Probing the Asian Tropopause Aerosol Layer during Summer Monsoon Using in situ Measurements

The monsoon deep convection provides a potential pathway for the transport of surface-emitted pollutants to the upper troposphere and lower stratosphere (UTLS). These pollutants are then trapped by the anticyclone forming a sustained pollution layer. Recent satellite studies have revealed enhanced aerosol mass in the in Asia (Asian Tropopause Aerosol Layer, ATAL) during the monsoon season. The enhanced aerosol layer, however, has not been confirmed by in situ measurements, in part due to the limitation of instrumentation that is capable to be deployed on balloons. With the development of a light-weight, high-sensitivity particle counter (printed optical particle spectrometer, POPS), we are able to measure the vertically-resolved aerosol number density and size distribution in real time. We deployed the POPS on balloons in the summer of 2015 in Kunming, China, a site on the edge of the Asian summer anticyclone region. The measurements showed an ATAL spanning from 15 to 19 km vertically. The particles in the ATAL were dominated by submicron particles, with their total number concentration reaching 30 cm-3 for aerosols with diameter of 140 nm to 3 μm. The particle number concentration of the ATAL was 3-4 times higher than that of the layer immediately below 15 km. Based on the result this pilot study, we will expand our measurements to the center of the Asian monsoon anticyclone in Lhasa, China in the summer of 2016. Additional balloon-borne measurements will be conducted at Houston following the Lhasa measurements in an attempt to sample the outflow from the Asian summer monsoon anticyclone. The in-situ size distribution data can be used to diagnose air motion in the UTLS region and constrain dynamic transport in global models.