Nitrous acid is a weak, unstable acid with the formula HNOâ‚‚ that plays an outsized role in atmospheric chemistry, indoor air quality, and even food science. Despite being unfamiliar to most people, it is one of the most important precursors of hydroxyl radicals in the lower atmosphere, and it forms readily in settings as ordinary as your kitchen. Its instability is part of what makes it consequential: nitrous acid breaks apart easily in sunlight, and the fragments it leaves behind drive chain reactions that shape air pollution, ozone formation, and human exposure to irritants.
What Nitrous Acid Actually Is
Nitrous acid is a simple molecule made of hydrogen, nitrogen, and oxygen. In water it behaves as a weak acid, with a measured pKa of about 3.16 at 25 °C, meaning it only partially gives up its hydrogen ion in solution.1PubMed. Ab initio procedure for aqueous-phase pKa calculation: the acidity of nitrous acid That makes it considerably weaker than its close relative nitric acid (HNO₃), which dissociates almost completely. In air, nitrous acid exists as a gas and can adopt two shapes: a trans form (where the hydrogen points away from the second oxygen) and a cis form (where it curls toward it). The trans form is slightly more stable.2PubMed Central. Theoretical study of the structures, stability and vibrational spectra of the nitrous acid complexes with CH4 In practical terms, the molecule is fragile. It decomposes on its own in solution and photolyzes rapidly in sunlight, which is exactly the property that makes it so important outdoors.
You will often see nitrous acid written as HONO in atmospheric science literature, a shorthand that reflects the way the atoms are bonded: hydrogen to oxygen to nitrogen to oxygen. This is worth knowing because HONO appears constantly in air quality research, and recognizing the abbreviation saves confusion with nitrous oxide (Nâ‚‚O, the dental anesthetic and greenhouse gas) or nitrogen dioxide (NOâ‚‚, the reddish-brown pollutant). These are all different molecules with very different behaviors.
Why Atmospheric Scientists Care So Much About It
The atmosphere cleans itself through a molecule called the hydroxyl radical (OH), which reacts with and breaks down pollutants ranging from carbon monoxide to volatile organic compounds. Nitrous acid is one of the most efficient sources of this radical. When sunlight hits HONO, it splits into OH and nitric oxide (NO) almost immediately. In urban areas, this photolysis reaction dominates the early-morning chemistry of the lower atmosphere.
Measurements taken during a field campaign in Milan found that HONO photolysis was by far the largest OH source in the early morning hours and could match or even surpass the total midday OH production from all other pathways combined.3Journal of Geophysical Research: Atmospheres. Impact of nitrous acid photolysis on the total hydroxyl radical budget during the Limitation of Oxidant Production/Pianura Padana Produzione di Ozono study in Milan A rural study in New York State confirmed this was not just a city phenomenon: HONO photolysis accounted for about a quarter of total radical production from the three main photolytic sources at the measurement site, and it dominated during the early morning near ground level.4Journal of Geophysical Research: Atmospheres. Summertime nitrous acid chemistry in the atmospheric boundary layer at a rural site in New York State – Section: Abstract
This matters because the hydroxyl radical sets the pace for ozone formation and smog production. Without enough OH, volatile organic compounds linger and nitrogen oxides accumulate. With a big pulse of OH from morning HONO photolysis, the photochemical engine that produces ground-level ozone kicks into gear earlier and runs harder. Modeling work over China estimated that including HONO chemistry in air-quality simulations boosted predicted surface ozone by roughly 6 to 13 percent across the country’s three most developed regions, and combining HONO with another reactive chlorine species raised ozone predictions by around 11 to 14 percent.5Atmospheric Chemistry and Physics. Combined impacts of nitrous acid and nitryl chloride on lower-tropospheric ozone: new module development in WRF-Chem and application to China Adding HONO sources to the model essentially erased a persistent bias in ozone forecasts, suggesting that earlier models had been systematically underestimating pollution because they left this chemistry out.
Where Nitrous Acid Comes From Outdoors
The outdoor sources of HONO are surprisingly varied, and researchers are still sorting out which ones matter most in different settings.
- Vehicle exhaust: Cars and trucks emit HONO directly. Measurements inside a road tunnel in Birmingham, UK, found a strong correlation between HONO levels and traffic density, with peak concentrations during the evening rush hour. The fleet-average ratio of HONO to total nitrogen oxides was just under 1 percent, but in areas with heavy traffic the direct vehicular source was likely the dominant contributor to boundary-layer HONO.6Atmospheric Chemistry and Physics. Nitrous acid (HONO) emissions under real-world driving conditions from vehicles in a UK road tunnel
- Surface reactions with NOâ‚‚: Nitrogen dioxide landing on wet or organic-rich surfaces can be converted into HONO. This heterogeneous chemistry is especially active at night, when NOâ‚‚ accumulates without being broken down by sunlight. In a field study near Marseille, nighttime HONO built up at rates of about 0.1 parts per billion per hour, reaching concentrations up to 1.2 ppb by dawn.7Atmospheric Research. Concentrations of nitrous acid, nitric acid, nitrite and nitrate in the gas and aerosol phase at a site in the emission zone during ESCOMPTE 2001 experiment
- Photosensitized reactions on organic aerosols: When sunlight hits particles coated with organic material, NOâ‚‚ on those surfaces gets converted to HONO much faster than in the dark. Laboratory experiments with humic acid aerosol showed that the reaction was essentially undetectable in darkness but ramped up strongly with increasing light intensity.8Atmospheric Chemistry and Physics. Light induced conversion of nitrogen dioxide into nitrous acid on submicron humic acid aerosol Follow-up work confirmed that organic photosensitizers reduce adsorbed NOâ‚‚ to produce HONO while simultaneously incorporating nitrogen into the organic material.9PubMed Central. Reduction and Photoreduction of NO2 in Humic Acid Films as a Source of HONO, ClNO, N2O, NOX, and Organic Nitrogen
- Soil: Microbes in soil produce HONO as a byproduct of nitrification and denitrification, the same nitrogen-cycling processes that release nitric oxide and nitrous oxide. Fertilized agricultural soils can be particularly strong emitters.10Environmental Science & Technology. Large Contribution of Nitrous Acid to Soil-Emitted Reactive Oxidized Nitrogen and Its Effect on Air Quality Global modeling suggests that soil HONO emissions are increasing as climate warming and fertilizer use intensify.11Nature Communications. Increasing soil nitrous acid emissions driven by climate and fertilization change aggravate global ozone pollution
The relative importance of these sources shifts with time of day. At night, the surface conversion of NOâ‚‚ dominates and HONO accumulates in the shallow nocturnal boundary layer. At sunrise, photolysis rapidly destroys this built-up HONO, producing the OH burst that jump-starts daytime chemistry. During midday, the photosensitized reactions on aerosols and soil emissions become more relevant because they can sustain HONO production even as photolysis is consuming it.
Nitrous Acid in Snow and Ice
One of the more surprising findings in HONO research is that snowpacks are prolific producers of the molecule. In polar regions, where nitrate trapped in snow grains absorbs ultraviolet light and breaks down, HONO is released from the snowpack surface into the overlying air. During the Arctic spring at Alert, Canada, photochemical production of HONO in the snowpack was intense enough that the resulting concentrations in the boundary layer made HONO photolysis the dominant OH source.12Geophysical Research Letters. Snowpack photochemical production of HONO: A major source of OH in the Arctic boundary layer in springtime
Measurements in coastal Antarctica tell a similar story. HONO fluxes from the snow surface were measured in the range of 0.5 to 3.4 × 10¹² molecules per square meter per second, declining at night in sync with the drop in solar radiation.13Atmospheric Chemistry and Physics. Snowpack nitrate photolysis drives the summertime atmospheric nitrous acid (HONO) budget in coastal Antarctica The practical consequence is that polar atmospheres are far more chemically active than you might expect from the cold and the sparse vegetation. HONO from snow drives oxidation chemistry that affects mercury cycling, ozone depletion events, and the removal of gaseous pollutants transported from lower latitudes.
Indoor Sources and Why Your Kitchen Matters
Step inside a building and the HONO story shifts from atmospheric science to personal exposure. Any combustion appliance that burns fuel in an enclosed space produces nitrogen oxides, and a fraction of those emerge as HONO or are quickly converted to it on indoor surfaces. Controlled experiments with an unvented gas combustion source showed that peak indoor HONO concentrations reached roughly 8 to 10 times background levels during operation.14PubMed. Performance evaluation of an in situ nitrous acid measurement system and continuous measurement of nitrous acid in an indoor environment Venting the exhaust reduced concentrations by a third or more, a useful reminder that range hoods matter for more than clearing cooking odors.
A study of natural gas cooking stoves in a residential kitchen in Kunming, China, put concrete numbers on the problem. Just a few minutes of cooking pushed indoor HONO above 100 ppb and nitrogen oxides above 1,000 ppb. The researchers calculated HONO emission factors and estimated that total cooking-related HONO emissions for the city were comparable in scale to traffic-related emissions.15PubMed. Emission characteristics of indoor HONO from residential natural gas cooking stoves in a household in Kunming, China For a gas-stove household, this means the kitchen is periodically a more intense HONO environment than a busy roadside.
The Thirdhand Smoke Connection
One of the more alarming discoveries involving indoor HONO is its role in creating carcinogenic compounds from tobacco residue. When someone smokes indoors, nicotine deposits on walls, furniture, carpets, and clothing and persists for months. HONO in indoor air reacts with this sorbed nicotine to produce tobacco-specific nitrosamines (TSNAs), which are potent carcinogens. Laboratory experiments found that exposing nicotine-laden cellulose surfaces to 60 ppb of HONO for just three hours produced a greater than tenfold increase in surface-bound TSNAs.16PubMed Central. Formation of carcinogens indoors by surface-mediated reactions of nicotine with nitrous acid, leading to potential thirdhand smoke hazards
The story gets worse when you consider human skin. Research on dermal exposure found that substrates pre-coated with human skin oils and sweat produced higher TSNA concentrations from the nicotine-HONO reaction than clean substrates did.17PubMed Central. Thirdhand Exposures to Tobacco-Specific Nitrosamines through Inhalation, Dust Ingestion, Dermal Uptake, and Epidermal Chemistry In other words, the chemistry of thirdhand smoke is not limited to walls and fabrics. It can happen on your skin. If you move into a home where someone once smoked, or hold a child who was in a smoker’s car, the nicotine on those surfaces can react with ambient HONO from gas stoves, candles, or outdoor infiltration to generate carcinogens right on contact surfaces.
Health Effects of Breathing It In
Given that HONO is a reactive, acidic gas, the question of whether breathing it causes harm is straightforward in principle but muddied in practice. Direct inhalation studies are limited. A controlled chamber study exposed asthmatic adults to HONO and found a small but statistically significant decrease in forced vital capacity, along with mild irritant symptoms affecting the respiratory tract and mucous membranes. The researchers concluded that the exposure altered lung mechanics slightly but did not induce significant airflow obstruction.18PubMed Central. Effect of nitrous acid on lung function in asthmatics: a chamber study
Epidemiological evidence is messier. A study of adults in their homes found that each additional part per billion of indoor HONO was associated with a roughly 1 percent drop in a standard lung function measure after adjusting for confounders, along with non-significant increases in respiratory symptoms.19Thorax. Indoor nitrous acid and respiratory symptoms and lung function in adults However, a study of infants in their first year of life found that HONO exposure was not independently linked to respiratory symptoms once NOâ‚‚ exposure was accounted for.20Epidemiology. Exposure to NO2 and Nitrous Acid and Respiratory Symptoms in the First Year of Life – Section: Results The difficulty is that HONO and NOâ‚‚ are almost always present together indoors, making it hard to separate their effects. People with gas stoves have higher exposure to both, and teasing apart which molecule drives which symptom remains an open problem in environmental epidemiology.
What is clearer is that the indirect effects of indoor HONO may matter more than the direct irritation. The thirdhand smoke pathway described above creates carcinogens that would not exist without HONO. And because HONO photolyzes in indoor light (albeit more slowly than in direct sunlight), it produces OH radicals indoors too, which can then react with cleaning products, fragrances, and other volatile organic compounds to generate secondary pollutants like formaldehyde. The total health burden of indoor HONO is therefore a mix of its own irritant properties and the downstream chemistry it sets in motion.
How Researchers Measure It
Measuring HONO accurately has been a persistent headache. The molecule is sticky, reactive, and present at very low concentrations in outdoor air (typically low parts per billion). Early measurement techniques using denuders and filters were plagued by artifacts: surfaces in the instrument would convert NOâ‚‚ to HONO, inflating the readings. This led to years of controversy over whether reported HONO levels in the atmosphere were real or instrumental noise.
The development of the LOPAP (long path absorption photometer) instrument helped resolve much of this uncertainty. The LOPAP works by absorbing HONO into a liquid reagent in a stripping coil, then detecting the resulting colored product in a long optical path. Crucially, the instrument uses a two-channel design that allows it to correct for interfering species. It was designed to be compact, sensitive, and suitable for continuous monitoring in settings from ambient air to smog chambers to indoor environments.21Environmental Science & Technology. A new instrument to measure gaseous nitrous acid (HONO) in the atmosphere Spectroscopic methods like differential optical absorption spectroscopy (DOAS) offer a complementary approach by measuring HONO along a long open light path, avoiding surface contact altogether. These improved tools are a large part of why the field’s understanding of HONO sources and concentrations has advanced so rapidly since the early 2000s.
Nitrous Acid in Food Preservation
Outside the atmosphere, nitrous acid has a long if largely unrecognized history in your refrigerator. When sodium nitrite is added to cured meats like bacon, ham, and hot dogs, the nitrite ion encounters the mildly acidic conditions of the meat matrix and converts in part to nitrous acid. This is where a lot of the curing chemistry actually happens. Nitrous acid reacts with myoglobin to produce the characteristic pink color of cured meat, and it generates reactive nitrogen species that inhibit the growth of dangerous bacteria, particularly Clostridium botulinum.
The bactericidal mechanism involves multiple stress pathways. Under the low-oxygen, low-pH conditions inside cured meat, nitrite-derived species including peroxynitrite act as strong oxidants that damage bacterial enzymes and membranes. The effectiveness of this system depends heavily on pH, oxygen levels, salt concentration, and the presence of ascorbate, which accelerates the formation of reactive intermediates.22PubMed. Mechanisms of the bactericidal effects of nitrate and nitrite in cured meats Nitrous acid is in many ways the working molecule behind nitrite curing, even though the ingredient label says “sodium nitrite.” The same chemistry that makes cured meat safe from botulism is also what produces nitrosamines when cured meat is cooked at high temperatures, which is why the relationship between processed meat and cancer risk has attracted so much scrutiny.
This dual nature of nitrous acid chemistry captures something true about the molecule generally. In the atmosphere, HONO photolysis produces the OH radicals that clean the air of pollutants but also drive ozone production. Indoors, gas stoves provide useful heat while generating HONO that contributes to thirdhand smoke chemistry. In food, nitrite-derived nitrous acid prevents deadly bacterial contamination while potentially promoting nitrosamine formation. The same reactivity that makes nitrous acid useful in one context makes it hazardous in another, and the practical question in every case is how to manage the conditions so you get more of the benefit and less of the risk.