important N2O sources in the global ocean relative to their volume. N2O production is related to low O2 concentrations and high primary productivity. However, when O2 is sufficiently low, canonical denitrification takes place and N2O consumption can be expected. N2O distribution in the ESP was analyzed over a wide latitudinal and longitudinal range (from 5° to 30° S and from 71–76° to ~ 84° W) based on ~ 890 N2O measurements. Intense N2O consumption, driving undersaturations as low as 40%, was always associated with secondary NO2– accumulation (SNM), a good indicator of suboxic/anoxic O2 levels. First, we explore relationships between ΔN2O and O2 based on existing data of denitrifying bacteria cultures and field observations. Given the uncertainties in the O2 measurements, a second relationship between ΔN2O and NO2– (> 0.75 μM) was established for suboxic waters (O2 < 8 μM). We reproduced the apparent N2O production (ΔN2O) along the OMZ in ESP with high reliability (r2 = 0.73 p = 0.01). Our results will contribute to the quantification of the N2O that is recycled in O2 deficient waters, and improve the prediction of N2O behavior under future scenarios of OMZ expansion and intensification.

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.

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.

Experiences with partial-flow filters to reduce the solid particle emissions from diesel engines

Four of these Particulate Reduction Systems (PMS) were tested on a passenger car and one of them on a HDV. Expectation of the research team was that they would reach at least a PM-reduction of 30% under all realistic operating conditions. The standard German filter test procedure for PMS was performed but moreover, the response to various operating conditions was tested including worst case situations. Besides the legislated CO, NOx and PM exhaust-gas emissions, also the particle count and NO2 were measured. The best filtration efficiency with one PMS was indeed 63%. However, under critical but realistic conditions filtration of 3 of 4 PMS was measured substantially lower than the expected 30 %, depending on operating conditions and prior history, and could even completely fail. Scatter between repeated cycles was very large and results were not reproducible. Even worse, with all 4 PMS deposited soot, stored in these systems during light load operation was intermittently blown-off. Due to these stochastic phenomena the behaviour of these systems is hardly predictable. Furthermore the provision of NO2, through catalysis ahead of the filter or in the filter matrix, is inherent in these systems. Some of this secondary NO2 is emitted. Cost/benefit ratio is high compared to full-flow filters and Diesel engines equipped with partial-flow filters are inferior to SI engines regarding global warming potential. Based on these findings it is concluded that the sustainable performance of partial-flow filters is not yet determined.

Revising the nitrogen cycle in the Peruvian oxygen minimum zone

The oxygen minimum zone (OMZ) of the Eastern Tropical South Pacific (ETSP) is 1 of the 3 major regions in the world where oceanic nitrogen is lost in the pelagic realm. The recent identification of anammox, instead of denitrification, as the likely prevalent pathway for nitrogen loss in this OMZ raises strong questions about our understanding of nitrogen cycling and organic matter remineralization in these waters. Without detectable denitrification, it is unclear how NH(4)(+) is remineralized from organic matter and sustains anammox or how secondary NO(2)(-) maxima arise within the OMZ. Here we show that in the ETSP-OMZ, anammox obtains 67% or more of NO(2)(-) from nitrate reduction, and 33% or less from aerobic ammonia oxidation, based on stable-isotope pairing experiments corroborated by functional gene expression analyses. Dissimilatory nitrate reduction to ammonium was detected in an open-ocean setting. It occurred throughout the OMZ and could satisfy a substantial part of the NH(4)(+) requirement for anammox. The remaining NH(4)(+) came from remineralization via nitrate reduction and probably from microaerobic respiration. Altogether, deep-sea NO(3)(-) accounted for only approximately 50% of the nitrogen loss in the ETSP, rather than 100% as commonly assumed. Because oceanic OMZs seem to be expanding because of global climate change, it is increasingly imperative to incorporate the correct nitrogen-loss pathways in global biogeochemical models to predict more accurately how the nitrogen cycle in our future ocean may respond.