We look at nitrous acid (HONO), a transient gas with a powerful effect on air quality, both indoors and outside.
Nitrous acid (HONO) exists in the air in tiny concentrations and is broken down almost as soon as it is formed, yet it has a major role in atmospheric chemistry. Despite its significance, many uncertainties remain in our understanding of it. We discuss the problem of air pollution today, the part played by nitrous acid (HONO), and its measurement with the AirYX HONO/NO2 Analyser (Model ICAD 200L).
The problem of air pollution persists
The Air Quality Stripes, visualisations of how air quality has changed in different cities since 1850, have gone from red to blue in recent years – at least in the UK, signalling improvements in air quality. However, pollution levels still exceed the World Health Organisation’s (WHO) Air Quality Guidelines, with it widely agreed that no safe level of exposure to air pollution exists.
Recent research found that in 2023/24 UK nitrogen dioxide and fine particulate matter levels exceeded the WHO air quality targets on 40 and 22 days of the year respectively, while the number of days that ozone exceeded targets had doubled. Speaking to the Royal College of Physicians (RCP), Dr Suzanne Bartington said that, “Exposure to air pollution is not a momentary problem – the impact lasts the course of a whole lifetime […] exposure to air pollution, even at low levels, in one part of our lives can go on to seriously affect our health in another”.
In a new report published by the RCP on Clean Air Day 2025, the body of medical professionals estimates that air pollution is still linked to around 30,000 deaths a year in the UK, and costs upwards of £27 billion in healthcare, productivity losses, and reduced quality of life. Furthermore, in the Environmental Improvement Plan released this December, the government acknowledges that “air pollution remains one of the biggest environmental threats to human health”.
What is HONO (nitrous acid)?
Nitrous acid is a compound that plays a significant role in air quality. It is produced during combustion processes, such as in diesel engines or during wildfires. However, the main formation mechanism is thought to be the reaction of nitrogen dioxide on the surfaces of particles, droplets, or infrastructure in the presence of water.
Nevertheless, in daytime, once formed, nitrous acid exists for just tens of minutes as it is broken down by sunlight (which also means that levels build up during the night). When nitrous acid undergoes photolysis, hydroxyl radicals are produced. These highly reactive molecules are key to the formation of ozone and, in turn, photochemical smog, as well the degradation of other air pollutants.
The effect of HONO on indoor air quality
HONO also has an important and very poorly understood role in indoor air quality. It is increasingly recognised as an indoor air pollutant with implications for health. It can be produced during everyday activities such as cooking with gas stoves, with concentrations spiking during or after cooking indoors.
Nitrogen dioxide (NO₂) emitted by combustion and outdoor infiltration can react on indoor surfaces (paint, glass, dust), especially with surface moisture, to also form HONO.
HONO is associated with respiratory irritation and reduced lung function, especially in homes with gas appliances. It also has the potential to form carcinogenic nitrosamines by reacting with amines, including tobacco smoke residues or other amine compounds, although this requires more research to establish how significant this is to human health. As interest grows around the effects of HONO on indoor air quality, achieving effective measurement raises a number of considerations.
Measuring HONO (nitrous acid)
Because of the importance of nitrous acid to indoor and outdoor air quality – and climate change too, given that pollutants such as ozone are also potent greenhouse gases, understanding its chemistry is vital. Data are fundamental to this understanding, but nitrous acid has traditionally been very difficult to measure. This is, in large part, due to its short atmospheric lifespan and very low concentrations (in the parts per billion range).
Our AirYX HONO/NO2 Analyser (Model ICAD 200L) provides a solution. It uses iterative cavity-enhanced direct optical absorption spectroscopy (ICAD). This involves injection of the sample into a cavity that has mirrors at either end. Light is bounced many times between the mirrors, generating a much longer light path than traditional DOAS, as well as relative to the footprint of the instrument.
However, in practice, some light is lost during cavity-enhanced DOAS, which reduces the theoretical light path slightly. This can lead to underestimation of the substance of interest, as the gas has less opportunity to absorb the light. ICAD incorporates an innovative algorithm that models this light path reduction, enabling accurate assessment of the concentration of the gas.
The AirYX ICAD 200L also uses a spectrum of light wavelengths (~360 to 390 nm) which, following transmission, are separated and analysed, confirming the identity of nitrous acid and differentiating it from compounds with similar absorption patterns, like water vapour.
Get in touch
To learn more about the AirYX HONO/NO2 Analyser (Model ICAD 200L), please get in touch with Lewis John, Cura Terrae Air’s UK Sales Manager for Scientific Applications.
