Introduction

Nitrogen dioxide (NO₂) is one of the important substances of the class of nitrous gases or nitrogen oxides. NO₂ is a reddish-brown gas with a sweet to pungent odour, which is also present to a minor extent as dimeric colourless N₂O₄.

Presence in the atmosphere

In the atmosphere, NO₂ (nitrogen dioxide) and NO (nitric oxide) (together they are called NO_x) are extremely tightly coupled during sunlit hours and rapidly interconvert with one another in the presence of ozone:

NO + O₃ –> NO₂ + O₂

NO₂ + hv –> NO + O

O + O₂ + M –> O₃ + M

Individually, NO and NO₂ have extremely short lifetimes of the order of seconds, however, by considering the two compounds together as NO_x the lifetime is lengthened to many hours. Hence, NO₂ and NO display strong diurnal cycles and their concentrations will display a seasonal cycle. In urban areas, NO_x can reach hundreds of ppbv, and in particularly polluted environments ppmv levels, whereas clean maritime air will have levels of only 5 – 10 pptv. The major loss processes for NO_x are conversion to HNO₃ via reaction with OH:

NO₂ + OH + M –> HNO₃ + M

and dry deposition of NOx. Nitric acid can be removed by wet and dry deposition, constituting a loss of NO_x from the atmosphere.

In the troposphere, in the presence of VOCs, NO_x can promote the formation of ozone (O₃).

For most sources, NO_x is emitted in the form of NO. Natural sources of NO are from soil processes and lightning discharge, while an ever increasing source is that from high-temperature combustion of of fossil fuels.

Natural sources of NO₂ are emissions from denitrifying bacteria in soil; oceans.

Anthropogenic sources are combustion of oil, gas and coal; atmospheric transformation of NO.

About 30% of the global budget of NO_x i.e. (NO₂ + NO), comes from fossil-fuel combustion, with almost 86% of the NO_x emitted in one form or the other into the planetary boundary layer from surface processes. Typical NO/NO₂ ratio in surface air are 0.2 – 0.5 during the day, tending to zero at night.

The photolysis of NO₂ to NO and the subsequent regeneration of NO₂ via reaction of NO with ozone (O₃) is sufficiently fast, in the moderate polluted environment, for the species to be in dynamic equilibrium .

Urban pollution

In all combustion processes, nitrogen oxides (NO_x) are formed by reaction of nitrogen and oxygen. The main combustion product is nitrogen monoxide (NO), a certain fraction of which reacts further with oxygen to form nitrogen dioxide. This reaction is exothermic so that cooling combustion exhaust gases promotes this secondary NO₂ formation. In ambient air, heating power stations, motor vehicles, industrial heating systems and building heating systems are the most important emission sources of NO₂.

Indoor pollution

Indoor NO₂ emissions are formed from combustion sources such as heating and cooking with solid fuel (wood, coal), liquid fuel (oil, kerosene) or gaseous fuel [town gas, natural gas, bottled gas (propane, butane)], especially in the initial heating phase. An unflued appliance that releases combustion gases directly into the indoor air can be a particularly strong source. In the literature, there are many reports on the results of NO₂ determinations in indoor air. On the basis of these results, the average concentrations may range from under 10 μg/m³ to 800 μg/m³ under different conditions. The concentration measured in indoor air is influenced by the frequency, duration and intensity of the indoor combustion processes. The air change rate with the outdoors and the NO₂ concentration of the ambient air affect indoor NO₂ concentrations. Further, decomposition reactions, i.e. those aided by indoor materials and surfaces, cause a decrease in NO₂ concentration.

What are the sources of NO₂?

Sources are such as heating and cooking with solid fuel (wood, coal), liquid fuel (oil, kerosene) or gaseous fuel.

How does NO₂ get into the air?

Indoor NO₂ emissions are formed from combustion sources.

What are the health effects of NO₂?

Chronic exposure to NO₂ can cause respiratory effects including:

· airway inflammation in healthy people

· increased respiratory symptoms in people with asthma

The influence of indoor air pollutants on health is important because the majority of people in the world spend more than 80% of their time indoors. The amount of time spent indoors depends upon on several factors including geographical region, job activities, and gender among other variables. Additionally, because home insulation is improving, this can result in greater retention of indoor air pollutants, such as (NO₂).

Evaluation, guideline and limit values

Commission Directive (EU) 2017/164 of 31 January 2017 establishing a fourth list of indicative occupational exposure limit values pursuant to Council Directive 98/24/EC, and amending Commission Directives 91/322/EEC, 2000/39/EC and 2009/161/EU gives following limit values for work places:

8 hours (Measured or calculated in relation to a reference period of 8 hours time-weighted average (TWA)): 960 µg/m³ or 500 ppb

Short-term (A limit value above which exposure should not occur and which is related to a 15-minute period unless otherwise specified.): 1910 µg/m³ or 1000 ppb

How can NO₂ be reduced?

While using a gas stove, it is advised to also use ventilation.

If venting isn’t possible, then replacing gas stoves with electric stove could be another option. Replacing gas stoves with electric ranges could greatly reduce the exposure to indoor NO₂.

It is important to keep gas stoves and heaters in good repair so they are not polluting extra NO₂.

How do we measure NO₂?

At Aristoteles Consulting, we measure NO₂ with continuously registering measuring devices. This devices are equipped with a Gas Sensitive Electrochemical (GSE) sensor. The measured data are saved by a data logger, which can be read out after the measurement to get information about the concentration and to create data tables and graphs showing time dependent variations.

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