Comparison of in situ N₂O and CH₄ measurements in the upper troposphere and lower stratosphere during STRAT and POLARIS

Nitrous oxide (N₂O) and methane (CH₄) were measured in the upper troposphere and lower stratosphere by multiple instruments aboard the NASA ER-2 aircraft during the 1995-1996 Stratospheric Tracers of Atmospheric Transport (STRAT) and 1997 Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) campaigns. Differences between coincidental, in situ measurements are examined to evaluate the agreement and variability in the agreement between these instruments during each flight. Mean N₂O measurement differences for each flight were much smaller than limits calculated from quoted values of N₂O measurement accuracy and for all but two flights were ≤8.7 ppb (3.5%). Mean CH₄ measurement differences for flights were similarly much smaller than calculated limits and for all but three flights were ≤65 ppb (4.4%). Typical agreement between instruments during flights averaged 6.2 ppb (2.5%) for N₂O and 43 ppb (2.9%) for CH₄. In contrast, for about half of the flights, the variability of N₂O and CH₄ measurement differences exceeded limits calculated from quoted values of measurement precision. The typical measurement difference variability (1σ) during a flight averaged ±8.0 ppb (3.2%) for N₂O and ±43 ppb (2.9%) for CH₄. For some flights, large differences or variations in differences are attributable to the poor measurement accuracy or precision of one instrument. It is demonstrated that small offsets between the computer clocks of these instruments can result in significant differences between their "coincidental" N₂O and CH₄ data, especially when there is high spatial variability in tracer abundance along a flight track.