Striking repercussions of the Australian "Black Summer" bushfires on the stratospheric composition and dynamical circulation

<p>Wildfire-driven pyro-convection (PyroCb) is capable of lofting combustion products into the stratosphere, polluting it with smoke aerosols at hemispheric and yearly scales. This realization has emerged after the record-breaking British Columbia PyroCb event in August 2017 that approached moderate volcanic eruptions in terms of stratospheric aerosol load perturbation. The Australian “Black Summer” bushfires in 2019/20 have surpassed the previous record by a factor of 3 and rivaled the strongest volcanic eruptions in the XXI century. Here we exploit a synergy of various satellite observations, ECMWF meteorological analysis and radiative transfer modeling to quantify the perturbation of stratospheric particulate and gaseous composition, dynamical circulation and radiative balance caused by the Australian New Year’s PyroCb outbreak. One of the most striking repercussions of this event was the generation of several persistent anticyclonic vortices that provided confinement to the PyroCb plumes and preserved them from rapid dilution in the environment. The most intense vortex measured 1000 km in diameter, persisted in the stratosphere for over 13 weeks and lifted a confined bubble of combustion gases, aerosols and moisture to 35 km altitude. It was accompanied by a synoptic-scale ozone hole with the total column reduction by about 30%. The startling consequences of the Australian event provide new insights into climate-altering potential of the wildfires, that have increased in frequency and strength over the recent years.</p>

Organic pollutants from Indonesian peatland fires: regional influences and its impact on lower the stratospheric composition

<p>In 2015, the particularly strong dry season in Indonesia, caused by an exceptional strong El Niño, led to severe peatland fires. Due to the high carbon content of peatland, these fires are characterised by high volatile organic compound (VOC) biomass burning emissions. The resulting primary and secondary pollutants are efficiently transported to the upper troposphere/lower stratosphere (UTLS) by the developing Asian monsoon anticyclone (ASMA) and the general upward transport in the intertropical convergence zone (ITCZ). In this study, we assess the importance of these VOC emissions for the composition of the lower troposphere and the UTLS by performing multiple chemistry simulations using the global atmospheric model ECHAM/MESSy (EMAC). In a first step, we find that EMAC properly captures the exceptional strength of the Indonesian fires based on the comparison of modelled columns of the biomass burning marker hydrogen cyanide (HCN) to spaceborne measurements from the Infrared Atmospheric Sounding Interferometer (IASI). In the lower troposphere, the increase in VOC levels is higher in Indonesia compared to other biomass burning regions. This directly impacts the oxidation capacity and leads to a high reduction in hydroxyl radicals (OH) and nitrogen oxides (NO<sub>x</sub>). In general, an increase in ozone (O<sub>3</sub>) is predicted close to the peatland fires. However, particular high concentrations of phenols lead to an O<sub>3</sub> depletion in eastern Indonesia. By employing the detailed in-cloud OVOC oxidation scheme Jülich Aqueous-phase Mechanism of Organic Chemistry (JAMOC), we find that the predicted changes are dampened and that by ignoring these processes, global models tend to overestimate the impact of such extreme pollution events. The upward transport in the ASMA and the ITCZ leads to elevated VOC concentrations in the UTLS region. This also results in a depletion of lower stratospheric O<sub>3</sub>. We find that this is caused by a high destruction of O<sub>3</sub> by phenoxy radicals and by the increased formation of NO<sub>x</sub> reservoir species, which dampen the chemical production of O<sub>3</sub>.</p>

Simulated disruptions of the Quasi-Biennial Oscillation

<p>The Quasi-Biennial Oscillation has exhibited remarkable stability over the observational record—until a well-documented 2015/16 disruption and an emerging disruption in 2020/21. The possibility that disruptions are more frequent in a changing climate is important to consider, as the QBO affects predictability, stratospheric composition, and surface weather. However, this possibility is challenging to assess for a variety of reasons. For instance, the 2015/16 disruption has been attributed to anomalous easterly momentum flux from extratropical waves. By comparison, the 2020/21 disruption involves anomalous westerly forcing, less likely to originate from the same mechanism.</p><p>We present a rich variety of QBO disruptions that spontaneously arise in integrations of the high-top NASA GISS Model E2.2. The disruptions loosely fall into several categories, some of which are analogous to the 2015/16 disruption and the 2020 disruption, as well as a previously undocumented possible disruption in 1988. Several factors appear to influence QBO disruptions in the model: natural variability, climate change, tropical SSTs, volcanic eruptions, and model physics/tuning. Although QBO representation is an ongoing challenge for models, the results point to a model-independent framework for assessment of disruptions.</p><p> </p>

Overview and update of the SPARC Data Initiative: Comparison of stratospheric composition measurements from satellite limb sounders

The SPARC Data Initiative (SPARC, 2017) performed the first comprehensive assessment of currently available stratospheric composition measurements obtained from an international suite of space-based limb sounders. The initiative's main objectives were (1) to assess the state of data availability, (2) to compile vertically resolved, monthly zonal mean trace gas and aerosol climatologies, and (3) to perform a detailed inter-comparison of these climatologies, summarising useful information and highlighting differences among datasets. The vertically-resolved climatologies of 26 different atmospheric constituents extending over the region from the upper troposphere to the lower mesosphere (300–0.1 hPa) are provided on a common latitude-pressure grid and include most major long-lived trace gases (O3, H2O, N2O, CH4, CCl3F, and CCl2F2), transport tracers (HF, SF6, HCl, CO, HNO3, NOy), and shorter-lived trace gases important to stratospheric chemistry including nitrogens (NO, NO2, NOx, N

The 2019/20 Australian wildfires generated a persistent smoke-charged vortex rising up to 35 km altitude

The Australian bushfires around the turn of the year 2020 generated an unprecedented perturbation of stratospheric composition, dynamical circulation and radiative balance. Here we show from satellite observations that the resulting planetary-scale blocking of solar radiation by the smoke is larger than any previously documented wildfires and of the same order as the radiative forcing produced by moderate volcanic eruptions. A striking effect of the solar heating of an intense smoke patch was the generation of a self-maintained anticyclonic vortex measuring 1000 km in diameter and featuring its own ozone hole. The highly stable vortex persisted in the stratosphere for over 13 weeks, travelled 66,000 km and lifted a confined bubble of smoke and moisture to 35 km altitude. Its evolution was tracked by several satellite-based sensors and was successfully resolved by the European Centre for Medium-Range Weather Forecasts operational system, primarily based on satellite data. Because wildfires are expected to increase in frequency and strength in a changing climate, we suggest that extraordinary events of this type may contribute significantly to the global stratospheric composition in the coming decades.

A nitric acid dataset from IASI for polar stratospheric denitrification studies

In this paper, we exploit the first 10-year data-record (2008–2017) of nitric acid (HNO3) total columns measured by the IASI-A/Metop infrared sounder, characterized by an exceptional daily sampling and a good vertical sensitivity in the mid-stratosphere (around 50 hPa), to monitor the causal relationship between the temperature decrease and the observed HNO3 loss that occurs each year in the Antarctic stratosphere during the polar night. Since the HNdepletion results from the formation of polar stratospheric clouds (PSCs) which trigger the development of the ozone (O3) hole, its continuous monitoring is of high importance. We verify here, from the 10-year time evolution of the pair HNO3-temperature (taken from reanalysis at 50 hPa), the recurrence of specific regimes in the cycle of IASI HNO3 and identify, for each year, the day and the 50 hPa-temperature (drop temperature) corresponding to the onset of denitrification in Antarctic winter. Although the measured HNO3 total column does not allow differentiating the uptake of HNO3 by different types of PSC particles along the vertical profile, an average drop temperature of ∼ 191 ± 3 K, consistent with the nitric acid trihydrate (NAT) formation temperature (close to 195 K at 50 hPa), is found. The spatial distribution and inter-annual variability of the drop temperature are briefly investigated and discussed in the context of previous PSCs studies. This paper highlights the capability of the IASI sounder to monitor the long-term evolution of the polar stratospheric composition and processes involved in the depletion of stratospheric O3.

Simplified SAGE II ozone data usage rules

High-quality satellite-based measurements are crucial to the assessment of global stratospheric composition change. The Stratospheric Aerosol and Gas Experiment II (SAGE II) provides the longest, continuous data set of vertically resolved ozone and aerosol extinction coefficients to date and therefore remains a cornerstone of understanding and detecting long-term ozone variability and trends in the stratosphere. Despite its stability, SAGE II measurements must be screened for outliers that are a result of excessive aerosol emitted into the atmosphere and that degrade inferences of change. Current methods for SAGE II ozone measurement quality assurance consist of multiple ad hoc and sometimes conflicting rules, leading to too much valuable data being removed or outliers being missed. In this work, the SAGE II ozone data set version 7.00 is used to develop and present a new set of screening recommendations and to compare the output to the screening recommendations currently used. Applying current recommendations to SAGE II ozone leads to unexpected features, such as removing ozone values around zero if the relative error is used as a screening criterion, leading to biases in monthly mean zonal mean ozone concentrations. Most of these current recommendations were developed based on “visual inspection”, leading to inconsistent rules that might not be applicable at every altitude and latitude. Here, a set of new screening recommendations is presented that take into account the knowledge of how the measurements were made. The number of screening recommendations is reduced to three, which mainly remove ozone values that are affected by high aerosol loading and are therefore not reliable measurements. More data remain when applying these new recommendations compared to the rules that are currently being used, leading to more data being available for scientific studies. The SAGE II ozone data set used here is publicly available at https://doi.org/10.5281/zenodo.3710518 (Kremser et al., 2020). The complete SAGE II version 7.00 data set, which includes other variables in addition to ozone, is available at https://eosweb.larc.nasa.gov/project/sage2/sage2_v7_table (last access: December 2019), https://doi.org/10.5067/ERBS/SAGEII/SOLAR_BINARY_L2-V7.0 (SAGE II Science Team, 2012; Damadeo et al., 2013).

Evaluation of calibrations for limb scattering sensors

<p>The Ozone Mapping and Profiler Suite represents a new generation of the US ozone measuring instruments aimed to monitor the ozone recovery associated to the reduction in levels of man-made ozone depleting substances regulated by the Montreal protocol. The first OMPS was launched on board of the Suomi NPP satellite in October 2011. The Limb Profiler is a part of the OMPS instrumental suite, and it collects solar radiances scattered from the atmospheric limb in the UV and VIS spectral ranges. The next OMPS Limb Profiler is scheduled to launch in 2022 on board of NASA/NOAA JPSS-2 mission. These limb scattering measurements allow to retrieve vertical ozone profiles from the tropopause up to the mesosphere with a high vertical resolution (~2 km). The expected ozone recovery is almost three times slower than the ozone loss observed in 1980s and 1990s. To detect such small trends in ozone concentration, the instrument calibrations should be extremely accurate. Comparisons of ozone retrievals from OMPS LP with the correlative satellite measurements from Aura MLS and ISS SAGE III revealed that OMPS LP retrievals accurately characterize the vertical ozone distribution in different atmospheric regions which are most sensitive to changes in the stratospheric composition and dynamics. Between 18 and 42 km the mean differences between LP and correlative measurements are within ±10%, except for the northern high latitudes where between 20 and 32 km biases exceed 10% due to the measurement errors. We also found a small positive drift of ~0.5%/yr against MLS with a pattern that is consistent with the ~150-meter drift (over 7 years) in sensor pointing detected by one of our altitude resolving methods. The spatial patterns in the ozone biases and drifts suggest that remaining errors in the LP ozone retrievals are due to errors in altitude registration and instrument calibrations. We present a study where we evaluate calibrations of the OMPS LP by converting ozone differences between OMPS LP and Aura MLS into differences in radiances. Then these radiance differences are compared with the LP measured radiances to determine errors in OMPS LP calibrations. Since the OMPS LP has three slits, some of the errors, like a drift in the altitude registration, should be common across all three slits, but other errors will be unique for each slit, helping to isolate different sources of errors. This approach can be extended to earlier ESA’s limb scattering missions, like SCIAMACHY and OSIRIS, since MLS has long overlap with the ENVISAT and Odin missions.</p>