Emilie Meyer
Peroxyacetyl nitrate (PAN) is a secondary pollutant and a component of photochemical smog. It is formed from other pollutants by chemical reactions in the atmosphere. PAN is produced when nitrogen dioxide (NO2) reacts with peroxyacetyl (PA) radicals, which are oxidated products of volatile organic compounds (VOCs). VOCs are present in vehicle exhaust gases and second-hand cigarette smoke. The action of sunlight on VOCs and nitrogen oxides can also form peroxyacetyl nitrate.
| Characteristics | Values |
|---|---|
| Formation | The reaction of nitrogen dioxide (NO2) with peroxyacetyl (PA) radical, the oxidated product of volatile organic compounds (VOCs) |
| Type of Pollutant | Secondary pollutant |
| Sources | Motor vehicles, tobacco smoke, and the burning of fossil fuels |
| Effects on Humans | Reduced respiratory function (including emphysema and impaired breathing) and eye irritation |
| Effects on Plants | Changes in color and growth, increased susceptibility to disease, and death |
| Stability | More stable than ozone, capable of long-range transport |
| Toxicity | Higher than that of ozone |
| Decomposition | Decomposes into peroxyethanoyl radicals and nitrogen dioxide gas (NO2) |
| Natural Concentration | Below 0.1 μg/m3 |
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What You'll Learn
- PAN is a secondary pollutant, not directly emitted from exhausts but formed from other pollutants
- VOCs and nitrogen oxides from sunlight can form peroxyacetyl nitrate
- VOCs and aldehydes in the presence of NO2 can also form PAN
- PAN is more stable than ozone, so it can be transported over long distances
- PAN is a reservoir gas, acting as a source and sink of ROx and NOx radicals
PAN is a secondary pollutant, not directly emitted from exhausts but formed from other pollutants
Peroxyacetyl nitrate (PAN) is a respiratory and eye irritant present in photochemical smog. It is produced from the gas-phase oxidation of volatile organic compounds (VOCs) and nitrogen oxides, which are emitted from vehicle exhaust gases and second-hand cigarette smoke. VOCs include methane, benzene, and chlorofluorohydrocarbons. PANs are secondary pollutants, meaning they are not directly emitted from power plants or internal combustion engines but are instead formed from other pollutants through chemical reactions in the atmosphere.
The process by which PAN forms begins with the oxidation of unburned non-methane hydrocarbons, which are converted into aldehydes, ketones, and dicarbonyls through free radical reactions catalysed by ultraviolet light from the sun. These compounds then undergo secondary reactions to create peroxyacyl radicals, the most common of which is peroxyacetyl. This peroxyacetyl radical can be formed from the free radical oxidation of acetaldehyde, various ketones, or the photolysis of dicarbonyl compounds.
The formation of PAN from these precursors is not dependent on the presence of specific hydrocarbons in the atmosphere, except for peroxyacetyl nitrate. PANs are both toxic and irritating, more so than ozone, as they dissolve more readily in water. This property makes them lachrymators, causing eye irritation at very low concentrations.
The presence of PANs in the atmosphere has significant implications for air quality and public health. As powerful irritants, they can cause respiratory and ocular discomfort, with potential long-term exposure effects that are not yet fully understood. Furthermore, PANs play a crucial role in tropospheric ozone production by transporting NOx to regions where it can efficiently produce ozone. This transport mechanism contributes to the overall chemistry of the atmosphere and the formation of smog.
In summary, PAN is a secondary pollutant that forms through the interaction of various pollutants and atmospheric conditions. Its presence in the atmosphere underscores the complex nature of air pollution and the need to understand the chemical transformations that occur after the initial emission of pollutants from sources such as vehicle exhausts.
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VOCs and nitrogen oxides from sunlight can form peroxyacetyl nitrate
Peroxyacetyl nitrate (PAN), a secondary pollutant, is not directly emitted as exhaust from internal combustion engines. However, it is formed from other pollutants, such as nitrogen oxides and volatile organic compounds (VOCs), through chemical reactions in the atmosphere.
VOCs, such as methane, benzene, and chlorofluorohydrocarbons, are organic compounds that are released from vehicle exhaust gases and second-hand cigarette smoke. Nitrogen oxides (NOx), on the other hand, are produced during the combustion of fossil fuels and are present in automobile exhaust gases.
When these pollutants interact with sunlight, they undergo a series of chemical reactions, leading to the formation of peroxyacetyl nitrate. Sunlight, particularly its ultraviolet component, catalyses free radical reactions that oxidize unburned non-methane hydrocarbons into aldehydes, ketones, and dicarbonyls. These intermediates then undergo secondary reactions to produce peroxyacyl radicals, with peroxyacetyl being the most common.
The presence of sunlight is essential for the formation of peroxyacetyl nitrate through this mechanism. The reaction between nitrogen oxides and VOCs in the atmosphere, facilitated by sunlight, results in the production of peroxyacetyl nitrate. This process is a significant contributor to photochemical smog, a brownish-grey haze that contains harmful pollutants such as ozone, nitrogen oxides, and peroxyacetyl nitrate.
In summary, VOCs and nitrogen oxides from engine exhaust can interact with sunlight to initiate a series of chemical reactions that ultimately lead to the formation of peroxyacetyl nitrate. This process contributes to air pollution and has detrimental effects on human health, the environment, and various ecosystems.
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VOCs and aldehydes in the presence of NO2 can also form PAN
Peroxyacetyl nitrate (PAN) is a prevalent peroxyacyl nitrate formed from the thermal equilibrium between organic peroxy radicals. It is produced by the gas-phase oxidation of volatile organic compounds (VOCs) or by aldehydes and other oxygenated VOCs oxidizing in the presence of nitrogen dioxide (NO2). VOCs are organic chemical compounds composed of carbon that can evaporate under normal indoor atmospheric conditions of temperature and pressure. They are found in both indoor and outdoor environments due to their presence in everyday products and materials. VOCs are released into the air during the manufacture, use, or volatilization of these products. Examples of VOCs include methane, benzene, and chlorofluorohydrocarbons.
Aldehydes, such as formaldehyde, are also organic compounds with boiling points ranging from 50°C to 260°C, allowing them to exist indoors in the gaseous phase. They are often associated with indoor environments, particularly in residential housing, schools, and offices. Aldehydes, along with VOCs, have been linked to adverse health effects, including respiratory symptoms such as asthma and wheezing.
The presence of NO2, a gaseous air pollutant composed of nitrogen and oxygen, is crucial for the formation of PANs. NO2 is formed when fossil fuels, such as coal, oil, methane gas, or diesel, are burned at high temperatures. This occurs during the combustion process in internal combustion engines, contributing to the formation of PANs in vehicle exhaust gases. NO2 also forms indoors when fuels like wood or gas are burned and from appliances that burn natural gas, liquefied petroleum gas, or kerosene without proper ventilation.
The combination of VOCs, aldehydes, and NO2 in the atmosphere leads to the production of PANs through chemical reactions. PANs are secondary pollutants, meaning they are not directly emitted as exhaust but are formed from other pollutants. The toxic and irritating nature of PANs contributes to their impact on human health, particularly as respiratory and eye irritants. Additionally, PANs play a role in tropospheric ozone production by transporting NOx to regions where ozone can be more efficiently produced.
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PAN is more stable than ozone, so it can be transported over long distances
Peroxyacetyl nitrate (PAN) is a peroxyacyl nitrate that is a secondary pollutant. It is not directly emitted as exhaust from internal combustion engines but is formed from other pollutants by chemical reactions in the atmosphere. These chemical reactions involve the gas-phase oxidation of volatile organic compounds (VOCs) or the oxidation of aldehydes and other oxygenated VOCs in the presence of NO2.
PAN is more stable than ozone at lower temperatures, allowing it to be transported over long distances. This characteristic of PAN is significant as it enables the transport of NOx to regions where it can efficiently produce ozone. PAN acts as a carrier for oxides of nitrogen (NOx) and contributes to ozone formation in the global troposphere. The stability of PAN at lower temperatures facilitates its long-range transport through cold regions of the atmosphere, while its decomposition occurs at warmer levels.
The formation of PAN is influenced by various factors, including the presence of VOCs, NOx, and other precursors. Measurements in German cities have shown PAN values up to 25 μg/m3, significantly higher than its natural concentration of below 0.1 μg/m3. PAN is a powerful respiratory and eye irritant, causing more eye irritation from photochemical smog than ozone due to its higher solubility.
While PAN concentrations have shown a decreasing trend in some urban areas, its chemistry continues to play a role in enhancing O3 formation. This impact on O3 production is particularly notable in the context of urban ozone photochemical pollution. Overall, the stability of PAN at lower temperatures allows it to be transported over long distances, contributing to ozone formation and influencing photochemical pollution in regions far from its urban or industrial origins.
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PAN is a reservoir gas, acting as a source and sink of ROx and NOx radicals
Peroxyacetyl nitrate (PAN) is a peroxyacyl nitrate and a secondary pollutant present in photochemical smog. It is formed from other pollutants by chemical reactions in the atmosphere and is not directly emitted as exhaust from internal combustion engines. PAN is produced in the atmosphere via the photochemical oxidation of hydrocarbons to peroxyacetic acid radicals, which react with nitrogen dioxide (NO2) to form PAN.
PAN is a relatively long-lived reservoir for both NOx and organic radicals. It plays a crucial role in the complex radical chemistry and O3 formation of the troposphere. PAN can promote or suppress O3 production by affecting radical cycling. It tends to suppress O3 production under low-NOx and low-ROx conditions but enhances O3 production by supplying RO2 radicals under conditions with sufficient NOx.
PAN is thermally unstable in the boundary layer, with a lifetime of only about 2 hours at a typical NO2/NO ratio of ~7 and an ambient temperature of 25 °C. However, its lifetime increases rapidly with decreasing temperature, approximately five times every 10 °C, leading to a lifetime of over a month in the mid-troposphere. This property makes it the principal reservoir for short-lived NOx, facilitating the transport and release of NOx to remote regions, which has significant implications for global O3 distribution.
PAN is able to transport these unstable compounds far away from their urban and industrial origins, which is important for tropospheric ozone production. PAN serves as a carrier for oxides of nitrogen (NOx) into rural regions, causing ozone formation in the global troposphere. The composition of PAN in a particular region depends heavily on which hydrocarbons are present in the atmosphere.
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Frequently asked questions
Peroxyacetyl nitrate is a peroxyacyl nitrate and a secondary pollutant present in photochemical smog. It is a powerful respiratory and eye irritant.
PAN is formed from other pollutants by chemical reactions in the atmosphere. It is produced when nitrogen dioxide (NO2) reacts with the peroxyacetyl (PA) radical, which is the oxidation product of volatile organic compounds (VOCs). VOCs are present in engine exhaust.
Examples of VOCs include methane, benzene, and chlorofluorohydrocarbons.
PAN is toxic and irritating. It causes eye irritation at concentrations of only a few parts per billion. At higher concentrations, it damages vegetation.
PAN is more stable than ozone and therefore more capable of long-range transport. It also has a higher toxicity and is a better solvent, making it more harmful to human health.
