Airborne tunable diode laser measurements of formaldehyde during TRACE‐P: Distributions and box model comparisons

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Abstract
Airborne measurements of CH2O were acquired employing tunable diode laser absorption spectroscopy during the 2001 Transport and Chemical Evolution Over the Pacific (TRACE‐P) study onboard NASA's DC‐8 aircraft. Above ∼2.5 km, away from the most extreme pollution influences and heavy aerosol loadings, comprehensive comparisons with a steady state box model revealed agreement to within ±37 pptv in the measurement and model medians binned according to altitude and longitude. Likewise, a near unity slope (0.98 ± 0.03) was obtained from a bivariate fit of the measurements, averaged into 25 pptv model bins, versus the modeled concentrations for values up to ∼450 pptv. Both observations suggest that there are no systematic biases on average between CH2O measurements and box model results out to model values ∼450 pptv. However, the model results progressively underpredict the observations at higher concentrations, possibly due to transport effects unaccounted for in the steady state model approach. The assumption of steady state also appears to contribute to the scatter observed in the point‐by‐point comparisons. The measurement‐model variance was further studied employing horizontal flight legs. For background legs screened using a variety of nonmethane hydrocarbon (NMHC) tracers, measurement and model variance agreed to within 15%. By contrast, measurement variance was ∼60% to 80% higher than the model variance, even with small to modest elevations in the NMHC tracers. Measurement‐model comparisons of CH2O in clouds and in the lower marine troposphere in the presence of marine aerosols suggest rather significant CH2O uptake by as much as 85% in one extreme case compared to expectations based on modeled gas phase processes.