Air pollution is not a new problem, but it has changed dramatically over the last century. Modern air pollution is now far less visually dramatic than the ‘great smog’ episodes of the 1950s, but it is no less dangerous to human health.
Given the amount and frequency with which humans consume air, access to clean air to breathe is of critical importance for health. Yet, the World Health Organisation reports that more than four million deaths every year are as a direct result of exposure to polluted air. One of the most important of these air pollutants is nitrogen dioxide (NO2).
Understanding nitrogen dioxide
NO2 plays a key role in ozone and secondary particle formation, acidification and eutrophication. It influences the oxidative capacity of the atmosphere with the potential to significantly impact climate, air, and water quality, and human health globally.
The World Meteorological Organisation Global Atmospheric Watch (WMO-GAW) Programme, a global monitoring network, has identified NO2 as an essential climate variable that needs to be monitored on a global scale because of its importance to atmospheric chemistry, air quality and climate. NO2 is emitted primarily through combustion of fossil fuels by transportation, industrial, and electricity generation activities, but is also produced as a result of wildfires and naturally from lightning strikes.
NO2 is a toxic gas and exposure to high amount fractions can lead to adverse health outcomes in the human respiratory system. As a result of these concerns, it is a regulated air pollutant in many parts of the world including Europe (Air Quality Directive 2008/50/EC) and the United States (Clean Air Act 1970).
Air pollution in cities
Of particular importance for human exposure to NO2 is in densely populated cities where emissions from motor vehicles are predominant. This seems set to become an even larger issue in the future due to increasing trends in urbanisation observed across the world, and the fact that exceedances of air quality legislation are driven by very high NO2 amount fractions in urban areas.
“NO2 is a toxic gas, it is a regulated air pollutant in many parts of the world including Europe”
Increasingly stringent vehicle emission standards, introduced over the last 30 years, were intended to produce substantial decreases in NO2 amount fractions. However, this has not happened in regions that have seen large increases in diesel vehicle ownership, like the UK and Europe, because the emission standards for diesel vehicles have not delivered the expected reductions under real world driving conditions. To address this situation new European legislation (Regulation 2016/646) has recently been introduced for on-road type approval real driving emission tests using portable emissions measurement systems (PEMS); the metrological traceability for which is currently lacking.
The importance of air monitoring
More accurate and lower uncertainty measurements of NO2 are needed in order to better understand population level exposure; to improve air quality models and emission inventories; to better discern long‑term trends; and to enforce air quality and vehicle emission legislation. This is essential for the timely evaluation of air pollution mitigation policies, and to improve our understanding of the influence of anthropogenic emissions on the climate system. There is also an appreciation in the scientific community of the added value that can be derived from more accurate measurements of NO2, including improvements in the refinement of atmospheric models and ground truthing of satellite retrieval data.
To improve future quality of life and to reduce the economic burden of the detrimental health outcomes from NO2 exposure, greater reductions in NO2 amount fractions are needed now. To enable this requires greater confidence in ambient and emission measurement data, which itself is strongly linked to the analytical methodology and the accuracy and quality of the reference materials used for calibration. As a regulated air pollutant, NO2 is unique in that it is the only one that is typically determined through an indirect measurement method and is not direct calibrated.
Measuring nitrogen dioxide
For the last five decades, the standard method employed to make routine NO2 measurements to demonstrate compliance with legislation has been the chemiluminescence (CLD) method. This technique was first developed in the 1970s and has many advantages including high sensitivity and a large linear range.
This technique measures NO2 as the difference between two independent measurement channels; one that detects only nitrogen monoxide (NO) and a second channel which detects the sum of NO and NO2. The second channel relies on the conversion of NO2 to NO to enable its detection. This conversion process is not one hundred percent selective for NO2 and can be affected by both positive and negative artifacts. This leads to biases that reduce the accuracy of the NO2 measurements and present challenges when comparing measurements to numerical model outputs or satellite retrieval data, because you are no longer comparing the same things. Despite this, it remains the most widespread measurement technique for measuring atmospheric NO2.
Due to recent advances in spectroscopic based instrumentation, new approaches to directly measure NO2 are now commercially available, e.g. cavity attenuated phase shift spectroscopy (CAPS), cavity ring down spectroscopy (CRDS), tuneable diode laser adsorption spectroscopy (TDLAS) and quantum cascade laser adsorption spectroscopy (QCLAS). There are several challenges that limit the more widespread uptake of these new techniques. Some of this is inertia to change the status quo and the cost of new capital items, but more is that the standard methods – that are specifically recommended or required to report compliance with legislation – refer to the CLD method directly. If the regulatory bodies were to endorse or require a direct standard method for reporting compliance for NO2, then there would likely be a substantial change in the market demand for direct NO2 analysers, which would be beneficial for achieving more accurate measurements.
Producing gas reference materials
Routine instruments used for making atmospheric gas measurements are typically calibrated using gas reference materials in high pressure cylinders, as this is technically and logistically straightforward. One of the main obstacles to direct NO2 calibration is the high reactivity of NO2 gas that presents challenges in the production of accurate and stable NO2 gas reference materials. Traceable reference materials are critical for maintaining consistent, long-term datasets and for ensuring comparability and, therefore, a level playing field.
Gas reference materials are typically produced according to ISO 6142, from pure compounds that are blended gravimetrically to produce reference materials at the desired amount fractions. Primary gas reference materials (PRMs) are produced by National Metrology Institutes (NMIs) and represent the highest point in the metrological traceability chain, providing traceability to the internationally recognised international system of units (SI) that is critical to underpin the accuracy and comparability of all gas measurements.
The lack of high purity pure NO2 means that NO2 gas reference materials are predominantly produced from nitrogen monoxide (NO), which is reacted in the cylinder with molecular oxygen to produce NO2. At high amount fractions this reaction is fast and provided there is an excess of oxygen will always go to completion thereby ensuring that all NO has been converted to NO2 within the cylinder.
For gas reference materials prepared following ISO 6142, the amount fraction of impurities in the prepared mixture are derived from the analysis of pure gases used for the preparation. For NO2, due to its high reactivity, analysis is not possible using the current preparation methods, as typically many more compounds are present in the prepared mixture than can be expected based on the purity of the pure source gases. This happens as a result of additional nitrogen containing compounds (NOy) being unintentionally created during the preparation process.
As a result, characterisation, quantification and minimisation of these additional compounds are required if the produced NO2 gas reference materials are to be of the highest accuracy and with the lowest uncertainty.
Research and innovation
Recent work in the NPL led European Metrology Programme for Innovation and Research (EMPIR) funded Metrology for Nitrogen Dioxide (MetNO2) project (www.empir.npl.co.uk/metno2), has focussed on addressing these issues and on understanding the behaviour of these impurities after they are formed.
The most important impurity is water due to the complicated reactions of water with NO2 that produces nitric acid (HNO3). HNO3 is observed to be abundant and ubiquitous in virtually all NO2 gas reference materials, meaning that controlling and minimising water during preparation of NO2 gas reference materials is of utmost importance. The characterisation of the abundance of HNO3 in NO2 gas reference materials is required to improve accuracy so that the NO2 that has been lost through conversion to HNO3 can be accounted for. Due to its ubiquity, accurate measurements of HNO3 are needed, but are challenging.
In the MetNO2 project, new instrumental capabilities and spectroscopic methods were developed for the measurement of HNO3, enabling these important measurements going forward. Capabilities developed in this project are being evaluated in an international pilot study (CCQM-P172) organised by the Gas Analysis Working Group (GAWG) of the Consultative Committee on Amount of Substance (CCQM) designed to demonstrate the comparability of capabilities for the measurement of HNO3 impurities in NO2 gas reference materials. The results of which should be available soon.
While HNO3 is the major impurity in low amount fraction gas reference materials needed for the calibration of ambient measurements, a different impurity – nitrogen tetroxide (N2O4), the dimer of NO2 – predominates in higher amount fraction gas reference materials needed for emission level measurements. These two molecules are in equilibrium, an equilibrium which favours N2O4 when there is a greater amount of NO2 present.
Accurate quantification of N2O4, and an improved understanding of this equilibrium chemistry, is key to the improved calibration of emission level NO2 amount fractions. Due to the amount fraction range of operating vehicle engines, both the low and high amount fractions gas reference materials are needed for the calibration of portable emission measurement systems (PEMS). This is one of the focusses of work package one, led by NPL, as part of the EMPIR funded Improved vehicle exhaust quantification by portable emission measurement systems metrology (MetroPEMS) project (www.metropems.ptb.de).
This project is being led by Physikalisch-Technische Bundesanstalt (PTB) in Germany and is more broadly focussed on addressing the lack of metrological traceability for PEMS measurements, with a focus on NO2 and fine particles. The work on NO2 in this project builds on the successes of the MetNO2 project and will further improve and extend current capabilities for the production of NO2 gas reference materials. The intention is to encompass a wider amount fraction range with lower uncertainties than is currently possible, which are needed to meet the legislative requirements for PEMS measurements.
Widespread dissemination of these improvements in measurements and reference materials to end users (e.g. speciality gas industry, instrument manufacturers and monitoring networks) is key ensuring the market gap is addressed. Thus, enabling the transition from the status quo of indirect measurement and indirect calibration to a future of direct measurement and direct calibration, with implications for substantial improvements in the measurements of NO2 and for the added value that such measurements provide, to be fully realised.
Extreme climate events impact air quality
Weather and air quality are inexplicably linked as air quality is highly affected by changes in the weather, especially in extreme conditions. Warmer weather and high-pressure weather systems in summer can result in heat waves, which often lead to poor air quality. This is a result of providing ideal conditions for some pollutants to form efficiently, such as ground-level ozone, while also inhibiting the dispersion of pollutants, allowing them to build up to dangerous levels.
Rising temperatures can also create favourable environmental conditions for droughts and wildfires. It has been impossible to avoid the intense media coverage of wildfires raging out of control in California, the Mediterranean region and Siberia, amongst others, that has dominated the headlines over the past summer. These are being reported as some of the worst in history, and climate change has been implicated as a key factor behind the increasing risk and extent of these wildfires. Their devasting impact on human life, and manmade and natural structures, is all too apparent. But, they also result in substantial emissions of air pollutants, including nitrogen oxides, carbon monoxide and particulates, which have serious impacts on atmospheric composition, chemistry and air quality.
Understanding these impacts is important for improving the accuracy of the outputs of atmospheric models, which are critical for predicting future climate and for driving policy decisions. Global continuous monitoring datasets of atmospheric composition, such as those collected by the World Meteorological Organisation Global Atmospheric Watch (WMO-GAW) programme, represent an important resource to track long-term changes in atmospheric composition. These long-term measurement records provide a baseline from which the impacts of perturbations caused by extreme events can be distinguished and studied.
A critical component to ensuring the integrity of these long-term records is the accuracy and stability of traceable reference materials that are used for calibration. These ensure that the temporal (year to year) and spatial (different measurement sites) comparability of the records are robust. It is vital that policy makers are supported through the delivery of accurate, reliable, traceable data sets which underpin climate action and policy development, as climate change is likely to intensify future impacts by increasing the frequency and duration of extreme weather events that exacerbate air pollution.
In the year of COP26, and with increasing numbers of extreme climate events observed across the globe, understanding the sources and impacts of air pollution and utilising metrology to support mitigation efforts is now more important than ever.