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Tropospheric levels of ozone and its precursors have risen in Asia since 2000. Satellite observations and chemistry–transport simulations suggest that transport of these pollutants to North America partly offsets benefits from stricter regulation. Rapid population growth and industrialization have driven substantial increases in Asian ozone precursor emissions over the past decade 1 , with highly uncertain impacts on regional and global tropospheric ozone levels. According to ozonesonde measurements 2 , 3 , tropospheric ozone concentrations at two Asian sites have increased by 1 to 3% per year since 2000, an increase thought to contribute to positive trends in the ozone levels observed at North America’s West Coast 4 , 5 . However, model estimates of the Asian contribution to North American ozone levels are not well-constrained by observations 6 , 7 . Here we interpret Aura satellite measurements of tropospheric concentrations of ozone and its precursor NO 2 , along with its largest natural source, stratospheric ozone, using the TM5 global chemistry–transport model. We show that tropospheric ozone concentrations over China have increased by about 7% between 2005 and 2010 in response to two factors: a rise in Chinese emissions by about 21% and increased downward transport of stratospheric ozone. Furthermore, we find that transport from China of ozone and its precursors has offset about 43% of the 0.42 DU reduction in free-tropospheric ozone over the western United States that was expected between 2005 and 2010 as a result of emissions reductions associated with federal, state and local air quality policies. We conclude that global efforts may be required to address regional air quality and climate change.

The downward transport of stratospheric air can deliver significant quantities of ozone to the upper troposphere. An analysis of satellite data suggests that year-to-year variations in stratospheric circulation can account for around half of the interannual variability in tropospheric ozone levels in the northern mid-latitudes. The downward transport of stratospheric ozone is an important natural source of tropospheric ozone, particularly in the upper troposphere, where changes in ozone have their largest radiative effect 1 . Stratospheric circulation is projected to intensify over the coming century, which could lead to an increase in the flux of ozone from the stratosphere to the troposphere 2 , 3 , 4 . However, large uncertainties in the stratospheric contribution to trends and variability in tropospheric ozone levels 5 , 6 , 7 make it difficult to reliably project future changes in tropospheric ozone 8 . Here, we use satellite measurements of stratospheric water vapour and tropospheric ozone levels collected between 2005 and 2010 to assess the effect of changes in stratospheric circulation, driven by El Niño/Southern Oscillation and the stratospheric Quasi-Biennial Oscillation, on tropospheric ozone levels. We find that interannual variations in the strength of the stratospheric circulation of around 40%—comparable to the mean change in stratospheric circulation projected this century 2 —lead to changes in tropospheric ozone levels in the northern mid-latitudes of around 2%, approximately half of the interannual variability. Assuming that the observed response of tropospheric ozone levels to interannual variations in circulation is a good predictor of its equilibrium response, we suggest that the projected intensification of the stratospheric circulation over the coming century could lead to small but important increases in tropospheric ozone levels.

Tropospheric NO2 and ozone simulations have large uncertainties, but their biases, seasonality, and trends can be improved with NO2 assimilations. We perform global top-down estimates of monthly NOx emissions using two Ozone Monitoring Instrument (OMI) NO2 retrievals (NASAv3 and DOMINOv2) from 2005 to 2016 through a hybrid 4D-Var/mass balance inversion. Discrepancy inNO2 retrieval products is a major source of uncertainties in the top-down NOx emission estimates. The different vertical sensitivities in the two NO2 retrievals affect both magnitude and seasonal variations of top-down NOx emissions. The 12-year averages of regional NOx budgets from the NASA posterior emissions are 37 % to 53 % smaller than the DOMINO posterior emissions. Consequently, the DOMINO posterior surface NO2 simulations greatly reduced the negative biases in China (by 15 %) and the US (by 22 %) compared to surface NO2 measurements. Posterior NOx emissions show consistent trends over China, the US, India, and Mexico constrained by the two retrievals. Emission trends are less robust over South America, Australia, western Europe, and Africa, where the two retrievals show less consistency. NO2 trends have more consistent decreases (by 26 %) with the measurements (by 32 %) in the US from 2006 to 2016 when using the NASA posterior emissions. The performance of posterior ozone simulations has spatial heterogeneities from region to region. On a global scale, ozone simulations using NASA-based emissions alleviate the double peak in the prior simulation of global ozone seasonality. The higher abundances of NO2 from the DOMINO posterior simulations increase the global background ozone concentrations and therefore reduce the negative biases more than the NASA posterior simulations using GEOS-Chem v12 at remote sites. Compared to surface ozone measurements, posterior simulations have more consistent magnitude and interannual variations than the prior estimates, but the performance from the NASA-based and DOMINO-based emissions varies across ozone metrics. The limited availability of remote-sensing data and the use of prior NOx diurnal variations hinder improvement of ozone diurnal variations from the assimilation, and therefore have mixed performance on improving different ozone metrics. Additional improvements in posterior NO2 and ozone simulations require more precise and consistent NO2 retrieval products, more accurate diurnal variations of NOx and VOC emissions, and improved simulations of ozone chemistry and depositions.

Over the past two decades, satellite instruments have provided unprecedented information on global air quality, and yet the remote sensing of surface ozone remains elusive. Here we propose a new method to infer spatial variability in surface ozone by combining multispectral ozone retrievals using radiances from the tropospheric emission spectrometer thermal infrared instrument and the ozone monitoring instrument ultratraviolet/visible instrument with a chemical reanalysis. We find that our inferred surface ozone in summertime China and the United States has regional biases of less than 4 ppb and a high spatial correlation when validated against independent surface measurements. Over the broader Asia region, our analysis results in a spatial pattern of summertime surface ozone that can largely be explained by a combination of the Asian monsoon circulation and NO x emissions. Our results show the potential of combining satellite measurements and chemical reanalyses to provide critical air quality information in regions of limited surface networks, thereby enhancing the global air quality observing system.

The distribution of gases such as ozone and water vapour in the stratosphere—which affect surface climate—is influenced by the meridional overturning of mass in the stratosphere, the Brewer–Dobson circulation. However, observation-based estimates of the global strength of this circulation are difficult to obtain. Here we present two calculations of the mean strength of the meridional overturning of the stratosphere. We analyse satellite data that document the global diabatic circulation between 2007–2011, and compare these to three reanalysis data sets and to simulations with a state-of-the-art chemistry–climate model. Using measurements of sulfur hexafluoride (SF 6 ) and nitrous oxide, we calculate the global mean diabatic overturning mass flux throughout the stratosphere. In the lower stratosphere, these two estimates agree, and at a potential temperature level of 460 K (about 20 km or 60 hPa in tropics) the global circulation strength is 6.3–7.6 × 10 9  kg s −1 . Higher in the atmosphere, only the SF 6 -based estimate is available, and it diverges from the reanalysis data and simulations. Interpretation of the SF 6 -data-based estimate is limited because of a mesospheric sink of SF 6 ; however, the reanalyses also differ substantially from each other. We conclude that the uncertainty in the mean meridional overturning circulation strength at upper levels of the stratosphere amounts to at least 100%. The strength of the global meridional overturning circulation in the stratosphere is uncertain. An analysis of satellite data, reanalyses and model simulations reveals a strength of 6.3–7.6 × 10 9  kg s −1 , but no convergence at higher altitudes.

Delineating the boundary between troposphere and stratosphere in a chemistry transport model requires a state variable for each air mass that maps out the ever shifting, overlapping three‐dimensional (3‐D) boundary at each time step. Using an artificial tracer, e90, with surface sources and 90 day decay time, the model e90 tropopause matches the 1‐D temperature lapse rate definition of the tropopause as well as the seasonal variation of ozone at this boundary. This approach works from equator to pole, over all seasons, unlike methods based on potential vorticity or ozone. By focusing on the time scales that separate stratosphere from troposphere, we examine the cause of ozone seasonality at the midlatitude tropopause, the oldest air in the troposphere (winter descent in the subtropics), and a north‐south bias in the age of air of the lowermost stratosphere as evaluated using a northern tracer. The tracer e90 is invaluable in 3‐D modeling, readily separating stratosphere from troposphere and a giving quantitative measure of the effective distance from the tropopause.

The circulation of the stratosphere, also known as the Brewer–Dobson circulation, transports water vapor and ozone, with implications for radiative forcing and climate. This circulation is typically quantified from model output by calculating the tropical upwelling vertical velocity in the residual circulation framework, and it is estimated from observations by using time series of tropical water vapor to infer a vertical velocity. Recent theory has introduced a method to calculate the strength of the global mean diabatic circulation through isentropes from satellite measurements of long-lived tracers. In this paper, we explore this global diabatic circulation as it relates to the residual circulation vertical velocity, stratospheric water vapor, and ozone at interannual timescales. We use a comprehensive climate model, three reanalysis data products, and satellite ozone data. The different metrics for the circulation have different properties, especially with regards to the vertical autocorrelation. In the model, the different residual circulation metrics agree closely and are well correlated with the global diabatic circulation, except in the lowermost stratosphere. In the reanalysis products, however, there are more differences throughout, indicating the dynamical inconsistencies of these products. The vertical velocity derived from the time series of water vapor in the tropics is significantly correlated with the global diabatic circulation, but this relationship is not as strong as that between the global diabatic circulation and the residual circulation vertical velocity. We find that the global diabatic circulation in the lower to middle stratosphere (up to 500 K) is correlated with the total column ozone in the high latitudes and in the tropics. The upper-level circulation is also correlated with the total column ozone, primarily in the subtropics, and we show that this is due to the correlation of both the circulation and the ozone with upper-level temperatures.

The Tropospheric Emission Spectrometer (TES) on Aura and Infrared Atmospheric Sounding Interferometer (IASI) on MetOp-A together provide a time series of 10 years of free-tropospheric ozone with an overlap of 3 years. We characterise the differences between TES and IASI ozone measurements and find that IASI's coarser vertical sensitivity leads to a small (< 5 ppb) low bias relative to TES for the free troposphere. The TES-IASI differences are not dependent on season or any other factor and hence the measurements from the two instruments can be merged, after correcting for the offset, in order to study decadal-scale changes in tropospheric ozone. We calculate time series of regional monthly mean ozone in the free troposphere over eastern Asia, the western United States (US), and Europe, carefully accounting for differences in spatial sampling between the instruments. We show that free-tropospheric ozone over Europe and the western US has remained relatively constant over the past decade but that, contrary to expectations, ozone over Asia in recent years does not continue the rapid rate of increase observed from 2004 to 2010.

The continued monitoring of the ozone layer and its long-term evolution leans on comparative studies of merged satellite records. Comparing such records presents unique challenges due to differences in sampling, coverage, and retrieval algorithms between observing platforms, all of which complicate the detection of trends. Here we examine the effects of broad nadir averaging kernels on vertically resolved ozone trends, using one record as an example. We find errors as large as 1 % per decade and displacements in trend profile features by as much as 6 km in altitude due to the vertical redistribution of information by averaging kernels. Furthermore, we show that averaging kernels tend to increase (by 10 %–80 %, depending on the location) the length of the record needed to determine whether trend estimates are distinguishable from natural variability with good statistical confidence. We conclude that trend uncertainties may be underestimated, in part because averaging kernels misrepresent decadal to multidecadal internal variability, and in part because the removal of known modes of variability from the observed record can yield residual errors. The study provides a framework to reconcile differences between observing platforms and highlights the need for caution when using records from instruments with broad averaging kernels to quantify trends and their uncertainties.

Quantifying changes in global and regional tropospheric ozone is critical for understanding global atmospheric chemistry and its impact on air quality and climate. Satellites now provide multi-decadal records of daily global ozone profiles, but previous studies have found large disagreements in satellite-based ozone trends, including in trends from different products based on the same spectral radiances. In light of these disagreements, it is critical to quantify to what degree the observed trend is attributable to measurement error for each product by comparing satellite-retrieved ozone to long-term measurements from ozonesondes. NASA's TRopospheric Ozone and its Precursors from Earth System Sounding (TROPESS) project provides satellite retrievals of ozone from a suite of instruments, including Cross-track Infrared Sounder (CrIS), Atmospheric Infrared Sounder (AIRS), and multispectral combinations such as AIRS and Ozone Monitoring Instrument (OMI) (joint AIRS+OMI) using a common algorithm. We compare these products to ozonesondes and find that the evolution of global tropospheric ozone satellite–sonde biases for TROPESS CrIS (0.21 ± 3.6 % decade−1, 2016–2021), AIRS (−0.41 ± 0.57 % decade−1, 2002–2022), and joint AIRS+OMI (1.1 ± 1.0 % decade−1, 2004–2022) are less than the magnitude of trends in global tropospheric ozone reported by the Tropospheric Ozone Assessment Report Phase 1 (TOAR-I). We further quantify the bias in regional trends, which tend to be higher but with a smaller number of sondes, which can impact the satellite–sonde bias and trend. Our work represents an important basis for the utility of using satellite data to quantify changes in atmospheric composition in future studies.