Entwined long-range transports of biomass burning aerosols over the South-West Indian Ocean: a case study of aerosol river in September 2017

Abstract. Biomass burning (BB) aerosols emitted over Southern Africa (SAF) and South America (SA) represent a major seasonal perturbation to the usually pristine atmosphere of the South-West Indian Ocean (SWIO) but remain understudied in this remote region. Following a multi-instrumental approach, we characterize BB plumes reaching Reunion Island (21° S, 55° E) during September 2017, combining ground-based measurements (sun-photometer, lidars, Fourier Transform Infrared spectrometer), spaceborne observations, CAMS EAC4 reanalysis, and the Lagrangian transport model FLEXPART. Aerosol optical depth at 550 nm over Reunion reached unusually high values (0.16–0.42) during the second half of September, with organic matter contributing up to 60 %. MODIS imagery revealed two large-scale smoke plumes originating from SAF and SA transported toward the SWIO, and CALIOP profiles showed smoke layers extending from 4 to 9 km of altitude above Madagascar. On September 19^th, a single layer was identified over Reunion between 2.8 and 4.7 km with an Ångström exponent (Å₃₅₅/₅₃₂) of 1.32±0.23, consistent with moderately aged BB particles essentially originating from SAF. On September 25^th, two vertically decoupled layers were identified: a lower layer (3.3–5 km, Å=1.45±0.12) associated with mixed aged aerosols of SAF and SA origin, and an upper, drier layer (5–9 km, Å=1.60±0.06) of potentially fresher SAF smoke, consistent with rapid convective uplift into the mid-troposphere. This study offers new insights concerning the dynamical processes that govern aerosol variability over Reunion Island and highlights its value as a strategic long-term observational site in the SWIO.