Chemical aging of atmospheric mineral dust during transatlantic transport

Abstract

Abstract. Transatlantic dust transport has many implications for the atmosphere, ocean and climate. We present a modeling study on the impact of the key processes (dust emissions flux, convection and dust aging parameterizations) that control the transatlantic dust transport. Typically, the Inter-Tropical Convergence Zone (ITCZ) acts as a barrier for the meridional dust transport. To characterize the dust outflow over the Atlantic Ocean, we address two regional phenomena: (i) dust interactions with the ITCZ (DIZ) and (ii) the adjacent dust transport over the Atlantic Ocean (DTA). In the DTA zone, the dust loading shows a steep and linear gradient westward over the Atlantic Ocean where particle sedimentation is the dominant removal process, whereas in the DIZ zone cloud interactions and wet deposition predominate. To study the different impacts of aging, we present two case studies that exclude condensation and coagulation, and include dust aging at various levels of complexity. For dust aging, we consider the uptake of inorganic acids on the surface of mineral particles that form salt compounds. Calcium, used as a proxy for the overall chemically reactive dust fraction, drives the dust-related neutralization reactions leading to higher dust aerosol optical depth (AOD). The aged dust particles are transferred to the soluble aerosol modes in the model and are mixed with other species that originate from anthropogenic and natural sources. The neutralization products (salts) take up water vapor from the atmosphere and increase the dust AOD under subsaturated conditions. We define the "direct effect of dust aging" to refer to the increase in AOD as a result of hygroscopic growth. On the other hand, the aged dust is more efficiently removed (wet and dry) because of the increase in particle size and hygroscopicity. This more efficient removal reduces the dust AOD over the DIZ zone. We define this as the "indirect effect of dust aging", complementary to the direct effect that is dominant in the DTA zone. Distinction of the two aging effects helps develop insight into the regional importance of dust–air-pollution interactions.