Total organic carbon and contribution from speciated organics in cloud water: Airborne data analysis from the CAMP2Ex field campaign
Authors: Stahl Connor , Crosbie Ewan , Bañaga Paola Angela , Betito Grace , Braun Rachel A. , Cainglet Zenn Marie , Cambaliza Maria Obiminda , Cruz Melliza Templonuevo , Dado Julie Mae , Hilario Miguel Ricardo A. , Leung Gabrielle Frances , MacDonald Alexander B. , Magnaye Angela Monina , Reid Jeffrey , Robinson Claire , Shook Michael A. , Simpas James Bernard , Visaga Shane Marie , Winstead Edward , Ziemba Luke , Sorooshian Armin 
Published: June 2021

Abstract. This work focuses on total organic carbon (TOC) and contributing species in cloud water over Southeast Asia using a rare airborne dataset collected during NASA’s Cloud, Aerosol and Monsoon Processes Philippines Experiment (CAMP2Ex), in which a wide variety of maritime clouds were studied, including cumulus congestus, altocumulus, altostratus, and cumulus. Knowledge of TOC levels and their contributing species is needed for improved modeling of cloud processing of organics and to understand how aerosols and gases impact and are impacted by clouds. This work relies on 159 samples collected with an Axial Cyclone Cloud water Collector at altitudes of 0.2–6.8 km that had sufficient volume for both TOC and speciated organic composition analysis. Species included monocarboxylic acids (glycolate, acetate, formate, and pyruvate), dicarboxylic acids (glutarate, adipate, succinate, maleate, and oxalate), methanesulfonate (MSA), and dimethylamine (DMA). TOC values range between 0.018–13.660 ppm C with a mean of 0.902 ppm C. The highest TOC values are observed below 2 km with a general reduction aloft. An exception is samples impacted by biomass burning for which TOC remains enhanced as high as 6.5 km (7.048 ppm C). Estimated total organic matter derived from TOC contributes a mean of 30.7 % to total measured mass (inorganics + organics). Speciated organics contribute (on carbon mass basis) an average of 30.0 % to TOC in the study region, and account for an average of 10.3 % to total measured mass. The order of the average contribution of species to TOC, in decreasing contribution of carbon mass, is as follows: acetate (14.7 ± 20.5 %), formate (5.4 ± 9.3 %), oxalate (2.8 ± 4.3 %), DMA (1.7 ± 6.3 %), succinate (1.6 ± 2.4 %), pyruvate (1.3 ± 4.5 %), glycolate (1.3 ± 3.7 %), adipate (1.0 ± 3.6 %), MSA (0.1 ± 0.1 %), glutarate (0.1 ± 0.2 %), maleate (< 0.1 ± 0.1 %). Approximately 70 % of TOC remains unaccounted for, thus highlighting the complex nature of organics in the study region; samples collected in biomass burning plumes have up to 95.6 % of unaccounted TOC mass based on the species detected. Consistent with other regions, monocarboxylic acids dominate the speciated organic mass (~75 %) and are about four times in greater abundance than dicarboxylic acids. Samples are categorized into four cases based on back-trajectory history revealing source-independent similarity between the bulk contributions of monocarboxylic and dicarboxylic acids to TOC (16.03 %–23.66 % and 3.70 %–8.75 %, respectively). Furthermore, acetate, formate, succinate, glutarate, pyruvate, oxalate, and MSA are especially enhanced during biomass burning periods, attributed to peat emissions transported from Sumatra and Borneo. Lastly, dust (Ca2+) and sea salt (Na+/Cl−) tracers exhibit strong correlations with speciated organics, thus supporting how coarse aerosol surfaces interact with these water-soluble organics.