Temporal variation and source identification of carbonaceous aerosols in Monrovia, Liberia

Abstract

This study examined the atmospheric Black Carbon (BC) and Organic Carbon (OC) data for two years from January 2019 to December 2020 in Monrovia, Liberia. This study is the first of its kind in Monrovia, Liberia and the Mano River Union region. The objective is to evaluate the temporal variation in BC and OC over Monrovia and locate the probable local and regional source locations contributing to BC and OC therein. We also presented here the relationship of BC and OC with meteorological parameters. The highest BC (∼1.4 µg m−3) and OC (∼14 µg m−3) concentrations were observed in January 2019 and 2020. A clear seasonal effect was found in both years, with high mean BC (∼4.9 µg m−3) and OC (∼10 µg m−3) during the dry season (November to April). Diurnal variation suggested high BC and OC during 5:00–7:00 AM, 12 noon, 2:00 PM, and 4:00–7:00 PM. Long-range transportation and local emission sources were studied using Conditional Bivariate Probability Function (CBPF), and Concentration Weighted Trajectory (CWT). CBPF indicated that higher BC and OC concentrations were experienced when the wind was blowing from the South (at >2 ms−1) and North-West (1–2 ms−1) where a lot of shipping activities are carried out. CWT for BC showed that the air mass passes over Senegal through Guinea Bissau via the Atlantic Ocean while for OC, on the Atlantic Ocean and Freeport of Monrovia, indicating shipping emissions as probable sources. The results of this study can help policymakers devise appropriate strategies to control the BC and OC emissions over Monrovia, Liberia.

Keywords

• Black carbon
• Organic carbon
• Monrovia
• Temporal variation
• Cape Mesurado
• Carbonaceous aerosols