The geometric mean concentrations were, respectively, 157 nmol/m3 indoors and 139 nmol/m3 outdoors . In the Connecticut and Virginia homes sampled by Leaderer et al., the average fine-particle ammonium concentrations in the summer were slightly smaller for air-conditioned homes, 78 nmol/m3 compared to homes without air conditioning, 97 nmol/m3 , possibly due to lower air-exchange rates in the air-conditioned homes. During winter, homes with kerosene heaters had much higher fine-particle ammonium concentrations, 126 nmol/m3 than homes without, 9.4 nmol/m3 . The average indoor concentration was about 2´ the outdoor value during winter in kerosene-heater homes. The elevated NH4 + levels in the kerosene-heater homes were matched by elevated SO4 2- levels in these homes. This observation suggests that acidic S emissions from kerosene combustion were neutralized by NH3 emitted from occupants and other indoor sources to produce particle phase ammonium sulfate. Johnson et al.139 used an aerosol mass spectrometer to make time-resolved measurements of ammonium levels in sub-micron particles both outdoors and inside a mixeduse laboratory at Drexel University. During mid-April sampling, the I/O ratio for ammonium was slightly less than that for sulfate . Avery et al.80 made more comprehensive AMS measurements of outdoor and indoor atmospheric aerosols during winter and summer in a Drexel classroom. In both seasons,rolling bench the indoor ammonium concentrations were low. Outdoor and indoor NH4 + levels averaged 0.63 µg/m3 and 0.07 µg/m3 , respectively, in the winter, and 0.44 µg/m3 and 0.05 µg/m3 , respectively, in the summer.
During winter and summer, the sulfate normalized NH4 + concentration was much larger outdoors than indoors, indicating loss of ammonia from particles during outdoor to indoor transport. The sulfate-normalized indoor/outdoor ratio for NH4 + tracked the sulfate-normalized outdoor NH4 + concentration in summer, but not in winter. This observation is consistent with evidence suggesting that a larger fraction of the outdoor aerosol comprises NH4NO3 in the winter compared to the summer. In the summer, with higher temperatures favoring ammonia and nitric acid gas rather than condensed-phase ammonium nitrate, the ammonium is present in the outdoor aerosol particles primarily as sulfate salts. Particles containing ammonium salts accumulate on indoor surfaces. The flux of particles to a surface can be estimated as the product of the airborne particle concentration and the particle deposition velocity. Actual measurements of accumulated ammonium salts on indoor surfaces are reported only in a few studies. Sinclair et al. measured accumulated ionic species on vertical and horizontal zinc and aluminum surfaces in mechanically ventilated telephone switching offices. Compared to other ionic species, the behavior of ammonium was complex. The zinc and aluminum surfaces, oriented both horizontally and vertically, quickly reached an NH4 + surface accumulation beyond which there was little further increase. The authors speculated that an equilibration process might be occurring on these surfaces whereby, as additional ammonium salts deposited, there was a compensating loss of gaseous NH3 from the surface. Ligocki et al. measured deposition velocities of various ionic constituents of particles to vertical and upward-facing horizontal surfaces in five California museums. The deposition velocities for sulfate, nitrate and chloride were much larger to horizontal surfaces than to vertical surfaces, as would be expected for particles influenced by gravitational setting.
However, for ammonium, the deposition velocities were similar for both surface orientations. Furthermore, ammonium had the lowest deposition velocity among the measured ionic species. The authors speculated that ammonium may have been lost from collection surfaces in the form of gaseous ammonia during the long exposure time. These independent indications of ammonia loss from surface-accumulated ammonium in two different types of buildings using different experimental methods reinforce one another and encourage further investigation. The vapor pressure of ammonia immediately above a saturated solution of an ammonium salt can be high. Saturated salt solutions can occur in aqueous surface films as the relative humidity decreases and the film loses water. Opila et al.calculated that, at 25 °C, the vapor pressure for NH3 is 4 Pa above a saturated pH 5.5 solution of 2SO4 and 0.1 Pa above a saturated pH 4.2 solution of NH4Cl. With a Henry’s law constant of KH = 59 M/atm, ammonia is almost two thousand times more water-soluble than CO2 and about forty times more soluble than SO2. It is also the most prominent basic gas found in indoor air. The pKa of the ammonium ion is 9.24 at 25 °C.Ammonia is more basic than CO2 is acidic. This fact, coupled with ammonia’s much greater water solubility, means that the time to equilibrate with liquid water is greater NH3 than for CO2. The upper trace in Figure 5 shows the equilibrium pH of pure liquid water as a function of its exposure to gaseous ammonia , in the absence of any other acidic or basic species. However, in actual indoor environments, carbon dioxide is always present. Figure 5 also has traces showing the equilibrium pH of liquid water as a function of gaseous ammonia when the CO2 concentration is either 400 or 1000 ppm.
Ammonia’s large water solubility, coupled with its moderate basicity, means that, molecule for molecule, ammonia has a much larger impact on the pH of condensed-phase water than does CO2. Specifically, only a small amount of NH3 is sufficient to neutralize the influence of CO2 when otherwise pure water is in equilibrium with these species. Quantitatively, 1.4 ppb NH3 neutralizes the influence of 100 ppm CO2. Humans are primary sources for both CO2 and NH3 indoors. In indoor environments, whenever the major determinant of carbon dioxide and ammonia concentrations is human occupancy, and under the assumption that these two species dominate acid-base influence, then condensed-phase water in equilibrium should have a close-to-neutral pH. Of course, that outcome can be altered by the presence of other gaseous acids and bases. Gaseous ammonia contributes to the neutralization of acids in airborne particles. Brauer et al. found that aerosol strong acidity was much lower indoors than outdoors, suggesting partial neutralization of acidic aerosol by indoor NH3. Liang and Waldman measured I/O ratios for aerosol strong acidity that were substantially lower than those for sulfate. They proceeded to use the ratio of these I/O values to estimate percent neutralization of indoor acidic aerosol and concluded, as had Brauer et al.,dry rack cannabis that acidic aerosols transported indoors were partially neutralized by NH3. Suh et al. found that the indoor/outdoor ratio of geometric mean concentrations for ammonium was greater than unity , whereas, for sulfate, the ratio was less than unity . This finding is consistent with neutralization of acidic aerosol components by indoor ammonia. Suh et al. measured indoor aerosol acidity levels that were substantially lower than corresponding outdoor levels and attributed this difference to neutralization by indoor NH3. Indoor NH3 levels > 50 ppb were calculated to be sufficient to completely neutralize H+ in the sampled homes in State College, Pennsylvania.Neutralization by ammonia of inhaled strongly acidic aerosols has also been reported. In an experiment with one subject, Larson et al.109 demonstrated that NH3 in the respiratory tract partially neutralized inhaled sulfuric acid aerosol. Sarangapani and Wexler developed a mathematical model to explore the neutralization of sulfate-containing aerosols by NH3 in the respiratory tract. The model predicts neutralization for particles smaller than 0.1 µm diameter, but for particles larger than 1.0 µm it predicts little neutralization. When the air inhaled is cool and humid, modeled neutralization is enhanced. In indoor air, a potential source of secondary organic aerosols is ozone reacting with terpenoids used to scent cleaning agents or air fresheners.Ammonia has been shown to influence the formation of secondary organic aerosols generated by ozone-initiated chemistry. In outdoor air, HNO3 and NH3 have been found at levels consistent with predictions for equilibrium partitioning with particulate NH4NO3. Such equilibrium conditions may not prevail indoors. Li and Harrison found, in the University of Essex buildings sampled, that the combined concentrations of nitric acid and ammonia were lower indoors than anticipated under equilibrium conditions. Presumably the findings reflect partial dissociation of ammonium nitrate coupled with loss of nitric acid to surfaces indoors. Similarly, for the three institutional buildings that they surveyed, Liang and Waldman found that the products of the measured gas-phase concentrations were in poor agreement with theory for half of the sampling periods.
During summer sampling, indoor ammonium nitrate levels were higher in air-conditioned buildings than in naturally ventilated buildings, and the authors attributed this observation to the cooler temperatures in air-conditioned buildings, and hence lower values for KAN. In their study of California museums, Ligocki et al. found that the I/O ratio for fine-mode ammonium nitrate was smaller than that of other fine-mode species that they measured. They attributed their observation to either dissociation of ammonium nitrate as it was transported from outdoors to indoors or to dissociation occurring during indoor sampling. Suh et al. reported measured values for the product of airborne nitric acid and ammonia concentrations that they described as “comparable to the dissociation constant.” However, Figure 4 in their paper indicates that a substantial proportion of the daytime values for the concentration product were below 3 ppb2 . Although corresponding temperatures are not reported, 3 ppb2 appears lower than expected for equilibrium conditions at the indoor temperatures that should have prevailed during the sampling period in State College, Pennsylvania. At an unoccupied m2 home in Clovis, California, Fischer et al. measured time-resolved outdoor and indoor concentrations of gas-phase NH3, HONO, HNO3 and SO2. During the experiments, indoor activities that might produce these species were restricted. Although there were no known indoor sources, measured concentrations of indoor NH3 tended to be slightly higher than corresponding outdoor concentrations, possibly as a consequence of NH4NO3 dissociation. Yet the concentration of gaseous nitric acid indoors was always very low, suggesting that HNO3 was lost to indoor surfaces to a larger extent than NH3. During these same periods, Lunden et al.measured nitrate and sulfate levels in indoor and outdoor aerosol particles at the Clovis house. The measured I/O ratios for nitrate were much lower than the measured I/O ratios for sulfate, suggesting that ammonium nitrate dissociated when outdoor particles were transported indoors. Additionally, they found that the increases in indoor NH3 correlated with increases in the difference between outdoor and indoor aerosol nitrate levels, consistent with outdoor NH4NO3 serving as the indoor ammonia source. Their calculations indicated that the rate of “evaporation” of NH4NO3 varied substantially with temperature and with the concentrations of gas-phase NH3 and HNO3. The time scale for evaporation indicated that this removal mechanism for NH4NO3 would often be comparable to that for air exchange or deposition to indoor surfaces. A mass-balance model that included a term for the rate of NH4NO3 dissociation successfully modeled the levels of indoor NH4NO3 and HNO3 during the measurement periods. However, the authors were unable to successfully model the measured NH3 concentrations, and they concluded that modeling NH3 levels was more complicated. They were unable to calculate a single deposition velocity that could describe removal of NH3 by indoor surfaces. They did calculate a best-fit deposition velocity for HNO3; the value obtained, 0.56 cm/s , was noted to be higher than the expected mass-transport limit, an outcome that the authors suggested was a consequence of “problems encountered when measuring nitric acid concentration.”López-Aparicio et al. measured NH3 among other species outside and inside the Baroque Library Hall in the National Library in Prague from the beginning of July 2009 until the end of March 2010. The room where the indoor measurements were made was normally unoccupied and had no readily identified sources of NH3. During the months July-October, the I/O ratio for NH3 was larger than one , while during the months November-March, the I/O ratio was less than one . The researchers speculated that NH4NO3 dissociation was responsible for a fraction of the indoor NH3, and that this process made a larger contribution during the warmer months when KAN was larger. This hypothesis was supported by the product of measured nitric acid and ammonia concentrations, which was smaller than that expected for equilibrium with NH4NO3 at the temperatures measured in the room. It should be noted that any contribution of NH4NO3 dissociation to indoor NH3 concentrations would be more difficult to observe in occupied settings, given typical NH3 emissions from human occupants.