AS1.11

Satellite observations have revealed an enhanced aerosol layer near the tropopause over Asia during the summer monsoon, called the Asian Tropopause Aerosol Layer (ATAL). The chemical composition of the ATAL is investigated here using offline ionic analysis of aerosols collected with a balloon-borne impactor near the tropopause region over India onboard extended duration balloon flights in the summer of 2017 and winter 2018. We found NO₃^- and NO₂^- dominant among other ions with values ranging between 87-343 ng/m³ during the summer campaign. In contrast, SO₄ levels were found above detection limit (>10 ng/m³) only in winter. In addition, we determined the origin of the air masses sampled during the flights through back trajectory analysis combined with convection. The results obtained therein were put into a context of large-scale transport and aerosol distribution with GEOS-Chem chemical transport model simulations. The first flight of summer 2017 sampled air mass within the Asian monsoon anticyclone (AMA), associated with smaller particle size found on stage 2 (particle size cut off > 0.15 microns) of the impactor, while the second flight sampled air mass at the edge of the AMA associated with larger particle size on stage 1 (particle size cut off between 2 and 0.5 microns). The sampled air masses in winter 2018 were affected by smoke from the Pacific Northwest fire event in Canada, which occurred 7 months prior to our campaign. Concentrations of SO₄^2-, NH4⁺, and Ca²⁺ were enhanced. Overall, our results suggest that nitrogen- containing particles represent a large fraction of aerosols populating the ATAL in agreement with aircraft measurements during the StratoClim campaign. Furthermore, GEOS-chem model simulations suggest that lightning NOx emissions had a minimal impact on the production of nitrate aerosols sampled during the two flights. 

How to cite: Vernier, H., Rastogi, N., Liu, H., Fairlie, D., Pandit, A., Bedka, K. M., Patel, A., Ratnam, M. V., Kumar, B. S., Gadhavi, H., Weinhold, F. G., Berthet, G., and Vernier, J.-P.: Exploring the ionic composition of the Asian Tropopause Aerosol Layer using medium duration balloon flights, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2323, https://doi.org/10.5194/egusphere-egu21-2323, 2021.

The Asian Tropopause Aerosol Layer (ATAL) has emerged over recent decades to play an increasingly prominent role in the upper troposphere and lower stratosphere above the Asian monsoon region. Although the effects of the ATAL on the surface and top-of-atmosphere radiation budget have been examined by several studies, the processes and effects by which the ATAL alters radiative transfer within the tropopause layer have been much less discussed. We have used a conditional composite approach to investigate aerosol mixing ratios and their impacts on radiative heating rates in the Asian monsoon tropopause layer in MERRA-2. We have then subsampled in time based on known volcanic eruptions and the evolution of emission and data assimilation inputs to the MERRA-2 aerosol analysis to isolate the ATAL contribution and compare it to radiative heating signatures in the monsoon anticyclone region after volcanic eruptions. The results indicate that the ATAL impact on radiative heating rates in this region is on the order of 0.1 K/day, similar to that associated with ozone variability in MERRA-2 but weaker than cloud radiative effects at these altitudes. We have validated these results and tested their sensitivity to variations in the vertical structure and composition of ATAL aerosols using offline radiative transfer simulations. The idealized simulations produce similar but slightly stronger responses of radiative heating rates to the ATAL and are in good agreement with previous estimates of the top-of-atmosphere radiative forcing. Although the ATAL perturbations inferred from MERRA-2 are only about 10% of mean heating rates at these levels, their spatial distribution suggests potential implications for both isentropic and diabatic transport within the monsoon anticyclone, which should be examined in future work. Our results are limited by uncertainties in the composition and spatiotemporal variability of the ATAL, and reflect only the conditions in this layer as represented by MERRA-2. Targeted observations and model simulations are needed to adequately constrain the uncertainties, particularly with respect to the relative proportions and contributions of nitrate aerosols, which are not included in the MERRA-2 aerosol analysis.

How to cite: Gao, J. and Wright, J.: Aerosol influences on radiative heating rates in the Asian tropopause aerosol layer, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5145, https://doi.org/10.5194/egusphere-egu21-5145, 2021.

Deployment of the high-altitude M55-Geophysica aircraft in Kathmandu during Summer 2017 within StratoClim campaign has yielded a wealth of unique high-resolution measurements in the Asian Monsoon Anticyclone (AMA). In a particular flight (F8, 10 August 2017) the aircraft flew at the cold-point tropopause level through active overshoots and their outflows minutes to hours old. The measurements reveal up to 2500 ppmv of ice water above 17 km in large aggregated ice crystals up to 700 µm in diameter. Smaller crystals were observed as high as 18.8 km (410 K). Tracer and thermodynamical measurements show manifestations of vigorous vertical motions and provide evidence for ongoing mixing of tropospheric and stratospheric air around the tropopause. We use an ensemble of airborne and satellite measurements inside and downwind of convective overshoots together with trajectory modeling to characterize the impact of overshooting convection on the thermodynamical structure and chemical composition of the Asian tropopause layer. The effect of cross-tropopause convective transport on the Asian lower stratospheric water vapour is discussed.

How to cite: Khaykin, S., Krämer, M., Moyer, E., Bucci, S., Afchine, A., Borrmann, S., Cairo, F., Clouser, B., D’Amato, F., Legras, B., Lykov, A., Mitev, V., Matthey, R., Rolf, C., Singer, C., Ulanovsky, A., Viciani, S., Volk, M., Yushkov, V., and Stroh, F.: Evolution of tracer and ice crystal distribution in the young plumes of overshooting turrets from the StratoClim golden flight, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9756, https://doi.org/10.5194/egusphere-egu21-9756, 2021.

Vertical distribution of aerosols and their composition in the lower troposphere is critically important for assessing the Earth’s radiation budget and their impact on monsoon circulation. We combine the extinction coefficient, particulate depolarization ratio obtained from CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) for period of 11 years (2008-2018) over the Indian region to provide an unprecedented climatological overview of the physical and optical characteristics of quasi-aerosol layers and their source and formation mechanism throughout its annual life cycle in the free troposphere. The key findings includes: i)The quasi aerosol layer over the Indian region are found to be persistent between 4-6 km during all seasons and occasionally reach above 6 km and exhibited strong seasonal and regional dependency, ii) Layer thickness varies between 2.0 -3.0 km corresponds to primary peak are more frequent of about 80-90 % of cases over all six regions and while  secondary layer occasionally forms (10-20 %), iii) The aerosol layer thickness increases by about 36.7 and 25% during summer and fall season compared to that of spring, and winter, iv) Layer-AOT showed year-to-year variations of up to a factor of two with a relative variability of about 15-23% (1σ), v) Trend in layer AOT is not very conspicuous and showed oscillatory pattern, vi) Depolarization ratios generally increase with height suggesting that the irregularity of aerosol shape increases with altitude, vii) The polluted dust and smoke are the major aerosol components of the observed quasi aerosol layer  between 4 to 6 km for spring and fall season while these are the polluted dust during winter and summer.

How to cite: Kumar, G., Vernier, J. P., Madhavan, B. L., Ansari, K., and Sinha, P. R.: Ubiquity of quasi-aerosol layers in the free troposphere over the Indian region: Results from multiyear satellite observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11003, https://doi.org/10.5194/egusphere-egu21-11003, 2021.