Vertical Differences of Sulfate Formation Mechanisms and Sources in the Urban Atmosphere Based on Tower Measurements

Abstract Sulfate particle (SO 4 2− ), the most abundant chemical component in tropospheric aerosols, induces haze formation and affects Earth's climate. However, the isotopic composition records of sulfate in vertical space are still lacking. This work performed simultaneous vertical measurements of sulfur isotopes of sulfate (δ 34 S‐SO 4 2− ) at 3 and 488 m heights on the Canton Tower in Guangzhou, south China. Although δ 34 S‐SO 4 2− of the two heights were close, a significant difference ( p < 0.05) in the estimated fractionation factor ( ε g→p ) from SO 2 to SO 4 2− was found between 3 m (6.7 ± 0.7‰) and 488 m (8.7 ± 0.9‰). The higher ε g→p values in the high‐altitude atmosphere were attributed to the increasing contributions of SO 2 with H 2 O 2 /O 3 and OH reactions, whereas SO 2 and NO 2 reaction enhanced sulfate formation near the ground. The quantified sulfate sources using δ 34 S were compared with and without the consideration of fractionation factors. The result revealed that the partitioned emission sources to sulfate by considering the fractionation effects were more logical. After considering the fractionation effects, it was found that the proportion of coal combustion to sulfate aerosols increased from the ground (53%) to 488 m (62%), suggesting that the air masses carrying coal‐derived components reached the upper air of Guangzhou through regional transport. Vehicles were also important sulfate sources with a contribution of 41 ± 9% near the ground. Our findings suggest that vertical observations are necessary for studying the full picture of the formation mechanism and sources of sulfate aerosols in the atmosphere.