HOx chemistry during INTEX-A 2004: Observation, model calculation, and comparison with previous studies

OH and HO₂ were measured with the Airborne Tropospheric Hydrogen Oxides Sensor (ATHOS) as part of a large measurement suite from the NASA DC-8 aircraft during the Intercontinental Chemical Transport Experiment-A (INTEX-A). This mission, which was conducted mainly over North America and the western Atlantic Ocean in summer 2004, was an excellent test of atmospheric oxidation chemistry. The HOx results from INTEX-A are compared to those from previous campaigns and to results for other related measurements from INTEX-A. Throughout the troposphere, observed OH was generally 0.95 of modeled OH; below 8 km, observed HO₂ was generally 1.20 of modeled HO₂. This observed-to-modeled comparison is similar to that for TRACE-P, another midlatitude study for which the median observed-to-modeled ratio was 1.08 for OH and 1.34 for HO₂, and to that for PEM-TB, a tropical study for which the median observed-to-modeled ratio was 1.17 for OH and 0.97 for HO₂. HO₂ behavior above 8 km was markedly different. The observed-to-modeled HO₂ ratio increased from ~1.2 at 8 km to ~3 at 11 km with the observed-to-modeled ratio correlating with NO. Above 8 km, the observed-to-modeled HO₂ and observed NO were both considerably greater than observations from previous campaigns. In addition, the observed-to-modeled HO₂/OH, which is sensitive to cycling reactions between OH and HO₂, increased from ~1.5 at 8 km to almost 3.5 at 11 km. These discrepancies suggest a large unknown HOx source and additional reactants that cycle HOx from OH to HO₂. In the continental planetary boundary layer, the observed-to-modeled OH ratio increased from 1 when isoprene was less than 0.1 ppbv to over 4 when isoprene was greater than 2 ppbv, suggesting that forests throughout the United States are emitting unknown HOx sources. Progress in resolving these discrepancies requires a focused research activity devoted to further examination of possible unknown OH sinks and HOx sources.

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An edited version of this paper was published by AGU. Copyright 2008 American Geophysical Union.
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Author Ren, X.
Olson, J.
Crawford, J.
Brune, W.
Mao, J.
Long, R.
Chen, Z.
Chen, G.
Avery, M.
Sachse, G.
Barrick, J.
Diskin, G.
Huey, L.
Fried, Alan
Cohen, R.
Heikes, B.
Wennberg, P.
Singh, H.
Blake, D.
Shetter, Richard E.
Publisher UCAR/NCAR - Library
Publication Date 2008-03-08T00:00:00
Digital Object Identifier (DOI) Not Assigned
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Topic Category geoscientificInformation
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Metadata Date 2025-07-17T15:59:19.089445
Metadata Record Identifier edu.ucar.opensky::articles:6331
Metadata Language eng; USA
Suggested Citation Ren, X., Olson, J., Crawford, J., Brune, W., Mao, J., Long, R., Chen, Z., Chen, G., Avery, M., Sachse, G., Barrick, J., Diskin, G., Huey, L., Fried, Alan, Cohen, R., Heikes, B., Wennberg, P., Singh, H., Blake, D., Shetter, Richard E.. (2008). HOx chemistry during INTEX-A 2004: Observation, model calculation, and comparison with previous studies. UCAR/NCAR - Library. https://n2t.org/ark:/85065/d7cf9q95. Accessed 10 August 2025.

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