HO_<i>x</i> chemistry during INTEX‐A 2004: Observation, model calculation, and comparison with previous studies

<jats:p>OH and HO<jats:sub>2</jats:sub> 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 HO<jats:sub>2</jats:sub> was generally 1.20 of modeled HO<jats:sub>2</jats:sub>. 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 HO<jats:sub>2</jats:sub>, 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 HO<jats:sub>2</jats:sub>. HO<jats:sub>2</jats:sub> behavior above 8 km was markedly different. The observed‐to‐modeled HO<jats:sub>2</jats:sub> 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 HO<jats:sub>2</jats:sub> and observed NO were both considerably greater than observations from previous campaigns. In addition, the observed‐to‐modeled HO<jats:sub>2</jats:sub>/OH, which is sensitive to cycling reactions between OH and HO<jats:sub>2</jats:sub>, increased from ∼1.5 at 8 km to almost 3.5 at 11 km. These discrepancies suggest a large unknown HO<jats:sub><jats:italic>x</jats:italic></jats:sub> source and additional reactants that cycle HO<jats:sub><jats:italic>x</jats:italic></jats:sub> from OH to HO<jats:sub>2</jats:sub>. 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 HO<jats:sub><jats:italic>x</jats:italic></jats:sub> sources. Progress in resolving these discrepancies requires a focused research activity devoted to further examination of possible unknown OH sinks and HO<jats:sub><jats:italic>x</jats:italic></jats:sub> sources.</jats:p>