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.