Mean outdoor values for palmitic and stearic acids were each in the approximate range 40-80 ng/m3. The mean I/O ratio for palmitic acid was 2.1 during cooler weather and 5.8 for warmer periods. The mean I/O ratio for stearic acid was 1.8 during cooler weather and 24 for warmer periods. Hasheminassab et al. subsequently reported on fine PM organic chemical composition for three of these study sites. They determined that “organic acids inside the retirement communities were dominated by indoor sources .” Total fine particle organic acid concentrations were in the approximate range 0.2-1.7 µg/m3 . Speciated concentrations were not reported. Human skin lipids contain a noteworthy abundance of n-alkanoic carboxylic acids, spanning a broad range of carbon numbers. Among the most prominent of these compounds are palmitic acid , myristic acid and stearic acid . Weitkamp et al. analyzed the fatty acids extracted from the hair of barber shop sweepings and detected the presence of nalkanoic carboxylic acids with carbon numbers ranging from 7 to 22 ; palmitic and stearic acids were especially abundant. Through the routine shedding of particles from the human envelope, one can anticipate that occupants are primary sources of these carboxylic acids in occupied spaces. The presence in indoor dust of squalene, a major skin lipid, reinforces the idea that occupants constitute emission sources of skin lipids to indoor environments. Daher et al. reported on the chemical characterization of both fine and coarse particles “inside the refectory of Santa Maria Delle Grazie Church, home of Leonardo Da Vinci’s ‘Last Supper.’” This highly controlled environment was well protected from the influence of outdoor air pollution. The investigators found, however, that “fatty acids … had high indoor-to-outdoor concentration ratios … showing a good correlation with indoor [fine-particle organic carbon mass concentrations],weed trimming tray implying a common indoor source.” In their supporting information, the authors report monthly concentrations of indoor n-alkanoic carboxylic acids from C14 through C29.
Averaged across all months, the three most abundant species were myristic , palmitic and stearic acids, with respective mean concentrations of 31, 27, 9.6 ng/m3 , which sum to 80% of the total for all n-alkanoic acids . Daher et al. noted that “potential indoor sources include skin emissions from visitors….” Kristensen et al.15 reported on time resolved measurements of gaseous and submicron particle-phase semivolatile organic compounds from a weeks-long sampling campaign in a normally occupied single-family home in northern California. That study identified cooking as an important source of indoor SVOCs, especially in the particle phase. The authors reported that, “the most abundant compounds related to cooking events include straight-chained saturated and unsaturated fatty acids .”Given this perspective, it should not be surprising that Liu et al.69 found dicarboxylic acids to be prominent organic components accumulated in indoor window films. Specifically, dicarboxylic acids with carbon numbers in the range 6 to 14 were the second or third most abundant class for most samples, behind monocarboxylic acids and comparable to n-alkanes . Among the dicarboxylic acids, azelaic acid, a product of ozone reacting with oleic acid, was generally the most abundant. Surface densities were highly variable across samples, with the highest reported value for azelaic acid being 7.3 µg m-2 on the indoor surface of an urban laboratory site in Toronto. Liu et al. inferred from their data that, “the greater accumulation of dicarboxylic acids in indoor rather than outdoor window films suggests indoor sources such as cooking.” With the high propensity to be in the condensed phase, it is worthwhile to consider whether dicarboxylic acids could materially influence the pH of indoor aqueous surface films. Consider the example of a surface film density of azelaic acid being 7.3 µg m-2. Assume that this abundance represents the sum of undissociated azelaic acid plus the two conjugate bases. Consider the influence on pH of surface water of this abundance of azelaic acid in isolation.
We do not have data on the abundance of water in the surface films studied by Liu et al. For exploration, consider three possibilities, corresponding to surface water thicknesses of 1 nm, 3 nm, and 10 nm. Also, assume that the surface water behaves thermodynamically like bulk water. Finally, neglect any substrate effects on aqueous film chemistry. This set of assumptions along with the reported pKa values in Table 17 allow for calculation of the equilibrium pH in the surface water. The results, in relation to the water film thickness, are pH = 3.0 for 1 nm, pH = 3.2 for 3 nm, and pH = 3.5 for 10 nm. Evidently, with such a highly favored aqueous phase, even the relatively weak azelaic acid can be sufficiently abundant to strongly acidify thin water films on indoor surfaces.In their classroom monitoring study, Liu et al.13 reported measurement results for 14 “diacid/hydroxycarbonyl acid ” compounds in the gas phase. Oxalic and malonic acid were reported as non-detectable indoors, even though there were substantial concentrations in outdoor air . The three most abundant diacids in indoor air reported in this study were succinic acid , glutaric acid , and adipic acid . The study by Liu et al.13 represents the most extensive and detailed set of gas-phase indoor organic acid data reported to date. Their supplemental information reports time average indoor and/or outdoor concentrations for 155 species. Table 18 reproduces the indoor and outdoor concentrations for the 18 species for which the time-averaged indoor concentration exceeded 10 ppt. Half of these species were reported as “not detected” in outdoor air. Among the remaining nine, the ratio of average indoor to average outdoor concentrations ranged from 4 to 25 , with a median ratio of 8. The consistently high I/O ratios reflects the importance of indoor emission sources for this group of abundant species. Wisthaler and Weschler326 have shown that these oxoacids are major secondary products of ozone/squalene chemistry, noting that squalene is a primary component of human skin lipids.In an extensive monitoring campaign undertaken in an ordinarily occupied single-family residence, Liu et al. reported on the gas-phase concentrations of a few other organic acids in addition to several n-alkanoic carboxylic acids. With tentative species identification, they reported that the time-average indoor concentration of acrylic acid was ppt during summer monitoring and 312 ppt during winter.
Analogously, glycolic acid was reported at 32 ppt for summer and 36 ppt for winter. Methanesulfonic acid was found to be present at an average abundance of 35 ppt in the summer and 115 ppt in the winter. In each case, the I/O ratio was well above 1.0, implicating indoor sources as important contributors to indoor concentrations. That study also reported an observation regarding a dicarboxylic acid: “Spikes of C2H3O4 + were observed during some occasions of sautéing in the summer.”The estimated average oxalic acid concentration in the summer season in the single-family residence was 16 ppt; in the winter, the average level was not stated, indicating that it was below the 10 ppt reporting threshold.In a follow-up investigation in Portugal, Alves et al. sampled PM10 inside and outside of a primary school classroom in the Aveiro city center during the winter and spring of 2011. They conducted detailed chemical analyses of composited samples, including measurements of diacids, cannabis grow setup ketoacids and aromatic acids. Table 20 records the reported indoor and outdoor concentrations for eight acidic species whose individual concentrations exceeded 10 ng/m3 . A striking feature is the extraordinarily high indoor concentration of malic acid. Alves et al. remarked that, “the fact that this acid is found in many sour or tart-tasting foods can eventually justify its detection at such high levels in indoor particles. The most common use of malic acid is in candy and potato chips.”Dehydroabietic acid and abietic acid are also known as “resin acids,” as they occur in tree resins. Resin acids occur in certain soaps. They are prominently emitted organic compounds from biomass burning. Noonan and coworkers have reported on indoor concentrations of abietic acid and dehydroabietic acid in PM2.5 samples collected in homes that used wood stoves for heat. The studies were conducted in association with a remediation program to improve the impact of wood stove use on ambient PM2.5 levels. Sampling in 16 homes, Ward and Noonan reported average ± standard deviation indoor concentrations before the remediation to be 80 ± 61 ng/m3 for dehydroabietic acid and 3.7 ± 5.7 ng/m3 for abietic acid. Corresponding results for 21 homes as reported by Noonan et al. were 102 ± 73 ng/m3 for dehydroabietic acid and 8.8 ± 20 ng/m3 for abietic acid. The higher concentrations after remediation were attributed by the study authors to the more effective heating of fuel prior to its combustion in the higher efficiency stoves, leading to enhanced release into indoor air of these semivolatile wood constituents. Many studies have reported outdoor concentrations in the gas and/or particle phase for dicarboxylic and other organic acids reflecting urban and regional air quality concerns.
In summarizing selected results here, we focus on sampling conducted in urban and suburban environments, rather than in the more remote portions of the atmosphere, because of the implicit connection of urban studies to larger numbers of indoor environments and therefore greater relative significance for indoor air quality concerns, including human exposure. An early report by Kawamura and Kaplan characterized outdoor dicarboxylic acids in gas plus particle phases in the Los Angeles area from sampling during summer and autumn of 1984. They concluded that “oxalic acid is the dominant species.” Considering the sum of C2-C6 plus C9 , the total average concentration ± standard deviation for 12 atmospheric samples was 8.3 ± 4.5 nmol/m3 . The three most prominent species were oxalic acid , succinic acid and adipic acid . An early study in Tokyo sampled at intervals between late spring and autumn 1989. In that study, dicarboxylic and ketocarboxylic acids were assessed for the particle-phase only, with no particle size cutoff. The total average mass concentration of n-alkanoic dicarboxylic acids spanning C2 to C10 was 440 ng/m3 with the three most prominent species being oxalic acid , malonic acid and succinic acid .Among the total of 24 reported acids, only two other species had reported average concentrations above 30 ng/m3 : pyruvic acid and glyoxylic acid . Altogether, diacid concentrations averaged 540 ng/m3 and ketoacids 98 ng/m3 .Rogge et al. conducted detailed organic chemical composition analysis for fine particles collected outdoors at uniform intervals for year 1982 at four sites in the Los Angeles area. The average concentration of total aliphatic dicarboxylic acids was 239 ng/m3 . The four most abundant species were succinic acid , malonic acid , azelaic acid , and glutaric acid . These four species contributed 70% of the total mass concentration reported for aliphatic dicarboxylic acids. Oxalic acid was not reported. Khwaja collected and analyzed seven atmospheric samples collected over two days during October 1991 in a semiurban area of New York state. They reported concentrations of oxocarboxylic, ketocarboxylic, and dicarboxylic acids in the particle phase. Average ± standard deviation levels were 231 ± 118 ng/m3 for oxalic acid, 119 ± 44 ng/m3 for succinic acid, 84 ± 20 ng/m3 for malonic acid, 59 ± 21 ng/m3 for pyruvic acid, and 44 ± 16 ng/m3 for glyoxalic acid. Several recent studies have reported particle-associated organic acids sampled from outdoor air in and near Beijing, China. 336-339 Results from one illustrative study are highlighted in Table 21, which reports a subset of species for which the annual average ambient concentration of the analyte in PM2.5 was above 10 ng/m3 . Several dicarboxylic acids are featured, with oxalic acid being the most abundant. Seasonally, the average ± standard deviation for total dicarboxylic acid concentrations varied from a low of 366 ± 261 ng/m3 in autumn to a high of 763 ± 701 ng/m3 in winter. Among the other prominent organic acids quantified in PM2.5 in Beijing are phthalic acid and terephthalic acid, whose structures and thermodynamic properties are illustrated in Figure 16 and its caption.Cooking is a major air pollutant emission source. Even though most cooking occurs indoors, because of the much greater overall research emphasis on outdoor air pollution, most studies on emissions of organic acids from cooking activities have focused on larger-scale cooking operations, e.g. as practiced in restaurants or in the food-preparation industry, rather than from residential cooking. Abdullahi et al. have reviewed emissions from cooking of particulate matter and associated chemical components.