Reply on RC1

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> The Amazon Basin is frequently influenced by the trans-Atlantic transport of African dust plumes during its wet season (January&ndash;April), which not only interrupts the near-pristine atmospheric condition in that season, but also provides nutrient inputs into the Amazon rainforest associated with dust deposition. The factors controlling the long-range transport (LRT) of African dust towards the Amazon Basin and consequently the overall impact of African dust over the Amazon Basin are not yet well understood. In this study, we use the chemical transport model GEOS-Chem to investigate the impact of the export of African mineral dust upon the Amazon Basin during the period of 2013&ndash;2017, constrained by multiple datasets obtained from AERONET, MODIS, as well as Cayenne site and the Amazon Tall Tower Observatory (ATTO) site in the Amazon Basin. With optimized particle mass size distribution (PMSD), the model well captures observed AOD regarding both the mean value as well as the decline rate of the logarithm of AOD over the Atlantic Ocean along the transport path (AOaTP), implying the consistence with observed export efficiency of African dust along the trans-Atlantic transport. With an annual emission of 0.73 &plusmn; 0.12 Pg a^-1, African dust entering the Amazon Basin has surface concentrations of 5.7 &plusmn; 1.3 &micro;g m^-3 (up to 15 &micro;g m^-3 in the northeast corner) during the wet season, accounting for 47 % &plusmn; 5.0 % (up to 70 %) of mass concentrations of total aerosols. The frequency of dust events in the Amazon Basin (defined as when surface dust concentrations reach the threshold of 9 &micro;g m^-3 on daily basis) in the wet season is around 18 % averaged over the basin, with maxima over 60 % at the northeast coast. During the dust events, AOD over most of the Amazon Basin is dominated by dust. Observed dust peaks over the Amazon Basin are generally associated with relatively higher African dust emissions (including Sahara and Sahel) and longer lifetime of dust along the trans-Atlantic transport, namely higher export efficiency of African dust across the Atlantic Ocean. Associated with dust deposition, we further estimate annual inputs of 52 &plusmn; 8.7, 0.97 &plusmn; 0.16 and 21 &plusmn; 3.6 mg m^-2 a^-1 for iron, phosphorus and magnesium deposited into the Amazon rainforest, respectively, which may well compensate the hydrologic losses of nutrients in the forest ecosystem.