Water-soluble low molecular weight organics in cloud water at Mt. Tai Mo Shan, Hong Kong

Abstract

Cloud-water samples collected at the summit of Mt. Tai Mo Shan (Mt. TMS, 957 m, a.s.l.), Hong Kong in autumn 2016 and spring 2017 were measured for molecular compositions and stable carbon isotope ratios (δ13C) of dicarboxylic acids, oxoacids and α-dicarbonyls. Oxalic acid (C2, 253–1680 μg L−1) was found as the most abundant diacid, followed by succinic acid (C4, 24–656 μg L−1) in autumn and phthalic acid (Ph, 27–363 μg L−1) in spring. Higher concentrations of Ph (192 ± 197 μg L−1) and terephthalic acid (tPh, 31 ± 15 μg L−1) were observed in autumn than those in spring, illustrating the enhanced contribution from fossil fuel combustion and plastic wastes burning. Stronger correlations for the shorter chain diacids (C2–C4) with NO3 −, nss-SO4 2− and nss-K+ in autumn (R2 ≥ 0.7) than spring suggested that these diacids were mainly produced via atmospheric photooxidation following anthropogenic emissions. The δ13C values of C2 (mean − 14.7‰), glyoxylic acid (ωC2, −12.2‰), pyruvic acid (Pyr, −15.5‰), glyoxal (Gly, −13.5‰) were much higher than those in atmospheric aerosols from isoprene and other precursors, indicating that diacids, oxoacids and α-dicarbonyls in cloud at Mt. TMS were significantly influenced by photochemical formation during the long-range atmospheric transport.