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Dry deposition is an important ozone sink that impacts ecosystem carbon and water cycling. Ozone dry deposition in forests is regulated by vertical transport, stomatal uptake, and non‐stomatal processes including chemical removal. However, accurate descriptions of these processes in deposition parameterizations are hindered by sparse observational constraints on individual sink terms. Here we quantify the contribution of canopy‐atmosphere turbulent exchange and chemical ozone removal by soil‐emitted nitric oxide (NO) to ozone deposition in a North‐Italian broadleaf deciduous forest. We apply a multi‐layer canopy exchange model to interpret campaign observations of nitrogen oxides (NOx = NO + NO2) and ozone exchange above and inside the forest canopy. Two state‐of‐science parameterizations of in‐canopy vertical diffusivity, based on above‐canopy wind speed or stability, do not reproduce the observed exchange suppressed by canopy‐top radiative heating, resulting in overestimated dry deposition velocities of 10%–19% during daytime. Applying observation‐derived vertical diffusivities in our simulations largely resolves this overestimation. Soil emissions are an important NOx source despite the observed high background NOx levels. Soil NOx emissions decrease the gradient between canopy and surface layer NOx mixing ratios, which suppresses simulated NOx deposition by 80% compared to a sensitivity simulation without soil emissions. However, a sensitivity analysis shows that the enhanced chemical ozone sink by reaction with soil‐emitted NO is offset by increased vertical ozone transport from aloft and suppressed dry deposition. Our results highlight the need for targeted observations of non‐stomatal ozone removal and turbulence‐resolving deposition simulations to improve quantification and model representation of forest ozone deposition. Plain Language Summary: Ozone is a harmful air pollutant that impacts human and ecosystem health. Ozone can be removed by forest ecosystems as a result of air transport into forests followed by plant ozone uptake or chemical removal, but quantifying these individual processes is difficult. We combine model simulations and treetop measurements to study the role of vertical forest‐atmosphere air transport and chemical ozone removal inside the forest. We find that our model can only reproduce surface ozone removal if we account for suppressed transport as derived from observations. The soil is a substantial source of nitric oxide (NO) that reacts with ozone. According to our analysis, the presence of a soil NO source does not lead to increased ozone removal because other ozone sinks are reduced. Our results suggest that individual ozone removal processes in forests can best be studied using targeted observations and models that better resolve forest‐atmosphere exchange. Key Points: We use a multi‐layer canopy‐atmosphere exchange model to interpret ozone flux observations inside and above a North‐Italian forestTwo state‐of‐science vertical exchange parameterizations do not capture in‐canopy stable stratification suppressing ozone depositionSoil nitric oxide emissions do not increase ozone deposition due to compensating effects by deposition and transport [ABSTRACT FROM AUTHOR]

In Colombia, industrialization and a shift towards intensified agriculture have led to increased emissions of air pollutants. However, the baseline state of air quality in Colombia is relatively unknown. In this study we aim to assess the baseline state of air quality in Colombia with a focus on the spatial and temporal variability in emissions and atmospheric burden of nitrogen oxides (NOx = NO + NO2) and evaluate surface NOx, ozone (O3) and carbon monoxide (CO) mixing ratios. We quantify the magnitude and spatial distribution of the four major NOx sources (lightning, anthropogenic activities, soil biogenic emissions and biomass burning), by integrating global NOx emission inventories into the mesoscale meteorology and atmospheric chemistry model WRF-Chem. The comparison with in situ measurements is bound to urban areas whereas the use of remote sensing data allows to also evaluate air quality in remote regions. WRF-Chem was set up for a domain centered over Colombia with a similar resolution as OMI observed NO2 vertical columns as well as the EDGAR anthropogenic emission inventory, both providing information on a ~20 km resolution. However, this apparently poses a challenge regarding comparison with these urban observations. Air mass factors were recalculated based on the vertical distribution of NO2 within WRF-Chem, with respect to the coarse (1° x 1°) a priori profiles. The main reason for recalculation is a more consistent satellite-model comparison but it also reduced the mean bias. WRF-Chem was, on average, able to provide good estimates for tropospheric NO2 columns with an averaged difference of 0.02 x 1015 molecules cm-2, which is < 5 % of the mean column. However, the simulated NO2 columns are overestimated in regions with abundant modeled lightning emissions and underestimated in regions where biomass burning emissions dominate in the model. This result reflects the high contribution by lightning emissions (1258 Gg N yr-1) and the low contribution by biomass burning emissions (104 Gg N yr-1) to total NOx emissions within the WRF-Chem domain. WRF-Chem was unable to capture NOx and CO urban pollutant mixing ratios, both in timing and magnitude. Yet, WRF-Chem was able to simulate the urban diurnal cycle of O3 satisfactory but with a systematic overestimation of 10 ppb due to the equally large underestimation of NO mixing ratios and, consequently, titration. This indicates that these city environments are in the NOx saturated regime with frequent O3 titration. We also applied an online meteorology-chemistry single column model (SCM) to evaluate how enhanced emissions and different representation of advection and mixing conditions could explain an improved representation of the observed O3 and NOx diurnal cycles. The SCM appears to indeed better represent the observed diurnal cycle of urban pollutant mixing ratios. But, interestingly, this result did not require an enhancement in the emissions, indicating that the role of boundary layer dynamics and advection should be considered besides the use of high-resolution models and emissions inventories to realistically simulate urban air quality. Results obtained in this study provide insight in the magnitude, distribution and temporal evolution of different sources of pollution in Colombia and its surrounding territories. This study not only identifies different source regions, but also shows the interannual variability of these sources during the last one and a half decade using satellite data. Furthermore, this study shows that relatively coarse anthropogenic emission inventories can give reasonable results regarding the diurnal cycle of urban pollutant mixing ratios with a careful consideration of advection and mixing conditions. It serves as a base to assess scenarios of future air quality in Colombia, or similar regions with distinct contrasting emission regimes and a limited air quality monitoring network, as a function of further industrialization and land use changes. [ABSTRACT FROM AUTHOR]

Ozone (O3) is a secondary air pollutant that negatively affects human and ecosystem health. Ozone simulations with regional air quality models suffer from unexplained biases over Europe, and uncertainties in the emissions of ozone precursor group nitrogen oxides (NOx = NO + NO2) contribute to these biases. The goal of this study is to use NO2 column observations from the OMI satellite sensor to infer top-down NOx emissions in the regional meteorology-chemistry model WRF-Chem, and to evaluate the impact on simulated surface O3 with in situ observations. We first perform a simulation for July 2015 over Europe and evaluate its performance against in situ observations from the AirBase network. The spatial distribution of mean ozone concentrations is reproduced satisfactorily. However, the simulated maximum daily 8-hour ozone concentration (MDA8 O3) is underestimated (mean bias error (MBE) = -14.2μgm-3), and its spread is too low. We subsequently derive satellite-constrained surface NOx emissions using a mass balance approach based on the relative difference between OMI and WRF-Chem NO2 columns. The method accounts for feedbacks through OH, NO2's dominant daytime oxidant. Our optimized European NOx emissions amount to 0.50TgN (for July 2015), 0.18TgN higher than the bottom-up emissions (which lacked agricultural soil NOx emissions). Much of the increases occur across Europe, in regions where agricultural soil NOx emissions dominate. Our best estimate of soil NOx emissions in July 2015 is 0.1TgN, much higher than the bottom-up 0.02TgN natural soil NOx emissions from the MEGAN model. A simulation with satellite-updated NOx emissions reduces the systematic bias between WRF-Chem and OMI NO2 (slope = 0.98, r² = 0.84), and reduces the low bias against independent surface NO2 measurements by 1.1μgm-3 (-56%). Following these NOx emission changes, daytime ozone is strongly affected, since NOx emission changes particularly affect daytime ozone formation. Monthly averaged simulated daytime ozone increases by 6.0μgm-3, and increases of >10μgm-3 are seen in regions with large emission increases. With respect to the initial simulation, MDA8 O3 has an improved spatial distribution, expressed by an increase in r² from 0.40 to 0.53, and a reduced mean bias (-7.4μg m-3, -48%). Overall, our results highlight the dependence of surface ozone on its precursor NOx and demonstrate that simulations of surface ozone benefit from constraining surface NOx emissions by satellite NO2 column observations. [ABSTRACT FROM AUTHOR]