Present-day Patagonian dust emissions: Mass flux constraints, meteorological triggers and the effect on phytoplankton biomass
Authors: Cosentino Nicolas , Gaiero Diego , Ruiz-Etcheverry Laura , Lambert Fabrice , Bia Gonzalo , Simonella Lucio , Coppo Renata , Torre Gabriela , Saraceno Martin , Tur Veronica 
Published: March 2021

&lt;p&gt;The magnitude of the climate forcing associated with mineral dust aerosols remains uncertain, due in part to a lack of direct observations on dust source areas. While modeling and satellite studies provide spatially extensive constraints, these studies must be supported by surface-validating, in situ dust monitoring. Our study focuses on Patagonia, the main source of dust to the southern oceans (&gt;45&amp;#186;S), a region of low biological productivity potentially susceptible to increased micronutrient fertilization through dust deposition and associated atmosphere&amp;#8208;to&amp;#8208;ocean CO&lt;sub&gt;2&lt;/sub&gt; flux. This mechanism is hypothesized to have contributed significantly to the last interglacial&amp;#8208;to&amp;#8208;glacial climatic transition. However, the dust&amp;#8208;phytoplankton biomass connection remains contentious for the present&amp;#8208;day climate system.&lt;/p&gt;&lt;p&gt;We analysed multi-year time series of surface dust-related visibility reduction (DRVR) and dust mass sampling at four downwind coastal monitoring sites, along with key meteorological time series at these same sites. We find that local DRVR across Patagonia is partly controlled by long-term water deficit (i.e., months), while same-day conditions play a smaller role, reflective of water retention properties of fine-grained dust-emitting soils in low-moisture conditions. This conclusion is supported independently by reanalysis data showing that large-scale dust outbreaks are usually associated with anomalously high long-term water deficit. By combining visibility data with surface dust sampling we were able to derive emission rates associated with regional patches of dust-emitting surfaces and test the skill of dust emission schemes to simulate dust activity close to the sources. Our results suggest that the inclusion of long-term hydrologic soil balance may improve the performance of dust emission schemes in climate models.&lt;/p&gt;&lt;p&gt;We also analyzed the impact of southernmost Patagonian dust emissions on southwestern Atlantic Ocean continental shelf and proximal open ocean satellite chlorophyll&amp;#8208;a concentration. We used the DRVR and mass flux time series of the southernmost site to model dust emission, transport, and deposition to the ocean. We then performed a dust event&amp;#8208;based analysis of chlorophyll&amp;#8208;a time series, using a novel approach by which time series are corrected for post&amp;#8208;depositional particle advection due to ocean currents. Finally, we performed total iron determinations, release experiments and iron solid speciation analysis in dust samples. Iron is a key micronutrient limiting phytoplankton biomass in high&amp;#8208;nutrient, low&amp;#8208;chlorophyll oceans such as offshore of the 200&amp;#8208;m isobath off Patagonia. We find no compelling evidence for an influence of dust as an enhancer of phytoplankton biomass either on shelf or proximal open ocean waters of the southwestern Atlantic Ocean. For open ocean waters this is consistent with a lack of source&amp;#8208;inherited bioavailable iron in dust samples. Future case studies addressing similar questions should concentrate on dust sources with identified high contents of bioavailable iron, particularly in the Southern Hemisphere where atmospheric processing of iron-bearing particles is weak.&lt;/p&gt;