Oxalate and perchlorate: two trace components in the environment
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Oxalic acid is the most abundant atmospheric dicarboxylic acid. Its salts are very hygroscopic and can act as cloud condensation nuclei. In this work, vapor pressure of oxalic acid was measured in the laboratory at atmospherically relevant temperatures and these data were used to explain the observed gas-aerosol partitioning of the summertime oxalic acid data collected from three major US cities. We inferred that below 50% relative humidity, the oxalic acid aerosol in the atmosphere behaves like pure oxalic acid. The median oxalate concentration is nearly an order of magnitude greater than gas phase oxalic acid concentration. Data analysis shows that the main mechanism of formation is aqueous phase photochemical production. There was no correlation between oxalic acid and traffic markers such as NOx and CO, suggesting that traffic emissions are not a significant source for oxalic acid or its precursors. Perchlorate has received much attention following its discovery in waterways in the US. Perchlorate interferes with the uptake of iodide by the thyroid gland affecting thyroid hormone production. A sensitive and selective method for determination of perchlorate in various matrices using ion chromatography-mass spectrometry (IC-MS) was developed. Perchlorate forms an ion-pair with a dicationic reagent; the ion-pair is detected by MS. The detection limit for perchlorate in real samples was 100 ng/L. A rapid and simple pretreatment technique for the determination of urinary perchlorate was developed. The sensitivity for measuring perchlorate is high using this method since there was no dilution of sample and the matrix effects are reduced. Perchlorate and iodide levels in seaweed samples were measured and their bioconcentration factors were calculated to assess the efficiency of seaweed as a source of iodide. We examined the possibility of the formation of perchlorate in the atmosphere and studied its mechanism. We showed that perchlorate is readily formed when chloride aerosol was exposed to electrical discharge. Many precipitation samples contain detectable levels of perchlorate. This strongly suggests that some perchlorate is formed in the atmosphere and a natural perchlorate background of atmospheric origin should exist.