2017 News & Events
The Asian Summer Monsoon contributes significantly to stratospheric aerosols with a far-reaching impact
10 July 2017
Scientists from CSD, the Cooperative Institute for Research and Environmental Sciences (CIRES), and partners recently published Efficient transport of tropospheric aerosol into the stratosphere via the Asian Summer Monsoon anticyclone in the Proceedings of the National Academy of Sciences journal.
Recent satellite observations have identified an enhanced aerosol layer near the tropopause over Asia during the June-September period of the Asian Summer Monsoon (ASM). This aerosol layer, the Asian Tropopause Aerosol Layer (ATAL), is composed of organics and sulfate from human activities. To improve understanding of the phenomenon, the research team conducted in situ measurements in Kunming, China during the summer of 2015, where they determined the vertical structure and size distribution of the ATAL.
Stratospheric aerosols, minute particles suspended in the stratospheric level of the atmosphere, can significantly affect the Earth's climate by reflecting sunlight – typically reducing the amount of energy reaching the lower atmosphere and the Earth's surface and having a cooling effect. Aerosols are classified into two types: natural and human-made. In the case of the ATAL, increased economic growth and larger populations have increased the amount of human-made pollutants in the lower stratosphere.
The researchers found that, due to the vertical movement of air associated with the ASM, some pollutants released at the surface were forming aerosols in the upper troposphere which ultimately enter the stratosphere. The ASM system acts as smokestack, sweeping pollutants from the lower troposphere to the upper troposphere and lower stratosphere, where they are transformed into stratospheric aerosols. While many stratospheric aerosols come through upwelling in the tropics, the ASM accounts for 15 percent of stratospheric aerosol in the North Hemisphere annually. To put that amount into context, small volcanic eruptions from 2000-2015 contributed a similar amount.
The NOAA-developed Portable Optical Particle Spectrometer (POPS) instrument measures aerosol properties accurately and is more easily deployed than other instruments with similar sensitivities, thereby enabling further understanding of the ASM beyond previous studies.
The researchers measured aerosol particle numbers and size distribution with NOAA-built POPS instruments deployed on weather balloons in Kunming, China in August, 2015. They used these measurements to verify that the results from a detailed aerosol model coupled with a global climate model were representative, and then they used the model output to determine whether these aerosols could have a global impact on climate. From the data collected during this study, the researchers concluded that the ASM significantly affects stratospheric aerosols and their importance may grow with increasing economic growth in the Asian region.
Pengfei Yu, Karen H. Rosenlof, Shang Liu, Hagen Telg, Troy D. Thornberry, Andrew W. Rollins, Robert W. Portmann, Zhixuan Bai, Eric A. Ray, Yunjun Duan, Laura L. Pan, Owen B. Toon, Jianchun Bian, and Ru-Shan Gao, Efficient transport of tropospheric aerosol into the stratosphere via the Asian Summer Monsoon anticyclone, Proceedings of the National Academy of Sciences, doi:10.1073/pnas.1701170114, 2017.
Abstract
An enhanced aerosol layer near the tropopause over Asia during the June-September period of the Asian summer monsoon (ASM) was recently identified using satellite observations. Its sources and climate impact are presently not well-characterized. To improve understanding of this phenomenon, we made in situ aerosol measurements during summer 2015 from Kunming, China, then followed with a modeling study to assess the global significance. The in situ measurements revealed a robust enhancement in aerosol concentration that extended up to 2 km above the tropopause. A climate model simulation demonstrates that the abundant anthropogenic aerosol precursor emissions from Asia coupled with rapid vertical transport associated with monsoon convection leads to significant particle formation in the upper troposphere within the ASM anticyclone. These particles subsequently spread throughout the entire Northern Hemispheric (NH) lower stratosphere and contribute significantly (∼15%) to the NH stratospheric column aerosol surface area on an annual basis. This contribution is comparable to that from the sum of small volcanic eruptions in the period between 2000 and 2015. Although the ASM contribution is smaller than that from tropical upwelling (∼35%), we find that this region is about three times as efficient per unit area and time in populating the NH stratosphere with aerosol. With a substantial amount of organic and sulfur emissions in Asia, the ASM anticyclone serves as an efficient smokestack venting aerosols to the upper troposphere and lower stratosphere. As economic growth continues in Asia, the relative importance of Asian emissions to stratospheric aerosol is likely to increase.