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Ozone is a potent air pollutant in the lower troposphere and an important short-lived climate forcer (SLCF) in the upper troposphere. Studies investigating long-term trends in tropospheric column ozone (TCO3) have shown large-scale spatiotemporal inconsistencies. Here, we investigate the long-term trends in lower tropospheric column ozone (LTCO3, surface-450 hPa sub-column) by exploiting a synergy of satellite and ozonesonde datasets and an Earth System Model (UKESM) over North America, Europe and East Asia for the decade 2008-2017. Overall, we typically find small LTCO3 linear trends with large uncertainty ranges from the Ozone Monitoring Instrument (OMI) and the Infrared Atmospheric Sounding Interferometer (IASI), while model simulations indicate a stable LTCO3 tendency. Trends in the satellite a priori datasets show negligible trends indicating year-to-year sampling is not an issue. The application of the satellite averaging kernels AKs) to the UKESM ozone profiles, accounting for the satellite vertical sensitivity and allowing for like-for41 like comparisons, has a limited impact on the modelled LTCO3 tendency in most cases. While, in relative terms, this is more substantial (e.g. in the order of 100%), the absolute magnitudes of the model trends show negligible change. However, as the model has a near-zero tendency, artificial trends were imposed on the model time-series (i.e. LTCO3 values rearranged from smallest to largest) to test the influence of the AKs but simulated LTCO3 trends remained small. Therefore, the LTCO3 tendency between 2008 and 2017 in northern hemispheric regions are likely small, with large uncertainties, and it is difficult to detect any small underlying linear trends due to inter-annual variability or other factors which require further investigation. [ABSTRACT FROM AUTHOR]

Using state-of-the-art satellite ozone profile products, and chemical transport model, we provide an updated estimate of the tropospheric ozone radiative effect (TO3RE) and observational constraint on its variability over the decade 2008–2017. Previous studies have shown the short-term (i.e. a few years) globally weighted average TO3RE to be 1.17±0.03 W/m², while our analysis suggests that the long-term (2008–2017) average TO3RE to be 1.21–1.28 W/m². Over this decade, the modelled/observational TO3RE linear trends show negligible change (i.e. ±0.1 %/year), so the tropospheric ozone radiative contribution to climate has remained stable with time. Two model sensitivity experiments fixing emissions and meteorology to one year (i.e. start year – 2008) show that ozone precursor emissions (meteorological factors) have had limited (substantial) impacts on the long-term tendency of globally weighted average TO3RE. Here, the meteorological variability in the tropical/sub-tropical upper troposphere is dampening any tendency in TO3RE from other factors (e.g. emissions, atmospheric chemistry). [ABSTRACT FROM AUTHOR]

Ozone is a potent air pollutant in the lower troposphere and an important short-lived climate forcer (SLCF) the upper troposphere. Studies using satellite data to investigate spatiotemporal variability of troposphere ozone (TO3) have predominantly focussed on the tropospheric column metric. This is the first study to investigate long-term spatiotemporal variability in lower tropospheric column ozone (LTCO3, surface-450 hPa sub-column) by merging multiple European Space Agency - Climate Change Initiative (ESA-CCI) products produced by the Rutherford Appleton Laboratory (RAL) Space. We find that in the LTCO3, the degrees of freedom of signal (DOFS) from these products varies with latitude range and season and is up to 0.65, indicating that the retrievals contain useful information on lower TO3. The spatial and seasonal variation of the RAL Space products are in good agreement with each other but there are systematic offsets of up to 3.0-5.0 DU between them. Comparison with ozonesondes shows that the Global Ozone Monitoring Experiment (GOME-1, 1996-2003), the SCanning Imaging Absorption spectroMeter for Atmospheric CartograpHY (SCIAMACHY, 2003-2010) and the Ozone Monitoring Instrument (OMI, 2005-2017) have stable LTCO3 records over their respective periods, which can be merged together. While GOME-2 (2008-2018) shows substantial drift in its bias with respect to ozonesondes. We have therefore constructed a robust merged dataset of LTCO3 from GOME-1, SCIAMACHY and OMI between 1996 and 2017. Comparing the LTCO3 differences between the 1996-2000 and 2013-2017 5-year averages, we find significant positive increases (3.0-5.0 DU) in the tropics/sub-tropics, while in the northern mid-latitudes, we find small scale differences in LTCO3. Therefore, we conclude that there has been a substantial increase in tropical/tropical LTCO3 during the satellite-era. [ABSTRACT FROM AUTHOR]

We present a novel approach to derive indirect global information on the hydroxyl radical (OH), one of the most important atmospheric oxidants, using state-of-art satellite trace gas observations (key sinks and sources of OH) and a steady-state approximation (SSA). This is a timely study as OH observations are predominantly from spatially sparse field and infrequent aircraft campaigns, so there is a requirement for further approaches to infer spatial and temporal information on OH and its interactions with important climate (e.g. methane, CH4) and air quality (e.g. nitrogen dioxide, NO2) trace gases. Due to the short lifetime of OH (~1.0 s), SSAs of varying complexities can be used to model its concentration and offer a tool to examine the OH budget in different regions of the atmosphere. Here, we use the well-evaluated TOMCAT three-dimensional chemistry transport model to identify atmospheric regions where different complexities of the SSAs are representative of OH. In the case of a simplified SSA (S-SSA), where we have observations of ozone (O3), carbon moNO[sub x]ide (CO), CH4and water vapour (H2O) from the Infrared Atmospheric Sounding Interferometer (IASI) on-board ESA's MetOp-A satellite, it is most representative of OH between 600 and 700 hPa (though suitable between 400-800 hPa) within ~20 % of TOMCAT modelled OH. The same S-SSA is applied to aircraft measurements from the Atmospheric Tomography Mission (ATom) and compares well with the observed OH concentrations within ~30 % yielding a correlation of 0.78. We apply the S-SSA to IASI data spanning 2008-2017 to explore the global long-term inter-annual variability of OH. Relative to the 10-year mean, we find that global annual mean OH anomalies ranged from -3.1 % to +4.4 %, with the largest spread in the tropics between -7.0 % and +7.7 %. Investigation of the individual terms in the S-SSA over this time period suggests that O3and CO were the key drivers of variability in the production and loss of OH. For example, large enhancement in the OH sink during the positive 2015/2016 ENSO event was due to large scale CO emissions from drought induced wildfires in South East Asia). The methodology described here could be further developed as a constraint on the tropospheric OH distribution as further satellite data becomes available in the future. [ABSTRACT FROM AUTHOR]

Mainland and maritime Southeast Asia are home to more than 655 million people, representing nearly 10 % of the global population. The dry season in this region is typically associated with intense biomass burning activity, which leads to a significant increase in surface air pollutants that are harmful to human health, including ozone (O3). Latitude-based differences in dry season and land use distinguish two regional biomass burning regimes: (1) burning on the peninsular mainland peaking in March and (2) burning across Indonesia peaking in September. The type and amount of material burned in each regime impacts the emissions of nitrogen oxides (NOx = NO + NO2) and volatile organic compounds (VOCs), which combine to produce ozone. Here, we use the nested GEOS-Chem atmospheric chemistry transport model (horizontal resolution of 0.25° x 0.3125°), in combination with satellite observations from the Ozone Monitoring Instrument (OMI) and ground-based observations from Malaysia, to investigate ozone photochemistry over Southeast Asia in 2014. Seasonal cycles of tropospheric ozone columns from OMI and GEOS-Chem peak with biomass burning emissions. Compared to OMI, the model has a mean annual bias of -11 % but tends to overestimate tropospheric ozone near areas of seasonal fire activity. We find that outside of these burning areas, the underlying photochemical environment is generally NOx-limited, dominated by anthropogenic NOx and biogenic non-methane VOC emissions. Pyrogenic emissions of NOx play a key role in photochemistry, shifting towards more VOC-limited ozone production and contributing about 30 % of the regional ozone formation potential during both biomass burning seasons. Using the GEOS-Chem model, we find that biomass burning activity coincides with widespread ozone exposure at levels that exceed world public health guidelines, resulting in 272 premature deaths on mainland Southeast Asia in March of 2014 and another 273 deaths across Indonesia in September. Despite a positive model bias, hazardous ozone levels are confirmed by surface observations during both burning seasons. [ABSTRACT FROM AUTHOR]

We have used satellite observations and a simulation from the TOMCAT chemistry transport model (CTM) to investigate the influence of the well-known winter-time North Atlantic Oscillation (NAO) on European tropospheric composition. Under the positive phase of the NAO (NAO+), strong westerlies tend to enhance transport of European pollution (e.g. nitrogen oxides, NOx, carbon monoxide, CO) away from anthropogenic source regions. In contrast, during the negative phase of the NAO (NAO-), more stable meteorological conditions lead to a build up of pollutants over these regions, relative to the winter-time average pollution levels. However, the secondary pollutant ozone shows the opposite signal of larger values during NAO+. NAO+ introduces Atlantic ozone-enriched air into Europe while under NAO- westerly transport of ozone is reduced yielding lower values over Europe. Furthermore, ozone concentrations are also decreased by chemical loss through the reaction with accumulated primary pollutants such as nitric oxide (NO) in NAO-. Peroxyacetyl nitrate (PAN), in the upper troposphere-lower stratosphere (UTLS) peaks over Iceland/Southern Greenland in NAO-, between 200-100 hPa, consistent with the trapping by an anticyclone at this altitude. Model simulations show enhanced PAN over Iceland/Southern Greenland in NAO- is associated with vertical transport of polluted air from the troposphere into the UTLS. Overall, this work shows that NAO circulation patterns are an important governing factor for European winter-time composition and air pollution. [ABSTRACT FROM AUTHOR]