Exploring Oxidation in the Remote Free Troposphere: Insights From Atmospheric Tomography (ATom)

Abstract

Earth's atmosphere oxidizes the greenhouse gas methane and other gases, thus determining their lifetimes and oxidation products. Much of this oxidation occurs in the remote, relatively clean free troposphere above the planetary boundary layer, where the oxidation chemistry is thought to be much simpler and better understood than it is in urban regions or forests. The NASA airborne Atmospheric Tomography study (ATom) was designed to produce cross sections of the detailed atmospheric composition in the remote atmosphere over the Pacific and Atlantic Oceans during four seasons. As part of the extensive ATom data set, measurements of the atmosphere's primary oxidant, hydroxyl (OH), and hydroperoxyl (HO 2 ) are compared to a photochemical box model to test the oxidation chemistry. Generally, observed and modeled median OH and HO 2 agree to within combined uncertainties at the 2σ confidence level, which is~±40%. For some seasons, this agreement is within~±20% below 6-km altitude. While this test finds no significant differences, OH observations increasingly exceeded modeled values at altitudes above 8 km, becoming~35% greater, which is near the combined uncertainties. Measurement uncertainty and possible unknown measurement errors complicate tests for unknown chemistry or incorrect reaction rate coefficients that would substantially affect the OH and HO 2 abundances. Future analysis of detailed comparisons may yield additional discrepancies that are masked in the median values.Plain Language Summary Chemistry in the vast, remote atmosphere destroys methane and other greenhouse gases. This chemistry is thought to be simple and well understood compared to that in polluted cities or in forests. From the NASA airborne Atmospheric Tomography study over remote oceans, comparisons of observed and modeled reactive gases show that the chemistry is generally understood to within the uncertainties of the measurement and model. However, for the atmosphere's primary reactive gas, hydroxyl, measured values exceed modeled values in the upper troposphere, pointing to errors in probably the measurements but possibly the model chemistry.

Constraining remote oxidation capacity with ATom observations

“…Global models tend to overestimate OH against constraints from methyl chloroform observations ( Shindell et al, 2006 ; Naik et al, 2013 ; Nicely et al, 2017 ), but we find here that tropospheric OH is successfully simulated within observational uncertainty (74% to 135%, 2 σ confidence level). This result from a global CTM is consistent with good agreement between OH measurements and a box model during NASA’s Pacific Exploratory Mission – Tropics (PEM-Tropic B) campaign in the clean remote Pacific ( Tan et al, 2001 ) and a similar analysis by Brune et al (2020) for ATom 1 through 4.…”

“…Causes of the remote model bias in NO y Figure 8 shows that the model NO y overestimate in winter is primarily caused by nitric acid (HNO 3 ). Excessive remote HNO 3 is a long-standing model deficiency (Bey et al, 2001;Staudt et al, 2003;Brunner et al, 2003Brunner et al, , 2005. The model bias identified here is unlikely to result from overestimated continental emissions due to the short lifetime of NO y against deposition (∼ 3 d in the Northern Hemisphere winter).…”

What the COVID-19 lockdown revealed about photochemistry and ozone production in Quito, Ecuador

“…Hence, from propagation of error, we estimate that uncertainty (1-σ) in the first source is 32% (considering uncertainties in J O3->O1D as well as in ozone and water vapor measurements reported in the methods section). For radical production due to formaldehyde and HONO photolysis (as well as for radical abundance), at the moment we assume model uncertainty reported in the literature (i.e., Brune et al, 2019 ) of 35%. Likewise, we estimate that uncertainty in other photochemical quantities (radical abundance and ozone production) is similar, but in the future these estimations need to be re-checked as measurements of physical and chemical quantities become available in the study area.…”

“…We used the Framework for 0-D Atmospheric Modeling ( Wolfe et al, 2016 ), F0AM v4, which is a complete MATLAB-based software loaded with six ready-to-use atmospheric chemistry mechanisms as well as with three options for resolving photolysis frequencies. The F0AM has been extensively used and proven to be a powerful tool for photochemical simulations (i.e., Brune et al, 2019 ). In the present case, we used the Master Chemical Mechanism ( Jenkin et al, 1997 ; Saunders et al, 2003 ), MCM v3.3.1, obtained via website ( http://mcm.leeds.ac.uk/MCM ).…”

The potential role of organics in new particle formation and initial growth in the remote tropical upper troposphere

“…Willis et al (2016) showed that marine organics contribute to the growth of newly formed particles in the summertime Arctic at low altitude; however, it was unclear if marine organics were involved in nucleation. Burkart et al (2017) found that particle growth in the remote Arctic was largely due to condensation of unidentified organic compounds, possibly of marine origin, associated with oxidation or photochemistry of the sea-surface micro-layer (Abbatt et al, 2019). Andreae et al (2018) proposed that oxidized biogenic volatile organic compounds (VOCs) were the source of recently formed particles found in the outflows and anvils of convective storms over Amazonia.…”