Origins of Particulate Organic Acids during High-Altitude Transport over the North China Plain: Results from Mount Tai and a Flight Campaign in Winter 2019

Measurements of atmospheric organic particle composition at higher altitudes are scarce. The present study discusses concentrations and sources of PM and organic constituents based on winter-time observations at Mount Tai and aircraft measurements above the North China Plain (NCP). For PM_2.5 at the mountain site, concentrations up to 94 μg m^–3 were measured. Correlations with surrounding cities indicated that, despite an observed boundary layer height below the sampling altitude (1534 m asl), polluted air masses from the plain ascended to the mountaintop, possibly due to orographic effects. Organic constituents showed mean concentrations ranging from ∼1 ng m^–3 for terpene-derived acids and some branched or unsaturated dicarboxylic acids but could reach up to ∼100 ng m^–3 for oxalic acid. A cluster heatmap revealed correlational relationships between the compounds that originate from sources, including traffic, combustion of coal, waste, and biomass as well as secondary formation. Average concentrations of most short-chain organic acids taken in the general upwind direction of Mount Tai at mean flight altitudes of ∼4000 m averaged 10–25% of the mountain ones outside haze periods, suggesting that chemical formation during high-altitude transport could have occurred. One flight sample showed high concentrations of up to 180 ng m^–3 for oxalic acid, which is comparable to mountain haze periods, corroborating earlier observations of elevated pollution layers in the area even at such altitudes. CAPRAM multiphase modeling reproduced selected mountain concentrations of malonic and succinic acids reasonably well, while an initial underestimation of oxalic acid could be reduced by including salt formation and thereby enhanced phase partitioning. Overall, this study addresses observational gaps at high altitudes over the NCP and suggests processes and sources that might also be relevant in other regions.