Bromide Variability in Wet Atmospheric Precipitation at a Coastal Location in Southwestern Europe

During the last decades, scientific interest in bromine (Br) has been fueled by its influence on the climate, namely via the depletion of ozone and elemental mercury. Today, there is compelling evidence that the heterogeneous/multiphase Br-reactive chemistry in the troposphere, extremely important in polar regions, is also abundant in other areas of the globe, including tidal coastal areas. This likely impacts the composition of wet precipitation for bromide (Br‒) and sustains Br transport over relatively large distances inland, enriching continental depositional environments. Thus, contributing to the quantification of Br‒ in rainwater concerning its main natural source (i.e., the ocean), and possible influential meteorological factors (e.g., wind speed/direction), seems important to better understand the role of the oceanogenic Br in paleoclimatology as a proxy for storminess. In this study, we used Br‒ concentrations ([Br‒]) in a set of twenty-one rainfall samples collected, on an event basis, between June 2018 and November 2019, at three sites in a coastal area of NW Portugal (Iberian Peninsula, IP; SW Europe), to investigate source–receptor relationships. The sampling sites were selected to represent a sea-to-inland gradient (~ 30 km long). Chemical analyses were performed by ion chromatography (anions) and inductively coupled plasma atomic emission spectrometry (cations). [Br‒] data, though without statistical outliers, showed high variability (12–375 µg/L), the median of the period being 155 µg/L, while the average of the recorded values was 161 ± 100 µg/L. In the sampling period, two [Br‒] peaks in rainfall were observed: in winter (January 30, 2019; 375 µg/L) and in summer (June 21, 2018; 329 µg/L), both at the innermost site. HYSPLIT model analysis suggests that the first event was related to air masses passing directly over the Atlantic before reaching the sampling area, while in the second, air masses arrived at the target region from the NE-to-E sector, after traveling through the IP landmass. PCA analysis (after z-score conversion of the values) allowed us to recognize three main groups of rainwater samples following similar tendencies on Br‒. It is expected that our results will be useful for ongoing and future research on (paleo)precipitation and (paleo)cyclogenesis.