Comment on acp-2021-837

Airborne in situ measurements of short-lived chlorocarbons and investigation of their pathways from northern hemispheric source regions into the lowermost stratosphere

This study identifies the fast (i.e., days–weeks) transport pathways that connect the Northern Hemisphere surface to the upper troposphere and lower stratosphere (UTLS) during northern summer by integrating a large (90 member) ensemble of Boundary Impulse Response tracers in the Whole Atmosphere Community Climate Model version 5. We show that there is a fast transport pathway that occurs over the southern slope of the Tibetan Plateau, northern India, the Arabian Sea, and Saudi Arabia; furthermore, we show that during July this pathway connects the Northern Hemisphere surface to the UTLS on a modal time scale of 5–10 days. A less efficient transport pathway is also identified over the western Pacific. A detailed budget analysis reveals that, while convective processes are responsible for transport to 200–300 hPa, the resolved dynamics, specifically the vertical eddy flux, dominate at 100–150 hPa. Transport variations are analyzed on weekly, monthly, and interannual time scales and are largely related to differences in the resolved dynamics in the UTLS.

<p>Air mass transport within the summertime Asian monsoon circulation provides a major source of anthropogenic pollution for the upper troposphere and lower stratosphere (UTLS). In our study, we investigate the quasi-horizontal transport of airmasses from the Asian summer monsoon anticyclone (ASMA) into the extratropical lower stratosphere and their chemical evolution. For that reason, we developed a method to identify and track the air masses exported from the monsoon. This method is based on the anomalously low potential vorticity (PV) of these air masses (tropospheric low–PV cutoffs) compared to the lower-stratosphere, and uses trajectory calculations and chemical fields from the Chemical Lagrangian Model of the Stratosphere (CLaMS). The results show evidence for frequent summertime transport from the monsoon anticyclone to mid-latitudes over the North Pacific, even reaching high latitude regions of Siberia and Alaska. Particularly, the most promising region and time for measurements of transported anticyclonic air masses that cross the tropopause, is the North Pacific from July to August. Most of the low–PV cutoffs related to air masses exported from the ASMA have lifetimes shorter than one week (about 90%) and sizes smaller than 1 percent of the northern hemisphere (NH) area. The chemical composition of these air masses is characterised by carbon monoxide, ozone and water vapour mixing ratios at an intermediate range between values typical for the monsoon anticyclone and the lower-stratosphere. The chemical evolution during transport within these low–PV cutoffs shows a gradual change from characteristics of the monsoon anticyclone to characteristics of the lower stratospheric background during about one week, indicating continuous mixing with the background atmosphere.</p>

Air mass transport within the summertime Asian monsoon circulation provides a major source of anthropogenic pollution for the upper troposphere and lower stratosphere (UTLS). Here, we investigate the quasi-horizontal transport of air masses from the Asian summer monsoon anticyclone (ASMA) into the extratropical lower stratosphere and their chemical evolution. For that reason, we developed a method to identify and track the air masses exported from the monsoon. This method is based on the anomalously low potential vorticity (PV) of these air masses (tropospheric low–PV cutoffs) compared to the lower-stratosphere, and uses trajectory calculations and chemical fields from the Chemical Lagrangian Model of the Stratosphere (CLaMS). The results show evidence for frequent summertime transport from the monsoon anticyclone to mid-latitudes over the North Pacific, even reaching high latitude regions of Siberia and Alaska. Most of the low–PV cutoffs related to air masses exported from the ASMA have lifetimes shorter than one week (about 90 %) and sizes smaller than 1 percent of the northern hemisphere (NH) area. The chemical composition of these air masses is characterised by carbon monoxide, ozone and water vapour mixing ratios at an intermediate range between values typical for the monsoon anticyclone and the lower-stratosphere. The chemical evolution during transport within these low–PV cutoffs shows a gradual change from characteristics of the monsoon anticyclone to characteristics of the lower stratospheric background during about one week, indicating continuous mixing with the background atmosphere.

Abstract. Eastward air-mass transport from the Asian summer monsoon (ASM) anticyclone in the upper troposphere and lower stratosphere (UTLS) often involves eastward-shedding vortices, which can cover most of the Japanese archipelago. We investigated the aerosol characteristics of these vortices by analysing data from two lidar systems in Japan, at Tsukuba (36.1∘ N, 140.1∘ E) and Fukuoka (33.55∘ N, 130.36∘ E), during the summer of 2018. We observed several events with enhanced particle signals at Tsukuba at 15.5–18 km of altitude (at or above the local tropopause) during August–September 2018, with a backscattering ratio of ∼ 1.10 and particle depolarization of ∼ 5 % (i.e. not spherical, but more spherical than ice crystals). These particle characteristics may be consistent with those of solid aerosol particles, such as ammonium nitrate. Each event had a timescale of a few days. During the same study period, we also observed similar enhanced particle signals in the lower stratosphere at Fukuoka. The upper troposphere is often covered by cirrus clouds at both lidar sites. Backward trajectory calculations for these sites for days with enhanced particle signals in the lower stratosphere and days without indicate that the former air masses originated within the ASM anticyclone and the latter more from edge regions. Reanalysis carbon monoxide and satellite water vapour data indicate that eastward-shedding vortices were involved in the observed aerosol enhancements. Satellite aerosol data confirm that the period and latitudinal region were free from the direct influence of documented volcanic eruptions and high-latitude forest fires. Our results indicate that the Asian tropopause aerosol layer (ATAL) over the ASM region extends east towards Japan in association with the eastward-shedding vortices and that lidar systems in Japan can detect at least the lower-stratospheric portion of the ATAL during periods when the lower stratosphere is undisturbed by volcanic eruptions and forest fires. The upper-tropospheric portion of the ATAL is either depleted by tropospheric processes (convection and wet scavenging) during eastward transport or is obscured by much stronger cirrus cloud signals.

<p>A robust but thin aerosol layer in summer extending geographically from Eastern Mediterranean to Western China, the Asian Tropopause Aerosol Layer (ATAL) was observed and verified by the CALIPSO lidar measurements. However, its source and forming mechanism is still under debate. In August 2018 and 2019, two experimental campaigns over the Tibetan Plateau were carried out at Golmud (GLM, 36.48°N, 94.93°E) and Qaidam (QDM, 37.74°N, 95.34°E), during which a balloon-borne Portable Optical Particle Counter (POPS) was used to measure the features of aerosol particulates. The in-situ measurements show a robust ATAL around the tropopause, ranging from 14 to 18 km a.s.l., with a maximum aerosol number density of 35–40 cm<sup>-3</sup> and a maximum aerosol mass concentration of 0.13 μg m<sup>-3</sup> for particles with diameters between 0.12 and 3 μm, and majority of the particulates (98%) are smaller than 0.4 μm in diameter.</p><p>Backward-trajectory simulations are conducted with the Massive-Parallel Trajectory Calculations (MPTRAC) model to investigate the possible sources and transport pathways of the observed particulates. The backward-trajectory analysis revealed that the air parcels arrived at the altitude of the ATAL through two separate pathways: 1) the uplift below the 360 K isentropic surface, where air parcels were first elevated to the upper troposphere and then joined the ASM anticyclonic circulation, which will take about 5–10 days; and 2) the quasi-horizontal transport along the anticyclonic circulation, located approximately between the 360 and 440 K isentropic surfaces. The dispersion of the volcanic aerosol from the volcanic eruption of Raikoke in June 2019 has enhanced the aerosol layer in the Tibetan Plateau upper troposphere and lower stratosphere (UTLS), but the ATAL was not concealed by the volcanic plume because the boundary of the Asian summer monsoon (ASM) anticyclone acted as a transport barrier which stopped most of the volcanic aerosol entering the ASM region. Only at most 20% of the aerosol particulates observed in the Tibetan Plateau UTLS was contributed by the Raikoke volcanic eruption. Comparing with the Nabro eruption in 2011, the influence of volcanic eruption on the ATAL significantly depends on the relative geographical location of the volcanic eruption and the ASM anticyclone, as well as the volcanic plume height.</p>

Halogens from long-lived anthropogenic substances contribute to the depletion of stratospheric ozone. Besides these long-lived substances, chlorinated and brominated substances with lifetimes of less than 6 months are additional sources of stratospheric halogens. These substances, also known as very short-lived substances (VSLSs), have both natural and anthropogenic origins. The contribution of chlorinated VSLSs (Cl-VSLSs) to stratospheric chlorine is a few percent. In comparison, brominated VSLSs (Br-VSLS) contribute to about a quarter of the stratospheric bromine. The relative contribution of VSLSs to stratospheric halogen loading is expected to increase as the Montreal Protocol controlled substances progressively decrease. Due to their short lifetimes, VSLSs rapidly release their halogen content into the lowermost stratosphere, a region where changes in ozone have a relatively large impact on surface climate. Here we present the global seasonal distribution of the two major Br-VSLSs CH2Br2 and CHBr3, which account for about 80 % of total organic Br-VSLS. Measurements from four High Altitude and Long Range Research Aircraft (HALO) missions, the HIAPER Pole-to-Pole Observations (HIPPO) mission, and the Atmospheric Tomography (ATom) mission were used for this purpose. Observational results show a similar seasonality of CH2Br2 in the free and upper troposphere of both hemispheres and less clear seasonality with larger variations for CHBr3. The distribution of CH2Br2 in the lowermost stratosphere suggests differences in hemispheric autumn, where the influx of tropospheric air seen in northern hemispheric summer to autumn is not evident in the Southern Hemisphere. However, the southern hemispheric database is insufficient to quantify this difference. The observed distributions were additionally compared to distributions based on model results of TOMCAT and CAM-Chem, both using the emission inventory of Ordóñez et al. (2012). Neither model was able to reproduce the seasonal distribution of CH2Br2 in the Southern Hemisphere. In contrast, both models show a pronounced seasonality of CHBr3 in both hemispheres, which is not confirmed by observations. The distributions of both substances in the lowermost stratosphere are overall well captured by the models, except for southern hemispheric autumn with considerably lower mixing ratios in the observations.

Abstract. In the frame of the SWOP (sounding water vapour, ozone, and particle) campaign during the Asian summer monsoon (ASM), ozone and water vapour profiles were measured by balloon-borne sensors launched from Lhasa (29.66° N, 91.14° E, elevation 3650 m), China, in August 2013. In total, 24 soundings were launched, nearly half of which show strong variations in the relationship between ozone and water vapour in the tracer–tracer correlation in the upper troposphere and lower stratosphere (UTLS). For each sounding, 20-day backward trajectories were calculated using the trajectory module of the Chemical Lagrangian Model of the Stratosphere (CLaMS) to analyse these variations. The trajectory calculations demonstrate that three tropical cyclones (tropical storm Jebi, typhoons Utor and Trami), which occurred over the western Pacific Ocean during August 2013, had a considerable impact on the vertical distribution of ozone and water vapour by uplifting marine air masses to altitudes of the ASM anticyclone. Air parcels subsequently arrived at the observation site via two primary pathways: firstly via direct horizontal transport from the location of the typhoon to the station within approximately 3 days, and secondly via transport following the clockwise wind flow of the ASM within a timescale of 1 week. Furthermore, the interplay between the spatial position of the ASM anticyclone and tropical cyclones plays a key role in controlling the transport pathways of air parcels from the boundary layer of the western Pacific to Lhasa in horizontal and vertical transport. Moreover, the statistical analysis shows that the strongest impact by typhoons is found at altitudes between 14.5 and 17 km (365–375 K). Low ozone values (50–80 ppbv) were observed between 370 and 380 K due to the strong vertical transport within tropical cyclones.

Abstract. Warm conveyor belts (WCBs) are coherent ascending airstreams in extratropical cyclones. They are a major source of moisture for the extratropical upper troposphere and lower stratosphere (UTLS), where moisture acts as a potent greenhouse gas and WCB-associated cirrus clouds contribute to cloud radiative forcing. However, the processes controlling WCB moisture transport and cloud properties are poorly characterised. Furthermore, recent studies have revealed (embedded) convection as a ubiquitous feature of WCBs, highlighting the importance of understanding their updraught and microphysical structure. We present a Lagrangian investigation of WCB moisture transport for a case from the WISE (Wave-driven ISentropic Exchange) campaign based on a convection-permitting simulation. Lagrangian non-dimensional metrics of the moisture budget suggest that the ascent timescale (τ600) strongly controls the end-of-ascent total moisture content, which is largest for slowly ascending trajectories (τ600≥20 h, 30 % of all WCB trajectories). This is due to relatively warm end-of-ascent temperatures and the strong temperature control on transported water vapour. Deviations from equilibrium water vapour condensate partitioning are largest for slow trajectories due to faster glaciation and lower ice crystal numbers. A local moisture transport minimum at intermediate τ600 results from a shift towards a riming-dominated precipitation formation pathway and decreasing outflow temperatures with decreasing τ600. The fastest trajectories (τ600≤5 h, 5 % of all WCB trajectories) transport the largest condensate mass to the UTLS due to less efficient condensate loss and produce the longest-lived outflow cirrus clouds. Models that parameterise convection may under-represent these processes, potentially impacting weather forecasts and climate predictions.

Defining the upper boundary of the Asian Tropopause Aerosol Layer (ATAL) using the static stability

The 2019 Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) field experiment obtained a diverse set of in situ and remotely sensed measurements before and during a pyrocumulonimbus (pyroCb) event over the Williams Flats fire in Washington State. This unique dataset confirms that pyroCb activity is an efficient vertical smoke transport pathway into the upper troposphere and lower stratosphere (UTLS). The magnitude of smoke plumes observed in the UTLS has increased significantly in recent years, following unprecedented wildfire and pyroCb activity observed worldwide. The FIREX-AQ pyroCb dataset is therefore extremely relevant to a broad community, providing the first measurements of fresh smoke exhaust in the upper troposphere, including from within active pyroCb cloud tops. High-resolution remote sensing reveals that three plume cores linked to localized fire fronts, burning primarily in dense forest fuels, contributed to four total pyroCb “pulses.” Rapid changes in fire geometry and spatial extent dramatically influenced the magnitude, behavior, and duration of pyroCb activity. Cloud probe measurements and weather radar identify the presence of large ice particles within the pyroCb and hydrometers below cloud base, indicating precipitation development. The resulting feedbacks suggest that vertical smoke transport efficiency was reduced slightly when compared with intense pyroCb events reaching the lower stratosphere. Physical and optical aerosol property measurements in pyroCb exhaust are compared with previous assumptions. A large suite of aerosol and gas-phase chemistry measurements sets a foundation for future studies aimed at understanding the composition of smoke plumes lifted by pyroconvection into the UTLS and their role in the climate system.

Sulfur compounds in the upper troposphere and lower stratosphere (UTLS) impact the atmosphere radiation budget, either directly as particles or indirectly as precursor gas for new particle formation. In situ measurements in the UTLS are rare, but are important to better understand the impact of the sulfur budget on climate. The BLUESKY mission in May/June 2020 explored an unprecedented situation. 1) The UTLS experienced extraordinary dry conditions in spring 2020 over Europe, in comparison to previous years and 2) the first lockdown of the COVID-19 pandemic caused major emission reductions from industry, ground, and airborne transportation. With the two research aircraft HALO and Falcon, 20 flights were conducted over Central Europe and the North Atlantic to investigate the atmospheric composition with respect to trace gases, aerosol, and clouds. Here, we focus on measurements of sulfur dioxide (SO2) and particulate sulfate (SO42-) in the altitude range of 8 to 14.5 km which show unexpectedly enhanced mixing ratios of SO2 in the upper troposphere and of SO42- in the lowermost stratosphere. In the UT, we find SO2 mixing ratios of (0.07 ± 0.01) ppb, caused by the remaining air traffic, reduced SO2 sinks due to low OH and low cloud fractions, and to a minor extend by uplift from boundary layer sources. Particulate sulfate showed elevated mixing ratios of up to 0.33 ppb in the LS. We suggest that the eruption of the volcano Raikoke in June 2019, which emitted about 1 Tg SO2 into the stratosphere in northern midlatitudes caused these enhancements, in addition to Siberian and Canadian wildfires and other minor volcanic eruptions. Our measurements can help to test models and lead to new insights in the distribution of sulfur compounds in the UTLS, their sources and sinks. Moreover, these results can contribute to improve simulations of the radiation budget in the UTLS with respect to sulfur effects.

We use the global three-dimensional Goddard Earth Observing System chemical transport model with the Universal tropospheric–stratospheric Chemistry eXtension mechanism to examine the contributions of the chemical processes to summertime O3 in the upper troposphere and lower stratosphere (UTLS) over the Tibetan Plateau and the South Asian monsoon region (TP/SASM). Simulated UTLS O3 concentrations are evaluated by comparisons with Microwave Limb Sounder products and net chemical production of O3 (NPO3) are evaluated by comparisons with model results in previous studies. Simulations show that the chemical processes lead to an increase in O3 concentration, which is opposite to the effect of O3 transport in the UTLS over the TP/SASM region throughout the boreal summer. NPO3 in UTLS over the TP/SASM region is the largest in summer. Elevated values (0.016–0.020 Tg year−1) of the seasonal mean NPO3 are simulated to locate at 100 hPa in the TP/SASM region, where the mixing ratios of O3 are low and those of O3 precursors (NOx, VOCs, and CO) are high. The high concentrations of O3 precursors (NOx, VOCs, and CO) together with the active photochemical reactions of NO2 in the UTLS over the TP/SASM region during summertime could be important reasons for the enhancement of $${\text{NP}}_{{{\text{O}}_{3} }}$$ over the studied region.

The impact of transient tropospheric forcing on the deep vertical mountain-wave propagation is investigated by a unique combination of in situ and remote sensing observations and numerical modeling. The temporal evolution of the upstream low-level wind follows approximately a [Formula: see text] shape and was controlled by a migrating trough and connected fronts. Our case study reveals the importance of the time-varying propagation conditions in the upper troposphere and lower stratosphere (UTLS). Upper-tropospheric stability, the wind profile, and the tropopause strength affected the observed and simulated wave response in the UTLS. Leg-integrated along-track momentum fluxes ([Formula: see text]) and amplitudes of vertical displacements of air parcels in the UTLS reached up to 130 kN m−1 and 1500 m, respectively. Their maxima were phase shifted to the maximum low-level forcing by ≈8 h. Small-scale waves ([Formula: see text] km) were continuously forced, and their flux values depended on wave attenuation by breaking and reflection in the UTLS region. Only maximum flow over the envelope of the mountain range favored the excitation of longer waves that propagated deeply into the mesosphere. Their long propagation time caused a retarded enhancement of observed mesospheric gravity wave activity about 12–15 h after their observation in the UTLS. For the UTLS, we further compared observed and simulated [Formula: see text] with fluxes of 2D quasi-steady runs. UTLS momentum fluxes seem to be reproducible by individual quasi-steady 2D runs, except for the flux enhancement during the early decelerating forcing phase.

Nitric oxide (NO) and nitrogen dioxide (NO2) are important trace gases in the upper troposphere and lower stratosphere (UTLS).  In the upper troposphere, NO is primarily formed by lightning and can react with other radicals to form a suite of reactive nitrogen (NOy) species and produce ozone.  In the lower stratosphere, NOx can be responsible for catalytic ozone destruction and can form halogen reservoirs such as ClNO2 and ClONO2.  Accurate measurements of NOx species are therefore important for understanding and diagnosing UTLS chemical regimes.  Further, understanding NO2 concentrations in the stratosphere and upper troposphere is critical for accurate satellite retrievals of tropospheric NO2. In-situ measurements of NOx species above 12-15 km are infrequent and challenging.  Where there are measurements of NOx species up to about 12 km, NO mixing ratios tend to be reasonably well-reproduced using global models.  However, NO2 measurements frequently agree well with models and photostationary state calculations up to about 8 km, but at higher altitudes routinely are significantly higher than expected relative to the measured NO.  Proposed sources of this discrepancy include measurement artifacts affecting the in-situ measurements, inaccuracies in rate constants at low temperatures, and missing chemistry.  The net result is significant uncertainty in the NO2 concentrations in the UTLS.Here we present results from the recent high-altitude aircraft campaign Asian Summer Monsoon Chemistry and CLimate Impact Project (ACCLIP) over southeast Asia in summer 2022.  We show measurements from the recently-developed NOAA NO laser-induced fluorescence instrument up to altitudes of almost 20 km.  This instrument makes direct NO measurements with precision and accuracy sufficient for measurements below one ppt with one second of integration, making it ideal for aircraft campaigns.  In order to measure NO2 with this instrument, we have developed a photolysis inlet which converts NO2 to NO using an LED centered at 395 nm.  What is novel about this NO2 conversion is that it is done in an unpressurized pylon of the aircraft and thus the sample remains at the UTLS ambient temperatures until after photolysis.  This thus significantly reduces the potential interferences from NOy species that can thermally decompose to NO2 at room temperature, eliminating a potential source of major artifact.  Our data show good agreement with photostationary state calculations performed for these flights.