Characterization of extensive rainwater sampling in a megacity: effects of drought and biomass burning events

Atmospheric blocking can cause prolonged droughts in inland Brazil, leading to wildfires that enhance fine particulate matter concentrations in these areas during winter and spring. Biomass smoke plumes can undergo long-range transport and interact with air masses, resulting in darkened, contaminated precipitation. This study presents an extensive analysis of rainwater composition in the Metropolitan Area of São Paulo (MASP) between 2020 and 2021, determining elemental and water-soluble ionic compositions. Subsequently, wet deposition of chemical species was estimated based on these concentrations and precipitation amounts. The most abundant elements (K, Na, Ca, and Mg) were linked to natural sources and biomass burning. Zinc was among the most abundant anthropogenically emitted elements and was previously associated with vehicular sources. Principal Component Analysis (PCA) indicated that biomass burning and vehicular emissions contributed significantly to wet deposition. Sea spray was a minor contributor; however, it increased during events in the wet season (driven by humid air masses brought in by easterly winds), thereby enhancing sodium enrichment. Notably, the potentially toxic Ni accounted for 26% of total deposition during sea spray events, suggesting the influence of industrial emissions carried by easterly winds and sea breezes. The samples influenced by biomass burning were collected following prefrontal winds that transported biomass-burning aerosols from the northern part of São Paulo state. During these events, the deposition of biomass-burning-related K and Rb accounted for 27% and 33% of the total deposition, respectively. A linear correlation was observed between SO2 and deposited sulfate, suggesting the influence of secondary formation processes from precursors emitted by industrial and vehicular sources. In conclusion, biomass burning was an important source influencing rainwater composition during the dry season, whereas sea spray played a more substantial role in the wet season, and its occurrence was notably associated with nickel deposition.