1. OH in the tropical upper troposphere and its relationships to solar radiation and reactive nitrogen Ru-Shan Gao, Karen Rosenlof, David Fahey, Paul Wennberg, Eric Hintsa, and Tom Hanisco and the STRAT team 1

2. OH in the tropical upper troposphere (TUT) Low concentration but important for pollutant degradation Direct source:O 3 + h  O 2 + O( 1 D)(R1) O( 1 D) + H 2 O  2OH(R2) Low O 3, low H 2 O conditions in the TUT  Low OH? - Kley et al., Science 1996 Recent work by Markus Rex – Effect of low OH in TUT Tightly coupled to HO 2 - Easier to model HO 2 /OH than OH 2

3. Box model results, Jaeglé et al., AE 2000 3

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5. Hanisco et al., JPC, 2001 OH in the lower stratosphere correlates with SZA 5

6. OH in the lower troposphere correlates with J( 1 D) Rohrer and Berresheim, Nature, 2006 6

7. 7 Measurements in TUT during STRAT

8. 8 OH measurements in TUT

9. O 3 + h  O 2 + O( 1 D), O( 1 D) + H 2 O  2OH 9

10. 10 [OH] vs. JO( 1 D) * [O 3 ] * [H 2 O]

11. Measurements in TUT during STRAT 11

12. The cycling time between OH and HO 2 is much shorter than the lifetime of HO x OH HO 2 : OH + CO (+ O 2 )  HO 2 + CO 2 (R3) HO 2 + NO  OH + NO 2 (R4) OH is controlled by R3, R4, and HO x sources and sinks O 3 +H 2 O is not the dominant HO x source Main HO x sink: NO y (high NO y ) or HO x self reactions (low NO y ) When NO y is low, NO determines [OH] 12

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15. Concluding remarks: 1) A simple functional description for [OH] in the tropical upper troposhpere (TUT) is accurate to ±50% - It appears that the TUT OH has no dependence on O 3 and H 2 O - Spread in [OH] at a given SZA needs further examination 2) Low [OH] air parcels do exist in TUT - O 3, NO, and NO y are very low in these air parcels - Low NO appears to be the main cause 3) Where to find low [OH] air parcels: Source: aged marine boundary air (low O 3, NO, and NO y ) Strong convection without lightning – marine convection Most likely place – the west Pacific warm pool Low O 3 can be an indicator of low NO and low OH! 15

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17. Measurements in TUT during STRAT 17

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