Abstract:
Particulate matter air pollution is a leading cause of
global mortality, particularly in Asia and Africa. Addressing the
high and wide-ranging air pollution levels requires ambient
monitoring, but many low- and middle-income countries
(LMICs) remain scarcely monitored. To address these data gaps,
recent studies have utilized low-cost sensors. These sensors have
varied performance, and little literature exists about sensor
intercomparison in Africa. By colocating 2 QuantAQ Modulair PM, 2 PurpleAir PA-II SD, and 16 Clarity Node-S Generation II
monitors with a reference-grade Teledyne monitor in Accra,
Ghana, we present the first intercomparisons of different brands of
low-cost sensors in Africa, demonstrating that each type of low-cost sensor PM2.5 is strongly correlated with reference PM2.5, but
biased high for ambient mixture of sources found in Accra. When compared to a reference monitor, the QuantAQ Modulair-PM has
the lowest mean absolute error at 3.04 μg/m3
, followed by PurpleAir PA-II (4.54 μg/m3
) and Clarity Node-S (13.68 μg/m3
). We
also compare the usage of 4 statistical or machine learning models (Multiple Linear Regression, Random Forest, Gaussian Mixture
Regression, and XGBoost) to correct low-cost sensors data, and find that XGBoost performs the best in testing (R2
: 0.97, 0.94, 0.96;
mean absolute error: 0.56, 0.80, and 0.68 μg/m3 for PurpleAir PA-II, Clarity Node-S, and Modulair-PM, respectively), but tree-based
models do not perform well when correcting data outside the range of the colocation training. Therefore, we used Gaussian Mixture
Regression to correct data from the network of 17 Clarity Node-S monitors deployed around Accra, Ghana, from 2018 to 2021. We
find that the network daily average PM2.5 concentration in Accra is 23.4 μg/m3
, which is 1.6 times the World Health Organization
Daily PM2.5 guideline of 15 μg/m3
. While this level is lower than those seen in some larger African cities (such as Kinshasa,
Democratic Republic of the Congo), mitigation strategies should be developed soon to prevent further impairment to air quality as
Accra, and Ghana as a whole, rapidly grow
