Evaluation of climate model aerosol seasonal and spatial variability over Africa using AERONET

Abstract

Abstract. The sensitivity of climate models to the characterization of African aerosol particles is poorly understood. Africa is a major source of dust and biomass burning aerosols and this represents an important research gap in understanding the impact of aerosols on radiative forcing of the climate system. Here we evaluate the current representation of aerosol particles in the Conformal Cubic Atmospheric Model (CCAM) with ground-based remote retrievals across Africa, and additionally provide an analysis of observed aerosol optical depth at 550 nm (AOD 550 nm ) and Ångström exponent data from 34 Aerosol Robotic Network (AERONET) sites. Analysis of the 34 long-term AERONET sites confirms the importance of dust and biomass burning emissions to the seasonal cycle and magnitude of AOD 550 nm across the continent and the transport of these emissions to regions outside of the continent. In general, CCAM captures the seasonality of the AERONET data across the continent. The magnitude of modeled and observed multiyear monthly average AOD 550 nm overlap within ±1 standard deviation of each other for at least 7 months at all sites except the Réunion St Denis Island site (Réunion St. Denis). The timing of modeled peak AOD 550 nm in southern Africa occurs 1 month prior to the observed peak, which does not align with the timing of maximum fire counts in the region. For the western and northern African sites, it is evident that CCAM currently overestimates dust in some regions while others (e.g., the Arabian Peninsula) are better characterized. This may be due to overestimated dust lifetime, or that the characterization of the soil for these areas needs to be updated with local information. The CCAM simulated AOD 550 nm for the global domain is within the spread of previously published results from CMIP5 and AeroCom experiments for black carbon, organic carbon, and sulfate aerosols. The model's performance provides confidence for using the model to estimate largescale regional impacts of African aerosols on radiative forcing, but local feedbacks between dust aerosols and climate over northern Africa and the Mediterranean may be overestimated.

On the Role of the Flaming to Smoldering Transition in the Seasonal Cycle of African Fire Emissions

“…The underestimation of CO columns exceeds the interannual variation in MOPITT observations, a feature that was also seen in other studies that used different atmospheric transport models and previous versions of the GFED data (Edwards et al, ; van Leeuwen et al, ). Additionally, the models driven by GFED cannot reproduce the late peak of satellite retrievals of NH 3 columns (Paulot et al, ) and aerosol optical depth (AOD; Horowitz et al, ; Magi et al, ; Tummon et al, ) over Africa. These tracers follow different chemical processes than CO and have shorter atmospheric lifetimes; therefore, the modeling biases are probably caused by the underestimated emissions of GFED in the late season rather than by modeling deficiencies.…”

Mass concentration, optical depth and carbon composition of particulate matter in the major southern West African cities of Cotonou (Benin) and Abidjan (Côte d'Ivoire)

“…9). Horowitz et al (2017) have suggested that biomass burning aerosol could make up a large fraction of the AOD in SWA during the burning season (December-February) from observations recorded in northern Benin (Djougou) and Nigeria (Ilorin). However, the coastal urban zone of SWA clearly lacks dedicated observations, which can in turn be used to understand local PM 2.5 variations better.…”

“…Indeed AOD is an indicator of the aerosol load in the atmospheric column and its spectral behaviour provides information on the shape of the aerosol size distribution (O'Neill et al, 2003) and so the aerosol type. The AOD in the Sahel region has been largely investigated (Horowitz et al, 2017;Léon et al, 2009;Mallet et al, 2008) thanks to the availability of the AERONET automatic sun photometer (Holben et al, 1998). Fig.…”

An Evaluation of the CHIMERE Chemistry Transport Model to Simulate Dust Outbreaks across the Northern Hemisphere in March 2014

“…Observational data for a March period based on available data are reported in the literature [92], unfortunately the exact data are not available: observed values are averaged from available data, but a first estimate shows that CHIMERE for March 2014 produces the same order of magnitude for dust concentrations, the main hot spots in Cape Verde (16. [94], and also identified in the Conformal Cubic Atmospheric Model (CCAM) [95] where the dust lifetime is longer than the ones calculated in previous modeling exercises.…”