Long-term trends and transport of surface pollutants to UTLS  during the Asian summer monsoon

Abstract

<p>Using MERRA2 reanalyses, we have examined the long-term (2000-2019) trends and transport of surface pollutants, CO, BC and OC from surface to the upper troposphere and lower stratosphere (UTLS) during the Asian summer monsoon.    We find a strong linear trend indicating an expansion and strengthening of the Asian Monsoon Anticyclone (AMA), in conjunction with increased concentration of CO, BC and OC in the UTLS, including the Aerosol Tropopause Aerosol Layer (ATAL). </p><p>The UTLS trend in CO can be tracked to increased upward transport primarily from surface sources near 25-35<sup>o</sup>N, in association with the expansion/strengthening of the AMA, and a northward displacement of ascending branch of the monsoon meridional circulation.  In contrast, near 25-35<sup>o</sup>N, BC and OC trends show significant reduction from surface to mid-troposphere, coupled a weak increase at UTLS (above 250 -100 hPa).  The reduction in surface and tropospheric BC and OC likely reflects reduced emission due to the clean air acts in East Asia.  Additionally, heavier rainfall associated with the enhanced ascent and wet scavenging may also contribute to the strong reduction in tropospheric BC and OC.  The increase in UTLS OC/BC appears to stem from increased and extended biomass burning near surface sources located in extratropical latitudes (70-130<sup>o</sup> E, 55-70<sup>o</sup> N).  The OC/BC aerosols are transported upward by vertical mixing over the source regions, and enter the tropical UTLS through horizonal diffusive processes.   Additionally, enhanced penetrative convection in the anomalous ascent regions during the peak monsoon season may also play a role in further enhancing the monsoon ascent, lifting ambient hydrophobic OC/BC and water vapor in the mid-to-upper troposphere to higher elevations, resulting in enhanced ice-cloud fraction, increased latent and radiative heating in the UTLS/ATAL region.</p><p> </p>