Effect of local and regional sources on the isotopic composition of nitrous oxide in the tropical free troposphere and tropopause layer

Measurements and models of the spatiotemporal variability of surface N 2 O mixing ratios and isotopic compositions are increasingly used to constrain the global N 2 O budget. However, large variability observed on the small spatial scales of soil chambers and shipboard sampling, which appears to be very sensitive to local environmental conditions, has made extrapolation to the global scale difficult. In this study, we present measurements of the isotopic composition of N 2 O ( δ 15 N bulk , δ 15 N α , δ 15 N β , and δ 18 O) from whole‐air samples collected at altitudes of 0.5 to 19km by the NASA DC‐8 and WB‐57 aircraft during the Costa Rica‐Aura Validation Experiment (CR‐AVE) and the Tropical Composition, Cloud and Climate Coupling Experiment (TC4) campaigns in January–February 2006 and July–August 2007, respectively. The vertical profiles of isotopic composition showed predictable, repeating patterns consistent with the influence of a surface source at lower altitudes and the influence of stratospheric photochemistry in the lower stratosphere. Their correlations with marine tracers at lower altitudes are consistent with a predominantly oceanic source, although a soil source cannot be ruled out. Measurements in a combustion plume revealed a strong depletion in 15 N at the central nitrogen atom (i.e., low δ 15 N α values), providing new information on N 2 O isotopic compositions from combustion. This new data set demonstrates that a coherent picture of the isotopic composition of tropospheric N 2 O is possible at currently attainable precisions and that its variations from 0.5 km to the lower stratosphere are a useful tool in investigating the sources and distributions of this important greenhouse gas.