Observations of HO_x and its relationship with NO_x in the upper troposphere during SONEX

Abstract

Abstract. 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.

Measurements of tropospheric HO<sub>2</sub> and RO<sub>2</sub> by oxygen dilution modulation and chemical ionization mass spectrometry

“…For example, when [NO] > 0.5 ppbV, (both 1-Hz and 1-min averaged [NO]) the model-predicted HO 2 + RO 2 mixing ratios are much lower than the actual observed radical concentrations, and to a lesser extent, the model-predicted HO 2 mixing ratios are lower than the observed data. This tendency for the measurement/model ratio to increase with increasing NO x has been discussed previously for measurements of hydroperoxy radicals and hydroxyl radicals (Faloona et al, 2000;Ren et al, 2005Ren et al, , 2006Olson et al, 2006), and for this reason we consider times when [NO] < 0.5 ppbV in the following analysis. The discrepancy between measurements and model with high NO x for the peroxy radical data from MIRAGE-Mex, INTEX-B and other more recent campaigns will be explored further in a separate paper.…”

Measurements of the sum of HO&lt;sub&gt;2&lt;/sub&gt;NO&lt;sub&gt;2&lt;/sub&gt; and CH&lt;sub&gt;3&lt;/sub&gt;O&lt;sub&gt;2&lt;/sub&gt;NO&lt;sub&gt;2&lt;/sub&gt; in the remote troposphere

“…Analysis of HO x observations by Faloona et al (2000) suggest that a model in which the HO 2 NO 2 equilibrium constant is reduced by a factor of five, or with HO 2 NO 2 formation suppressed entirely, provides a more accurate representation of upper tropospheric HO x observations than the standard model. We estimate that inclusion of the IR photolysis of HO 2 NO 2 should have nearly the same effect on HO 2 NO 2 concentrations at the altitudes where most of the observations described by Faloona et al (2000) were collected. The main consequence is a decrease in the modelled abundance of HO 2 NO 2 .…”

Constraining the concentration of the hydroxyl radical in a stratocumulus-topped marine boundary layer from sea-to-air eddy covariance flux measurements of dimethylsulfide

“…To approximate the diel profile of OH, we need to know the sources and sinks of the HO x (OH + HO 2 ) system because the OH:HO 2 ratio is usually a steady-state constant (Faloona et al 2000). To a first order, the availability solar flux, and hence the ozone photolysis frequency, J O( 1 D), limits OH production since the subsequent reaction between O( 1 D) and water vapor is rapid.…”