The South Asian monsoon—pollution pump and purifier

Abstract

Air pollution is growing fastest in monsoon-affected South Asia. During the dry winter monsoon, the fumes disperse toward the Indian Ocean, creating a vast pollution haze, but their fate during the wet summer monsoon has been unclear. We performed atmospheric chemistry measurements by aircraft in the Oxidation Mechanism Observations campaign, sampling the summer monsoon outflow in the upper troposphere between the Mediterranean and the Indian Ocean. The measurements, supported by model calculations, show that the monsoon sustains a remarkably efficient cleansing mechanism by which contaminants are rapidly oxidized and deposited to Earth's surface. However, some pollutants are lofted above the monsoon clouds and chemically processed in a reactive reservoir before being redistributed globally, including to the stratosphere.

Separating Emission and Meteorological Drivers ofMid‐21st‐Century Air Quality Changes in IndiaBased on Multiyear Global‐RegionalChemistry‐Climate Simulations

“…But, to what direction and extent the pollution‐related meteorological condition will change in future is still largely unquantified. Indian summer monsoon is well known to have a strong control over the distribution of surface pollutants over India (e.g., De Laat & Lelieveld, ; Lelieveld et al, ). However, a quantitative analysis, especially using multiple variables and a combined index, is less common in previous studies.…”

SO2 Observations and Sources in the Western Pacific Tropical Tropopause Region

“…Simulations by Brühl et al () using the ECHAM/MESSy Atmospheric Chemistry model (EMAC) of the 2002–2011 period generally show tropical average values (20°S–20°N) at 16‐ to 18‐km altitude with SO 2 almost always less than 50 pptv, and during volcanically quiescent periods (e.g., 2002–2004) the modeled zonal mean was frequently less than 10 or 20 pptv. Lelieveld et al (, see the supporting information in that paper) present results from the EMAC model from July to August 2015 and calculate that at the 100‐hPa level (~17 km) in the western Pacific, SO 2 is near 10 pptv.…”

“…However, liquid droplets are not an infinite sink for SO 2 , but rather the SO 2 removal rate depends on the availability of the aqueous oxidants hydrogen peroxide and O 3 , and the oxidation rate is strongly pH dependent. Lelieveld et al () have shown that more than 100 pptv SO 2 can be found in the UT in the outflow regions of South Asian summer monsoon convection. Evaluation of the transport and chemical mechanisms in models therefore relies on measurements of SO 2 in regions such as the western Pacific where deep convection is a dominant feature.…”

Study of SO Pollution in the Middle East Using MERRA‐2, CAMS Data Assimilation Products, and High‐Resolution WRF‐Chem Simulations

“…ME emits about 10% of the total global anthropogenic SO (Klimont et al, ). These emissions could be involved in the monsoon circulation and have a global effect on atmospheric composition (Lelieveld et al, ). SO conventional emission inventories have a high level of uncertainty over the ME, and air‐quality observations are sparse.…”