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 (R
2: 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.