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The Antarctic ozone “hole” was discovered in 1985, and man-made ozone- depleting substances (ODS) are its primary cause. Following reductions of ODSs under the Montreal Protocol, signs of ozone recovery have been reported, based largely on observations and broad yet compelling model-data comparisons. While such approaches are highly valuable, they don't provide rigorous statistical detection of the temporal and spatial structure of Antarctic ozone recovery in the presence of internal climate variability. Here, we apply pattern-based detection and attribution methods as employed in climate change studies to separate anthropogenically forced ozone responses from internal variability, relying on trend pattern information as a function of month and height. The analysis uses satellite observations together with single-model and multi-model ensemble simulations to identify and quantify the month-height Antarctic ozone recovery “fingerprint”. We demonstrate that the data and simulations show remarkable agreement in the fingerprint pattern of the ozone response to decreasing ODSs since 2005. We also show that ODS forcing has enhanced ozone internal variability during the austral spring, influencing detection of forced responses and their time of emergence. Our results provide robust statistical and physical evidence that actions taken under the Montreal Protocol to reduce ODSs are indeed resulting in the beginning of Antarctic ozone recovery, defined as increases in ozone consistent with expected month-height patterns. 

Abstract Volcanic eruptions and wildfires can impact stratospheric chemistry. We apply tracer‐tracer correlations to satellite data from Atmospheric Chemistry Experiment—Fourier Transform Spectrometer and the Halogen Occultation Experiment at 68 hPa to consistently compare the chemical impact on HCl after multiple wildfires and volcanic eruptions of different magnitudes. The 2020 Australian New Year (ANY) fire displayed an order of magnitude less stratospheric aerosol extinction than the 1991 Pinatubo eruption, but showed similar large changes in mid‐latitude lower stratosphere HCl. While the mid‐latitude aerosol loadings from the 2015 Calbuco and 2022 Hunga volcanic eruptions were similar to the ANY fire, little impact on HCl occurred. The 2009 Australian Black Saturday fire and 2021 smoke remaining from 2020 yield small HCl changes, at the edge of the detection method. These observed contrasts across events highlight greater reactivity for smoke versus volcanic aerosols at warm temperatures. 

Abstract Following the Hunga Tonga–Hunga Ha'apai (HTHH) eruption in January 2022, significant reductions in stratospheric hydrochloric acid (HCl) were observed in the Southern Hemisphere mid‐latitudes during the latter half of 2022, suggesting potential chlorine activation. The objective of this study is to comprehensively understand the loss of HCl in the aftermath of HTHH. Satellite measurements and a global chemistry‐climate model are employed for the analysis. We find strong agreement of 2022 anomalies between the modeled and the measured data. The observed tracer‐tracer relations between nitrous oxide (N₂O) and HCl indicate a significant role of chemical processing in the observed HCl reduction, especially during the austral winter of 2022. Further examining the roles of chlorine gas‐phase and heterogeneous chemistry, we find that heterogeneous chemistry emerges as the primary driver for the chemical loss of HCl, and the reaction between hypobromous acid (HOBr) and HCl on sulfate aerosols is the dominant loss process. 

Abstract The Hunga Tonga‐Hunga Ha'apai (Hunga) volcanic eruption in January 2022 injected a substantial amount of water vapor and a moderate amount of SO₂into the stratosphere. Both satellite observations in 2022 and subsequent chemistry‐climate model simulations forced by realistic Hunga perturbations reveal large‐scale cooling in the Southern Hemisphere (SH) tropical to subtropical stratosphere following the Hunga eruption. This study analyzes the drivers of this cooling, including the distinctive role of anomalies in water vapor, ozone, and sulfate aerosol concentration on the simulated climate response to the Hunga volcanic forcing, based on climate simulations with prescribed chemistry/aerosol. Simulated circulation and temperature anomalies based on specified‐chemistry simulations show good agreement with previous coupled‐chemistry simulations and indicate that each forcing of ozone, water vapor, and sulfate aerosol from the Hunga volcanic eruption contributed to the circulation and temperature anomalies in the SH stratosphere. Our results also suggest that (a) the large‐scale stratospheric cooling during the austral winter was mainly induced by changes in dynamical processes, not by radiative processes, and that (b) the radiative feedback from negative ozone anomalies contributed to the prolonged cold temperature anomalies in the lower stratosphere (∼70 hPa level) and hence to long lasting cold conditions of the polar vortex. 

Abstract. Pyrocumulonimbus clouds (pyroCbs) generated by intense wildfires can serve as a direct pathway for the injection of aerosols and gaseous pollutants into the lower stratosphere, resulting in significant chemical, radiative, and dynamical changes. Canada experienced an extremely severe wildfire season in 2023, with a total area burned that substantially exceeded those of previous events known to have impacted the stratosphere (such as the 2020 Australian fires). This season also had record-high pyroCb activity, which raises the question of whether the 2023 Canadian event resulted in significant stratospheric perturbations. Here, we investigate this anomalous wildfire season using retrievals from multiple satellite instruments, ACE-FTS (Atmospheric Chemistry Experiment – Fourier transform spectrometer), OMPS LP (Ozone Mapping and Profiler Suite Limb Profiler), and MLS (Microwave Limb Sounder), to determine the vertical extents of the wildfire smoke along with chemical signatures of biomass burning. These data show that smoke primarily reached the upper troposphere, and only a nominal amount managed to penetrate the tropopause. Only a few ACE-FTS occultations captured elevated abundances of biomass-burning products in the lowermost stratosphere. OMPS LP aerosol measurements also indicate that any smoke that made it past the tropopause did not last long enough or reach high enough to significantly perturb stratospheric composition. While this work focuses on Canadian wildfires given the extensive burned area, pyroCbs at other longitudes (e.g., Siberia) are also captured in the compositional analysis. These results highlight that despite the formation of many pyroCbs in major wildfires, those capable of penetrating the tropopause are extremely rare; this in turn means that even a massive area burned is not necessarily an indicator of stratospheric effects. 

The 2019 to 2020 Australian summer wildfires injected an amount of organic gases and particles into the stratosphere unprecedented in the satellite record since 2002, causing large unexpected changes in HCl and ClONO 2 . These fires provided a novel opportunity to evaluate heterogeneous reactions on organic aerosols in the context of stratospheric chlorine and ozone depletion chemistry. It has long been known that heterogeneous chlorine (Cl) activation occurs on the polar stratospheric clouds (PSCs; liquid and solid particles containing water, sulfuric acid, and in some cases nitric acid) that are found in the stratosphere, but these are only effective for ozone depletion chemistry at temperatures below about 195 K (i.e., largely in the polar regions during winter). Here, we develop an approach to quantitatively assess atmospheric evidence for these reactions using satellite data for both the polar (65 to 90°S) and the midlatitude (40 to 55°S) regions. We show that heterogeneous reactions apparently even happened at temperatures at 220 K during austral autumn on the organic aerosols present in 2020 in both regions, in contrast to earlier years. Further, increased variability in HCl was also found after the wildfires, suggesting diverse chemical properties among the 2020 aerosols. We also confirm the expectation based upon laboratory studies that heterogeneous Cl activation has a strong dependence upon water vapor partial pressure and hence atmospheric altitude, becoming much faster close to the tropopause. Our analysis improves the understanding of heterogeneous reactions that are important for stratospheric ozone chemistry under both background and wildfire conditions. 

Abstract. Polar stratospheric clouds (PSCs) play a key role in the polar chemistry of the stratosphere. Nitric acid trihydrate (NAT) particles have been shown to lead to denitrification of the lower stratosphere. While the existence of large NAT particles (NAT “rocks”) has been verified by many measurements, especially in the Northern Hemisphere (NH), most current chemistry–climate models use simplified parameterizations, often based on evaluations in the Southern Hemisphere where the polar vortex is stable enough that accounting for NAT rocks is not as important as in the NH. Here, we evaluate the probability density functions of various gaseous species in the polar vortex using one such model, the Whole Atmosphere Community Climate Model (WACCM), and compare these with measurements by the Michelson Interferometer for Passive Atmospheric Sounding onboard the Environmental Satellite (MIPAS/Envisat) and two ozonesonde stations for a range of years and in both hemispheres. Using the maximum difference between the distributions of MIPAS and WACCM as a measure of coherence, we find better agreement for HNO3 when reducing the NAT number density from the standard value of 10−2 used in this model to 5×10-4 cm−3 for almost all spring seasons during the MIPAS period in both hemispheres. The distributions of ClONO2 and O3 are not greatly affected by the NAT density. The average difference between WACCM and ozonesondes supports the need to reduce the NAT number density in the model. Therefore, this study suggests using a NAT number density of 5×10-4 cm−3 for future simulations with WACCM. 

In 1967, scientists used a simple climate model to predict that human-caused increases in atmospheric CO 2 should warm Earth’s troposphere and cool the stratosphere. This important signature of anthropogenic climate change has been documented in weather balloon and satellite temperature measurements extending from near-surface to the lower stratosphere. Stratospheric cooling has also been confirmed in the mid to upper stratosphere, a layer extending from roughly 25 to 50 km above the Earth’s surface (S 25 − 50 ). To date, however, S 25 − 50 temperatures have not been used in pattern-based attribution studies of anthropogenic climate change. Here, we perform such a “fingerprint” study with satellite-derived patterns of temperature change that extend from the lower troposphere to the upper stratosphere. Including S 25 − 50 information increases signal-to-noise ratios by a factor of five, markedly enhancing fingerprint detectability. Key features of this global-scale human fingerprint include stratospheric cooling and tropospheric warming at all latitudes, with stratospheric cooling amplifying with height. In contrast, the dominant modes of internal variability in S 25 − 50 have smaller-scale temperature changes and lack uniform sign. These pronounced spatial differences between S 25 − 50 signal and noise patterns are accompanied by large cooling of S 25 − 50 (1 to 2 ° C over 1986 to 2022) and low S 25 − 50 noise levels. Our results explain why extending “vertical fingerprinting” to the mid to upper stratosphere yields incontrovertible evidence of human effects on the thermal structure of Earth’s atmosphere.