Abstract
Increasing energy consumption and food production worldwide results in anthropogenic emissions of reactive nitrogen into the atmosphere. To date, however, little information is available on tropical urban environments where inorganic nitrogen is vastly transported and deposited through precipitation on terrestrial and aquatic ecosystems. To fill this gap, we present compositions of water stable isotopes in precipitation and atmospheric nitrate (δ18O-H2O, δ2H-H2O, δ15N-NO3-, and δ18O-NO3-) collected daily between August 2018 and November 2019 in a tropical urban atmosphere of central Costa Rica. Rainfall generation processes (convective and stratiform rainfall fractions) were identified using stable isotopes in precipitation coupled with air mass back trajectory analysis. A Bayesian isotope mixing model using δ15N-NO3- compositions and corrected for potential 15N fractionation effects revealed the contribution of lightning (25.9 ± 7.1%), biomass burning (21.8 ± 6.6%), gasoline (19.1 ± 6.4%), diesel (18.4 ± 6.0%), and soil biogenic emissions (15.0 ± 2.6%) to nitrate wet deposition. δ18O-NO3- values reflect the oxidation of NOx sources via the ·OH + RO2 pathways. These findings provide necessary baseline information about the combination of water and nitrogen stable isotopes with atmospheric chemistry and hydrometeorological techniques to better understand wet deposition processes and to characterize the origin and magnitude of inorganic nitrogen loadings in tropical regions.
Highlights
Since the beginning of the industrialization era, human activities have dramatically increased the amounts of reactive nitrogen emitted to the atmosphere and deposited to the terrestrial and aquatic ecosystems, modifying the biogeochemical cycle of nitrogen (Harrison 2018; Kanakidou et al 2016; Michalski et al 2011)
We propose that δ15N-NO3- and δ18O-NO3- values in atmospheric nitrate are controlled by temporal nitrogen emission source changes and reaction chemistry processes, and by the proportions of convective versus stratiform rainfall
Air mass trajectory analysis supports the possible transport of NOx emissions from cultivated areas from the Caribbean and Pacific lowlands to the Central Valley. These results highlight the advantage of combining water and nitrogen stable isotopes with atmospheric chemistry and hydrometeorology to study the wet deposition of secondary atmospheric pollutants like nitrate
Summary
Since the beginning of the industrialization era, human activities have dramatically increased the amounts of reactive nitrogen emitted to the atmosphere and deposited to the terrestrial and aquatic ecosystems, modifying the biogeochemical cycle of nitrogen (Harrison 2018; Kanakidou et al 2016; Michalski et al 2011). Other reactive nitrogen emissions include biomass burning (~5 Tg N/yr), biogenic soil processes (~18 Tg N/yr) and lighting (up to 5 Tg N/yr). Emissions of nitrogen oxides (NOx = NO + NO2) are of interest as they are important drivers of atmospheric chemistry since tropospheric ozone production is controlled by their availability (Fang et al 2011; Wallington et al 2019). Identifying the mechanisms controlling the transformation of NOx into nitrate and its deposition (e.g., through precipitation) is important to underpin the potential impacts on the nitrogen cycles from local to regional and global scales
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