Stratospheric influence on the northern North American free troposphere during TOPSE: 7 Be as a stratospheric tracer

We use 7 Be, with HNO 3 and O 3 , to identify air masses sampled from the NCAR C‐130 during TOPSE that retained clear evidence of stratospheric influence. A total of 43 such air masses, spread fairly evenly across the February to May sampling period, and 40°N–86°N latitude range, were encountered. South of 55°N, nearly all clear stratospheric influence was restricted to altitudes above 6 km. At higher latitudes stratospherically influenced air masses were encountered as low as 2 km. Approximately 12% of all TOPSE sampling time at altitudes above 2 km was spent in stratospherically impacted air, above 6 km this increased to more than half of the time. Because it is not certain how much of this stratospherically influenced air irreversibly injected mass (and chemical compounds) into the troposphere, we estimate the stratospheric fraction of O 3 in high latitude TOPSE samples based on a linear relationship to 7 Be and compare it to in situ O 3 . This analysis indicates that the stratospheric source can account for a dominant fraction (>85%) of in situ O 3 throughout TOPSE, but that the stratospheric contribution was nearly constant through the 4 month campaign. In February and March the 7 Be based estimates of stratospheric O 3 account for 10–15% more O 3 than was measured, but by April and May there is up to about 10% more O 3 than expected from the stratospheric source. This trend suggests that a seasonal transition from O 3 depletion to photochemical production in the high latitude North American troposphere is the major cause of the springtime increase in O 3 .