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 tropospher e and lower stratosphere by multiple instruments aboard the NASA ER-2 aircr aft during the 1995-1996 Stratospheric Tracers of Atmospheric Transport (ST RAT) 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 bet ween these instruments during each flight. Mean N2O measurement differences for each flight were much smaller than limits calculated from quoted value s of N2O measurement accuracy and for all but two flights were less than or equal to 8.7 ppb (3.5%). Mean CH4 measurement differences for flights were similarly much smaller than calculated limits and for all but three flight s were less than or equal to 65 ppb (4.4%). Typical agreement between instr uments 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 an d CH4 measurement differences exceeded limits calculated from quoted values of measurement precision. The typical measurement difference variability ( lo) during a flight averaged +/-8.0 ppb for N2O and +/-43 ppb (2.9%) for CH 4. For some flights, large differences or variations in differences are att ributable 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 "coincident al" N2O and CH4 data, especially when there is high spatial variability in tracer abundance along a flight track.