Although the above cited studies have discussed the product of gaseous nitric acid and ammonia concentrations as a test of equilibrium between these gas-phase species and particulate NH4NO3, in indoor environments the mass of NH4NO3 accumulated on surfaces is anticipated to be much larger than the mass associated with indoor airborne particles. The complicating features of this additional compartment for ammonium and nitrate have not been fully incorporated into efforts to predict or interpret indoor ammonia behavior. Nevertheless, the rapid loss of nitric acid to indoor surfaces, which is not matched by ammonia loss, is likely to result in products of the gas-phase species that are smaller than would be predicted for equilibrium with aerosol NH4NO3. Additional measurements of ammonium concentrations on indoor surfaces, coupled with measurements of gaseous nitric acid and ammonia, would contribute to better understanding of indoor ammonia chemistry.By raising the pH of surface-associated aqueous films, ammonia has the potential to influence the partitioning of various species between the gas phase and indoor surfaces. In chamber experiments , Webb et al. found that elevated ammonia levels promoted the desorption of nicotine from nylon carpet, but not from painted gypsum board. In a more detailed subsequent series of studies, Ongwandee and colleagues examined the impact of NH3 and CO2 on the sorption of N-containing organics to mineral and real-world surfaces. In the experiments most directly relevant to acids and bases in ordinary indoor environments,roll bench the investigators found that the sorption of nicotine to polyester curtain and to carpet increased as the RH increased.
Ammonia at high levels suppressed the sorption of nicotine to carpet at 50% and 90% RH but not at 0% RH. We stress that the ammonia concentrations in these studies were much larger than those routinely observed indoors . This point is particularly relevant for situations where NH3 appears to be competing with N-containing basic organics for surface sites: the behavior at high concentrations may not be directly predictive of effects at much lower levels. Nonetheless, these studies illustrate different mechanisms, including pH modification, through which NH3 can influence the sorption of N-containing organics to real room surfaces. During the HOMEChem campaign, an ammonia cleaner was used on surfaces in the living room and kitchen of the test house either before or after mopping with a vinegar solution.A rapid decrease was observed in the gas-phase concentrations of acidic species when ammonia surface cleaning preceded vinegar mopping. A final note concerns the potential for ammonia to contribute to discoloration of interior surfaces. Updyke et al. demonstrated that when filter samples of secondary organic aerosol generated from both O3-initiated and OH reactions with biogenic and anthropogenic precursors were exposed to 100 ppb of NH3 in humid air, the samples changed from initially white to a red brown color. The extent to which this browning occurred varied with the SOA precursors and ranged from no color change for SOA from isoprene to a strong color change for SOA derived from limonene. In the latter case, the light absorption coefficients for wavelengths 300-700 nm were comparable to values measured for brown carbon from biomass burning. The authors hypothesize that “browning” begins when NH3 reacts with a carbonyl group in SOA constituents forming hemiaminals that subsequently dehydrate into primary imines. Not only is such chemistry anticipated to occur indoors, but it may also occur on indoor surfaces soiled with SOA formed from reactions between ozone and terpenoids or sesquiterpenes.
Such SOA may be close to colorless when first deposited on indoor surfaces, but over time, in the presence of NH3, the chemicals could become “brown,” contributing to discoloration of lightly colored indoor surfaces.Sulfur dioxide and sulfate are prominent contributors to atmospheric acidity. Sulfur, originating as a minor constituent of coal, is oxidized to SO2 when the fuel is burned. In the atmosphere, sulfur can be oxidized from +IV to +VI . That oxidation process is an important factor in the acidifying influence of atmospheric sulfur for two reasons. First, whereas SO2 is moderately soluble in water, sulfuric acid is highly soluble and – in the atmosphere – is almost entirely found in the condensed phase. Second, although both acids are diprotic, sulfuric acid is a much stronger acid than is sulfurous acid . Consequently, atom for atom, the conversion of S to S substantially increases the acidic potency of airborne S. Sulfur dioxide and sulfate play important roles in the acid-base properties of indoor environments, too. In this subsection, we’ll first consider SO2 and its related S species and then discuss sulfate and associated S compounds. We explore sources, dynamic behavior, and fates, especially considering the role affecting the pH of condensed-phase water indoors. The role of sulfate contributing to aerosol strong acidity is further considered in §3.8.Over the past few decades, atmospheric levels of sulfur dioxide have declined in the United States and in Europe owing in large part to reduced sulfur emissions from coal combustion. In 2018, the US national average SO2 concentration, as measured across a network of 287 outdoor air monitoring stations, was 14 ppb . In India, SO2 emissions trended upwards between 1996 and 2010. In China, the temporal patterns of emission rates and concentrations have exhibited variability, with an overall decreasing trend emerging during the past several years.
Over the past three decades, a slight reduction of ambient SO2 has been reported from a monitoring station in South Korea, with an overall mean SO2 abundance of 5.5 ppb for 14 y of sampling during the period 1987-2013. Global anthropogenic SO2 emissions are estimated to have increased between 2000 and 2006 with a declining trend subsequently, through 2011. The presence of SO2 in outdoor air constitutes a major source for SO2 in buildings, it being transported indoors along with ventilation air. In the absence of indoor emission sources, SO2 concentrations in buildings are observed to be lower than the corresponding outdoor concentrations. Table 8 presents a summary from one major US study, in 1977-1978, of measured SO2 levels in residences and outdoors in circumstances in which outdoor air was thought to be the most important indoor SO2 source. If an average I/O ratio of 0.4 is assumed to prevail,drying rack cannabis then the average indoor SO2 concentration in the US in 2018 is estimated to have been approximately 6 ppb for homes with no indoor sources of SO2. Sulfur dioxide is emitted indoors when sulfur-containing fuels are burned and the combustion byproducts are released directly into the indoor space. One potentially important indoor emission source is unvented kerosene space heaters. Even though household-grade kerosene is low in sulfur , unvented combustion of kerosene for space heating can have a discernible impact on indoor SO2 levels. For example, Leaderer et al. measured 24-h average SO2 levels in homes in Virginia and Connecticut during summer and winter . For the wintertime measurements, the average indoor SO2 level in kerosene-heater homes was 16 ppb, 20´ higher than the average level of 0.8 ppb inside homes without kerosene heaters and 4´ higher than the concurrently measured average outdoor level. The maximum 24- h average indoor level in a kerosene-heater home in that study was 107 ppb. Coal used for space heating and cooking can also contribute to elevated indoor SO2 levels. The potential is even greater than with kerosene space heaters because coal has a higher sulfur content than kerosene. Empirical data are not abundant; however, Seow et al. reported a median 24-h average indoor SO2 concentration of 907 µg/m3 for 42 households that used “smokeless” coal as a residential fuel in Yunnan Province, China. Sulfur dioxide interacts with indoor surfaces. In the absence of indoor sources, the associated net loss to surfaces causes concentrations indoors to be lower than corresponding outdoor levels. Indoor surface reactions would also diminish the contributions of indoor emission sources to indoor concentrations. Although not the subject of many systematic recent investigations, studies of SO2 interactions with indoor surface materials were widely undertaken in the 1960s and 1970s.One exception to the historical pattern is the more recent work by Grøntoft and Raychaudhuri, who assessed the humidity dependence of SO2 uptake on a variety of indoor surface materials.
The acidic properties of sulfur dioxide parallel those of carbon dioxide. Sulfur dioxide is moderately soluble ;aqueous SO2 forms diprotic sulfurous acid upon combining with a water molecule. Dissociation liberates the H+ ion and forms bisulfite from a first dissociation reaction, and then sulfite from a second. The respective acid dissociation constants are pKa = 1.86 for H2SO3 and 7.17 for HSO3 – . Compared with carbon dioxide, SO2 has a larger Henry’s law constant; and sulfurous acid is much stronger than carbonic acid. The net consequences are that SO2 can contribute to more acidification of condensed water than does CO2, even with much smaller gas-phase abundance. This point is illustrated in Figure 6, which shows the equilibrium pH for water exposed to SO2 with and without the simultaneous presence of CO2. In this situation, for SO2 levels below about 3 ppb, the pH is a function of both the CO2 and the SO2 levels. For indoor CO2 levels in the range 400-1000 ppm, and with SO2 at or below 5 ppb, the pH spans a range 5.0-5.6. For higher SO2 levels, as might occur from unvented indoor combustion of kerosene, the pH becomes largely independent of the CO2 level and declines to approximately 4.4 at 100 ppb SO2. An interesting feature regarding the fate of indoor SO2 is suggested from a detail in the empirical study of Spengler et al.As displayed in Table 8, the indoor/outdoor SO2 ratio in Kingston, TN , was markedly lower than in any of the other five cities studied . The authors noted that all the homes in Kingston were air conditioned. Keeping windows closed rather than open would have the tendency to decrease I/O ratios simply by reducing the air-exchange rate, a, as is evident from equation . However, it is also plausible that condensing water on air conditioner coils could constitute an additional sink for SO2. For example, water in equilibrium with 1 ppb of SO2 and 800 ppm CO2 would, at a pH of 5.26, contain 3.3 µM of bisulfite. If the air conditioner were to condense and drain 1 L per hour of liquid water equilibrated with 1 ppb of SO2 at pH 5.26, that would correspond to a removal rate of 210 µg/h of SO2. That amount is equivalent to the removal of SO2 by means of 80 m3 h-1 of ventilation, a ventilation rate that could occur in a closed home with air conditioner use. Consequently, aqueous removal of S could constitute a meaningful loss mechanism associated with air conditioning in humid climates. As yet, there are no experimental data with which to directly test this inference.In the atmosphere, over a time scale of hours to days, the sulfur in SO2 is oxidized from S to S, as in sulfate. The oxidation process can occur in the gas phase, initiated by the hydroxyl radical, or in the aqueous phase, e.g. in cloud droplets. Atmospheric sulfate is an important contributor to urban and regional air pollution, prominently featured in issues as seemingly disparate as acid deposition, visibility impairment, and cardiovascular health risk. A major attribute of inorganic S is that it is essentially nonvolatile. It is found in the atmosphere concentrated in condensed-phase components, e.g., in cloud water, rain drops, and particulate matter. When aerosol sulfate is formed from gaseous SO2, most of the sulfur mass is concentrated in particles with diameters in the range 0.1-1 µm. Because particles in this size range penetrate and persist well indoors, and because indoor sources of sulfate are uncommon , fine-mode aerosol sulfate has been used as an indicator of outdoor air’s influence on indoor fine-particle concentrations183 and on personal exposures to fine particle mass. Oxidation of sulfur from S to S strongly influences acidification. Whereas SO2 is moderately soluble and so is substantially gaseous, to a good approximation all of the highly soluble sulfuric acid formed in the atmosphere will be transferred to the condensed phase. In addition, sulfurous acid and bisulfite are considerably weaker acids than their respective counterparts, sulfuric acid and bisulfate .