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Trends and seasonal variability of atmospheric NO2 and HNO3 concentrations across three major African biomes inferred from long-term series of ground-based and satellite measurements.

Authors :
Ossohou, M.
Galy-Lacaux, C.
Yoboué, V.
Hickman, J.E.
Gardrat, E.
Adon, M.
Darras, S.
Laouali, D.
Akpo, A.
Ouafo, M.
Diop, B.
Opepa, C.
Source :
Atmospheric Environment. Jun2019, Vol. 207, p148-166. 19p.
Publication Year :
2019

Abstract

Abstract In the framework of the INDAAF (International Network to study Deposition and Atmospheric chemistry in AFrica) programme, part of the ACTRIS European Research Infrastructure for the long-term observation of Aerosols, Cloud, and Trace gases, this paper aims to study trends and seasonal variability of surface atmospheric NO 2 and HNO 3 concentrations, and OMI (Ozone Monitoring Instrument) NO 2 over 6 sites in tropical Africa. Sites are located in west and central Africa to represent the major African biomes: dry savanna (Banizoumbou, Niger and Katibougou, Mali), wet savanna (Djougou, Benin and Lamto, Côte d'Ivoire) and forest (Bomassa, Republic of Congo and Zoétélé, Cameroon). Ground-based NO 2 and HNO 3 concentrations were obtained over the period 1998–2015 using INDAAF passive samplers at a monthly basis, and NO 2 Vertical Column Densities (VCDs) from OMI for a 1° grid cell around each sites were obtained from 2005 to 2015. Mean annual NO 2 concentrations ranged from 2.3 ± 1.2 to 0.9 ± 0.4 ppb from dry savannas to forests, representing a north south gradient. In dry savannas, we observe two concentration peaks of NO 2 appearing at the beginning and the end of the wet season both for ground-based and satellite measurements, whereas at wet savannas and forest sites, NO 2 concentrations are highest in the dry season. The seasonality of surface NO 2 observations provide further evidence for a large role of microbial soil NOx emissions in dry savannas and of biomass burning NOx emissions in wet savanna and forest sites. Mean annual HNO 3 concentrations ranged from 0.4 to 0.5 ppb in dry and wet savannas to 0.2–0.3 ppb in forest. In dry ecosystems, higher HNO 3 concentrations are measured in the early wet season, consistent with NO 2 results. The analysis of a long-term dataset of surface O 3 concentrations indicates that HNO 3 production can mainly be explained by the photooxidation of NO x. Mann-Kendall and Seasonal Kendall statistical tests showed that NO 2 surface concentrations have a significant decreasing seasonal and annual trends at multiple sites (p-value < 0.05) ranging from −2.96% yr−1 (at Zoétélé) to −0.64% yr−1 (at Banizoumbou). HNO 3 results indicate no trends except at Bomassa (1.07% yr−1). The decreasing NO 2 ground-based concentration trends observed in wet savannas sites are correlated with OMI NO 2 decreasing trends at these sites. Trends obtained for NO 2 concentrations in wet savanna and forest ecosystems are consistent with trends of NOₓ biomass burning emissions. Highlights • Long term data of surface and column nitrogenous gases from major African biomes. • North-South latitudinal negative gradient in surface concentrations of NO 2 and HNO 3. • Microbial soil emissions of NOₓ are predominant in dry savannas. • Biomass burning emissions of NOx are predominant in wet savanna and forest sites. • NO 2 annual and seasonal decreasing trends at most of west and central Africa sites. [ABSTRACT FROM AUTHOR]

Details

Language :
English
ISSN :
13522310
Volume :
207
Database :
Academic Search Index
Journal :
Atmospheric Environment
Publication Type :
Academic Journal
Accession number :
135977011
Full Text :
https://doi.org/10.1016/j.atmosenv.2019.03.027