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.
Highlights
The last 2 decades have witnessed an acceleration in research to unravel the nature of the organic fraction of airborne particles, including speciation (Hallquist et al, 2009; Kanakidou et al, 2005), with implications for how particles impact air quality, public health, and the planet’s energy balance
Stahl et al.: Total organic carbon and the contribution from speciated organics in cloud water proximately 70 % of total organic carbon (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
We test two hypotheses: (i) the relative contribution of organic acids to TOC will exceed what was observed in the surface layer over Metro Manila owing to more aged air masses aloft compared with the surface layer in Metro Manila, which is exposed to fresher emissions; and (ii) clouds impacted by biomass burning emissions will exhibit chemical profiles shifted to higher TOC concentrations and with a greater portion of that TOC accounted for by organic acids
Summary
The last 2 decades have witnessed an acceleration in research to unravel the nature of the organic fraction of airborne particles, including speciation (Hallquist et al, 2009; Kanakidou et al, 2005), with implications for how particles impact air quality, public health, and the planet’s energy balance. Based on these points, we test two hypotheses: (i) the relative contribution of organic acids to TOC will exceed what was observed in the surface layer over Metro Manila owing to more aged air masses aloft compared with the surface layer in Metro Manila, which is exposed to fresher emissions; and (ii) clouds impacted by biomass burning emissions will exhibit chemical profiles shifted to higher TOC concentrations and with a greater portion of that TOC accounted for by organic acids. Than 1 % of total aerosol mass, with oxalate being the most abundant acid (Stahl et al, 2020); (iii) organic acid concentrations are more enhanced during biomass burning periods (Hilario et al, 2020a), especially succinate and oxalate (Braun et al, 2020; Stahl et al, 2020); and (iv) wet deposition samples clearly show the influence of biomass burning tracer species on cloud composition (Ma et al, 2021)
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