Britter, R., 2015. The rise of low cost sensing for managing air pollution in cities. Environment International 75,[199][200][201][202][203][204][205]. Online link: http://dx

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Over the past decade, a range of sensor technologies became available on the market, enabling a revolutionary shift in air pollution monitoring and assessment. With their cost of up to three orders of magnitude lower than standard/reference instruments, many avenues for applications have opened up. In particular, broader participation in air quality discussion and utilisation of information on air pollution by communities has become possible. However, many questions have been also asked about the actual benefits of these technologies. To address this issue, we conducted a comprehensive literature search including both the scientific and grey literature. We focused upon two questions: (1) Are these technologies fit for the various purposes envisaged? and (2) How far have these technologies and their applications progressed to provide answers and solutions? Regarding the former, we concluded that there is no clear answer to the question, due to a lack of: sensor/monitor manufacturers' quantitative specifications of performance, consensus regarding recommended end-use and associated minimal performance targets of these technologies, and the ability of the prospective users to formulate the requirements for their applications, or conditions of the intended use. Numerous studies have assessed and reported sensor/monitor performance under a range of specific conditions, and in many cases the performance was concluded to be satisfactory. The specific use cases for sensors/monitors included outdoor in a stationary mode, outdoor in a mobile mode, indoor environments and personal monitoring. Under certain conditions of application, project goals, and monitoring environments, some sensors/monitors were fit for a specific purpose. Based on analysis of 17 large projects, which reached applied outcome stage, and typically conducted by consortia of organizations, we observed that a sizable fraction of them (~ 30%) were commercial and/or crowd-funded. This fact by itself signals a paradigm change in air quality monitoring, which previously had been primarily implemented by government organizations. An additional paradigm-shift indicator is the growing use of machine learning or other advanced data processing approaches to improve sensor/monitor agreement with reference monitors. There is still some way to go in enhancing application of the technologies for source apportionment, which is of particular necessity and urgency in developing countries. Also, there has been somewhat less progress in wide-scale monitoring of personal exposures. However, it can be argued that with a significant future expansion of monitoring networks, including indoor environments, there may be less need for wearable or portable sensors/monitors to assess personal exposure. Traditional personal monitoring would still be valuable where spatial variability of pollutants of interest is at a finer resolution than the monitoring network can resolve.

21 22The complexity of ambient particle composition considerably complicates pinpointing specific 23 causal associations between exposure to particles and adverse human health effects, the 24 contribution of different sources to ambient particles at different locations, and the consequent 25 formulation of policy action to most cost-effectively reduce harm caused by airborne particles. 26Nevertheless, the coupling of increasingly sophisticated measurements and models of particle 27 composition and epidemiology continue to reveal associations between particle components 28 and sources (and at lower concentrations) and a wide range of adverse health outcomes. This 29 article describes the current metrics, legislation and policies for protection of public health 30 from ambient particles, and reviews the current approaches to source apportionment of 31 particles and the latest evidence for their health effects. A particular focus is placed on particles 32 in the ultrafine fraction. The review concludes with an extended evaluation of emerging 33 challenges and future requirements in methods and metrics for understanding health outcomes, 34 and in the policy context. 35 36 For the research described here it is usually only the particle phase that is being discussed, to 44 which the word aerosol is sometimes erroneously applied (an aerosol being the combination of 45 the particles and the gas in which they are suspended). Instead, in line with correct usage, the 46 terminologies particle or particulate matter (abbreviated to PM), rather than aerosol, are used. 47 48A link between poor air quality and mortality has been recognised for centuries, becoming 49 particularly manifest as urbanisation and industrialisation intensified 3,4 . The source of air 50 pollution was formerly dominated by widespread coal and other solid-fuel burning, plus other 51 toxic emissions from largely unregulated industrial processes. In developed countries at least, 52 the extent of air pollution from such sources declined markedly with Clean Air Acts and 53 similar 'smoke control' legislation introduced from the mid-20 th century 5 , although these 54 remain important sources of air pollution in some parts of the world. From the latter part of the 55 20 th century, the coupling of increasingly sophisticated measurements of atmospheric 56 composition and epidemiological methods has continued to reveal associations between a 57 range of air pollutants (and at lower concentrations) and adverse health outcomes 4 . There is 58 recognition also of the multitude of sources and complex chemistry now contributing to poor 59 air quality, and the wider geographic scales of influence 5 . 60 61In the contemporary context, the deleterious impact of PM on air quality and health is 62 recognised by the World Health Organisation who publish advisory air quality guidelines for 63 ambient concentrations of PM (and other ground-level pollutants); see Table 1 6 . Many 64 countries or political blocs such as the European Union (EU) have developed polic...

Ultrafine particles (UFPs; diameter less than 100 nm) are ubiquitous in urban air, and an acknowledged risk to human health. Globally, the major source for urban outdoor UFP concentrations is motor traffic. Ongoing trends towards urbanisation and expansion of road traffic are anticipated to further increase population exposure to UFPs. Numerous experimental studies have characterised UFPs in individual cities, but an integrated evaluation of emissions and population exposure is still lacking. Our analysis suggests that the average exposure to outdoor UFPs in Asian cities is about four-times larger than that in European cities but impacts on human health are largely unknown. This article reviews some fundamental drivers of UFP emissions and dispersion, and highlights unresolved challenges, as well as recommendations to ensure sustainable urban development whilst minimising any possible adverse health impacts.

Low-cost sensor technology can potentially revolutionise the area of air pollution monitoring by providing high-density spatiotemporal pollution data. Such data can be utilised for supplementing traditional pollution monitoring, improving exposure estimates, and raising community awareness about air pollution. However, data quality remains a major concern that hinders the widespread adoption of low-cost sensor technology. Unreliable data may mislead unsuspecting users and potentially lead to alarming consequences such as reporting acceptable air pollutant levels when they are above the limits deemed safe for human health. This article provides scientific guidance to the end-users for effectively deploying low-cost sensors for monitoring air pollution and people's exposure, while ensuring reasonable data quality. We review the performance characteristics of several low-cost particle and gas monitoring sensors and provide recommendations to end-users for making proper sensor selection by summarizing the capabilities and limitations of such sensors. The challenges, best practices, and future outlook for effectively deploying low-cost sensors, and maintaining data quality are also discussed. For data quality assurance, a two-stage sensor calibration process is recommended, which includes laboratory calibration under controlled conditions by the manufacturer supplemented with routine calibration checks performed by the end-user under final deployment conditions. For large sensor networks where routine calibration checks are impractical, statistical techniques for data quality assurance should be utilised. Further advancements and adoption of sophisticated mathematical and statistical techniques for sensor calibration, fault detection, and data quality assurance can indeed help to realise the promised benefits of a low-cost air pollution sensor network.

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