Abstract
Abstract. Southern West Africa (SWA) is influenced by large numbers of aerosol particles of both anthropogenic and natural origins. Anthropogenic aerosol emissions are expected to increase in the future due to the economical growth of African megacities. In this paper, we investigate the aerosol optical depth (AOD) in the coastal area of the Gulf of Guinea using sun photometer and MODIS satellite observations. A network of lightweight handheld sun photometers have been deployed in SWA from December 2014 to April 2017 at five different locations in Côte d'Ivoire and Benin. The handheld sun photometer measures the solar irradiance at 465, 540 and 619 nm and is operated manually once per day. Handheld-sun-photometer observations are complemented by available AERONET sun photometer observations and MODIS level 3 time series between 2003 and 2019. MODIS daily level 3 AOD agrees well with sun photometer observations in Abidjan and Cotonou (correlation coefficient R=0.89 and RMSE = 0.19). A classification based on the sun photometer AOD and Ångström exponent (AE) is used to separate the influence of coarse mineral dust and urban-like aerosols. The AOD seasonal pattern is similar for all the sites and is clearly influenced by the mineral dust advection from December to May. Sun photometer AODs are analyzed in coincidence with surface PM2.5 concentrations to infer trends in the particulate pollution levels over conurbations of Abidjan (Côte d'Ivoire) and Cotonou (Benin). PM2.5-to-AOD conversion factors are evaluated as a function of the season and the aerosol type identified in the AE classification. The highest PM2.5 concentrations (up to 300 µg m−3) are associated with the advection of mineral dust in the heart of the dry season (December–February). Annual means are around 30 µg m−3, and 80 % of days in the winter dry season have a value above 35 µg m−3, while concentrations remain below 16 µg m−3 from May to September. No obvious trend is observed in the 2003–2019 MODIS-derived PM2.5 time series. However the short dry period (August–September), when urban-like aerosols dominate, is associated with a monotonic trend between 0.04 and 0.43 µgm-3yr-1 in the PM2.5 concentrations over the period 2003–2017. The monotonic trend remains uncertain but is coherent with the expected increase in combustion aerosol emissions in SWA.
Highlights
The increasing trend in the anthropogenic emissions in Africa (Liousse et al, 2014) gives rise to the question of the impact of human activities on air quality, the monsoon system and the regional climate
Those differences can explain that the “merge” product used in this study has a large bias during the dry season in the northern part for the inland sites, so the north–south aerosol optical depth (AOD) gradient in this area remains difficult to assess based on satellite products
The PM2.5/AOD ratio retrieved in the long dry season for the urban-like category is affected by a larger uncertainty due to a limited impact of urban-like aerosol during the long dry season compared to coarse dust
Summary
The increasing trend in the anthropogenic emissions in Africa (Liousse et al, 2014) gives rise to the question of the impact of human activities on air quality, the monsoon system and the regional climate. The summer wet season corresponds to the continental intrusion of the southwesterly monsoon winds carrying moist air and precipitation During this period, biomass-burning emissions in central Africa can be advected to SWA by easterly wind and can impact the local air quality of coastal conurbations (Menut et al, 2018). In a companion paper (Djossou et al, 2018), the AOD measurements obtained in the downtowns of the major cities of Abidjan (Côte d’Ivoire) and Cotonou (Benin) were presented along with the surface observations of the PM2.5 mass concentration and carbonaceous aerosol composition. We report additional AOD measurements over SWA using lightweight handheld and automatic sun photometers with the purpose of validating the MODIS-derived AOD at the regional scale and investigating further the use of AOD for local pollution assessment. The last section presents the interannual trends in PM2.5 derived from the MODIS observations
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