AIR QUALITY IN CITIES – AND THE VITAL ROLE OF MONITORING


As a manufacturer of air quality sensors, Alphasense has been intimately involved with the development of air quality management strategy around the world, and in the following article, Technical Director and air quality expert Dr John Saffell explains the evolving and vitally important role of air quality monitoring.


Background


The world’s urban population has grown rapidly from 751 million in 1950 to 4.2 billion in 2018. According to the United Nations, in 2018 55% of the world’s population lived in urban areas, and this is expected to increase to 68% by 2050. Projections show that urbanisation, coupled with global population growth could add 2.5 billion people to urban areas by 2050, with around 90% of this increase taking place in Asia and Africa.


The main sources of urban air pollution are combustion products from motor vehicles, heat and power generation, industrial facilities, municipal waste incineration, residential cooking, heating, and lighting, plus gases and volatile organic compounds (VOCs) from landfi lls, wastewater treatment plants and intensive farming. Although urban air quality has improved in some developed countries, according to the World Health Organisation (WHO), air pollution is getting worse in most low and middle-income cities. Contributing factors include increased power demand and the soaring use of private vehicles.


Urban air pollution is generally poorly dispersed, particularly on busy roads in built-up areas where road canyons retain the vehicle combustion products. This results in pollution hot-spots where the levels of key pollutants signifi cantly exceed WHO guideline levels.


When considering ambient air quality, it is important to differentiate between the four main issues: greenhouse gases (GHGs), urban air quality, indoor air quality and nuisance odours. These are of course inter-related, but their causes differ and mitigation measures vary accordingly. Climate change is caused by GHGs such as methane (CH4


but these gases are not the most important from a health perspective. Diesel vehicles emit less CO2


can be used as a metric to judge whether infrastructure expenditure has been well spent.


Low cost sensors


Highlighting the disadvantages of fi xed station monitoring, researchers from Cambridge University deployed low cost Alphasense sensors in the backpacks of (cycling and walking) students to ‘map’ air pollution across the city. The results showed that some parts of the city experienced NO2


levels over ten times that measured by the reference station.


Air quality monitoring is substantially improved when continuous monitoring is undertaken using high density networks. Pollution hot-spots can be identifi ed on a temporal and spatial basis, helping to select the best mitigation measures and then locating sensors at, for example, places where new traffi c management measures are implemented. Air quality networks can be strategically located before, during and after urban construction projects. The high spatial and temporal resolution of air quality networks is already helping air quality modellers to improve their algorithms and better predict the effect of future infrastructure projects.


) and carbon dioxide (CO2 ), per mile than petrol vehicles, which is why they


could be considered more ‘climate friendly,’ but diesel vehicles are generally responsible for more nitrogen dioxide (NO2


) and particulates, which is why many cities are setting up Low Emission Zones that limit diesel vehicle access.


Globally, around 3.8 million premature deaths are attributed to ambient air pollution every year, 80% of which, according to the WHO, are due to heart disease and strokes. A further 20% are from respiratory illnesses and cancers related to exposure to fi ne particulate matter (PM2.5). Gases also represent a signifi cant risk to human health; these include nitrogen dioxide (NO2


), ozone (O3 ), sulfur dioxide (SO2


High density local air quality data has long been the dream of city and town planners, but reference stations are prohibitively expensive, and in the past the performance of low cost ambient air quality sensors was unacceptable. However, Alphasense has invested heavily in research and development in this area, and is now routinely supplying high quality reliable gas and particle sensors to partners who manufacture air quality networks. The key issues addressed by this work included sensitivity, stability, selectivity and repeatability. This work has delivered a number of important and unique innovations. Every sensor manufactured by Alphasense for the last two decades has been tested and the records are retained in-house. Consequently, every air quality sensor is supplied with the detailed performance for that individual sensor.


Learning more from air quality networks ) and carbon monoxide (CO), as well as


volatile organic compounds (VOCs). Most of these pollutants are not visible, so monitoring is essential.


Air quality monitoring


Traditional air quality monitoring stations are large mains powered housings with reference grade analysers that require regular calibration and maintenance. Data from these stations are essential because they can be used for enforcement and are accepted in a court of law.


However reference air quality monitoring stations are expensive to construct and maintain. In addition, these large structures are often diffi cult to locate and require planning permission from the local authority. Importantly, they only provide air quality data from one fi xed location, which may not be representative of local air quality. Urban air quality maps therefore depend heavily on air quality modelling with only a few accurate data points.


Clean air is a fundamental human right, and urban infrastructure investment should improve air quality. Hence effective air quality monitoring


AET October / November 2019 www.envirotech-online.com


Spatially dense monitors provide greater insight into the air quality status of a city, extending beyond better data granularity. For example, if one monitor in a network shows an elevated reading, the cause is mostly likely a local point source of pollution, but if all closely located monitors show elevated values, then that would indicate a distant (far fi eld) pollution source. Also, since many gases and particle sizes are normally monitored, investigators are able to identify specifi c pollution types – for example, adding a CO2


sensor and particle size monitor can help identify the type of combustion sources.


It is also possible to track the movement of pollution through a network, particularly when the readings are supplemented by wind measurements. In these circumstances, the visualisation of data is key; for example, some network operators have developed maps which display live colour-coded data displays from which it is possible to capture the display over several hours or days and create brief animations that dramatically help to visualise the movement of an air pollution plume.


In addition, networks benefi t from improved operational reliability because one monitor showing unusual readings might indicate a problem with that monitor, whereas a group of monitors all showing unusual readings is more


110 AQMesh pods deliver hyperlocal data to the Breathe London website


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