Abstract
Measurements of several short‐lived photochemical species (e.g., OH, HO2, and CH2O) were obtained from the DC‐8 and P3‐B aircraft during the NASA Transport and Chemical Evolution over the Pacific (TRACE‐P) campaign. To assess fast photochemical theory over the east Asian coast and western Pacific, these measurements are compared to predictions using a photochemical time‐dependent box model constrained by coincident measurements of long‐lived tracers and physical parameters. Both OH and HO2 are generally overpredicted by the model throughout the troposphere, which is a different result from previous field campaigns. The calculated‐to‐observed ratio of OH shows an altitude trend, with OH overpredicted by 80% in the upper troposphere and by 40–60% in the middle troposphere. Boundary layer and lower tropospheric OH ratios decrease from middle tropospheric values to 1.07 for the DC‐8 and to 0.70 for the P3‐B. HO2 measured on the DC‐8 is overpredicted by a median of 23% and shows no trend in the agreement with altitude. Three subsets of data which compose 12% of the HO2 measurements represent outliers with respect to calculated‐to‐observed ratios: stratospherically influenced air, upper tropospheric data with NO > 135 pptv, and data from within clouds. Pronounced underpredictions of both HO2 and OH were found for stratospherically influenced air, which is in contrast to previous studies showing good agreement of predicted and observed HOx in the stratosphere. Observational evidence of heterogeneous uptake of HO2 within low and middle tropospheric clouds is presented, though there is no indication of significant HO2 uptake within higher‐altitude clouds. Model predictions of CH2O are in good agreement with observations in the median for background concentrations, but a large scatter exists. Factors contributing to this scatter are examined, including the limited availability of some important constraining measurements, particularly CH3OOH. Some high concentrations of CH2O near the coast are underpredicted by the box model as a result of the inherent neglect of transport effects of CH2O and its precursors via the steady state assumption; however, these occurrences are limited to ∼1% of the data. For the vast majority of the atmosphere, transport is unimportant in the budget of CH2O, which may be considered to be in steady state.
Highlights
For the vast majority of the atmosphere, transport is unimportant in the budget of CH2O, which may be considered to be in steady state
[2] NASA’s Transport and Chemical Evolution over the Pacific (TRACE-P) field campaign was conducted along the Asian Pacific Rim and the western Pacific during February – April 2001 [TRACE-P Science Team, 2003]
Methanol was measured at these altitudes during TRACE-P at a median value of 895 pptv, and at these concentrations, the model calculations show the impact on CH2O predictions is on the order of 10%, similar to results presented by Frost et al [2002]
Summary
[2] NASA’s Transport and Chemical Evolution over the Pacific (TRACE-P) field campaign was conducted along the Asian Pacific Rim and the western Pacific during February – April 2001 [TRACE-P Science Team, 2003]. Though this process initially consumes OH, further reactions of products from hydrocarbon oxidation such as formaldehyde (CH2O) can in turn produce multiple HO2 radicals Species such as peroxides that are typically considered to be in equilibrium with HOx can become local sources when transport-induced nonequilibrium conditions occur, such as the convective transport of abundant peroxides from the boundary layer into the free troposphere [Jaegleet al., 1997; Prather and Jacob, 1997]. [9] In addition to the required constraints described above, the model constrains the following species when measurements are available: Hydrogen peroxide (H2O2), methyl hydrogen peroxide (CH3OOH), nitric acid (HNO3), and peroxy acetyl nitrate (PAN). If unavailable, these species are calculated to be in diurnal photochemical equilibrium. Short-lived nitrogen (NO + NO2 + NO3 + 2N2O5 + HONO + HNO4) is photochemically partitioned at each time step, while the total short-lived nitrogen is held constant to a value such that predicted NO matches the measurement at the time of observation
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