title: Airborne in situ measurements of short-lived chlorocarbons and investigation of their pathways from northern hemispheric source regions into the lowermost stratosphere
Creators: name: Lauther, Valentin
Publisher: name: Bergische Universität Wuppertal
Year: 2021
Subjects: Gas Chromatography
Mass Spectrometry
airborne measurements
Asian Monsoon
UTLS
Cl-VSLS
CH2Cl2
CHCl3
HALO
Adsorbens

description: Chlorine containing very short-lived species (Cl-VSLS) are predicted to threaten the timely recovery of the stratospheric ozone layer to pre-1980 values. In addition, the contribution of Cl-VSLS to the reduction of ozone in the upper troposphere and lower stratosphere (UTLS) has an impact on surface climate. To the present day, there is no direct observational evidence for a fast transport pathway of Cl-VSLS from the source regions into the UTLS because in situ measurements of Cl-VSLS in the UTLS are complex and thus extremely rare. The aim of the present work is to identify and describe such fast transport pathways on the basis of airborne in situ observations in the UTLS above western Europe and the Atlantic Ocean. In order to investigate transport processes and the distribution of Cl-VSLS in the UTLS, the airborne ﬁve channel trace gas analyzer HAGAR-V (High Altitude Gas Analyzer - 5 channel version) was modiﬁed. For the two channel GC/MS module (two gas chromatographs (GC) coupled to one quadrupole mass spectrometer (MS)) of HAGAR-V a new sample preconcentration unit as well as a novel type of fast ramping temperature controlled GC column oven was developed and characterized. On its ﬁrst successful deployment on the research aircraft HALO (High Altitude and LOng range) during the WISE (Wave-driven ISentropic Exchange) campaign in autumn 2017, the GC/MS module measured the two major Cl-VSLS (CH<sub>2</sub>Cl<sub>2</sub> and CHCl<sub>3</sub>) and ﬁve long-lived species (CH<sub>3</sub>Cl, CFC-11, CFC-113, HFC-125, and HFC-134a) at a time resolution of 3 minutes and average precisions of about 1–4 % of tropospheric background depending on species. This was achieved by using only one of the two GC/MS channels. The novel data set is analyzed for optimal processing routines to optimize the accuracy and the precision. The use of two diﬀerent in-ﬂight calibration gases provides several options for the calculation of mixing ratios. In general, a quadratic relationship between mixing ratios and MS detector response yields the most accurate results. Several in-ﬂight diagnostic modes are used to derive speciﬁc data corrections and processing methods depending on ﬂight and species. A comparison between CFC measurements with HAGAR-V’s GC/MS and ECD (Electron Capture Detector) modules shows a generally good agreement with only insigniﬁcant diﬀerences between the two modules. WISE measurements with the MS module in the lowermost stratosphere (LMS) reveal distinct entries of CH<sub>2</sub>Cl<sub>2</sub>-rich and CH<sub>2</sub>Cl<sub>2</sub>-poor air when correlated with the long-lived tracer N<sub>2</sub>O. The CH<sub>2</sub>Cl<sub>2</sub>-rich air exhibits mixing ratios up to 130 % higher than CH<sub>2</sub>Cl<sub>2</sub>-poor air. These measurements are analyzed with the help of artiﬁcial tracers of air mass origin and backward trajectories calculated with the Chemical Lagrangian Model of the Stratosphere (CLaMS). In addition, the measurements are qualitatively compared to ground-based observations of the Advanced Global Atmospheric Gases Experiment (AGAGE) network as well as to low-level aircraft measurements from East Asia. It is concluded that the WISE measurements of CH<sub>2</sub>Cl<sub>2</sub>-poor air in the northern hemisphere (NH) UTLS reﬂect the tropical CH<sub>2</sub>Cl<sub>2</sub> surface background seasonality from along the western part of the Inter-Tropical Convergence Zone (ITCZ). Fast convection above the region of Central America (maximum averaged diabatic ascent rate 24 K/18 h) uplifts CH<sub>2</sub>Cl<sub>2</sub>-poor air from the boundary layer (BL) to the tropical tropopause layer (TTL) from where it isentropically enters the extratropical UTLS on a relatively direct path. Transport times along this pathway range between two weeks and two months and are short enough for the Cl-VSLS to enter the UTLS with barely reduced mixing ratios compared to the BL. This transport pathway primarily contributes to CH<sub>2</sub>Cl<sub>2</sub>poor air in the NH LMS (mainly between ~ 345 K and ~ 375 K of potential temperature). In addition, the major hurricane Maria could be identiﬁed to have signiﬁcantly contributed to the transport of CH<sub>2</sub>Cl<sub>2</sub>-poor air into the UTLS along this pathway. This study presents for the ﬁrst time an observation-based transport analysis for CH<sub>2</sub>Cl<sub>2</sub>, connecting the BL along the western ITCZ to the NH LMS via convection above Central America. Measurements of CH<sub>2</sub>Cl<sub>2</sub>-rich air in the LMS are identiﬁed to almost exclusively originate in southern and eastern Asia. It is shown that these air masses were transported via convective updraft of the Asian summer monsoon (ASM). Key part of this transport pathway into the NH LMS is the additional slow upwelling within the ASM anticyclone (ASMA) to potential temperature levels of 380 K or more. The transport of CH<sub>2</sub>Cl<sub>2</sub>-rich air continues eastward along the subtropical jet stream and enters the extratropical LMS above the Paciﬁc or the Atlantic Ocean. Compared to the transport of CH<sub>2</sub>Cl<sub>2</sub>-poor air from the western ITCZ into the NH LMS, transport via the ASMA is relatively slow (on average two months). However, it is fast enough to cause tropospheric intrusions of particularly high CH<sub>2</sub>Cl<sub>2</sub> mixing ratios into the NH LMS. In general, out of young (&lt; 6 months) air masses those originating from southern and eastern Asia dominate the larger part of the NH LMS in summer with air parcel fractions ≥ 50 %, thereby strongly impacting the trace gas distribution in the LMS with CH<sub>2</sub>Cl<sub>2</sub>-rich air. The present work provides the ﬁrst direct evidence for the strong impact of CH<sub>2</sub>Cl<sub>2</sub>-rich air from Asian sources on the NH LMS on the basis of airborne in situ observations. CHCl<sub>3</sub> shows similar but less pronounced features as CH<sub>2</sub>Cl<sub>2</sub> when correlated with N<sub>2</sub>O. It is very likely that relatively CHCl<sub>3</sub>-rich air in the LMS originated from similar (anthropogenic) sources in southern and eastern Asia as the CH<sub>2</sub>Cl<sub>2</sub>-rich air. In contrast to CH<sub>2</sub>Cl<sub>2</sub>, CHCl<sub>3</sub> mixing ratios in the UT and the tropopause region can be equally high as the relatively CHCl<sub>3</sub>-rich air from southern and eastern Asia in the LMS. Other sources thus likely contribute with relatively CHCl<sub>3</sub>-rich air to the CHCl<sub>3</sub> distribution in the LMS. However, at least in the LMS, samples of CH<sub>2</sub>Cl<sub>2</sub>-poor air from the western ITCZ also contain CHCl<sub>3</sub>-poor air. It is concluded that the extraordinarily high Asian CH<sub>2</sub>Cl<sub>2</sub> and CHCl<sub>3</sub> emissions strongly enhance CH<sub>2</sub>Cl<sub>2</sub> mixing ratios in the NH LMS but enhance those of CHCl<sub>3</sub> to a lesser degree (enhancement &gt; 100 % and &gt; 50 %, respectively).