Tropospheric ozone production and chemical regime analysis during the COVID-19 lockdown over Europe

The COVID-19 (Coronavirus disease 2019) European lockdowns have lead to a significant reduction in the emissions of primary pollutants such as NO (nitric oxide) and NO2 (nitrogen dioxide). As most photochemical processes are related to nitrogen oxide (NOx ≡ NO + NO2) chemistry, this event has presented an exceptional opportunity to investigate its effects on air quality and secondary pollutants, such as tropospheric ozone (O3). In this study, we present the effects of the COVID-19 lockdown on atmospheric trace gas concentrations, net ozone production rates (NOPR) and the dominant chemical regime throughout the troposphere based on three different research aircraft campaigns across Europe. These are the UTOPIHAN campaigns in 2003 and 2004, the HOOVER campaigns in 2006 and 2007 and the BLUESKY campaign in 2020, the latter performed during the COVID-19 lockdown. We present in situ observations and simulation results from the ECHAM5/MESSy Atmospheric Chemistry model which allows for scenario calculations with business as usual emissions during the BLUESKY campaign, referred to as "no-lockdown scenario". We show that the COVID-19 lockdown reduced NO and NO2 mixing ratios in the upper troposphere by around 55 % compared to the no-lockdown scenario due to reduced air traffic. O3 production and loss terms reflected this reduction with a deceleration in O3 cycling due to reduced mixing ratios of NOx while NOPRs were largely unaffected. We also study the role of methyl peroxyradicals forming HCHO (αCH3O2) to show that the COVID-19 lockdown shifted the chemistry in the upper troposphere/tropopause region to a NOx limited regime during BLUESKY. In comparison, we find a VOC limited regime to be dominant during UTOPIHAN.

Would El Niño enhance or suppress the migrating diurnal tide in the MLT region?

Previous observations and simulations are controversial as to whether El Niño will increase or decrease the diurnal tide (DW1) in the upper mesosphere and lower thermosphere (MLT) region. This study revisited the linear response of the MLT DW1 to El Niño during the winter (December-January-February) based on 19-year satellite observations of Sounding of the Atmosphere using Broadband Emission Radiometry (SABER). The MLT DW1 temperature amplitudes decreased by ~10 % during four El Niño winters from 2002 to 2020, consistent with the results from the simulation of the Specified-Dynamics version of the Whole Atmosphere Community Climate Model (SD-WACCM). According to the multiple linear regression analysis, the linear effects of El Niño-Southern Oscillation (ENSO) on tropical MLT DW1 are negative in both SABER observations and SD-WACCM simulations. In the SD-WACCM simulation, Hough mode (1, 1) dominates the DW1 tidal variation in the tropical MLT region. The consistency between the (1, 1) mode in the tropopause region and in the MLT region, as well as the downward phase progression from 15 to 100 km, indicates the direct upward propagation of DW1 from the excitation source in the troposphere. During 7 of 8 El Niño winters from 1979 to 2014, the anomalous amplitudes of the (1, 1) mode are negative in both the tropopause region and MLT region. The suppressed DW1 heating rates in the tropical troposphere (average over ~0–16 km and 35° S–35° N) during the El Niño events contribute to the decreased DW1 tide. The mesospheric latitudinal zonal wind shear anomalies during El Niño winters would lead to a narrower waveguide and prevent the vertical propagation of the DW1 tide. The gravity wave drag excited by convection also plays a role in modulating the MLT DW1 amplitude.

UAS Chromatograph for Atmospheric Trace Species (UCATS) – a versatile instrument for trace gas measurements on airborne platforms

UCATS (the UAS Chromatograph for Atmospheric Trace Species) was designed and built for observations of important atmospheric trace gases from unmanned aircraft systems (UAS) in the upper troposphere and lower stratosphere (UTLS). Initially it measured major chlorofluorocarbons (CFCs) and the stratospheric transport tracers nitrous oxide (N2O) and sulfur hexafluoride (SF6), using gas chromatography with electron capture detection. Compact commercial absorption spectrometers for ozone (O3) and water vapor (H2O) were added to enhance its capabilities on platforms with relatively small payloads. UCATS has since been reconfigured to measure methane (CH4), carbon monoxide (CO), and molecular hydrogen (H2) instead of CFCs and has undergone numerous upgrades to its subsystems. It has served as part of large payloads on stratospheric UAS missions to probe the tropical tropopause region and transport of air into the stratosphere; in piloted aircraft studies of greenhouse gases, transport, and chemistry in the troposphere; and in 2021 is scheduled to return to the study of stratospheric ozone and halogen compounds, one of its original goals. Each deployment brought different challenges, which were largely met or resolved. The design, capabilities, modifications, and some results from UCATS are shown and described here, including changes for future missions.

An aerosol classification scheme for global simulations using the K-means machine learning method

A machine learning K-means algorithm is applied to data of seven aerosol properties from a global aerosol simulation using EMAC-MADE3. The aim is to partition the aerosol properties across the global atmosphere in specific aerosol regimes. K-means is an unsupervised machine learning method with the advantage that an a priori definition of the aerosol classes is not required. Using K-means, we are able to quantitatively define global aerosol regimes, so-called aerosol clusters, and explain their internal properties as well as their location and extension. This analysis shows that aerosol regimes in the lower troposphere are strongly influenced by emissions. Key drivers of the clusters’ internal properties and spatial distribution are, for instance, pollutants from biomass burning/biogenic sources, mineral dust, anthropogenic pollution, as well as their mixing. Several continental clusters propagate into oceanic regions. The identified oceanic regimes show a higher degree of pollution in the northern hemisphere than over the southern oceans. With increasing altitude, the aerosol regimes propagate from emission-induced clusters in the lower troposphere to roughly zonally distributed regimes in the middle troposphere and in the tropopause region. Notably, three polluted clusters identified over Africa, India and eastern China, cover the whole atmospheric column from the lower troposphere to the tropopause region. A markedly wide application potential of the classification procedure is identified and further aerosol studies are proposed which could benefit from this classification.

On the occurrence of strong vertical wind shear in the tropopause region: a 10-year ERA5 northern hemispheric study

A climatology of the occurrence of strong wind shear in the upper troposphere–lower stratosphere (UTLS) is presented, which gives rise to defining a tropopause shear layer (TSL). Strong wind shear in the tropopause region is of interest because it can generate turbulence, which can lead to cross-tropopause mixing. The analysis is based on 10 years of daily northern hemispheric ECMWF ERA5 reanalysis data. The vertical extent of the region analyzed is limited to the altitudes from 1.5 km above the surface up to 25 km, to exclude the planetary boundary layer as well as strong wind shear in higher atmospheric layers like the mesosphere–lower thermosphere. A threshold value of St2=4×10-4s-2 of the squared vertical shear of the horizontal wind is applied, which marks the top end of the distribution of atmospheric wind shear to focus on situations which cannot be sustained by the mean static stability in the troposphere according to linear theory. This subset of the vertical wind shear spectrum is analyzed for its vertical, geographical, and seasonal occurrence frequency distribution. A set of metrics is defined to narrow down the relation to planetary circulation features, as well as indicators for momentum-gradient-sharpening mechanisms. The vertical distribution reveals that strong vertical wind shear above the threshold occurs almost exclusively at tropopause altitudes, within a vertically confined layer of about 1–2 km in extent directly above the local lapse rate tropopause. The TSL emerges as a distinct feature in the tropopause-based 10-year temporal and zonal mean climatology, spanning from the tropics to latitudes around 70∘ N, with average occurrence frequencies on the order of 1 %–10 %. The horizontal distribution of the strong vertical wind shear near the tropopause exhibits distinctly separated regions of occurrence, which are generally associated with jet streams and their seasonality. At midlatitudes, strong wind shear values occur most frequently in regions with an elevated tropopause and at latitudes around 50∘ N, associated with jet streaks within northward-reaching ridges of baroclinic waves. At lower latitudes in the region of the subtropical jet stream, which is mainly apparent over the east Asian continent, the occurrence frequency of strong wind shear near the tropopause reaches maximum values of about 30 % during winter and is tightly linked to the jet stream seasonality. The interannual variability of the occurrence frequency for strong wind shear might furthermore be linked to the variability of the zonal location and strength of the jet. The east-equatorial region features a bi-annual seasonality in the occurrence frequencies of strong vertical wind shear near the tropopause. During the summer months, large areas of the tropopause region over the Indian Ocean are up to 70 % of the time exposed to strong wind shear, which can be attributed to the emergence of the tropical easterly jet. During winter, this occurrence frequency maximum shifts eastward over the maritime continent, where it is exceptionally pronounced during the DJF 2010/11 La Niña phase, as well as quite weak during the El Niño phases of 2009/10, 2014/15, and 2015/16. This agrees with the atmospheric response of the Pacific Walker circulation cell in the El Niño–Southern Oscillation (ENSO) ocean–atmosphere coupling.

Characterization of Cirrus Clouds in the Mid-Latitude Tropopause Region

<p>Cirrus clouds contribute to the general radiation budget of the Earth, playing an important role in climate projections. Of special interest are optically thin cirrus clouds close to the tropopause due to the fact that they are difficult to capture and thus their impact is not yet well understood. This study presents a characterization of cirrus clouds observed by the limb sounder GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere) aboard the German research aircraft HALO during the WISE (Wave-driven ISentropic Exchange) campaign in September/October 2017. This campaign took place in Shannon, Ireland (52.70°N, 8.86°W).  We developed an optimized cloud detection method and derived macro-physical characteristics of the detected cirrus clouds: cloud top height, cloud bottom height, vertical extent and cloud top position with respect to the tropopause. The fraction of cirrus clouds detected above the tropopause (> 0 km) is in the order of 13% to 27%, depending on the detection method and the definition of the tropopause. In general, good agreement with the clouds predicted by the ERA5 reanalysis dataset is obtained. However, cloud occurrence is ≈50% higher in the observations for the region close to and above the tropopause. Cloud bottom heights are also detected above the tropopause. Considering the uncertainties for the tropopause height, cloud top height and cloud bottom height determination we could not find unambiguous evidence for the formation of cirrus layers above the tropopause. In addition, for a better understanding of the tropopause cirrus properties and life conditions, two cirrus cases observed during two scientific flights were selected from  the observations and compared with cirrus simulations performed with the 3D Lagrangian microphysical model  CLaMS-Ice, which is based on the two-moment bulk  cirrus model by Spichtinger and Gierens (2009) (doi: 10.5194/acp-9-685-2009). The model is fed by backward trajectories computed from high resolution ERA5 data (hourly, spatial grid 30 km). This contribution summarizes and extends on work described by Bartolome Garcia et al. (2020) (doi:10.5194/amt-2020-394).</p>

Volcanic aerosol heating in the tropical tropopause region and associated water vapour changes

<p>Large volcanic eruptions affect the distribution of atmospheric water vapour, for instance through cooling of the surface, warming of the lowermost stratosphere, and increasing the upwelling in the tropical tropopause region.</p><p>To better understand the volcanic impact on the tropical tropopause region and associated changes in the water vapour distribution in the stratosphere we employ a combination of short term convection-resolving global simulations with ICON and long term low resolution ensemble simulations with the MPI-ESM1.2-LR EVAens<strong>, </strong>both with prescribed volcanic forcing. With the EVAens a long term statistical analysis of the water vapour trends during the build-up and decay of a volcanic aerosol layer is made possible. The impact of the heating in the cold point regions is studied for five different eruption magnitudes. Stratospheric water vapour changes are analyzed in simulations with synthetic and observation based aerosol profiles showing that the distance of the aerosol profile from the cold point region can be more important for the water vapour entry into the stratosphere than the emitted amount of sulfur.</p><p>Whereas the EVAens is ideal to investigate the slow ascent of water vapour into the stratosphere the 10 km high resolution simulations with ICON allow insights into the convective changes after volcanic eruptions going beyond the limitations parameterizations usually impose on the model data.</p>

On the occurrence of enhanced vertical wind shear in the tropopause region: A ten year ERA5 northern hemispheric study

A climatology of the occurrence of enhanced wind shear in the UTLS is presented, which gives rise to define a tropopause shear layer (TSL). Enhanced wind shear in the tropopause region is of interest because it can generate turbulence which can lead to cross-tropopause mixing. The analysis is based on ten years of daily northern hemispheric ECMWF ERA-5 reanalysis data. The vertical extent of the region analysed is limited to the altitudes from 1.5 km above the surface up to 25 km, to exclude the planetary boundary layer as well as enhanced wind shear in higher atmospheric layers like the mesosphere/lower thermosphere. A threshold value of S2t = 4 · 10−4 s−2 is applied, which marks the top end of the spectrum of atmospheric wind shear to focus on situations which cannot be sustained by the mean static stability in the troposphere according to linear wave theory. This subset of the vertical wind shear spectrum is analysed for its vertical, geographical, and seasonal occurrence frequency distribution. A set of metrics is defined to narrow down the relation to planetary circulation features, as well as indicators for momentum gradient sharpening mechanisms. The vertical distribution reveals that large shear values occur almost exclusively at tropopause altitudes, within a vertically confined layer of about 1–2 km extent directly above the local lapse rate tropopause (LRT). The TSL emerges as a distinct feature in the tropopause-based 10 year temporal and zonal mean climatology, spanning from the tropics to latitudes around 70° N, with average occurrence frequencies of the order of 1 %–10 %. The horizontal distribution of the tropopause based enhanced vertical wind shear exhibits distinctly separated regions of occurrence, which are generally associated with jet streams and their seasonality. At midlatitudes, enhanced wind shear values occur most frequently in regions with an elevated tropopause and at latitudes around 50° N, associated with jet streaks within northward reaching ridges of baroclinic waves. At lower latitudes in the region of the subtropical jet stream, which is mainly apparent over the East Asian continent, the occurrence frequency of enhanced tropopause-based wind shear reaches maximum values of about 30 % during winter and is tightly linked to the jet stream seasonality. The interannual variability of the occurrence frequency for enhanced wind shear might furthermore be linked to the variability of the zonal location and strength of the jet. The east-equatorial region features a bi-annual seasonality in the occurrence frequencies of tropopause based enhanced vertical wind shear. During the summer months, large areas of the tropopause region over the Indian ocean are up to 70 % of the time exposed to large values of wind shear, which can be attributed to the emergence of the tropical easterly jet. During winter, this occurrence frequency maximum shifts eastward over the maritime continent, where it is exceptionally pronounced during the 2011 la Niña year, as well as quite weak during the El Niño phases of 2010 and 2015/2016. This agrees with the atmospheric response of the Pacific Walker circulation cell in the ENSO ocean-atmosphere coupling.

UAS Chromatograph for Atmospheric Trace Species (UCATS) – a versatile instrument for trace gas measurements on airborne platforms

UCATS (the UAS Chromatograph for Atmospheric Trace Species) was designed and built for observations of important atmospheric trace gases from unmanned aircraft systems (UAS) in the upper troposphere and lower stratosphere (UT/LS). Initially it measured major chlorofluorocarbons (CFCs) and the stratospheric transport tracers nitrous oxide (N2O) and sulfur hexafluoride (SF6), using gas chromatography with electron capture detection. Compact ozone (O3) and water vapor (H2O) instruments were added to enhance science missions on platforms with relatively small payloads. Over the past decade, UCATS has been reconfigured to measure methane (CH4), carbon monoxide (CO), and molecular hydrogen (H2) instead of CFCs and has undergone numerous upgrades to its subsystems. It has served as part of large payloads on stratospheric UAS missions to probe the tropical tropopause region and transport of air into the stratosphere, in piloted aircraft studies of greenhouse gases, transport, and chemistry in the troposphere, and will soon return to the study of stratospheric ozone depletion, one of the original goals for UCATS. Each deployment brought different challenges, which were largely met or resolved. The design, capabilities, modifications and some results from UCATS are shown and described here, including changes for upcoming missions.