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The concentration of nitrogen oxides (NOx) plays a central role in controlling air quality. On a global scale, the primary sink of NOxis oxidation to form HNO3. Gas-phase HNO3photolyses slowly with a lifetime in the troposphere of 10 days or more. However, several recent studies examining HONO chemistry have proposed that particle-phase HNO3undergoes photolysis 10–300 times more rapidly than gas-phase HNO3. We present here constraints on the rate of particle-phase HNO3photolysis based on observations of NOxand HNO3collected over the Yellow Sea during the KORUS-AQ study in summer 2016. The fastest proposed photolysis rates are inconsistent with the observed NOxto HNO3ratios. Negligible to moderate enhancements of the HNO3photolysis rate in particles, 1–30 times faster than in the gas phase, are most consistent with the observations. Small or moderate enhancement of particle-phase HNO3photolysis would not significantly affect the HNO3budget but could help explain observations of HONO and NOxin highly aged air.

Acetone is one of the most abundant oxygenated volatile organic compounds (VOCs) in the atmosphere. The oceans impose a strong control on atmospheric acetone, yet the oceanic fluxes of acetone remain poorly constrained. In this work, the global budget of acetone is evaluated using two global models: CAM‐chem and GEOS‐Chem. CAM‐chem uses an online air‐sea exchange framework to calculate the bidirectional oceanic acetone fluxes, which is coupled to a data‐oriented machine‐learning approach. The machine‐learning algorithm is trained using a global suite of seawater acetone measurements. GEOS‐Chem uses a fixed surface seawater concentration of acetone to calculate the oceanic fluxes. Both model simulations are compared to airborne observations from a recent global‐scale, multiseasonal campaign, the NASA Atmospheric Tomography Mission (ATom). We find that both CAM‐chem and GEOS‐Chem capture the measured acetone vertical distributions in the remote atmosphere reasonably well. The combined observational and modeling analysis suggests that (i) the ocean strongly regulates the atmospheric budget of acetone. The tropical and subtropical oceans are mostly a net source of acetone, while the high‐latitude oceans are a net sink. (ii) CMIP6 anthropogenic emission inventory may underestimate acetone and/or its precursors in the Northern Hemisphere. (iii) The MEGAN biogenic emissions model may overestimate acetone and/or its precursors, and/or the biogenic oxidation mechanisms may overestimate the acetone yields. (iv) The models consistently overestimate acetone in the upper troposphere‐lower stratosphere over the Southern Ocean in austral winter. (v) Acetone contributes up to 30–40% of hydroxyl radical production in the tropical upper troposphere/lower stratosphere. Acetone is widely observed in the Earth's atmosphere, with mixing ratios ranging from parts‐per‐trillion levels in the stratosphere to parts‐per‐billion levels in polluted regions. Acetone is directly emitted from a wide variety of natural and anthropogenic sources and is also produced from the photochemical oxidation of a number of precursors. The role of the ocean is complicated; acetone is produced in the ocean from the photolysis of colored dissolved organic materials or from biological processes but is also removed via microbial uptake. Previous studies have found that the direction and magnitude of oceanic acetone fluxes vary dramatically with seasons and locations. In this work, we use a data‐oriented machine‐learning approach to predict the surface seawater concentration of acetone, leveraging in situ acetone measurements in the surface seawater around the globe. This machine learning‐based approach shows promising potential and can be expanded to the bottom‐up oceanic emissions of other climate‐relevant compounds. We develop an online air‐sea exchange module for acetone, with ocean biogeochemistry represented using data‐oriented machine learningTwo separate global acetone simulations are compared to global‐scale multiseasonal airborne observationsGlobal models consistently overestimate acetone in the upper troposphere over the Southern Ocean in austral winter