Constraining remote oxidation capacity with ATom observations.

Area of Science:

• Atmospheric Chemistry and Physics
• Climate Science
• Environmental Monitoring

Background:

• Global oxidation capacity, determined by hydroxyl radical (OH) concentration, regulates the atmospheric lifetime of key trace gases like methane and carbon monoxide (CO).
• Current models often underestimate methane lifetime and CO concentrations, suggesting an overestimation of OH, particularly over remote oceans where oxidant chemistry is poorly validated due to limited observational data.
• The NASA ATom aircraft campaign provides crucial in-situ measurements to address these knowledge gaps in remote oceanic atmospheric chemistry.

Purpose of the Study:

• To evaluate the oxidation capacity over remote oceans and assess the performance of the GEOS-Chem chemical transport model in simulating these conditions.
• To investigate the drivers of OH production and loss, including nitrogen oxides (NOx) and volatile organic compounds (VOCs), over remote oceanic regions.
• To identify and quantify discrepancies between model simulations and observational data for OH, OH reactivity (OHR), acetaldehyde, and peroxyacetic acid (PAA).

Main Methods:

• Utilized observational data from the first two NASA ATom aircraft campaign deployments (July-August 2016 and January-February 2017) over remote oceans.
• Compared in-situ measurements of OH, NOx, ozone, water vapor, ozone photolysis frequencies, and VOCs with simulations from the GEOS-Chem chemical transport model.
• Analyzed OH reactivity (OHR) by comparing measured components (cOHRobs) with model simulations (cOHRmod) and investigated potential missing sources of VOCs.

Main Results:

• The GEOS-Chem model accurately simulates the magnitude and vertical profile of remote OH concentrations within measurement uncertainties, with minimal bias in OH production drivers except for wintertime NOx.
• A significant enhancement in OH reactivity (OHR) below 3 km was observed during the ATom-1 deployment, which was not captured by model simulations or the sum of measured reactive components, suggesting missing reactive VOCs.
• The model underestimates acetaldehyde and PAA concentrations throughout the troposphere, particularly during Northern Hemisphere summer, despite accounting for known ocean VOC sources. Additional unknown precursors are needed to resolve this bias.

Conclusions:

• The GEOS-Chem model demonstrates good performance in simulating remote oceanic OH concentrations and their production drivers, but significant underestimation of acetaldehyde and PAA points to missing VOC sources or sinks.
• The observed OHR enhancement suggests the presence of unmeasured reactive VOCs over remote oceans, which are not fully represented by current models, impacting the accuracy of simulated oxidation capacity.
• Resolving the model bias in acetaldehyde and PAA, and consequently the OHR, is unlikely to fully address previously reported global model biases in OH and methane lifetime, highlighting the need to investigate OH sources and sinks over land.