Sensitivity of transatlantic dust transport to chemical aging and related atmospheric processes

Abstract

We present a sensitivity study on transatlantic dust transport, a process which has many implications for the atmosphere, the ocean and the climate. We investigate the impact of key processes that control the dust outflow, i.e., the emission flux, convection schemes and the chemical aging of mineral dust, by using the EMAC model following Abdelkader et al. (2015). To characterize the dust outflow over the Atlantic Ocean, we distinguish two geographic zones: (i) dust interactions within the Intertropical Convergence Zone (ITCZ), or the dust–ITCZ interaction zone (DIZ), and (ii) the adjacent dust transport over the Atlantic Ocean (DTA) zone. In the latter zone, the dust loading shows a steep and linear gradient westward over the Atlantic Ocean since particle sedimentation is the dominant removal process, whereas in the DIZ zone aerosol–cloud interactions, wet deposition and scavenging processes determine the extent of the dust outflow. Generally, the EMAC simulated dust compares well with CALIPSO observations; however, our reference model configuration tends to overestimate the dust extinction at a lower elevation and underestimates it at a higher elevation. The aerosol optical depth (AOD) over the Caribbean responds to the dust emission flux only when the emitted dust mass is significantly increased over the source region in Africa by a factor of 10. These findings point to the dominant role of dust removal (especially wet deposition) in transatlantic dust transport. Experiments with different convection schemes have indeed revealed that the transatlantic dust transport is more sensitive to the convection scheme than to the dust emission flux parameterization. To study the impact of dust chemical aging, we focus on a major dust outflow in July 2009. We use the calcium cation as a proxy for the overall chemical reactive dust fraction and consider the uptake of major inorganic acids (i.e., H2SO4, HNO3 and HCl) and their anions, i.e., sulfate (SO42−), bisulfate (HSO4−), nitrate (NO3−) and chloride (Cl−), on the surface of mineral particles. The subsequent neutralization reactions with the calcium cation form various salt compounds that cause the uptake of water vapor from the atmosphere, i.e., through the chemical aging of dust particles leading to an increase of 0.15 in the AOD under subsaturated conditions (July 2009 monthly mean). As a result of the radiative feedback on surface winds, dust emissions increased regionally. On the other hand, the aged dust particles, compared to the non-aged particles, are more efficiently removed by both wet and dry deposition due to the increased hygroscopicity and particle size (mainly due to water uptake). The enhanced removal of aged particles decreases the dust burden and lifetime, which indirectly reduces the dust AOD by 0.05 (monthly mean). Both processes can be significant (major dust outflow, July 2009), but the net effect depends on the region and level of dust chemical aging

Profiling of Saharan dust from the Caribbean to western Africa – Part 1: Layering structures and optical properties from shipborne polarization/Raman lidar observations

“…The assumption of a universal dust depolarization ratio of about 0.3 may not be generally valid. Aging of dust particles caused by cloud evolution and subsequent evaporation processes or by chemical reactions on the dust particle surface (Abdelkader et al, 2015(Abdelkader et al, , 2017 may change the chemical composition and shape characteristics (towards more smoothed, more spherical, less irregularshaped dust particles) and thus may lead to a significant decrease of the dust depolarization ratio during long-range transport. Furthermore, a stronger removal of large dust particles compared to small particles by gravitational settling may change the dust particle size distribution significantly.…”

“…The internally mixed non-dust aerosol contributions will be counted as dust contributions. Chemical aging and successive water uptake at high humidity may significantly change the overall (dust and non-dust) particle extinction coefficient (Abdelkader et al, 2017) and thus the retrieval of dust mass concentrations from the lidar-derived particle extinction coefficients.…”

Long-term study on coarse mode aerosols in the Amazon rain forest with the frequent intrusion of Saharan dust plumes

“…3. 5 15 Relating to (3): Wet deposition is the dominant aerosol loss mechanism in tropical latitudes because of their intense precipitation (Huang et al, 2009;Martin et al, 2010a;Abdelkader et al, 2017). According to Fig.…”

“…This oscil-20 lating atmospheric state is determined by the position of the intertropical convergence zone (ITCZ) and the corresponding trade wind circulation as well as the strong seasonality in biomass burning in Africa and South America (Martin et al, 2010a;2010b;Fuchs and Cermak, 2015;Abdelkader et al, 2017). Figure 1 serves as a general illustration of the large-scale atmospheric circulation as well as the predominant aerosol and trace gas emissions patterns in the Atlantic region that govern the overall atmospheric seasonality in the 25 Amazon Basin.…”

“…For the wet season, the NE trajectories meet the ITCZ rain belt at about 03° N. This means that for the transatlantic passage north of 03° N (i.e., in the subtropical latitudes) no major wet deposition of the transported aerosol load is expected. Once the BTs intersect the rain belt south of 03° N, aerosol scavenging becomes a substantial aerosol loss mechanism, which acts as a barrier for southwards transport (Abdelkader et al, 2017) and "effi-10 ciently scrubs out aerosols" from the advected air masses (Andreae et al, 2012). The dry season scenario is different: Here the ITCZ rain belt is located mostly north of 02° N. This implies that the SE trajectories mostly bypass the most intense rain fields.…”