The lower average values for HONO in AC homes may partially reflect HONO loss to the air conditioner condensate. Dividing these same 58 homes between those with gas stoves and those without, average HONO concentrations were 0.8 ± 0.8 ppb in non-gas-stove homes and 4.0 ± 2.8 ppb in gasstove homes . During winter months, Leaderer et al. measured average HONO concentrations to be 6.8 ± 6.1 ppb in kerosene-heater homes and 3.5 ± 3.6 ppb in nonkerosene heater homes .For the homes without kerosene heaters, average wintertime HONO concentrations were 2.4 ± 3.1 ppb in non-gas-stove homes and 5.5 ± 3.8 ppb in gas-stove homes . All of this evidence points to unvented combustion as contributing to measurable increases in indoor HONO levels.In 99 homes in Upland, CA, and San Bernardino County, Lee et al.measured average HONO concentrations of 4.6 ± 4.3 ppb, considerably higher than the outdoor levels of 0.9 ± 2.3 ppb. Homes with gas ranges had higher indoor NO2 and HONO concentrations than those without. Indoor concentrations of HONO were positively correlated with NO2, with HONO levels occurring at approximately 17% of the NO2 levels. HONO concentrations were inversely correlated with O3 concentrations. A similar inverse correlation between HONO and O3 was reported by Weschler et al.based on spot measurements made in a Burbank telecommunications office. In both studies, the authors suggest that this observation may be a result of ozone-initiated oxidation of nitrite ions in aqueous surface films; the concentration of nitrite ions in indoor aqueous solutions is linked to gas-phase HONO concentrations . Semi-continuous measurements of HONO concentrations were made in an unoccupied school classroom in France,planting racks using wet chemical sampling and subsequent quantification with high performance liquid chromatography. Five experiments were conducted with controlled injections of NO2 under different lighting and relative humidity conditions.
With average indoor NO2 levels in the range 28-46 ppb and indoor RH levels in the range 30-60%, average indoor HONO levels were 5.1-6.2 ppb. Mendez et al. developed a description for HONO formation that assumes NO2 is first sorbed to surface sites, which are limited in number, and that NO2 then reacts with water to produce HONO and HNO3. Key parameters were fitted based on measurements in one experiment, and, with these fitted parameters, the model reasonably predicted the measured HONO concentrations in the other four experiments. As part of a study in a Syracuse home, Zhou et al. made time-resolved measurements of HONO concentrations during baseline conditions and cooking events. Mean ± standard deviation HONO concentrations were 4.3 ± 2.2 ppb during baseline conditions, rising to 19.5 ± 10.5 ppb during cooking . A short-term peak concentration of 50 ppb was measured. To our knowledge, this is the first study to report indoor baseline HONO concentrations larger than indoor baseline NO2 concentrations . Collins et al. measured time-resolved HONO levels inside and outside a Toronto home in November using a high-resolution time-of-flight chemical ionization mass spectrometer with acetate as the reagent ion. They found that, while indoor NO2 levels varied over a large range depending on outdoor levels, indoor HONO concentrations varied over a relatively narrow range and did not correlate with NO2 concentrations. Perturbation experiments were conducted in the kitchen using a burner on a gas stove and opening/closing windows and a door. During these perturbations, NO2 emitted by the gas burner only weakly affected HONO levels. Flushing the kitchen via open windows and a door reduced HONO levels during the high ventilation period, but when windows and door were closed, HONO returned to a gas-phase concentration close to its pre-airing value. The temporal responses of HONO were similar to those of small carboxylic acids in these airing experiments.
The authors concluded that gas-phase HONO was in equilibrium with, and strongly controlled by, surface sources. This inference was further supported by nitrite levels measured on various impermeable vertical surfaces in the kitchen and the upstairs of the home. Nitrite levels averaged approximately 1012 molecules cm-2 ; the authors cautioned that this value should be considered a lower limit. HONO measurements that were made during venting experiments as part of the HOMEChem campaign in Austin TX158 substantiate the Toronto home findings by Collins et al.When the Austin test house was vented, gas-phase HONO concentrations decreased from ~ 4 ppb to about 1 ppb. When windows were then closed, the HONO concentration returned to a level close to that measured before venting. See §4.6 for further discussion. It is interesting to compare the influence of HONO to that of HNO3 on the pH of aqueous surface films or bulk water. To begin, consider that the equilibrium pH of water exposed to 800 ppm of CO2 and 20 ppb of NH3 is 7.12. Adding 5 ppb of gaseous HONO to this mix would decrease the equilibrium pH to 6.53, whereas adding 0.1 ppb of HNO3 would decrease the equilibrium pH to 3.48. So, although measured indoor concentrations of HONO tend to be 10- 100´ larger than those of HNO3, the expected influence of HONO on pH is considerably weaker, based on analyses for equilibrium conditions. Taken together, these studies illustrate a strong direct contribution from indoor combustion to indoor HONO concentrations, a contribution from the partial transformation of NO2 to HONO on indoor surfaces, the potential for ozone to decrease indoor HONO levels via oxidation of nitrite ions in aqueous solution, and the ability of indoor basic surfaces to serve as large reservoirs for nitrous acid. More measurements of nitrite ions on indoor surfaces, as well as of the time-dependent pH of aqueous films on different indoor surfaces, would improve our understanding of the reported and inferred dynamics of this inorganic acid.During a study conducted in a 79-m3 stainless steel climate chamber, Brauer et al. examined the impact of human occupants on indoor HONO concentrations.
At a high air-exchange rate , four human occupants had only a small effect on HONO concentrations resulting from the addition of NO2 to the chamber. However, at a much lower air-exchange rate , the measured indoor HONO concentration with occupants was reduced to 40% of its value without humans in the chamber . When the subjects left the chamber, HONO levels returned to levels previously observed for the empty chamber.Direct removal by breathing could not account for the observed HONO removal rate. Reaction of HONO with NH3 emitted by the occupants also did not explain the observed reduction of indoor HONO levels. Brauer et al. speculated that “the effect of increased surface area is a plausible explanation for our observations.”One can estimate the potential magnitude of HONO removal by exposed skin, hair and clothing of the four subjects in the chamber. Assume that the deposition velocity for HONO to human surfaces is similar to that measured for ozone and assume a body surface area of 1.8 m2 for each human in the chamber. Then four humans would remove HONO at a rate equivalent to ventilating with clean air at 58 m3 /h or 0.73 h-1 in the 79 m3 chamber. Such a removal by human occupants is predicted to yield a reduction of approximately 60% in HONO concentration at a chamber air exchange rate of 0.5 h-1 , which is consistent in scale with the reduction shown in Figure 4 of Brauer et al.Conversely, the effect of removal on human surfaces would only be expected to reduce the indoor concentrations of HONO by about 5% for the high air-exchange rate condition of 12 h-1. If HONO loss does occur on the occupant envelope,sub irrigation cannabis important questions remain to be answered. Is this phenomenon transient, terminating when equilibrium partitioning is achieved? Or is HONO being irreversibly sorbed by skin and clothing? Based on 48-h measurements of O3 and NO2 in a Southern California museum gallery, coupled with their model of indoor chemistry, Nazaroff and Cass200 predicted that O3/NO2 chemistry would generate NO3 and N2O5 at substantial net rates. Weschler et al.suggested that under certain circumstances, O3/NO2 chemistry would generate indoor nitrate radical concentrations comparable to outdoor nighttime levels and that subsequent chemistry could be a substantial source of indoor nitric acid. Using a detailed chemical model, Sarwar et al. estimated an indoor nitrate radical concentration of 0.15 ppt for “base case” indoor conditions. Using a detailed model of gas-phase indoor chemistry, Carslaw predicted low NO3 concentrations under indoor conditions that included elevated concentrations of terpenes and unsaturated alkenes, which rapidly consume nitrate radicals. Carslaw noted that an anticipated consequence is formation of RO2·radicals, and subsequent production of organic acids. Nøjgaard made the first time-integrated measurements of the sum ‘NO3 + N2O5’ based on concentrations of an oxidation product of the NO3/cyclohexene reaction. Eleven separate measurements were made in an unoccupied 60 m3 conference room in Copenhagen, DK, during August. There were no indoor sources of O3 or NO2; these species originated outdoors.
The sum ‘NO3 + N2O5’ ranged from 1 to 58 ppt, and was influenced by the fraction of time mechanical ventilation occurred, levels of O3 and NO2, lighting and time of day. For the four samples collected during daylight hours, the sum of NO3 + N2O5 ranged from 3 to 10 ppt or approximately 0.6 to 1.4 ppt of NO3 given the measured cooccurring NO2 concentrations. These measured values are larger than Carslaw’s modeled estimates of NO3 levels under typical indoor conditions. Nøjgaard speculated that this discrepancy might be due to actual NO concentrations being lower than those used in the model and concluded by calling for time-resolved measurements of indoor NO3, such as could be achieved using cavity ring down spectroscopy. Arata et al. made the first real-time indoor NO3 measurements in the kitchen of a single family home during simulated-use conditions. Experiments included cooking with a butane stove in the presence of deliberately released ozone. At an enhanced O3 level of 40 ppb, researchers ignited the stove, operated it for about five minutes to boil water, and turned it off. After O3 had titrated the NO in the kitchen air, the N2O5 level began to increase, reaching a value of 190 ppt, while the NO3 concentrations leveled off at about 3 ppt. Based on simultaneous measurements of NO2, O3 and NO3, they estimated total nitrate radical reactivity with volatile organic compounds to be 0.8 s-1 . Using a box-model they calculated a peak NO3 production rate of 7 ppb h-1 . The model’s output indicated that reaction of N2O5 with indoor surfaces, producing nitric acid, accounted for 20% of NO3 loss during the period of peak NO3 production. More generally, these studies indicate that, under conditions with elevated indoor levels of O3, combustion events can result in meaningful levels of nitric acid, organonitrates and various oxidized VOC – even when measured residual NO3 concentrations are relatively low.Isocyanic acid is moderately acidic and moderately soluble . It has been recognized as a gas-phase acid in the outdoor atmosphere since 2008. More recently, experiments have demonstrated that gas-phase oxidation of nicotine by hydroxyl radicals generates HNCO. Measurements using an acetate CIMS in a chamber and in a Toronto home have explored indoor sources of HNCO. 238 The chamber studies indicated a molar ratio of HNCO/CO in side stream cigarette smoke of 2.7 × 10-3 . In a home, the background HNCO concentration was 0.15 ppb, about twice the outdoor level. A single cigarette’s side stream smoke increased the HNCO concentration to about 1.5 ppb. In chamber experiments, there was evidence for photochemical production of HNCO from cigarette smoke, doubling the concentration in about 30 minutes at an OH concentration of 1.1 × 107 molecules/cm3. However, in the home there was no evidence of photochemistry influencing the HNCO concentration. Simultaneous, time-resolved measurements of HNCO and CO indicated that partitioning to indoor surfaces was a significant sink for indoor HNCO. Isocycanic acid reacts with ammonia to form urea. Among halogenated acids, chlorine-containing species are the most noteworthy. In the atmosphere, hydrochloric acid is a prominent atmospheric inorganic strong acid. Important sources of atmospheric HCl are the combustion of fuels and wastes that contain chlorine, which include coal, bio-fuels, and plastics. Hydrochloric acid is also generated from acid-displacement reactions in which other atmospheric strong acids, such as HNO3, react with sea-salt aerosol, with the net effect represented by HNO3 + NaCl ® HCl + NaNO3. In a global emission inventory of HCl, combustion and sea-salt dechlorination were the largest sources.