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Constraining remote oxidation capacity with ATom observations.

ABSTRACT: The global oxidation capacity, defined as the tropospheric mean concentration of the hydroxyl radical (OH), controls the lifetime of reactive trace gases in the atmosphere such as methane and carbon monoxide (CO). Models tend to underestimate the methane lifetime and CO concentrations throughout the troposphere, which is consistent with excessive OH. Approximately half of the oxidation of methane and non-methane volatile organic compounds (VOCs) is thought to occur over the oceans where oxidant chemistry has received little validation due to a lack of observational constraints. We use observations from the first two deployments of the NASA ATom aircraft campaign during July-August 2016 and January-February 2017 to evaluate the oxidation capacity over the remote oceans and its representation by the GEOS-Chem chemical transport model. The model successfully simulates the magnitude and vertical profile of remote OH within the measurement uncertainties. Comparisons against the drivers of OH production (water vapor, ozone, and NO _y concentrations, ozone photolysis frequencies) also show minimal bias, with the exception of wintertime NO _y . The severe model overestimate of NO _y during this period may indicate insufficient wet scavenging and/or missing loss on sea-salt aerosols. Large uncertainties in these processes require further study to improve simulated NO _y partitioning and removal in the troposphere, but preliminary tests suggest that their overall impact could marginally reduce the model bias in tropospheric OH. During the ATom-1 deployment, OH reactivity (OHR) below 3 km is significantly enhanced, and this is not captured by the sum of its measured components (cOHR_obs) or by the model (cOHR_mod). This enhancement could suggest missing reactive VOCs but cannot be explained by a comprehensive simulation of both biotic and abiotic ocean sources of VOCs. Additional sources of VOC reactivity in this region are difficult to reconcile with the full suite of ATom measurement constraints. The model generally reproduces the magnitude and seasonality of cOHR_obs but underestimates the contribution of oxygenated VOCs, mainly acetaldehyde, which is severely underestimated throughout the troposphere despite its calculated lifetime of less than a day. Missing model acetaldehyde in previous studies was attributed to measurement uncertainties that have been largely resolved. Observations of peroxyacetic acid (PAA) provide new support for remote levels of acetaldehyde. The underestimate in both model acetaldehyde and PAA is present throughout the year in both hemispheres and peaks during Northern Hemisphere summer. The addition of ocean sources of VOCs in the model increases cOHR_mod by 3% to 9% and improves model-measurement agreement for acetaldehyde, particularly in winter, but cannot resolve the model summertime bias. Doing so would require 100 Tg yr^-1 of a long-lived unknown precursor throughout the year with significant additional emissions in the Northern Hemisphere summer. Improving the model bias for remote acetaldehyde and PAA is unlikely to fully resolve previously reported model global biases in OH and methane lifetime, suggesting that future work should examine the sources and sinks of OH over land.

SUBMITTER: Travis KR

PROVIDER: S-EPMC7939060 | biostudies-literature | 2020 Jul

REPOSITORIES: biostudies-literature

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Constraining remote oxidation capacity with ATom observations.

Travis Katherine R KR Heald Colette L CL Allen Hannah M HM Apel Eric C EC Arnold Stephen R SR Blake Donald R DR Brune William H WH Chen Xin X Commane Róisín R Crounse John D JD Daube Bruce C BC Diskin Glenn S GS Elkins James W JW Evans Mathew J MJ Hall Samuel R SR Hintsa Eric J EJ Hornbrook Rebecca S RS Kasibhatla Prasad S PS Kim Michelle J MJ Luo Gan G McKain Kathryn K Millet Dylan B DB Moore Fred L FL Peischl Jeffrey J Ryerson Thomas B TB Sherwen Tomás T Thames Alexander B AB Ullmann Kirk K Wang Xuan X Wennberg Paul O PO Wolfe Glenn M GM Yu Fangqun F

Atmospheric chemistry and physics 20200703 13

The global oxidation capacity, defined as the tropospheric mean concentration of the hydroxyl radical (OH), controls the lifetime of reactive trace gases in the atmosphere such as methane and carbon monoxide (CO). Models tend to underestimate the methane lifetime and CO concentrations throughout the troposphere, which is consistent with excessive OH. Approximately half of the oxidation of methane and non-methane volatile organic compounds (VOCs) is thought to occur over the oceans where oxidant ...[more]

PMID: 33688335

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Project description:Ribulose 1,5-bisphosphate (RuBP) carboxylase/oxygenase (RuBisCO, or Rubisco) catalyzes a key reaction by which inorganic carbon is converted into organic carbon in the metabolism of many aerobic and anaerobic organisms. Across the broader Rubisco protein family, homologs exhibit diverse biochemical characteristics and metabolic functions, but the evolutionary origins of this diversity are unclear. Evidence of the timing of Rubisco family emergence and diversification of its different forms has been obscured by a meager paleontological record of early Earth biota, their subcellular physiology and metabolic components. Here, we use computational models to reconstruct a Rubisco family phylogenetic tree, ancestral amino acid sequences at branching points on the tree, and protein structures for several key ancestors. Analysis of historic substitutions with respect to their structural locations shows that there were distinct periods of amino acid substitution enrichment above background levels near and within its oxygen-sensitive active site and subunit interfaces over the divergence between Form III (associated with anoxia) and Form I (associated with oxia) groups in its evolutionary history. One possible interpretation is that these periods of substitutional enrichment are coincident with oxidative stress exerted by the rise of oxygenic photosynthesis in the Precambrian era. Our interpretation implies that the periods of Rubisco substitutional enrichment inferred near the transition from anaerobic Form III to aerobic Form I ancestral sequences predate the acquisition of Rubisco by fully derived cyanobacterial (i.e., dual photosystem-bearing, oxygen-evolving) clades. The partitioning of extant lineages at high clade levels within our Rubisco phylogeny indicates that horizontal transfer of Rubisco is a relatively infrequent event. Therefore, it is possible that the mutational enrichment periods between the Form III and Form I common ancestral sequences correspond to the adaptation of key oxygen-sensitive components of Rubisco prior to, or coincident with, the Great Oxidation Event.

Dataset Information

Constraining remote oxidation capacity with ATom observations.

Publications

Constraining remote oxidation capacity with ATom observations.

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