Prominently featured constituents are the unsaturated fatty acids and dicarboxylic acids. We’ve already summarized the few instances in which organic acids have been directly measured indoors in relation to cooking as an emission source. Here, we highlight some studies that have assessed emission factors of organic acids associated with cooking. Because the cooking activities were at restaurant or industrial scale, these results should be considered as indicative rather than quantitatively accurate with regard to emission factors expected from residential cooking. The first major study to quantify emission factors of particle-associated organic acids from cooking focused on hamburger meat. The study contrasted griddle frying with charbroiling, and considered the effect of fat content of the meat. Prominent among the emitted compounds were two unsaturated fatty acids: palmitoleic and especially oleic acid. Emission factors were about an order of magnitude higher for oleic acid than for palmitoleic acid across fat content and cooking style. The highest reported emission factor was for charbroiling “regular” beef over a natural-gas flame: 570 mg of oleic acid emitted per kg of meat cooked. With “extralean” beef , the oleic acid emission factor declined to 82 mg/kg when charbroiled. A 50:50 mixture of regular and extralean beef, when fried, emitted 11 mg/kg of oleic acid. Emission factors were also reported for three dicarboxylic acids: succinic, glutaric, and adipic. Summed across the three species, the emission factors were 40 mg/kg for charbroiling regular beef, 22 mg/kg for charbroiling extralean beef,grow rack systems and 5 mg/kg for frying beef with intermediate fat content. Schauer et al. reported more detailed chemical characterization from industrial-scale hamburger charbroiling, using beef with 20% fat.
Prominent among the organic species quantified in fine particle emissions was oleic acid, with an average emission factor of 214 mg emitted per kg of meat cooked. Other emission factors reported for unsaturated organic acids were 18 mg/kg for palmitoleic acid and 32 mg/kg for linoleic acid. Among dicarboxylic acids, emission factors were reported for adipic acid at 2 mg/kg and suberic acid at 4 mg/kg. A subsequent investigation by Schauer et al. assessed organic emissions of industrial-scale cooking with seed oils. Specific cooking processes were “vegetables stir-fried in soybean oil, vegetables stir-fried in canola oil, and potatoes deep fried in hydrogenated soybean oil.” The authors found that “carbonyls and fatty acids make up a significant portion of the organic compounds emitted from all three seed oil cooking procedures.”Reported emission factors for the three most prominent n-alkenoic acids across the three types of procedures were in the respective ranges 1.9-6.3 mg/kg for oleic acid, 1.8-4.2 mg/kg for linoleic acid, and 0.08-0.31 mg/kg for linolenic acid. He et al. assessed organic composition of fine particles emitted from Chinese commercial restaurants that respectively used “Hunan” and “Cantonese” styles of cooking. They reported emission factors normalized as mass of analyte per mass of particles collected . Across the two cooking styles, for n-alkenoic acids, the summed emission factors were in the range 11-12% and the most abundant compounds were 9,12-octadecadienoic acid and 9-octadecenoic acid. Total contributions of dicarboxylic acids to fine PM mass were in the range 0.62-0.85% . The most abundant species was azelaic acid followed by suberic acid. Zhao et al. assessed the organic composition of fine particles emitted from restaurants with four different styles of Chinese cooking: Cantonese, Sichuan, Dongbei, and Hunan. They reported emission factors as mass of analyte per mass of particulate organic matter.
Among the unsaturated fatty acids, oleic acid was the most abundant species, with average emission factors in the range 14 mg/g to 29 mg/g . Linoleic acid emissions were in the range 3 mg/g to 14 mg/g . Emission factors also were reported for eight dicarboxylic acids. The totals range from 1.1 mg/g to 3.2 mg/g . Among the dicarboxylic acids, the most abundant species was azelaic acid, accounting for roughly half of the total. Zhao et al. contrasted these emissions from Chinese restaurants with “Western style fast-food cooking” as practiced in China. The reported emission factors were much higher for the Western style of cooking. For example, oleic acid emissions were 76 mg/g for Western-style fast food cooking compared with an average of 20 mg/g for the Chinese styles. For quantified dicarboxylic acids, the Western-style cooking emission factor was 58 mg/g compared to an average of 2 mg/g for the Chinese styles. Another important feature of cooking that might be associated with indoor organic acids is the potential for elevated health risks from exposure to cooking oil fumes. In particular, epidemiological research has revealed an excess incidence of lung cancer among never smoking Chinese women that is associated with high-temperature cooking. Qu et al. have found that the “condensates of volatile emissions from rapeseed and soybean cooking oils [are] genotoxic in short-term tests.”Shields et al.studied the mutagenicity of substances emitted from “a variety of cooking oils heated to the temperatures typically used in wok cooking.”They reported that “the mutagenicity of individual volatile emission condensates was correlated with linolenic acid content.” They also determined that “condensates from heated linolenic acid, but not linoleic or eruric acid, were highly mutagenic.”Other emitted organic compounds of interest in this study included 1,3-butadiene, benzene, and acrolein.In general, the incomplete combustion of carbonaceous materials will generate organic acids among many other partially oxidized compounds. We have already discussed some evidence regarding organic acid emissions associated with heating and cooking.
Other indoor combustion activities that could emit organic acids are considered here, in brief because the empirical evidence regarding emissions remains limited. Kuo et al. measured emission factors associated with incense burning. They measured carboxylates in PM2.5. They tested four samples of incense and evaluated the emission factors in units of µg of analyte per g of incense.The sum of the remaining quantified carboxylates was 37 µg/g, of which the four most prominent were oxalate , maleate , phthalate and succinate . The gaseous acetic acid emissions exceeded the PM2.5 acetate emission rate. The combustion and heating of tobacco remains a major indoor emission source of organic contaminants. In recent studies, emission rates were measured for a suite of organic acids for different types of tobacco products including conventional cigarettes and e-cigarettes346 and heat-not-burn tobacco cigarettes. The emission rates were assessed in units of mass emitted per time. In all, results were reported for 18-19 organic acids. Summed over all reported species,rolling flood tables organic acid emission factors ranged from 24 µg/h for heat-not-burn cigarettes without menthol to 323 µg/h for conventional cigarettes. The most prominent species from conventional cigarettes were palmitic acid , linoleic acid , myristic acid , eicosanoic acid , docosanoic acid , dodecanoic acid , and tetracosanoic acid .These seven species comprised 80% of the emissions of organic acids for conventional cigarettes. Although the apportionment varies, these are also the more prominent of the species emitted from the other tobacco products, summing to proportions in the range of 79-84% of the respective totals.Fox et al. have suggested that particle-associated organic acids may be suitable markers of airborne bacteria in occupied spaces. They note that “muramic acid … is found in almost all bacteria whereas 3-hydroxy fatty acids … are found only in Gram-negative bacteria.” They demonstrated that the abundance of these compounds in elementary school classrooms was elevated to levels well above those in outdoor air when the classrooms were occupied, yet were lower than outdoor levels when the classrooms were unoccupied. Abundances were small, with average indoor concentrations during occupancy of 0.9-3.5 picomoles/m3 for C10, C12, C14, andC16 fatty acids, and 7 picomoles/m3 for muramic acid. With a molecular weight of 251 g/mol, the corresponding average mass concentration of muramic acid would be 1.8 ng/m3 . Terpenes and related compounds are commonly found at elevated levels indoors. Among the common indoor sources are cleaning products and air fresheners. Indoor reactions between ozone and terpenes have been demonstrated to produce secondary particulate matter indoors. Glasius et al. have shown that reactive chemistry between ozone and terpenes produces dicarboxylic, oxocarboxylic, and hydroxyketocarboxylic acids that contribute to secondary organic aerosol .
Fick et al. demonstrated that the ozonation of a-pinene in a model ventilation system generated norpinic acid, pinic acid, norpinonic acid, and pinonic acid. Larsen et al. studied the chemical products resulting from ozonation of b-pinene under “conditions relevant for indoor environments.” They quantified “thirteen reaction products … in SOA, most of which being multifunctional carboxylic acids and carbonyls. Cis-pinic acid was the most abundant compound ….”Ozone-initiated reactions with unsaturated organic compounds can transform already existing organic acids and also create new organic acids indoors. Among the unsaturated organic compounds that are commonly present in occupied indoor spaces are squalene and fatty acids found in skin lipids.Also prominent are fatty acids that originate from cooking and from materials used in manufacturing and maintenance of indoor furnishings, such as linseed and tung oil. Thornberry and Abbatt studied the product yields and kinetics for ozonation of surfacebound unsaturated fatty acids: oleic, linoleic, and linolenic acid. Volatile products generated included the aldehydes hexanal, nonanal, and nonenal. Wisthaler and Weschler investigated the reactions of ozone with skin oils, focusing on the loss of ozone and the production of volatile products. They concluded that “reactions between ozone and human skin lipids reduce the mixing ratio of ozone in indoor air, but concomitantly increase the mixing ratios of volatile products and, presumably, skin surface concentrations of less volatile products.” Zhou et al. investigated the condensed-phase end products resulting from the ozonation of surface-bound squalene and skin oils. In the investigation of skin oil, the researchers reported that “upon oxidation with 50 ppb ozone for 90 min, there is a rapid loss of alkene, fatty acid, and triglyceride signals resulting from efficient multiphase ozonolysis. Oxygenated products [were identified], including a variety of carboxylic acids.”In the investigation of squalene, they found that, under dry conditions, “major condensed-phase end products were levulinic acid and succinic acid . Under humid conditions , the major end products were 4-oxopentanal, 4-oxobutanoic acid, and LLA.”Note that levulinic acid is also known as 4- oxopentanoic acid. Liu et al., in their classroom study, identified oxopentanoic acid, along with oxobutanoic acid and succinic acid as abundant indoor acids with high I/O ratios . These studies illuminate an important point, that the acids in indoor air not only exert their influence on indoor air composition through altering the pH of condensedphase water, some also are active participants in oxidative chemistry.Airborne particles may contain condensed water. The pH of this water is influenced by the presence of acidic and basic gases. Soluble minerals can also affect the pH of the aqueous phase in airborne particles, especially in the coarse size mode . In turn, the pH of aerosol water can influence important aspects of atmospheric chemistry, such as the phase partitioning and fate of acidic and basic gases, the dissolution of metals, and the rates of acid-catalyzed chemical reactions. Also, visibility impairment and acid deposition processes are affected by particle-phase acidity. Health concerns are associated with inhalation exposure to particle-phase strong acids. Considering airborne particulate matter overall, current scientific and public policy efforts focus on fine particle mass concentration, as measured through PM2.5 . Notwithstanding the current emphasis, there is a substantial history of research that aims to understand whether certain components of the particle phase might be specific causal agents of adverse health effects. Some of this work has focused on aerosol strong acids. A phase shift of acidic species from gas to particle would change the deposition pattern upon inhalation. Gaseous acids tend to be removed by dissolution in the upper airways whereas acids associated with fine particles can penetrate and deposit deeply in the lung. In an epidemiologic study that utilized data from 24 communities in the US and Canada, Dockery et al. wrote that, “Children living in the community with the highest levels of particle strong acidity were significantly more likely … to report at least one episode of bronchitis in the past year compared to children living in the least-polluted community.”However, they also went on to state that, “neither asthma, wheeze, cough, nor phlegm, were associated with levels of particle strong acidity for these children living in a non-urban environment.” A decade earlier, Lippmann provided this strong caution about the state of knowledge, writing that “we cannot adequately describe the nature and extent of the effects of the inhalation of acidic pollutants on human health at this time.