In situ observations of CH2Cl2 and CHCl3 show efficient transport pathways for very short-lived species into the lower stratosphere via the Asian and the North American summer monsoon

Efficient transport pathways for ozone-depleting very short-lived substances (VSLSs) from their source regions into the stratosphere are a matter of current scientific debate; however they have yet to be fully identified on an observational basis. Understanding the increasing impact of chlorine-containing VSLSs (Cl-VSLSs) on stratospheric ozone depletion is important in order to validate and improve model simulations and future predictions. We report on a transport study using airborne in situ measurements of the Cl-VSLSs dichloromethane (CH2Cl2) and trichloromethane (chloroform, CHCl3) to derive a detailed description of two transport pathways from (sub)tropical source regions into the extratropical upper troposphere and lower stratosphere (Ex-UTLS) in the Northern Hemisphere (NH) late summer. The Cl-VSLS measurements were obtained in the upper troposphere and lower stratosphere (UTLS) above western Europe and the midlatitude Atlantic Ocean in the frame of the WISE (Wave-driven ISentropic Exchange) aircraft campaign in autumn 2017 and are combined with the results from a three-dimensional simulation of a Lagrangian transport model as well as back-trajectory calculations. Compared to background measurements of similar age we find up to 150 % enhanced CH2Cl2 and up to 100 % enhanced CHCl3 mixing ratios in the extratropical lower stratosphere (Ex-LS). We link the measurements of enhanced CH2Cl2 and CHCl3 mixing ratios to emissions in the region of southern and eastern Asia. Transport from this area to the Ex-LS at potential temperatures in the range of 370–400 K takes about 6–11 weeks via the Asian summer monsoon anticyclone (ASMA). Our measurements suggest anthropogenic sources to be the cause of these strongly elevated Cl-VSLS concentrations observed at the top of the lowermost stratosphere (LMS). A faster transport pathway into the Ex-LS is derived from particularly low CH2Cl2 and CHCl3 mixing ratios in the UTLS. These low mixing ratios reflect weak emissions and a local seasonal minimum of both species in the boundary layer of Central America and the tropical Atlantic. We show that air masses uplifted by hurricanes, the North American monsoon, and general convection above Central America into the tropical tropopause layer to potential temperatures of about 360–370 K are transported isentropically within 5–9 weeks from the boundary layer into the Ex-LS. This transport pathway linked to the North American monsoon mainly impacts the middle and lower part of the LMS with particularly low CH2Cl2 and CHCl3 mixing ratios. In a case study, we specifically analyze air samples directly linked to the uplift by the Category 5 Hurricane Maria that occurred during October 2017 above the Atlantic Ocean. CH2Cl2 and CHCl3 have similar atmospheric sinks and lifetimes, but the fraction of biogenic emissions is clearly higher for CHCl3 than for the mainly anthropogenically emitted CH2Cl2; consequently lower CHCl3 : CH2Cl2 ratios are expected in air parcels showing a higher impact of anthropogenic emissions. The observed CHCl3 : CH2Cl2 ratio suggests clearly stronger anthropogenic emissions in the region of southern and eastern Asia compared to those in the region of Central America and the tropical Atlantic. Overall, the transport of strongly enhanced CH2Cl2 and CHCl3 mixing ratios from southern and eastern Asia via the ASMA is the main factor in increasing the chlorine loading from the analyzed VSLSs in the Ex-LS during the NH late summer. Thus, further increases in Asian CH2Cl2 and CHCl3 emissions, as frequently reported in recent years, will further increase the impact of Cl-VSLSs on stratospheric ozone depletion

Massive ozone production from South American wild fires observed during SOUTHTRAC

During the SOUTHTRAC mission, which took place in September and November 2019, the Germanresearch aircraft HALO performed several cross sections from the equator to the southern tip ofsouth America. The flight legs were flown along the coast of Brazil at typical altitudes of 13-14 km.During the northbound flight on October, 7th 2019 massive enhancements of pollutants wereobserved at these altitudes. Notably, in-situ observations show continuously elevated CO valuesexceeding 200 ppbv over a flight distance of more than 1000 km. These massive enhancements wereaccompanied by strongly elevated NO and NOy as well as CO2 and could be attributed to the large firesin South America during this time. These fires occurred in conjunction with convection overArgentina and Brazil, which led to efficient vertical transport. Lagrangian and chemical model analysisconfirmed the potential impact of convection and biomass burning to the observed enhancements ofozone and pollutants.Comparing the tracer observations to previous flights in exactly the same region three weeks earlier,we could estimate the ozone production due to the biomass burning. Weestimate an ozone production in the polluted air masses of almost 30%of the observed ozone mixing ratio. Given the large extent of the polluted area over 15 degrees oflatitude this may have an impact on the local energy budget of the tropopause region

Organic and inorganic bromine measurements around the extratropical tropopause and lowermost stratosphere (Ex-LMS): Insights into transport pathways and total bromine

We report on measurements of total bromine (Brtot) in the upper troposphere and lower stratosphere (UTLS) taken from the German High Altitude and LOng range research aircraft (HALO) over the North Atlantic, Norwegian Sea and north-western Europe in September/ October 2017 during the WISE (Wave-driven ISentropic Exchange) research campaign. Brtot is calculated from measured total organic bromine (Brorg) (i.e., the sum of bromine contained in CH3Br, the halons and the major very short-lived brominated substances) added to inorganic bromine (Bryinorg), evaluated from measured BrO and photochemical modelling. Combining these data, the weighted mean [Brtot] is 19.2 &#177; 1.2 ppt in the extratropical lower stratosphere (Ex-LS) of the northern hemisphere. The inferred average Brtot for the Ex-LS is slightly smaller than expected for the middle stratosphere in 2016 (~19.6 ppt (ranging from 19-20 ppt) as reported by the WMO/UNEP Assessment (2018)). However, it reflects the expected variability in Brtot in the Ex-LS due to influxes of shorter lived brominated source and product gases from different regions of entry. A closer look into Brorg and Bryinorg as well as simultaneously measured transport tracers (CO, N2O, ...) and an air mass lag-time tracer (SF6), suggests that a filament of air with elevated Brtot protruded into the extratropical lowermost stratosphere (Ex-LMS) from 350-385 K and between equivalent latitudes of 55-80&#730;N (high bromine filament &#8211; HBrF). Lagrangian transport modelling shows the multi-pathway contributions to Ex-LMS bromine. According to CLaMS air mass origin simulations, contributions to the HBrF consist of predominantly isentropic transport from the tropical troposphere (also with elevated [Brtot] = 21.6 &#177; 0.7 ppt) as well as a smaller contribution from an exchange across the extratropical tropopause which are mixed into the stratospheric background air. In contrast, the surrounding LS above and below the HBrF has less tropical tropospheric air, but instead additional stratospheric background air. Of the tropical tropospheric air in the HBrF, the majority is from the outflow of the Asian monsoon anticyclone and the adjacent tropical regions, which greatly influences concentrations of trace gases transported into the Ex-LMS in boreal summer and fall. The resulting increase of Brtot in the Ex-LMS and its consequences for ozone is investigated through the TOMCAT/SLIMCAT model simulations. However, more extensive monitoring of total stratospheric bromine in more aged air (i.e., in the middle stratosphere) as well as globally and seasonally is required in addition to model simulations to fully understand its impact on Ex-LMS ozone and the radiative forcing of climate.</p

Organic and inorganic bromine measurements around the extratropical tropopause and lowermost stratosphere: Insights into the transport pathways and total bromine

Abstract. We report on measurements of total bromine (Brtot) in the upper troposphere and lower stratosphere taken during 15 flights with the German High Altitude and LOng range research aircraft (HALO). The research campaign WISE (Wave-driven ISentropic Exchange) included regions over the North Atlantic, Norwegian Sea and north-western Europe in fall 2017. Brtot is calculated from measured total organic bromine (Brorg) added to inorganic bromine (Bryinorg), evaluated from measured BrO and photochemical modelling. Combining these data, the weighted-mean [Brtot] is 19.2 ± 1.2 ppt in the northern hemispheric lower stratosphere (LS) in agreement with expectations for Brtot in the middle stratosphere (Engel and Rigby et al. (2018)). The data reflects the expected variability in Brtot in the LS due to variable influx of shorter-lived brominated source and product gases from different regions of entry. A closer look into Brorg and Bryinorg, as well as simultaneously measured transport tracers (CO and N2O) and an air mass lag-time tracer (SF6), suggests that bromine-rich air masses persistently protruded into the lowermost stratosphere (LMS) in boreal summer, creating a high bromine region (HBrR). A subsection, HBrR*, has a weighted average of [Brtot] = 20.9 ± 0.8 ppt. The most probable source region is former air from the tropical upper troposphere and tropopause layer (UT/TTL) with a weighted mean [Brtot] = 21.6 ± 0.7 ppt. CLaMS Lagrangian transport modelling shows that the HBrR air mass consists of 51.2 % from the tropical troposphere, 27.1 % from the stratospheric background, and 6.4 % from the mid-latitude troposphere (as well as contributions from other domains). The majority of the surface air reaching the HBrR is from the Asian monsoon and its adjacent tropical regions, which greatly influences trace gas transport into the LMS in boreal summer and fall. Tropical cyclones from Central America in addition to air associated with the Asian monsoon region contribute to the elevated Brtot observed in the UT/TTL. TOMCAT global 3–D model simulations of a concurrent increase of Brtot show an associated O3 change of −2.6 ± 0.7 % in the LS and −3.1 ± 0.7 % near the tropopause. Our study of varying Brtot in the LS also emphasizes the need for more extensive monitoring of stratospheric Brtot globally and seasonally to fully understand its impact on LMS O3 and its radiative forcing of climate, as well as in aged air in the middle stratosphere to elucidate the stratospheric trend in bromine

Redistribution of total reactive nitrogen in the lowermost Arctic stratosphere in winter 2015/2016: In-situ observations of nitrification, denitrification and particulate nitrate

The Arctic winter 2015/2016 was characterized by extremely low temperatures in the stratosphere and by a very strong polar vortex, accompanied by extended fields of Polar Stratospheric Clouds. During this winter, aircraft-based measurements were carried out with the research aircraft HALO (High Altitude and Long-Range Research Aircraft) from Kiruna/Sweden and Oberpfaffenhofen/Germany.Total reactive nitrogen and its distribution between the gas and particle phases are key parameters for understanding processes controlling the ozone budget in the polar winter stratosphere. Tracer-tracer correlations were applied to study the vertical redistribution of gas-phase total reactive nitrogen. The extended observation period from December to March provided the opportunity to study the changing distribution of reactive nitrogen in the lowermost Arctic stratosphere during the course of the winter. In early winter, during December, the lowermost Arctic stratosphere did not show any indications for disturbed conditions.The situation changed during the observational period in January and February. Tracer-tracer correlations showed elevated levels of total reactive nitrogen of up to 6 nmol/mol. These observations could be interpreted by evaporation of polar stratospheric particles falling down from the stratosphere above and leading to a nitrification of the lowermost stratosphere. During some periods up to 60 % of the observed total reactive nitrogen can be attributed to evaporating particles. The observation of gas phase nitrification was accompanied by the occurrence of particulate nitrate in extended regions at altitudes between about 10 and 14 km. Usually, the occurrence of particulate nitrate is rare at such altitudes. The diameter of these particles was estimated to range between about 9 and 18 µm.During the late-winter observation period, no indications for polar stratospheric cloud particles at flight altitude were found. However, extended regions with elevated gas-phase concentrations of total reactive nitrogen were still observed. In late winter, the subsidence of air masses from the polar vortex became increasingly important for the distribution of total reactive nitrogen in the lowermost stratosphere. Air masses with substantial denitrification of up to 5 nmol/mol were observed. In these cases, up to 50 % of the undisturbed total reactive nitrogen was missing. Concurrently lower ozone concentrations were observed, indicating destruction of ozone at higher altitudes.Nitrification and denitrification of the lowermost stratosphere during the course of the winter are linked by heterogeneous processes in the above-lying stratosphere. Simulations with the CLaMS model confirm and complement the findings of the in-situ observations. They also suggest that the observations have been representative of the vortex-wide redistribution of total reactive nitrogen. The aircraft-based in-situ measurements provided a comprehensive picture of the temporal evolution of the distribution of total reactive nitrogen over the entire winter period 2015/2016

Redistribution of total reactive nitrogen in the lowermost Arctic stratosphere during the cold winter 2015/2016

Abstract. During winter 2015/2016 the Arctic stratosphere was characterized by extraordinarily low temperatures in connection with the occurrence of extensive polar stratospheric clouds. From mid of December 2015 until mid of March 2016 the German research aircraft HALO (High Altitude and Long–Range Research Aircraft) was deployed to probe the lowermost stratosphere in the Arctic region within the POLSTRACC (Polar Stratosphere in a Changing Climate) mission. More than twenty flights have been conducted out of Kiruna/Sweden and Oberpfaffenhofen/Germany, covering the whole winter period. Besides total reactive nitrogen (NOy), observations of nitrous oxide, nitric acid, ozone and water were used for this study. Total reactive nitrogen and its partitioning between gas- and particle phase are key parameters for understanding processes controlling the ozone budget in the polar winter stratosphere. The redistribution of total reactive nitrogen was evaluated by using tracer–tracer correlations. In January air masses with extensive nitrification were encountered at altitudes between 12 and 15 km. The excess NOy amounted up to about 6 ppb. During several flights, along with gas–phase nitrification, indications for extensive occurrence of nitric acid containing particles at flight altitude were found. These observations support the assumption of sedimentation and subsequent evaporation of nitric acid containing particles leading to redistribution of total reactive nitrogen. Remnants of nitrified air masses have been observed until mid of March. Between end of February and mid of March also de-nitrified air masses have been observed in connection with high potential temperatures. Using tracer–tracer correlations, missing total reactive nitrogen was estimated to amount up to 6 ppb. This indicates the downward transport of air masses that have been denitrified during the earlier winter phase. Observations within POLSTRACC, at the bottom of the vortex, reflect heterogeneous processes from the overlying Arctic winter stratosphere. The comparison of the observations with CLaMS model simulations confirm and complete the picture arising from the present measurements. The simulations confirm, that the ensemble of all observations is representative for the vortex–wide vertical NOy-redistribution

Redistribution of total reactive nitrogen in the lowermost Arctic stratosphere in winter 2015/2016: In-situ observations of nitrification, denitrification and particulate nitrate

The Arctic winter 2015/2016 was characterized by extremely low temperatures in the stratosphere and by a very strong polar vortex, accompanied by extended fields of Polar Stratospheric Clouds. During this winter, aircraft-based measurements were carried out with the research aircraft HALO (High Altitude and Long-Range Research Aircraft) from Kiruna/Sweden and Oberpfaffenhofen/Germany.Total reactive nitrogen and its distribution between the gas and particle phases are key parameters for understanding processes controlling the ozone budget in the polar winter stratosphere. Tracer-tracer correlations were applied to study the vertical redistribution of gas-phase total reactive nitrogen. The extended observation period from December to March provided the opportunity to study the changing distribution of reactive nitrogen in the lowermost Arctic stratosphere during the course of the winter. In early winter, during December, the lowermost Arctic stratosphere did not show any indications for disturbed conditions.The situation changed during the observational period in January and February. Tracer-tracer correlations showed elevated levels of total reactive nitrogen of up to 6 nmol/mol. These observations could be interpreted by evaporation of polar stratospheric particles falling down from the stratosphere above and leading to a nitrification of the lowermost stratosphere. During some periods up to 60 % of the observed total reactive nitrogen can be attributed to evaporating particles. The observation of gas phase nitrification was accompanied by the occurrence of particulate nitrate in extended regions at altitudes between about 10 and 14 km. Usually, the occurrence of particulate nitrate is rare at such altitudes. The diameter of these particles was estimated to range between about 9 and 18 µm.During the late-winter observation period, no indications for polar stratospheric cloud particles at flight altitude were found. However, extended regions with elevated gas-phase concentrations of total reactive nitrogen were still observed. In late winter, the subsidence of air masses from the polar vortex became increasingly important for the distribution of total reactive nitrogen in the lowermost stratosphere. Air masses with substantial denitrification of up to 5 nmol/mol were observed. In these cases, up to 50 % of the undisturbed total reactive nitrogen was missing. Concurrently lower ozone concentrations were observed, indicating destruction of ozone at higher altitudes.Nitrification and denitrification of the lowermost stratosphere during the course of the winter are linked by heterogeneous processes in the above-lying stratosphere. Simulations with the CLaMS model confirm and complement the findings of the in-situ observations. They also suggest that the observations have been representative of the vortex-wide redistribution of total reactive nitrogen. The aircraft-based in-situ measurements provided a comprehensive picture of the temporal evolution of the distribution of total reactive nitrogen over the entire winter period 2015/2016