Stable isotopic characterization of nitrate wet deposition in the tropical urban atmosphere of Costa Rica.

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.

Isotope composition of carbon dioxide and methane in a tropical urban atmosphere.

This work presents a weekly carbon isotope composition analysis (June 2017-January 2018) of carbon dioxide (CO2) and methane (CH4) in a tropical urban atmosphere (Central Valley, Costa Rica). δ13C values of CO2 and CH4 ranged from -12.2 to -5.9 ‰, and from -51.6 to -46.3 ‰, respectively. Mixing ratios of CO2 and CH4 varied from 384.2 to 528.5 ppmv, and from 1.860 to 2.613 ppmv, respectively. δ13C spatial variation and mixing ratios of CO2 and CH4 were influenced by the atmospheric stability and air circulation patterns in the metropolitan area. Low δ13C values and large mixing ratios were observed in the southwestern area of the valley during the rainiest period (September-November). Preliminary linear relationships between reciprocal CO2 mixing ratios and δ13C values indicate that CO2 emissions in the Central Valley are probably related to respiration processes and fossil fuel combustion, although CO2 enriched in 13C from volcanic degassing was also detected. Under stable atmospheric conditions, CH4 data seems to reflect the influence of emissions near the sampling sites. These preliminary results based on the carbon isotope technique demonstrate potential for carrying out atmospheric studies at tropical urban locations with different terrain characteristics and atmospheric mixing conditions.