Liquid water can be a component of airborne particles. This feature is understood to be important in several atmospheric processes, including the roles of aerosols influencing climate, the phase partitioning of water-soluble organic compounds, and the formation of secondary organic aerosol. Liquid water in particles is prominent, even in the absence of clouds. “Liquid water is predicted to be the most abundant particle-phase species in the atmosphere, 2-3 times total aerosol dry mass globally.” Notwithstanding its influence on atmospheric physical and chemical processes, and despite its relative abundance compared to dry aerosol constituents, the condensed phase normally represents a small proportion of tropospheric water molecules. At a temperature of 20 °C and relative humidity of 50%, the mass concentration of water vapor is 8.4 g/m3 and. Outside of fog and clouds, the abundance of aerosol liquid water is commonly at least five orders of magnitude smaller, usually below 100 µg/m3 . Meng et al.used thermodynamic modeling combined with extensive year 1987 measurements of aerosol chemical characterization to estimate the liquid water content of PM2.5 and PM10 for three urban sites near Los Angeles, California, considering separately winter and summer seasons. Using sampling durations of 4-7 h, the median aerosol liquid water contents in PM10 were generally in the range 4-17 µg/m3. The 90th percentile values for PM10 by location and season ranged from 42 µg/m3 at Long Beach during summer to 143 µg/m3 at Long Beach during winter. Nguyen et al. applied thermodynamic modeling to estimate the aerosol liquid water from aerosol mass spectrometry data in several field campaigns. Note that the AMS mainly measures sub-micron particles and so would not capture completely the liquid water associated with PM2.5 or PM10. Nguyen et al. report that “campaign average ALW mass amounts are 12, 11, and 3 µg/m3 for urban, urban downwind, and rural sites, respectively.” Parworth et al. reported an average of 19 µg/m3 for the water content associated with PM2.5 for wintertime conditions in Fresno, California.

Diurnal variability produced lower values in the afternoon and higher values during overnight and early morning periods. Indoors,cannabis drying recent studies are starting to provide some information about aerosol liquid water and its potential significance. Water-soluble organic compounds and indoor aqueous chemistry is highlighted in the work of Duncan et al.They made the important observation that “even a 1 nm water film on indoor surfaces, a film consistent with simple water adsorption, will provide more than 1000 times the volume of liquid water as is found in aerosols in outdoor air .” Note that 3 µg/m3 of aerosol water, chemically equilibrated with air, provides a contribution to the liquid water content of only L* = 3 ´ 10-9 L m-3, considerably smaller than the range expected to prevail indoors or than the contributions of the other forms of water that we have highlighted in this review. DeCarlo et al. inferred an important role for aerosol liquid water in their study of third-hand tobacco smoke. They made an interesting and potentially important observation regarding the role of heating, ventilation and air-conditioning systems influencing water in particles: “In the summertime, warm air with varying amounts of water content is brought into the building, mixed with recirculated air, and conditioned to cooler temperatures … for the supply airstream. This process leads to deliquescence and significant uptake of water by aerosol particles, as RH values will increase to above 90% in the supply air …. Even with the subsequent decrease in RH of the rooms, all of the indoor aerosol will maintain the aqueous phase because the indoor RH does not drop low enough to drive off the water. … In the wintertime, the temperature gradient is reversed with colder, drier outdoor air drawn into the HVAC system mixed with recirculating air and heated to temperatures approaching 38 °C …. This process effectively effloresces the aerosol particles, drying them and resulting in the loss of the aqueous phase in the aerosol.” The first quantitative determination of aerosol liquid water indoors was recently reported by Avery et al.Their study site was a university classroom in Philadelphia, PA. They monitored chemical composition of sub-micron particles indoors and outdoors during both winter and summer periods, using aerosol mass spectrometry.

Aerosol liquid water content was then computed using a thermodynamic model. A key finding was much higher abundance of aerosol liquid water outdoors than indoors, during both summer and winter periods. “Aerosol liquid water in winter has an average outdoor and indoor concentration of 2.6 ± 3.6 µg m-3 outdoors and only 0.11 ± 0.06 µg m-3 indoors. In summer, the decrease in concentrations upon transport indoors is much smaller, and similar to aerosol species at 2.7 ± 2.5 µg m-3 outdoors and 0.53 ± 0.24 indoors.” Water is an important constituent in indoor environments for many reasons. Among these are the partitioning and dynamic behavior of acids and bases. As reviewed in this section, water is manifest indoors in several forms: as water vapor, in bulk condensed liquid, sorbed to interior materials, in surface films, and in particulate matter. The abundance of water vapor is large, on the order of grams per m3 , but the direct influence of water vapor on indoor acids and bases is small. Bulk condensed water can be as large in abundance as water vapor. Acids and bases can partition into bulk condensed water from the gas phase and undergo acid-base chemistry therein. The thermodynamics of this system are generally well understood, but much of the bulk water may not become equilibrated owing to mass-transport limitations. Sorbed water can also be abundant indoors at a scale comparable to water vapor. As described by sorption isotherms, the abundance of sorbed water tends to increase monotonically with increasing relative humidity under equilibrium conditions. However, equilibrium may not be consistently attained for water sorption in indoor environments. Furthermore, the properties of acids and bases in water sorbed to common indoor materials are largely not understood. Surface-film water and aerosol water are far less abundant than the other forms of indoor water; but, water in these forms is highly accessible to gaseous species. Hence, some important acid-base processes may be modulated to meaningful extents by water in these less abundant forms. In this major portion of the review, we describe the state of knowledge regarding specific acids and bases indoors, emphasizing species that can be airborne, either as gases or in the particle phase. We organize the material according to species or groups of species that share core chemical characteristics.

We consider in separate subsections inorganic acids and organic acids. Among the inorganic gaseous acids, we discuss carbon dioxide , sulfur oxides , nitrogen oxides , and chlorinated acids . Particle-phase strong acidity is described in §3.8. Among the organic acids, we discuss n-alkanoic monocarboxylic acids , as well as dicarboxylic, n-alkenoic acids, and several other organic acids . The most important airborne basic species is ammonia; it is the subject of §3.2. Amine bases other than ammonia along with amino acids are the subjects of §3.9. Nicotine, an important indoor base resulting from tobacco smoking and vaping, is the topic of §3.10. In reviewing the states-of-knowledge for acids and bases, we summarize information about indoor concentrations along with the sources and sinks that account for their abundance. We devote substantial attention to the key physicochemical properties that influence phase partitioning and fates indoors. We highlight key reasons for concerns about the presence of these species indoors,greenhouse benches including possible effects on human health and well being and also material damage concerns. Two thermodynamic properties consistently influence indoor dynamic behavior of acids : the water-air partitioning coefficient and the propensity to donate a proton in aqueous solution. Another important attribute, especially for organic compounds, is the tendency to partition into condensed-phase weakly polar organic matter. These properties are quantified through the Henry’s law constant, KH, the acidity constant, pKa , and the octanol-air partition coefficient, Koa.Human ammonia emissions occur from breath, skin, flatulence, urine and feces; rates are highly variable among individuals. Over time, microbes transform urea in urine and feces to NH3; hence, diapers and unflushed toilets also are NH3 sources. As reported in Lee and Longhurst, early estimates of human emission rates included 540 g NH3-N y-1 person-1 ; 250 g NH3-N y-1 person-1 ; and 1300 g NH3-N y-1 person-1 .Based on typical NH3 concentrations in blood , “alveolar blood-gas equilibration alone should lead to an NH3 level of 15-40 ppb in exhaled air.”Special experimental techniques are required to disentangle breath emissions from skin emissions. Larson et al. concluded from a series of clever breath sampling experiments that the NH3 concentration in exhaled breath “is determined largely by the last segment of the respiratory tract traversed.” When the last segment traversed was the mouth , the exhaled concentration spanned the range 40-740 ppb with a central tendency of about 240 ppb; when it was the nose , the exhaled concentration was 10-90 ppb with a central tendency of 35 ppb. The higher level in the mouth was partially attributed to bacterial decomposition of urea in saliva. Norwood et al. studied the influence of different oral hygiene regimes on NH3 in exhaled breath.

A distilled water rinse or tooth brushing followed by a water rinse had little effect on NH3 levels. In contrast, an acidic oral rinse reduced the concentration in exhaled breath by more than 90% in all volunteers; breath levels returned to 50% of initial value within an hour. The acidic rinse presumably lowers saliva pH, increasing the ratio of NH4 + /NH3 in saliva and thereby decreasing the fraction of NH3 that volatilizes to breath. Using a cavity ring-down spectrometer, Schmidt et al. measured concentrations of NH3 in breath exhaled through the nose and though the mouth of 20 healthy subjects. The values for nose exhalation agree with those reported by Larson et al., while those for mouth exhalation are in better agreement with Norwood et al. Schmidt et al. observed that an acidic mouth rinse reduced the median level for nose- and mouth-breath to 21 ppb. Based on a review of the literature through 2014, Mochalski et al. estimated a breath emission rate of 91 nmol min-1 person-1 , which corresponds to an average breath concentration of approximately 210 ppb at a volumetric breathing rate of 15 m3 /day. Schmidt et al. measured NH3 emissions from skin of 20 subjects. They reported a median NH3 emission rate of 0.3 ng cm-2 min-1 from the forearms of subjects who had washed their skin and tried to minimize sweating prior to measurements. In their review, Mochalski et al. estimated a total human skin emission rate of 514 nmol min-1 person-1 . In subsequent experiments, Furukawa et al. reported a median emission rate of 270 ng cm-2 h-1 from the forearms of five male and five female volunteers. This average value is 15 times larger than that reported by Schmidt et al. The larger emission rates may have been due to the sampling method, which entailed passive samplers that were sealed to the skin; sweating likely occurred during the 1-hour sampling period, enhancing NH3 emission. Furukawa et al. also measured emission rates at 12 other body locations and summed the emissions from different body locations to obtain whole-body emission rate estimates . For males the range of these estimates spanned a factor three ; for females, somewhat lower values spanned a factor of two . The average whole-body skin-emission rate of NH3, likely enhanced by the sampling method, was estimated to be 5.9 ± 3.2 mg h-1, equivalent to 43 ± 23 g NH3-N y-1 person-1 . Recently, human ammonia emissions have been measured under a variety of conditions in carefully controlled chamber experiments. In eighteen experiments, most with two male and two female volunteers, NH3 emissions were quantified at different temperatures, relative humidities, fraction of exposed skin, and absence/presence of ozone. The investigators found a strong positive correlation between NH3 emission rates and temperature. For fully clothed adults and seniors, the calculated emission rate was 0.41 mg h-1 person-1 at 25 °C, 0.77 mg h-1 person-1 at 27 °C, and 1.4 mg h-1 person-1 at 29 °C. Emission rates also increased with an increase in exposed skin. Relative humidity had only a moderate impact on emission rates, while ozone had no detectable influence.