In the presence of liquid water, each molecule of SO2 that is converted to sulfuric acid can liberate up to two aqueous H+ ions along with the sulfate ion . When considered per atom of sulfur, the net effect of the oxidation reaction occurring in the presence of liquid water is to change the most preferred state from gaseous to aqueous , with the associated liberation of H+ ions. Guo et al. studied the pH of fine-mode particulate matter in the southeastern United States. They found mean pH at four study sites to be in the range 1.1-1.3 in summer and at two study sites to be 1.8 and 2.2 in winter. The authors noted that “in the southeastern USA, inorganic ions [in fine-mode aerosol] are currently dominated by sulfate and ammonium.”Figure 7 illustrates the influence that the oxidation of S to S could have on pH of condensed-phase water indoors. For example, with 1 ppb of SO2 in an indoor environment , water would have an equilibrium pH of 5.37. Given an assumed liquid water content of 1 L per 400 m3 of interior volume , converting half of that SO2 to sulfuric acid that is fully transferred to the water would lower the pH to 4.77. Complete oxidation would further reduce the pH to 4.49. Previous discussion has summarized some of the evidence indicating that sulfur oxidation reactions may occur on certain indoor surfaces. Conversely, there is not strong evidence to suggest that significant gasphase oxidation of S to S occurs indoors. In indoor air, whereas SO2 would primarily exist as a gas, indoor sulfate is predominantly in airborne particles. Like SO2, evidence supports a view that, in most circumstances, the principal source of indoor sulfate is supply from outdoor air via ventilation. Table 9 provides a summary of measurement results from one major field campaign investigating indoor and outdoor sulfate levels in residences. In that study, the use of unvented kerosene space heaters was associated with an indoor sulfate concentration several times higher than in homes without unvented kerosene heaters.In a source-apportionment study for fine particles and associated elements in residences in New York state, Koutrakis et al.reported that, “for homes with kerosene heaters, approximately 40-50% of the sulfur was found to be contributed by kerosene burning.” Ruiz et al.also reported substantially elevated indoor concentrations of fine-particle sulfate and SO2 in homes in Santiago, Chile that had kerosene heaters.

In another study,commercial greenhouse supplies “increased indoor concentrations of sulfates were found to be associated with smoking and also with gas stoves.”The associated sulfate increase was 0.046 µg/m3 per cigarette for smoking in a fully air-conditioned house and 1.1 µg/m3 from use of a gas stove. In the case of smoking, the surprising origin of sulfur was inferred to be the matches used to light cigarettes, rather than the cigarettes themselves. For gas cooking, a likely source is sulfur-containing odorants, such as methyl mercaptan , added to the fuel for safety.Wallace and Williams reported measurements of fine-particle sulfur for 36 households in Research Triangle Park, NC. They monitored each home for seven consecutive days in each of four seasons, relying on 24-h average sample results. They concluded that “sulfur has few indoor sources” in the houses they studied. The average indoor to outdoor fine-particle sulfate ratios per household varied across the range 0.26 to 0.87 . With summer AC use, mean I/O ratio was 0.50 as compared with 0.62-0.63 for the other three seasons. Based on the reported means for 36 houses , the mean fine-particle sulfate levels were 3.4 µg/m3 indoors and 6.2 µg/m3 outdoors, with a corresponding average I/O ratio of 0.56.184 Besides ventilation, the major removal process for SO2 from indoor air is deposition onto interior surfaces, as parameterized using the deposition velocity and with its influence quantified by equation . The possibility that SO2 is also removed incidentally by aqueous scrubbing when air conditioning causes water condensation was discussed. For fine-particle sulfate, in addition to ventilation, there would generally be two important mechanisms to consider for removal from indoor air. These are deposition to indoor surfaces and active removal by particle filtration in the mechanical ventilation or central air handling system. Less common, but worth noting, would be removal by means of a portable recirculating air filter.The deposition of fine-particle sulfate to indoor surfaces has two important distinctions from the case of gaseous SO2. First, fine-mode particles can be assumed to adhere without limit to indoor surfaces that they contact. As a result, the rate of uptake of sulfate particles is purely mass-transport limited, whereas for SO2, both mass transport and surface chemistry could influence the overall rate. Second, fine mode particles are transported by diffusive processes much more slowly than are gas molecules. Consequently, the mass-transport limited deposition velocity of SO2 to indoor surfaces would be much larger than the deposition velocity of fine-mode particle sulfate. Overall empirical evidence suggests a somewhat larger deposition velocity for SO2 compared to fine particle sulfate, with the former increasing with increasing RH.

Sinclair et al.reported on studies of the concentrations and fates of ionic substances in telephone switching offices in Wichita, KS; Lubbock, TX; Newark, NJ; and Neenah, WI. The fine-mode sulfate deposition velocities at these four sites were in the range 0.004- 0.005 cm/s. Riley et al.modeled the indoor/outdoor relationship for fine particle sulfate for different prototypical building types. For two residential scenarios, the predicted I/O values were 0.44 for a continuously operating central air system and 0.95 with high levels of natural ventilation. For offices, with mechanical ventilation, the predicted ratios were 0.18 with a high efficiency particle filter and 0.72 when particle filtration efficiency was of a lower grade . All of the experimental evidence from field monitoring studies described thus far is based on time-integrated methods, with sampling typically conducted for a 24-h period followed by chemical analysis in the laboratory. The lack of finer-scale time resolution limits the ability to infer potentially important dynamic processes from the experimental evidence. During the past few years, aerosol mass spectrometry has begun to be utilized in studies of indoor environments.In a mixed-use university laboratory space in Philadelphia, PA, the median indoor sulfate mass in submicron particles was measured to be 0.92 µg/m3 , with a corresponding I/O ratio of 61%. In a classroom in the same university, the I/O ratio based on mean concentrations was measured to be 31% for summer and 33% for winter, with corresponding indoor mean concentrations of 0.43 µg/m3 and 0.28 µg/m3, respectively. In these studies, the researchers treat fine particle sulfate as a nonvolatile marker of the influence of outdoor particles on indoor concentrations. They compare the I/O ratio for other chemical components, including ammonium, nitrate, and organic molecules,cannabis dry rack to that of sulfur to make novel inferences about dynamic processes affecting indoor aerosol composition and concentrations.As is apparent from Table 12, indoor air nitric acid concentrations tend to be very low and, depending on the measurement method, are often indistinguishable from zero. Salmon et al. measured HNO3 concentrations at five museums in the Los Angeles area during summer and winter months. Mean seasonal concentrations ranged from < 0.04 to 0.6 ppb . The mean nitric acid concentration measured in two University of Essex buildings was 0.94 µg/m3 ; the corresponding outdoor level was 4.6 µg/m3 . In six Boston homes, the mean summer level was 0.84 ppb contrasted with 0.03 ppb for five homes in winter.

In a New Jersey daycare facility, nursing home, and home for the elderly, mean HNO3 concentrations were in the range of 0.2 to 0.4 ppb.For 229 12-h samples collected In 47 homes in State College PA, the geometric mean indoor HNO3 concentration was 0.2 ppb . Fischer et al.made outdoor and indoor measurements of nitric acid, with 30-minute temporal resolution, at an unoccupied home in Clovis, CA. Although the outdoor levels were as high as 3 ppb, and indoor NH4NO3 dissociation was an additional HNO3 source, the indoor HNO3 levels were normally lower than the uncertainty in the instrument offset . Although concentrations of nitric acid tend to be low indoors, its influence on pH could still be substantial. For example, the equilibrium pH of water exposed to 800 ppm of CO2 is 5.46. Water equilibrated with both 800 ppm CO2 and 0.1 ppb HNO3 would have a pH of 1.83; with 800 ppm CO2 and 0.5 ppb HNO3, equilibrated water would have a pH of 1.48. Ammonia often co-occurs with CO2 indoors. The equilibrium pH of water exposed to 800 ppm of CO2 and 20 ppb of NH3 is 7.12. Maintaining an abundance of 0.1 ppb of gaseous HNO3 added to this mix would decrease the equilibrium pH to 3.48, while sustained exposure to 0.5 ppb of HNO3 would decrease the equilibrium pH to 3.13.However, these properties also mean that the time required for nitric acid to equilibrate with bulk water becomes unrealistically large as the equivalent thickness of surface water increases. Consequently, equilibrium calculations involving gaseous nitric acid indoors should be regarded as suggestive rather than as quantitatively accurate.Gaseous nitric acid can react with gaseous ammonia, contributing ammonium nitrate to airborne particles. To a lesser extent, nitrous acid can be a precursor for nitrite salts in airborne particles. Table 13 summarizes results from selected studies that have measured indoor and outdoor concentrations of nitrate ions in airborne particles.

Only one study has reported measurements of nitrite levels in indoor airborne particles.In that work, average indoor winter concentrations of nitrite were roughly four times larger than summer concentrations . Interestingly, in both winter and summer, the I/O ratios were substantially larger than unity , indicating an indoor source for particulate nitrite in these homes.Several studies that have measured outdoor and indoor NO2 or HNO3 have simultaneously measured the levels of nitrate in outdoor and indoor particles.Indoor levels of particle associated nitrate are typically in the range of 1-10 nmol/m3 . The presence of unvented gas combustion appliances appears to have little influence on the indoor concentration of particulate nitrate.In a daycare facility and a nursing home, indoor particulate nitrate concentrations were higher than co-occurring outdoor levels, but in a home for the elderly, indoor levels were lower than outdoor levels.In their study of 47 homes in State College PA, Suh et al.measured indoor levels of particulate nitrate that were typically higher than co-occurring outdoor levels. They speculated that the reason for the higher levels of nitrate indoors was the HNO3-NH3 reaction. Conversely, dissociation of ammonium nitrate as particles are transported from outdoors to indoors is a recognized occurrence.153 Sinclair et al.found that there was a nitrate artifact with Teflon membrane filters used to sample fine and coarse mode airborne particles and, consequently, they did not report nitrate levels in their studies. This outcome occurred despite washing the filters prior to use. In hindsight, the phenomenon might have been caused by NH3/HNO3/NH4NO3 partitioning between air and filter surfaces. In the 1980s, Pitts et al., using differential optical absorption spectroscopy , identified and measured ppb levels of HONO in a mobile laboratory after injecting NO2 sufficient to establish low ppm levels. In a subsequent study, HONO levels were measured to be as high as ~ 50 ppb in a mobile home during periods when a gas-fired kitchen stove and kerosene- or propane-fueled space heaters were operated.In the early 1990s, Febo and Perrino measured elevated HONO concentrations in a residence in the suburbs of Rome and in automobile cabins. Since then, indoor concentrations of HONO have been measured in numerous studies, although not as many as for NO and NO2. This body of research has been summarized by Gligorovski and by Collins et al.Of note are studies in multiple homes that have been made using integrated measurements, sometimes with passive diffusion samplers for intervals as long as two weeks. For example, in ten Albuquerque, NM homes with gas cooking, Spengler et al. measured average HONO concentrations of 4.7 ± 2.3 ppb.They found a correlation between indoor concentrations of HONO and NO2, with the indoor HONO level between 5% and 15% of indoor NO2. During summer months in homes in Connecticut and Virginia, Leaderer et al. measured average HONO concentrations of 1.6 ± 2.1 ppb in air-conditioned homes and 3.5 ± 2.6 ppb in non-air-conditioned homes .