Total organic carbon and the contribution from speciated organics in cloud water: airborne data analysis from the CAMP<sup>2</sup>Ex field campaign

Abstract

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 masses 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), methanesulfonic acid (MSA), and dimethylamine (DMA). TOC values range between 0.018 and 13.66 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 at altitudes 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 a 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 (±1 standard deviation): 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 %), and maleate (< 0.1 ± 0.1 %). Approximately 70 % of TOC remains unaccounted for, highlighting the complex nature of organics in the study region; in samples collected in biomass burning plumes, up to 95.6 % of TOC mass is unaccounted for based on the species detected. Consistent with other regions, monocarboxylic acids dominate the speciated organic mass (∼ 75 %) and are about 4 times more abundant 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, which is attributed to peat emissions transported from Sumatra and Borneo. Lastly, dust (Ca2+) and sea salt (Na+/Cl-) tracers exhibit strong correlations with speciated organics, supporting how coarse aerosol surfaces interact with these water-soluble organics.

Insights into tropical cloud chemistry in Réunion (Indian Ocean): results from the BIO-MAÏDO campaign

“…Oxalic acid is the smallest dicarboxylic acid with concentrations much lower than formic and acetic acids ranging from 0.27 to 1.78 µmol L −1 . The contribution of light acids has also been observed in recent studies in marine environments, where those species dominate the organic contribution to TOC (Boris et al, 2018;Stahl et al, 2021). Finally, malonic, succinic, and malic acids account for ∼ 70 % of the less concentrated dicarboxylic acid fraction.…”

“…Fe(II) concentration is quantified by UV-visible spectroscopy (λ = 562 nm), based on the rapid complexation of iron with ferrozine (Stookey, 1970). The total iron content (Fe(tot)) is detected after the reduction to Fe(II) by the addition of ascorbic acid.…”

“…Since the late 1990s, substantial efforts have been made to further understand the chemical composition and reactivity of dissolved matter in cloud droplets through in situ mea-surements and laboratory investigations (Aleksic et al, 2009;Bianco et al, 2017;Brege et al, 2018;Brüggemann et al, 2005;Cini et al, 2002;Deguillaume et al, 2014;Fomba et al, 2015;Gilardoni et al, 2014;Gioda et al, 2013;Herckes et al, 2013Herckes et al, , 2002Hutchings et al, 2009;Lee et al, 2012;Li et al, 2020;van Pinxteren et al, , 2020Renard et al, 2020;Sorooshian et al, 2007;Stahl et al, 2021;Vaïtilingom et al, 2013;Viana et al, 2014). Some of these studies were focused on the role of clouds in the life cycle of inorganic compounds (i.e.…”

Particulate Oxalate‐To‐Sulfate Ratio as an Aqueous Processing Marker: Similarity Across Field Campaigns and Limitations

“…Size-resolved data show that this agreement is greatest between 0.32-1 μm (Figure S3a in Supporting Information S1), where OXL and SO42-masses mostly reside (Cruz et al, 2019). An increase in supermicrometer OXL:SO 4 2− (Figure S3a in Supporting Information S1) suggests the enhancement of OXL via gas-particle partitioning of OXL and/or its precursors onto coarse particles as documented for the CHECSM region (Stahl et al, 2020b(Stahl et al, , 2021. These results suggest the ratio may be applicable to the mixed layer for submicrometer particles.…”

“…These high-OXL samples were mostly sampled within the free troposphere (>5 km) (Figure S5b in Supporting Information S1), altitudes of which were rarely sampled in other field campaigns with AToM being the exception. A few reasons can explain the high-altitude, high-OXL samples during CAMP 2 Ex: (a) OXL's lengthier chemical formation pathways compared to SO 4 2− (Ervens, 2015;Sorooshian, Lu, et al, 2007), (b) inefficient scavenging of gaseous precursors as air masses are transported upward (Heald et al, 2005), and (c) gas-phase OXL and/or its precursors partitioning onto dust particles (Stahl et al, 2020b(Stahl et al, , 2021Sullivan & Prather, 2007). As the PILS sampled PM 4 during CAMP 2 Ex, we hypothesize that the enhanced OXL is due to gas-particle partitioning of OXL and/or its precursors onto coarse mode particles such as dust or sea salt (Mochida et al, 2003;Rinaldi et al, 2011;Sullivan & Prather, 2007;Turekian et al, 2003), evidenced by a prominent coarse mode peak (D p ∼ 2.5 μm) in the size distributions of high-OXL samples (Figure S5 in Supporting Information S1).…”