Assessment of indoor HONO formation mechanisms based on in situ measurements and modeling

Modelling indoor radical chemistry during the HOMEChem campaign

In the indoor environment, occupants are exposed to air pollutants originating from continuous indoor sources and exchange with the outdoor air, with the highest concentration episodes dominated by activities performed indoors such as cooking and cleaning. Here we use the INdoor CHEMical model in Python (INCHEM-Py) constrained by measurements from the House Observations of Microbial and Environmental Chemistry (HOMEChem) campaign, to investigate the impact of a bleach cleaning event and cooking on indoor air chemistry. Measurements of the concentrations of longer-lived organic and inorganic compounds, as well as measured photolysis rates, have been used as input for the model, and the modelled hydroxyl (OH) radicals, hydroperoxyl radicals, and nitrous acid (HONO) concentrations compared to the measured values. The peak modelled OH, , and HONO concentrations during cooking and cleaning activities are about 30%, 10%, and 30% higher than the observations, respectively, within experimental uncertainties. We have determined rates for the rapid loss of HONO formed through cooking activities onto a wet surface during the cleaning events and also for the subsequent slow release of HONO from the cleaned surface back into the gas-phase. Using INCHEM-Py we have also predicted peak concentrations of chlorine (Cl) atoms, (0.75-2.3) × 105 atom per cm3 at the time of cleaning. Model predictions of the Cl atom and OH radical reactivities were also explored, showing high Cl atom reactivity throughout the day, peaking around 5000-9000 s-1. The OH reactivity was found to increase from a background value close to urban outdoor levels of 20-40 s-1, to levels exceeding observations in outdoor polluted areas following cooking and cleaning activities (up to 160 s-1). This underlines the high oxidation capacity of the indoor atmospheric environment through determining the abundance of volatile organic compounds.

Commercial kitchen operations produce a diverse range of gas-phase reactive nitrogen species

Gas-phase reactive nitrogen species (Nr) are important drivers of indoor air quality. Cooking and cleaning are significant direct sources indoors, whose emissions will vary depending on activity and materials used. Commercial kitchens experience regular high volumes of both cooking and cleaning, making them ideal study locations for exploring emission factors from these sources. Here, we present a total Nr (tNr) budget and contributions of key species NO, NO2, acidic Nr (primarily HONO) and basic Nr (primarily NH3) using novel instrumentation in a commercial kitchen over a two-week period. In general, highest tNr was observed in the morning and driven compositionally by NO, indicative of cooking events in the kitchen. The observed HONO and basic Nr levels were unexpectedly stable throughout the day, despite the dynamic and high air change rate in the kitchen. After summing the measured NOx, HONO and Nr,base fractions, there was on average 5 ppbv of Nr unaccounted for, expected to be dominated by neutral Nr species. Using co-located measurements from a proton transfer reaction mass spectrometer (PTR-MS), we propose the identities for these major Nr species from cooking and cleaning that contributed to Nr,base and the neutral fraction of tNr. When focused specifically on cooking events in the kitchen, a vast array of N-containing species was observed by the PTR-MS. Reproducibly, oxygenated N-containing class ions (C1-12H3-24O1-4N1-3), consistent with the known formulae of amides, were observed during meat cooking and may be good cooking tracers. During cleaning, an unexpectedly high level of chloramines was observed, with monochloramine dominating the profile, as emitted directly from HOCl based cleaners or through surface reactions with reduced-N species. For many species within the tNr budget, including HONO, acetonitrile and basic Nr species, we observed stable levels day and night despite the high air change rate during the day (>27 h-1). The stable levels for these species point to large surface reservoirs which act as a significant indoor source, that will be transported outdoors with ventilation.

A review of indoor nitrous acid (HONO) pollution: Measurement techniques, pollution characteristics, sources, and sinks

Indoor air quality is of major concern for human health and well-being. Nitrous acid (HONO) is an emerging indoor pollutant, and its indoor mixing ratios are usually higher than outdoor levels, ranging from a few to tens of parts per billion (ppb). HONO exhibits adverse effects to human health due to its respiratory toxicity and mutagenicity. Additionally, HONO can easily undergo photodissociation by ultraviolet light to produce hydroxyl radicals (OH•), which in turn trigger a series of further photochemical oxidation reactions of primary or secondary pollutants. The accumulation of indoor HONO can be attributed to both direct emissions from combustion sources, such as cooking, and secondary formation resulting from enhanced heterogeneous reactions of NOx on indoor surfaces. During the day, the primary sink of indoor HONO is photolysis to OH• and NO. Moreover, adsorption and/or reaction on indoor surfaces, and diffusion to the outside atmosphere contribute to HONO loss both during the day and at night. The level of indoor HONO is also affected by human occupancy, which can influence household factors such as temperature, humidity, light irradiation, and indoor surfaces. This comprehensive review article summarized the research progress on indoor HONO pollution based on indoor air measurements, laboratory studies, and model simulations. The environmental and health effects were highlighted, measurement techniques were summarized, pollution levels, sources and sinks, and household influencing factors were discussed, and the prospects in the future were proposed.

Cooking, Bleach Cleaning, and Air Conditioning Strongly Impact Levels of HONO in a House

The relative importance of common activities on indoor nitrous acid (HONO) mixing ratios was explored during high time resolution, month-long measurements by chemical ionization mass spectrometry in a previously unoccupied house. Indoor HONO varied from 0.2 to 84.0 ppb (mean: 5.5 ppb; median 3.8 ppb), an order of magnitude higher than simultaneously measured outdoor values, indicating important indoor sources. They agree well with simultaneous measurements of HONO by Laser-Photofragmentation/Laser-Induced Fluorescence. Before any combustion activities, the mixing ratio of 3.0 ± 0.3 ppb is indicative of secondary sources such as multiphase formation from NO₂. Cooking (with propane gas), especially the use of an oven, led to significant enhancements up to 84 ppb, with elevated mixing ratios persisting for a few days due to slow desorption from indoor surface reservoirs. Floor bleach cleaning led to prolonged, substantial decreases of up to 71-90% due to reactive processes. Air conditioning modulated HONO mixing ratios driven by condensation to wet surfaces in the AC unit. Enhanced ventilation also significantly lowered mixing ratios. Other conditions including human occupancy, ozone addition, and cleaning with terpene, natural product, and vinegar cleaners had a much smaller influence on HONO background levels measured following these activities.

Evaluation and impact factors of indoor and outdoor gas-phase nitrous acid under different environmental conditions

As an important indoor pollutant, nitrous acid (HONO) can contribute to the concentration of indoor OH radicals by photolysis via sunlight penetrating into indoor environments, thus affecting the indoor oxidizing capability. In order to investigate the concentration of indoor HONO and its impact factors, three different indoor environments and two different locations in urban and suburban areas were selected to monitor indoor and outdoor pollutants simultaneously, including HONO, NO, NO₂, nitrogen oxides (NO_x), O₃, and particle mass concentration. In general, the concentration of indoor HONO was higher than that outdoors. In the urban area, indoor HONO with high average concentration (7.10 ppbV) was well-correlated with the temperature. In the suburban area, the concentration of indoor HONO was only about 1-2 ppbV, and had a good correlation with indoor relative humidity. It was mainly attributed to the heterogeneous reaction of NO₂ on indoor surfaces. The sunlight penetrating into indoor environments from outside had a great influence on the concentration of indoor HONO, leading to a concentration of indoor HONO close to that outdoors.

Indoor acids and bases

Numerous acids and bases influence indoor air quality. The most abundant of these species are CO2 (acidic) and NH3 (basic), both emitted by building occupants. Other prominent inorganic acids are HNO3 , HONO, SO2 , H2 SO4 , HCl, and HOCl. Prominent organic acids include formic, acetic, and lactic; nicotine is a noteworthy organic base. Sources of N-, S-, and Cl-containing acids can include ventilation from outdoors, indoor combustion, consumer product use, and chemical reactions. Organic acids are commonly more abundant indoors than outdoors, with indoor sources including occupants, wood, and cooking. Beyond NH3 and nicotine, other noteworthy bases include inorganic and organic amines. Acids and bases partition indoors among the gas-phase, airborne particles, bulk water, and surfaces; relevant thermodynamic parameters governing the partitioning are the acid-dissociation constant (Ka ), Henry's law constant (KH ), and the octanol-air partition coefficient (Koa ). Condensed-phase water strongly influences the fate of indoor acids and bases and is also a medium for chemical interactions. Indoor surfaces can be large reservoirs of acids and bases. This extensive review of the state of knowledge establishes a foundation for future inquiry to better understand how acids and bases influence the suitability of indoor environments for occupants, cultural artifacts, and sensitive equipment.

Impacts of six potential HONO sources on HOx budgets and SOA formation during a wintertime heavy haze period in the North China Plain

The Weather Research and Forecasting/Chemistry (WRF-Chem) model updated with six potential HONO sources (i.e., traffic, soil, biomass burning and indoor emissions, and heterogeneous reactions on aerosol and ground surfaces) was used to quantify the impact of the six potential HONO sources on the production and loss rates of OH and HO₂ radicals and the concentrations of secondary organic aerosol (SOA) in the Beijing-Tianjin-Heibei (BTH) region of China during a winter heavy haze period of Nov. 29-Dec. 3, 2017. The updated WRF-Chem model well simulated the observed HONO concentrations at the Wangdu site, especially in the daytime, and well reproduced the observed diurnal variations of regional-mean O₃ in the BTH region. The traffic emission source was an important HONO source during nighttime but not significant during daytime, heterogeneous reactions on ground/aerosol surfaces were important during nighttime and daytime. We found that the six potential HONO sources led to a significant enhancement in the dominant production and loss rates of HO_x on the wintertime heavy haze and nonhaze days (particularly on the heavy haze day), an enhancement of 5-25 μg m^-3 (75-200%) in the ground SOA in the studied heavy haze event, and an enhancement of 2-15 μg m^-3 in the meridional-mean SOA on the heavy haze day, demonstrating that the six potential HONO sources accelerate the HO_x cycles and aggravate haze events. HONO was the key precursor of primary OH in the BTH region in the studied wintertime period, and the photolysis of HONO produced a daytime mean OH production rate of 2.59 ppb h^-1 on the heavy haze day, much higher than that of 0.58 ppb h^-1 on the nonhaze day. Anthropogenic SOA dominated in the BTH region in the studied wintertime period, and its main precursors were xylenes (42%), BIGENE (31%) and toluene (21%).

Evidence for Gas-Surface Equilibrium Control of Indoor Nitrous Acid

Nitrous acid (HONO) is an important component of indoor air as a photolabile precursor to hydroxyl radicals and has direct health effects. HONO concentrations are typically higher indoors than outdoors, although indoor concentrations have proved challenging to predict using box models. In this study, time-resolved measurements of HONO and NO₂ in a residence showed that [HONO] varied relatively weakly over contiguous periods of hours, while [NO₂] fluctuated in association with changes in outdoor [NO₂]. Perturbation experiments were performed in which indoor HONO was depleted or elevated and were interpreted using a two-compartment box model. To reproduce the measurements, [HONO] had to be predicted using persistent source and sink processes that do not directly involve NO₂, suggesting that HONO was in equilibrium with indoor surfaces. Production of gas phase HONO directly from conversion of NO₂ on surfaces had a weak influence on indoor [HONO] during the time of the perturbations. Highly similar temporal responses of HONO and semivolatile carboxylic acids to ventilation of the residence along with the detection of nitrite on indoor surfaces support the concept that indoor HONO mixing ratios are controlled strongly by gas-surface equilibrium.

Impact of the spectral and spatial properties of natural light on indoor gas-phase chemistry: Experimental and modeling study

The characteristics of indoor light (intensity, spectral, spatial distribution) originating from outdoors have been studied using experimental and modeling tools. They are influenced by many parameters such as building location, meteorological conditions, and the type of window. They have a direct impact on indoor air quality through a change in chemical processes by varying the photolysis rates of indoor pollutants. Transmittances of different windows have been measured and exhibit different wavelength cutoffs, thus influencing the potential of different species to be photolysed. The spectral distribution of light entering indoors through the windows was measured under different conditions and was found to be weakly dependent on the time of day for indirect cloudy, direct sunshine, partly cloudy conditions contrary to the light intensity, in agreement with calculations of the transmittance as a function of the zenithal angle and the calculated outdoor spectral distribution. The same conclusion can be drawn concerning the position within the room. The impact of these light characteristics on the indoor chemistry has been studied using the INCA-Indoor model by considering the variation in the photolysis rates of key indoor species. Depending on the conditions, photolysis processes can lead to a significant production of radicals and secondary species.

Reactive indoor air chemistry and health-A workshop summary

The chemical composition of indoor air changes due to the reactive nature of the indoor environment. Historically, only the stable parent compounds were investigated due to their ease of measurement by conventional methods. Today, however, scientists can better characterize oxidation products (gas and particulate-phase) formed by indoor chemistry. An understanding of occupant exposure can be developed through the investigation of indoor oxidants, the use of derivatization techniques, atmospheric pressure detection, the development of real-time technologies, and improved complex modeling techniques. Moreover, the connection between exposure and health effects is now receiving more attention from the research community. Nevertheless, a need still exists for improved understanding of the possible link between indoor air chemistry and observed acute or chronic health effects and long-term effects such as work-related asthma.

Emerging developments in the standardized chemical characterization of indoor air quality

Despite the fact that the special characteristics of indoor air pollution make closed environments quite different from outdoor environments, the conceptual ideas for assessing air quality indoors and outdoors are similar. Therefore, the elaboration of International Standards for air quality characterization in view of controlling indoor air quality should resort to this common basis. In this short review we describe the possibilities of standardization of tools dedicated to indoor air quality characterization with a focus on the tools permitting to study the indoor air chemistry. The link between indoor exposure and health as well as the critical processes driving the indoor air quality are introduced. Available International Standards for the assessment of indoor air quality are depicted. The standards comprise requirements for the sampling on site, the analytical procedures, and the determination of material emissions. To date, these standardized procedures assure that indoor air, settled dust and material samples are analyzed in a comparable manner. However, existing International Standards exclusively specify conventional, event-driven target-screening using discontinuous measurement methods for long-lived pollutants. Therefore, this review draws a parallel between physico-chemical processes in indoor and outdoor environments. The achievements in atmospheric sciences also improve our understanding of indoor environments. The community of atmospheric scientists can be both ideal and supporter for researchers in the area of indoor air quality characterization. This short review concludes with propositions for future standardization activities for the chemical characterization of indoor air quality. Future standardization efforts should focus on: (i) the elaboration of standardized measurement methods and measurement strategies for online monitoring of long-lived and short-lived pollutants, (ii) the assessment of the potential and the limitations of non-target screening, (iii) the paradigm shift from event-driven investigations to systematic approaches to characterize indoor environments, and (iv) the development of tools for policy implementation.