An observationally constrained evaluation of the oxidative capacity in the tropical western Pacific troposphere

Abstract

Hydroxyl radical (OH) is the main daytime oxidant in the troposphere and determines the atmospheric lifetimes of many compounds. We use aircraft measurements of O3, H2O, NO, and other species from the Convective Transport of Active Species in the Tropics (CONTRAST) field campaign, which occurred in the tropical western Pacific (TWP) during January–February 2014, to constrain a photochemical box model and estimate concentrations of OH throughout the troposphere. We find that tropospheric column OH (OHCOL) inferred from CONTRAST observations is 12 to 40% higher than found in chemical transport models (CTMs), including CAM‐chem‐SD run with 2014 meteorology as well as eight models that participated in POLMIP (2008 meteorology). Part of this discrepancy is due to a clear‐sky sampling bias that affects CONTRAST observations; accounting for this bias and also for a small difference in chemical mechanism results in our empirically based value of OHCOL being 0 to 20% larger than found within global models. While these global models simulate observed O3 reasonably well, they underestimate NOx (NO + NO2) by a factor of 2, resulting in OHCOL ~30% lower than box model simulations constrained by observed NO. Underestimations by CTMs of observed CH3CHO throughout the troposphere and of HCHO in the upper troposphere further contribute to differences between our constrained estimates of OH and those calculated by CTMs. Finally, our calculations do not support the prior suggestion of the existence of a tropospheric OH minimum in the TWP, because during January–February 2014 observed levels of O3 and NO were considerably larger than previously reported values in the TWP.

BrO and inferred Br<sub><i>y</i></sub> profiles over the western Pacific: relevance of inorganic bromine sources and a Br<sub><i>y</i></sub> minimum in the aged tropical tropopause layer

“…2c, k and 4c). Thus, current box-modeling results constrained with CONTRAST measurements and effective heterogeneous recycling are consistent with the proposed tropical rings of atomic halogens, where atomic bromine dominates inorganic bromine partitioning under the low ozone and ambient temperatures prevailing in the TTL SaizLopez and Fernandez, 2016). Kreycy et al (2013), proposed that BrONO 2 is relatively unstable, and recommend increasing BrONO 2 photolysis and decreasing BrONO 2 formation.…”

Global atmospheric chemistry – which air matters

“…Models A and B have much greater frequency of O 3 occurrence from 50-150 ppb, and half the models (B, D, E) show a reasonable frequency of O 3 at 10 ppb and less, as might be expected in the tropical Pacific boundary layer (Kley et al, 1996;Singh et al, 1996;Nicely et al, 2016). For CO, model A shows unusually low abundances.…”

“…Other approaches for indirectly comparing chemical models without transport have been developed (e.g., neural networks in Nicely et al, 2017). We choose the A-run approach as it will allow us to more directly compare modeled reactivities based on the primary CTM-CCM coding and still allows for all models to be initialized with the same chemical composition.…”

“…The goal here is to test their simulated chemical heterogeneity. While a box model could be designed (using 3-D meteorology) to address the first three needs (e.g., Nicely et al, 2016), it cannot address the last. More simply, all the necessary Earth system components are already built in to the CTMs-CCMs, and our approach of testing the modeled climatologies includes that of testing the Earth system components (e.g., emissions, transport, chemistry, scavenging, air-sea exchange, and land-surface interactions).…”

“…Unfortunately, many of these produced a biased, non-climatological sampling, for example, by chasing pollution plumes (Hsu et al, 2004) or by measuring only in clear skies (Nicely et al, 2016). The Pacific Exploratory Missions, PEM-Tropics and PEM-West, were notable in providing a mostly unbiased, exploratory sampling of specific regions in the remote Pacific with a full chemical payload measuring most of the key species Raper et al, 2001;Davis et al, 2003).…”