Observations of HOx and its relationship with NOx in the upper troposphere during SONEX (original) (raw)

2000, Journal of Geophysical Research

Simultaneous measurements of the oxides of hydrogen and nitrogen made during the NASA Subsonic Assessment, Ozone and Nitrogen Oxide Experiment (SONEX) afforded an opportunity to study the coupling between these two important families throughout the free troposphere and lowermost stratosphere. Moreover, the suite of measurements made during the campaign was unprecedented in its completeness, thus providing a uniquely detailed picture of the radical photochemistry that drives oxidation and ozone production in this part of the atmosphere. On average, observed hydrogen oxides (HOx = OH + HO2) agree well with both instantaneous and diel steady-state models; however, there is a persistent deviation of the observations that correlates with the abundance of nitrogen oxides (NOx = NO + NO2) in the sampled air mass. Specifically, the observed HO• tends to exceed the model predictions in the presence of high NO• concentrations, by as much as a factor of 5 (>500 pptv NOx), and is sometimes as little as half that expected by steady state at lower NOj levels. While many possibilities for these discrepancies are discussed, it is argued that an instrumental artifact is not probable and that the discrepancy may bespeak a shortcoming of our understanding of HO• chemistry. The consistently elevated HO• in the presence of elevated NOx leads directly to greater ozone production than expected, thereby extending the NOx-limited regime of the upper troposphere. These results could thus have bearing on the predicted impacts of increasing NO• emissions into this region of the atmosphere from, for example, the growth of global air traffic. DC-8 aircraft was the main platform for the study, and it operated for over 120 hours, including two transcontinental flights and 12 others based from Bangor, Maine, Shannon, Ireland, and the Azores. A summary of the mission can be found in the work of Singh et al. [1999]. The central importance of OH and HO2 in the chemistry of the atmosphere has been known for several decades [Levy, 1972; Logan et al., 1981], but it is only recently that in situ, simultaneous measurements of both of these extremely reactive and, consequently, very short-lived species have become available to test our understanding of the oxidative cycles in the atmosphere [Wennberg et al., 1995; Poppe et al., 1997; Folkins et al., 1997; Brune et al., 1998]. Comparisons of measurements made in the planetary boundary layer with steady-state models have been somewhat inconclusive in assessing whether or not an accurate understanding of the chemical cycles that give rise to the instantaneous levels of HOx in the troposphere is extant, or for that matter, if steady state is an applicable assumption [Crosley, 1995]. Experiments in the planetary boundary layer have shown that in general, the observations of OH are lower than expected [Perner et al., 1987; Eisele et al., 1996; Comes et al., 1997; Armerding et al., 1997]. This led many researches to invoke the presence of some unbeknown chemical sink, most probably hydrocarbon in form, which was not included in the chemical box models. Comes et al. [ 1997] even notice that the deviations of the model in their experiment in the Canary Islands seem to correspond to the NO:NO2 ratio, but the relationship is not fully developed, and they too suggest a missing biogenic hydrocarbon sink of OH. 3771 3772 FALOONA ET AL.: HOx AND ITS RELATIONSHIP TO NOx IN THE UT Meanwhile, stratospheric observations have shown the budget of HOx to be fairly well understood in that region of the atmosphere [Stimpfie et al., 1989; Wennberg et al., 1995; Pickett and Peterson, 1996]. The former two in situ studies did, however, help confirm the absence of two important heterogeneous processes in photochemical models at the time: the hydrolysis of N205 to HNO3 and the nighttime conversion of NOx species to nitrous acid (HONO) on sulfate aerosols. The latter mechanism provides a longer wavelength photolysis source of HOx in the early morning at high solar zenith angles, and tentative evidence of this was observed in the upper troposphere during SONEX as well [Jaegld et al., this issue]. Further measurements of HO• in the upper troposphere and lower stratosphere (UT/LS) provided evidence of other previously unrecognized players in the HOx budget in these dry environments, namely, the photolysis of acetone and peroxides [Singh et al., 1995; Jaegld et al., 1997; Folkins et al., 1998]. Brune et al. [1998] observed evidence of these precursors convectively lofted from Asia and transported to the upper troposphere over northern California. Jaegld et al. [1997] demonstrated this effect over the central United States in the data from the NASA Subsonic Aircraft Contrail and Cloud