Air quality monitoring in Nairobi, Kenya

Lowering the barriers to effective air quality monitoring in Nairobi, Kenya

Traditionally seen as the preserve of large, well-funded organizations capable of investing in complicated analyzer systems, air quality monitoring plays a vital role in providing an accurate picture of what exactly is floating around in the air around us. With people in countless cities around the world suffering from poor health as a result of airborne pollutants, a new way forward is needed – cost-effective monitoring hardware that’s simple to deploy and use. We take a look at how this approach has already shone a light on the pollution problems in Kenya’s capital, Nairobi.

A densely populated urban sprawl with a congested and poorly maintained road network, Nairobi is no stranger to air pollution and the health problems it brings, particularly in the form of respiratory diseases. A great deal of research has been conducted on the topic in recent years, with worrying results. For example, a study carried out in 2013 indicated that respiratory infections are the cause of more than 14 million illnesses in Kenya. But where do the roots of this problem lie?

Pollution from a variety of sources

Nairobi’s air quality problem can be traced back to a number of different sources. One of the prime offenders is traffic – many cars in Nairobi are old and run on low-quality fuels with a high sulfur content, resulting in sulfur dioxide (SO2), particulate matter (PM), and carbon monoxide (CO) emissions. On top of this there is the problem of wood and biomass burning in and around the city – for example the use of solid-fuel burners for cooking – as well as emissions from industrial activities.

As well as the sources of pollution, it is important to bear in mind the impact that climate and weather have on Nairobi’s air quality. Prevailing winds transport pollutants and dust from one part of the city to another, while seasonal variations in precipitation have a huge impact on the level of air quality.

A simple yet effective approach to air quality monitoring and measurement

In order to better understand the temporal and spatial variations of Nairobi’s air quality, in January 2018 Vaisala installed a compact environmental station consisting of a Vaisala AQT420 air quality transmitter and a WXT536 weather transmitter (Figure 1) at the headquarters of the Kenya Meteorological Department. The station is compact, easy to install and operate, and cost-effective. The goal was to demonstrate that it is possible to establish a measurement network that provides valuable insights into air quality without investing significant amounts of capital in expensive and complex equipment.

Figure 1. The compact Vaisala AQT420 environmental station installed at the Kenya Meteorological Department.

The Vaisala station measured the key meteorological (pressure, temperature, humidity, wind, precipitation) and air quality (NO2, SO2, CO, O3, PM2.5, and PM10) parameters over a period of 25 days in January 2018.

Revealing the true nature of air pollution in Nairobi

The measurements gathered by the AQT420 showed that daily air quality averages in Nairobi consistently failed to meet World Health Organization (WHO) guidelines. Most concerning were the recorded levels of SO2, which surpassed the WHO guideline of 20 g/m3 by a factor of five to ten, as illustrated in Figure 2a. Heavy exposure to high levels of SO2 not only damages the human respiratory system, but also causes acid rain, which damages vegetation.

The levels of particulate matter – fine particles (PM2.5) from traffic and burning waste, and particles (PM10) from desert dust and loose soil – were also far in excess of WHO guidelines (Figure 2b). As well as revealing these worrying trends, the air quality measurements also provided a valuable picture of the daily pollution cycles in Nairobi. Unsurprisingly, the levels of particulates, SO2, CO2, and NO2 peak during morning rush hour periods, while ozone peaks can be seen in the early afternoon due to the intense level of UV radiation.

The power of complementing air quality measurements with a compact weather station was demonstrated when investigating the reason for the exceptionally low particle counts on the day of January 4th. The graph showing hourly particulate and precipitation measurements (Figure 3.) tells the story of the events happening over the course of that particular day. A midnight rain shower cleaned the air and made the ground wet, preventing dusty conditions. The rising sun then dried the soil, leading to the increase in PM2.5 readings seen in the morning hours; another small afternoon rain shower then cleared the air for the evening.

What next for Nairobi?

The measurements taken by the Vaisala AQT420 station have demonstrated the viability of establishing a reliable and easy-to-operate air quality monitoring network in a location where no monitoring infrastructure currently exists. The investment required to establish such a network would be roughly equivalent to the cost of a single reference-level station

For Nairobi, this type of network would be extremely valuable in terms of identifying the most severely polluted areas and targeting and verifying the impact of pollution-control actions. Furthermore, having accurate long-term monitoring in place will increase understanding of the city’s air pollution cycles and the impact of weather and climate.