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.
Keywords: Carbon-13; Costa Rica; carbon dioxide; carbon emissions; greenhouse gases; methane; tropical urban atmosphere.