Vertical distribution and radiative effects of mineral dust and biomass burning aerosol over West Africa during DABEX

Abstract

[1] This paper presents measurements of the vertical distribution of aerosol extinction coefficient over West Africa during the Dust and Biomass-burning Aerosol Experiment (DABEX)/African Monsoon Multidisciplinary Analysis dry season Special Observing Period Zero (AMMA-SOP0). In situ aircraft measurements from the UK FAAM aircraft have been compared with two ground-based lidars (POLIS and ARM MPL) and an airborne lidar on an ultralight aircraft. In general, mineral dust was observed at low altitudes (up to 2 km), and a mixture of biomass burning aerosol and dust was observed at altitudes of 2-5 km. The study exposes difficulties associated with spatial and temporal variability when intercomparing aircraft and ground measurements. Averaging over many profiles provided a better means of assessing consistent errors and biases associated with in situ sampling instruments and retrievals of lidar ratios. Shortwave radiative transfer calculations and a 3-year simulation with the HadGEM2-A climate model show that the radiative effect of biomass burning aerosol was somewhat sensitive to the vertical distribution of aerosol. In particular, when the observed low-level dust layer was included in the model, the absorption of solar radiation by the biomass burning aerosols increased by 10%. We conclude that this absorption enhancement was caused by the dust reflecting solar radiation up into the biomass burning aerosol layer. This result illustrates that the radiative forcing of anthropogenic absorbing aerosol can be sensitive to the presence of natural aerosol species.

Measurements of aerosol properties from aircraft, satellite and ground‐based remote sensing: a case‐study from the Dust and Biomass‐burning Experiment (DABEX)

“…It is also important to recognise that the aircraft covered a horizontal path of about 100 km during the profile (see the bold line in Figure 1) so the FAAM AOD estimate is not a true column measurement. On four other flights during DABEX, FAAM AODs were also within 10% of AOD estimates from Banizoumbou (Johnson et al, 2008b). The comparison here gives us confidence that the aircraft scattering and absorption measurements are of reasonable magnitude, and are not grossly affected by problems with sampling efficiency (as had been a concern when sampling dust aerosol during previous measurement campaigns with the Met Office C-130 aircraft (Haywood et al, 2003b)).…”

“…Lidar and aircraft in situ measurements revealed an interesting vertical structure, with dust aerosol at low altitudes (typically below 2 km) and biomass-burning aerosol mixed with some dust aerosol in elevated layers between 2 and 5 km (Heese and Weigner, 2008;Johnson et al, 2008b). Aircraft-observed size distributions in the biomass-burning layers were similar to those from SAFARI-2000 except that the concentration of coarse particles (e.g.…”

“…The column-meanÅngström exponent of 0.84 is about halfway between the value in the dust layer (near zero) and the value of 1.7 that was estimated for the biomassburning aerosol component in Johnson et al (2008a). Mie calculations showed that there was a roughly linear relationship between theÅngström exponent and the proportion of extinction associated with biomass-burning aerosol (Johnson et al, 2008b), assuming mineral dust and biomass burning were the only sources of aerosol in the atmosphere. Therefore, theÅngström exponent of 0.84 in this regression would suggest that the dust and biomass-burning aerosol contributed equally (50 ± 7%) to the AOD at 550 nm.…”

Impacts of Aerosol Shortwave Radiation Absorption on the Dynamics of an Idealized Convective Atmospheric Boundary Layer

“…How shortwave (SW) radiation and aerosols interact in the atmosphere is one of the largest uncertainties in climate prediction (Satheesh and Ramanathan 2000;Tripathi 2005;Johnson et al 2008). Aerosols modify the vertical profile of radiative heating by absorbing and scattering radiation (Quijano et al 2000).…”

Vertical distributions of aerosol optical properties during the spring 2016 ARIAs airborne campaign in the North China Plain

“…Such variable aerosol vertical distributions can alter the optical properties of aerosols such as AOD, thus affecting the regional radiation balance (Liu et al, 2012) and even the global radiative forcing estimation (Zhang et al, 2013). A number of field programs have been carried out to measure the vertical distribution of dust or biomass burning aerosols with airborne and surface-based instruments (Johnson et al, 2008). The observations, combined with a radiative transfer model, allow the accurate calculation of radiative effects including aerosol optical properties (Gadhavi and Jayaraman, 2006) and absorption of solar radiation at the top of the atmosphere (Meloni et al, 2005).…”