1. Introduction
Air pollution has been shown to be detrimental to human health, including on occupational health in West Africa [
1]. Air pollutants can be classed as primary (e.g., nitrogen oxides (NOx); particulate matter (PM)), or as secondary if they are as consequences of chemical reactions in the lower atmosphere (e.g., ozone (O
3)) [
2]. The atmosphere is a medium in which air pollutants are dispersed away from their sources [
3] and as meteorological parameters such as temperature, and humidity vary daily, it is important to consider their relationship with air pollutants.
Several studies have presented the influence of meteorological parameters on air pollution. For instance, some studies have observed that temperature and sunshine duration had the strongest influence on the local surface O
3 concentration while the impacts of relative humidity and precipitation were weak and the impact of wind speed varied greatly between the cities in the Shanxi Province in China [
4]. In the study, if local surface O
3 concentration in a city in the Shanxi Province was significantly correlated with meteorological parameters that impacted photochemical reactions (e.g., temperature and sunshine duration), then the O
3 pollution was regarded to be mainly brought about by local photochemical build up; otherwise, regional wind direction and speed were the main attributes [
4]. All the monitoring stations used for the study were located in urban areas, therefore the meteorological interactions between urban, and rural areas that affect the photochemical processes that determine the O
3 production were not included in the results.
Studies on the association of PM concentration levels and the meteorological parameters are common as PM is considered impactful [
5,
6,
7]. The results from the evaluation of the temperature and humidity effects on PM concentrations in Auckland, New Zealand showed that the temperature values had a negative correlation with the PM
10 concentration values over a diurnal period and that the relative humidity generally presented a positive correlation with PM
10, but this correlation ceased beyond the 75% relative humidity value [
5]. The researchers posited that this is because with increasing humidity levels, moisture particles increasingly grow in size until they reach a threshold where dry deposition happens, therefore reducing the PM
10 concentrations in the atmosphere. The natural deposition of PM is affected by relative humidity and atmospheric PM concentration increases as the moisture particles adhere to PM [
5]; this study was carried out over an eight-week period. The influences of temperature, relative humidity, wind speed, and wind direction on PM
10 concentrations were evaluated in a study in urban and rural environments in İzmir, Türkiye [
6]. The levels of relative humidity were found to be the most influencing factors on the PM
10 concentration levels in both the urban and rural environments, however the recorded temperature values were not found to have any statistically significant effect on the PM
10 concentration levels. The researchers indicated that incorporating further meteorological parameters such as atmospheric pressure and precipitation would improve the regression models presented in the study [
6].
Air pollution studies in West Africa have been carried out in the past, but fewer studies exist on the correlations between meteorological parameters and air pollution. In [
8], the correlation between temperature, humidity and PM (PM
1, PM
2.5, PM
10) was studied using data from five monitoring centres in five states (Osun, Kebbi, FCT, Delta, Lagos) in Nigeria, West Africa. One of the five stations was located further north of the country where there are short rainy seasons (four months), compared to one in the centre of the country (7 months of the rainy season) and the three of the stations in the south west of the country (8 months). The results indicated strong correlations between all the PM sizes (PM
1, PM
2.5, PM
10) and relative humidity in Delta. However, for the other states, the correlations were weak. The studies presented weak correlations between the PM sizes (PM
1, PM
2.5, PM
10) and the ambient temperature values for all five sites in the states [
8]. The studies, however, were carried out over different periods at the five sites, ranging from 2 (Kebbi) to 7 months (Abuja).
The influences of the wind direction and speed, rainfall, ambient temperature and relative humidity on the PM
2.5, and PM
10 concentration values were presented in a study at an urban site in Port Harcourt, West Africa [
9]. It was reported that the wind speed, rainfall and ambient temperature all significantly affected the PM
2.5, and PM
10 concentration values but with weak correlations. The observed relative humidity values showed a weak but significant correlation with PM
10 concentration values and a weak but insignificant correlation with PM
2.5 concentration values. The study was carried out over a period of 8 months. A similar study at Akure, West Africa [
10] found weak correlations between the values of wind speed, humidity, temperature and the PM
10 and PM
2.5 concentration values. An earlier study at Ile-Ife, West Africa [
11] had similar results but only 162 samples of PM (PM
2.5, and PM
10) were collected over 10 months of the study.
The results from the studies of the relationships between meteorological parameters and air pollution can aid the development of air quality management plans [
9], especially in West Africa where there is a dearth of local air quality monitoring stations [
8]. The present study contributes to these by evaluating the relationships between temperature and humidity for not just PM
10 and PM
2.5 concentrations but also the NO, NO
2, O
3, and PM
1 concentrations which are air pollutants that have been uncommonly studied in the region. Secondly, the study uses hourly data covering the complete dry and rainy seasons over a period of 12 months; typically, previous studies in this region have not presented such complete continuous data. Lastly, the location (Akoka/Lagos, the largest city in West Africa [
1]) and use of calibrated sensors allow for a robust assessment of the results. The next section presents the materials and methods used for the study. A detailed assessment of the results, including the measures of central tendency and the temporal evolution of the air pollution then follows. The comparison of the results to previous studies is presented next and the conclusions section also presents suggestions for future work in this area.
4. Discussion
The results of the analyses of the relationships between the values of the concentrations of the air pollutants (NO2, O3, NO, PM1, PM2.5, PM10), and the measured ambient temperature and humidity values during the rainy and wet seasons are compared to published work in this area in this section. Most of the work in literature has examined mainly PM2.5, and PM10 concentration values, thus there is limited data on correlations between ambient temperature and humidity values and NO2, O3, NO, and PM1 concentration values.
The results indicate that for the rainy season, the recorded values of the mean concentration for the air pollutants (NO
2, O
3, NO, PM
1, PM
2.5, PM
10) are at best weakly correlated with both the recorded mean ambient temperature and humidity values and all of these values are at most statistically highly significant except for the correlation between NO and ambient humidity which is not statistically significant (
Table 12). The study at Akure [
10] is the closest comparison with the present work but only PM
2.5, and PM
10 concentration values were measured and these were determined to have a weak correlation with both the ambient temperature and humidity values and were all statistically insignificant. Thus, for the rainy season, the air pollutant concentration values in the present work and in published literature correlate weakly with both the ambient temperature and humidity.
The results for the dry season are less consistent (
Table 12). For the air pollutants (O
3, NO, PM
1, PM
2.5, PM
10), the concentrations values are at most weakly correlated with the ambient temperature and humidity values and these are statistically highly significant at best, except for the correlation between O
3 and ambient humidity (moderate) and the statistical significance of the relationship between PM
10 and ambient temperature (insignificant). However, the concentration of NO
2 responds moderately to changes in the ambient temperature and humidity and these are highly significant. A previous study in Lagos found weak correlations between the PM (PM
1, PM
2.5, PM
10) concentrations and the ambient temperature and humidity values [
8]. The observed PM (PM
2.5, PM
10) concentrations at Akure are also weakly correlated with the ambient temperature and humidity values and these are not statistically significant [
10]. The relationship between the PM and humidity can depend on the rate of particulate absorption in the atmosphere, washout due to rainfall, and dry deposition of the particles due to high humidity [
5,
10,
23]. The ambient temperature level can advance the photochemical reaction between particles and gases and atmospheric dispersion proceeds more effectively under hot air masses [
10]. In summary for the dry season, the air pollutant concentrations present a weak correlation with the ambient temperature and humidity for the work presented here and for those published in literature, except for the observed NO
2 concentration which correlate moderately with temperature and humidity.
Other studies exist in the literature which did not clearly delineate the rainy and dry seasons whilst evaluating the correlation effects of the meteorological parameters on the air pollutant concentrations. The studies at IIe-Ife showed that the concentration of PM
2.5 is weakly correlated to both the ambient temperature and humidity values and these are not statistically significant [
11]. For PM (PM
2.5, PM
10) concentrations in Port Harcourt [
9], the relationship with ambient temperature and humidity is weak and statistically highly significant, however this is negligible and insignificant for ambient humidity and negligible and significant for PM
10. Therefore, other studies from West Africa have shown that PM has a weak relationship with ambient temperature and humidity values (
Table 12).
5. Conclusions
This work scrutinized the relationship between the ambient humidity, ambient temperature and air pollution during the rainy and dry seasons in Lagos, West Africa. The climate in Lagos was defined as Tropical Savannah, with the rainy (winter) season lasting from April to September and the dry (summer) season lasting from October to March.
The results from the study indicate that the monthly mean concentration values of all the pollutants (NO2, O3, NO, PM1, PM2.5, PM10) are higher during the dry season than those during the rainy season. The lack of wet removal due to less rainfall and the dispersion of pollutants in the air parcels from the north-eastern desert regions during the dry season might account for some of these higher pollutant concentration levels.
5.1. Summary
In summary, during the year, the concentration of the air pollutants (NO2, O3, NO, PM1, PM2.5, PM10) tend to increase or decrease in response to the ambient temperature or humidity levels rather weakly, though during the dry season this response could be moderate for NO2, and O3. A high proportion (~70%) of the particulate matter pollutants concentrations is due to fine particles with diameters generally 2.5 µm or smaller. Thus, the PM2.5 and NO2 concentration levels exceeded those of the WHO air quality guidelines nearly 90% of the time during the test period.
The effects of NO2, O3, NO, and PM1 concentrations in this region have rarely been examined and this study adds to the knowledge.
5.2. Limitations of Study
A total of 7885 h of data over 12 months were used for this study, however a study over a longer period, possibly a decade, and including other meteorological parameters such as wind speed and rainfall patterns, might be needed to examine these relationships more extensively over several seasons. This is because sudden sustained high busts in emissions levels, unaccompanied by meteorological changes (as occurred in July 2020 for NO2 emissions during the study) can skew the data. The results would also inform air pollution dispersion models better.
5.3. Practical Implications
To use the results from these types of studies for policy development, care should be taken to avoid inferring causation from correlation; the details of the data must be examined. For example, as a consequence of the moderate (rather than negligible or weak) correlations indicated during the dry season, examinations of data from the months of January and February indicated consistently high ambient temperature values, low ambient humidity values and high concentration values of all air pollutants, and these could have implications for intervention measures for people with chronic respiratory conditions and or those prone to high temperature/dry environments.