In Situ Measurements of Chemical Tracers in the Stratosphere: CO, N₂O, and CH₄

• Main Author: Herman, Robert Laird
• Format: Thesis
• Language: English
• Published: 1998
• Subjects: Arctic
• Online Access: https://thesis.library.caltech.edu/16148/; https://thesis.library.caltech.edu/16148/1/Herman_RL_1998.pdf; https://resolver.caltech.edu/CaltechTHESIS:07272023-210618418

Stratospheric transport is studied with measurements of long-lived tracers. The Aircraft Laser Infrared Absorption Spectrometer II (ALIAS II) was designed and built to make in situ measurements of atmospheric trace gases at altitudes ranging from 7 km to 30 km. With this spectrometer, nitrous oxide (N₂O) and methane (CH₄) mixing ratios were measured on six high­ altitude balloon flights from the tropics (7°S), mid-latitudes (34°N), and high latitudes (65°N). Comparisons of measured mixing ratios with model predictions are used to quantify meridional transport rates in the stratosphere. The mean time scale for entrainment of mid-latitude stratospheric air into the tropics is estimated to be 7⁺¹⁰₋₇ months for altitudes between the tropical tropopause and 20 km, and 16⁺¹⁸₋₈ months for 20 to 28 km. These results suggest that most of the stratospheric entrainment into the tropics occurs at altitudes roughly below 20 km, and that the tropical stratosphere is dynamically isolated from the mid-latitude stratosphere at altitudes between 20 and 28 km. Faster transport in the lower stratosphere was studied with measurements of atmospheric carbon monoxide (CO) by the ALIAS instrument, predecessor to ALIAS II. This instrument measured CO on fifty eight flights of the NASA ER-2 high-altitude aircraft from October, 1995, to September, 1997, during two aircraft campaigns: Stratospheric Tracers of Atmospheric Transport (STRAT) and Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS). These flights covered altitudes up to 22 km and latitudes from 3°S to 90°N. CO has a photochemical lifetime of only several months in the upper troposphere and lower stratosphere, so its mixing ratio is sensitive to rapid transport. These measurements indicate that quasi-horizontal poleward outflow from the tropical stratosphere occurs on rapid time scales of roughly one month for altitudes between 16 and 20 km.