Thomas B. Ryerson

Recent observations of nitrous acid (HONO) in the remote troposphere show much higher concentrations than can be explained through known sources, with important implications for air quality and climate. Laboratory evidence and modelling of field observations suggests that nitrate aerosol photolysis is the likely mechanism providing the additional HONO, offering a rapid route for recycling of NO_x from nitric acid (HNO₃). Previous studies of the global impact of this chemistry have used either very restricted HONO data or a “top-down” approach to parameterize the HONO source by reconciling simulated and observed NO_x concentrations. Here, we use multiple, independent tropospheric HONO observations from different locations to parameterize nitrate photolysis, and evaluate its impacts on global atmospheric chemistry using GEOS-Chem. The simulations improve agreement between modelled and observed HONO concentrations relative to previous studies, decreasing the model bias by 5 %–20 %. The remaining (and large) underestimate of HONO in the model is due predominantly to an underestimate of total nitrate aerosol (−95 %) and is reduced to 20 % when accounting for low model nitrate. Despite the low bias in the model HONO, we find that nitrate aerosol photolysis leads to substantial global increases in NO_x, O₃ and OH concentrations, likely beyond the observational constraints. The additional source of NO_x (∼ 48 Tg N yr⁻¹ globally) is comparable to total NO_x emissions from all sources (∼ 55 Tg yr⁻¹). These HONO observations in the remote troposphere, thus imply a large uncertainty in the NO_x budget and an incomplete understanding of atmospheric chemistry. Improved techniques to measure HONO at the low concentrations typical of remote areas, coupled with more measurements in these areas and improved process level understanding of nitrate photolysis are needed to provide quantitative assessment of its potentially global-scale atmospheric impacts.

Studying tropospheric ozone over the remote areas of the planet, such as the open oceans and the polar regions, is crucial to understand the role of ozone as a global climate forcer and regulator of atmospheric oxidative capacity. A focus on the pristine oceanic and polar regions complements the available land-based datasets and provides insights into key photochemical and depositional loss processes that control the concentrations and spatiotemporal variability in ozone as well as the physicochemical mechanisms driving these patterns. However, an assessment of the role of ozone over the oceanic and polar regions has been hampered by a lack of comprehensive observational datasets. Here, we present the first comprehensive collection of ozone data over the oceans and the polar regions. The overall dataset consists of 77 ship cruises/buoy-based observations and 48 aircraft-based campaigns. The dataset, consisting of more than 630 000 independent ozone measurement data points covering the period from 1977 to 2022 and an altitude range from the surface to 5000 m (with a focus on the lowest 2000 m), allows systematic analyses of the spatiotemporal distribution and long-term trends over the 11 defined ocean/polar regions. The datasets from ships, buoys, and aircraft are complemented by ozonesonde data from 29 launch sites or field campaigns and by 21 non-polar and 17 polar ground-based station datasets. The datasets contain information on how long the observed air masses were isolated from land, as estimated by backward trajectories from the individual observation points. To extract observations representative of oceanic conditions, we recommend using a subset of the data with an isolation time of 72 h or longer, from the analysis with coincident radon observations. These filtered oceanic and polar data showed typically flat diurnal cycles at high latitudes, whereas daytime decreases in ozone (11 %–16 %) were observed at lower latitudes. The ship/buoy- and aircraft-based datasets presented here will supplement the land-based ones in the TOAR-II (Tropospheric Ozone Assessment Report Phase II) database to provide a fully global assessment of tropospheric ozone. The described dataset is available at https://doi.org/10.17596/0004044 (Kanaya et al., 2025).

The subtropics are influenced by stratosphere-troposphere exchange processes through the subtropical jet streams and tropopause folding events, which are commonly identified by the opposing gradients of ozone (O₃) and carbon monoxide (CO) and thus their ratio. Here, we used airborne observations of CO and O₃, as well as the global three-dimensional ECHAM5/MESSy Atmospheric Chemistry (EMAC) model, to investigate whether there is another important mechanism that conditions the subtropics. We show that high O₃–CO ratios extend deeply into the troposphere in the subtropics, which is evident in both in situ observations and model results. Tropospheric photochemistry leads to similar O₃–CO ratios as those for stratospheric air diluted into the troposphere. In the upper tropical troposphere, frequent deep convective events produce lightning that leads to high concentrations of nitrogen oxides (NO_x ≡ NO+NO₂), which drive O₃ production and which further catalyze the recycling of hydroxyl (OH) radicals, which reduces CO. These lightning-affected air masses can be transported from the tropics into the subtropics via the Hadley circulation. We have excluded NO production through lightning in a sensitivity run of the EMAC model and see an annual relative reduction of the O₃–CO ratio of up to almost 50 % in the tropics and up to 40 % in the northern subtropics, with even larger seasonal variability and major effects on the vertical profiles of O₃ and CO. We therefore show that photochemistry is an additional key factor alongside stratosphere-troposphere mixing in determining O₃-rich and CO-poor air masses in the troposphere.

The subtropics are influenced by stratosphere-troposphere exchange processes through the subtropical jet streams and tropopause folding events, which are commonly identified by the opposing gradients of ozone (O₃) and carbon monoxide (CO) and thus their ratio. Here, we used airborne observations of CO and O₃, as well as the global three-dimensional ECHAM5/MESSy Atmospheric Chemistry (EMAC) model, to investigate whether there is another important mechanism that conditions the subtropics. We show that high O₃–CO ratios extend deeply into the troposphere in the subtropics, which is evident in both in situ observations and model results. Tropospheric photochemistry leads to similar O₃–CO ratios as those for stratospheric air diluted into the troposphere. In the upper tropical troposphere, frequent deep convective events produce lightning that leads to high concentrations of nitrogen oxides (NO_x ≡ NO+NO₂), which drive O₃ production and which further catalyze the recycling of hydroxyl (OH) radicals, which reduces CO. These lightning-affected air masses can be transported from the tropics into the subtropics via the Hadley circulation. We have excluded NO production through lightning in a sensitivity run of the EMAC model and see an annual relative reduction of the O₃–CO ratio of up to almost 50 % in the tropics and up to 40 % in the northern subtropics, with even larger seasonal variability and major effects on the vertical profiles of O₃ and CO. We therefore show that photochemistry is an additional key factor alongside stratosphere-troposphere mixing in determining O₃-rich and CO-poor air masses in the troposphere.

The subtropics are influenced by stratosphere-troposphere exchange processes through the subtropical jet streams and tropopause folding events, which are commonly identified by the opposing gradients of ozone (O₃) and carbon monoxide (CO) and thus their ratio. Here, we used airborne observations of CO and O₃, as well as the global three-dimensional ECHAM5/MESSy Atmospheric Chemistry (EMAC) model, to investigate whether there is another important mechanism that conditions the subtropics. We show that high O₃–CO ratios extend deeply into the troposphere in the subtropics, which is evident in both in situ observations and model results. Tropospheric photochemistry leads to similar O₃–CO ratios as those for stratospheric air diluted into the troposphere. In the upper tropical troposphere, frequent deep convective events produce lightning that leads to high concentrations of nitrogen oxides (NO_x ≡ NO+NO₂), which drive O₃ production and which further catalyze the recycling of hydroxyl (OH) radicals, which reduces CO. These lightning-affected air masses can be transported from the tropics into the subtropics via the Hadley circulation. We have excluded NO production through lightning in a sensitivity run of the EMAC model and see an annual relative reduction of the O₃–CO ratio of up to almost 50 % in the tropics and up to 40 % in the northern subtropics, with even larger seasonal variability and major effects on the vertical profiles of O₃ and CO. We therefore show that photochemistry is an additional key factor alongside stratosphere-troposphere mixing in determining O₃-rich and CO-poor air masses in the troposphere.

We present global airborne observations of acetyl peroxynitrate (CH ₃ C(O)OONO ₂ , PAN) in the remote troposphere from the Atmospheric Tomography (ATom) campaign. These observations show that biomass burning is the dominant source of PAN in the Southern Hemisphere (SH). In the Northern Hemisphere, anthropogenic emissions from Asia and Europe also contribute significantly to PAN over the Pacific and Atlantic Oceans. Model simulations underestimate PAN in the lower troposphere, in part, due to the underestimation of local production driven by acetaldehyde oxidation and (the ratio of acetyl peroxy radicals reacting with NO ₂ relative to other pathways). The significant impacts of biomass burning evident in the ATom PAN observations suggest that improving model treatment of plume transport and the conversion of NO _x to PAN in biomass burning plumes is a viable focus for better simulating PAN. Global observations of PAN provide a benchmark for the evaluation of satellite observations and model simulations of PAN.

Recent observations of nitrous acid (HONO) in the remote troposphere found much higher concentrations than can be explained through known sources, with important implications for air quality and climate. Laboratory evidence and modelling of field observations suggests that nitrate aerosol photolysis is the likely mechanism providing the additional HONO, offering a rapid route for recycling of NO_x from nitric acid (HNO₃). Previous studies of the global impact of this chemistry have used either very restricted HONO data or a “top-down” approach to parameterize the HONO source by reconciling simulated and observed NO_x concentrations. Here, we use multiple, independent tropospheric HONO observations from different locations to parameterize nitrate photolysis, and evaluate its impacts on global atmospheric chemistry using GEOS-Chem. The simulations improve agreement between modelled and observed HONO concentrations relative to previous studies, decreasing the model bias by 5–20 %. The remaining (and large) underestimate of HONO in the model is due predominantly to an underestimate of total nitrate aerosol (-95 %) and is reduced to 20 % when accounting for low model nitrate. Despite the low bias in the model HONO, we find that nitrate aerosol photolysis leads to substantial global increases in NO_x, O₃ and OH concentrations, likely beyond the observational constraints. The additional source of NO_x (~48 Tg N yr^-1 globally) is comparable to total NO_x emissions from all sources (~55 Tg yr^-1). These HONO observations in the remote troposphere, thus imply a large uncertainty in the NO_x budget and an incomplete understanding of atmospheric chemistry.

The subtropics are influenced by stratosphere-troposphere exchange processes through the subtropical jet streams and tropopause folding events, which are commonly identified by the opposing gradients of ozone (O₃) and carbon monoxide (CO) and thus their ratio. Here, we used airborne observations of CO and O₃, as well as the global three-dimensional ECHAM5/MESSy Atmospheric Chemistry (EMAC) model, to investigate whether there is another important mechanism that conditions the subtropics. We show that high O₃–CO ratios extend deeply into the troposphere in the subtropics, which is evident in both in situ observations and model results. Tropospheric photochemistry leads to similar O₃–CO ratios as those for stratospheric air diluted into the troposphere. In the upper tropical troposphere, frequent deep convective events produce lightning that leads to high concentrations of nitrogen oxides (NO_x ≡ NO+NO₂), which drive O₃ production and which further catalyze the recycling of hydroxyl (OH) radicals, which reduces CO. These lightning-affected air masses can be transported from the tropics into the subtropics via the Hadley circulation. We have excluded NO production through lightning in a sensitivity run of the EMAC model and see an annual relative reduction of the O₃–CO ratio of up to almost 50 % in the tropics and up to 40 % in the northern subtropics, with even larger seasonal variability and major effects on the vertical profiles of O₃ and CO. We therefore show that photochemistry is an additional key factor alongside stratosphere-troposphere mixing in determining O₃-rich and CO-poor air masses in the troposphere.

The subtropics are influenced by stratosphere-troposphere exchange processes through the subtropical jet streams and tropopause folding events, which are commonly identified by the opposing gradients of ozone (O₃) and carbon monoxide (CO) and thus their ratio. Here, we used airborne observations of CO and O₃, as well as the global three-dimensional ECHAM5/MESSy Atmospheric Chemistry (EMAC) model, to investigate whether there is another important mechanism that conditions the subtropics. We show that high O₃–CO ratios extend deeply into the troposphere in the subtropics, which is evident in both in situ observations and model results. Tropospheric photochemistry leads to similar O₃–CO ratios as those for stratospheric air diluted into the troposphere. In the upper tropical troposphere, frequent deep convective events produce lightning that leads to high concentrations of nitrogen oxides (NO_x ≡ NO+NO₂), which drive O₃ production and which further catalyze the recycling of hydroxyl (OH) radicals, which reduces CO. These lightning-affected air masses can be transported from the tropics into the subtropics via the Hadley circulation. We have excluded NO production through lightning in a sensitivity run of the EMAC model and see an annual relative reduction of the O₃–CO ratio of up to almost 50 % in the tropics and up to 40 % in the northern subtropics, with even larger seasonal variability and major effects on the vertical profiles of O₃ and CO. We therefore show that photochemistry is an additional key factor alongside stratosphere-troposphere mixing in determining O₃-rich and CO-poor air masses in the troposphere.