# Summer smog

As summer smog (also smog , ozone smog or LA -Smog ) is defined as the air exposure to high concentrations of ozone and other photochemical oxidants . Summer smog is caused by the photochemical oxidation of carbon monoxide (CO), methane (CH 4 ) and volatile hydrocarbons ( VOC ) in the presence of nitrogen oxides and water vapor as catalysts , i.e. not too high in sunny weather that is not too cool. The formation mechanism was identified as the cause of the so-called Los Angeles smog around 1950, in particular by a research group led by Arie Jan Haagen-Smit at Caltech . The LA smog was recognized as a special phenomenon in 1943. At that time, industrial production increased particularly rapidly there due to the war. In contrast, mere smog , from smoke (smoke) and fog (mist), an older and rather wintry phenomenon.

The local ozone pollution is determined by air measuring stations and regularly displayed and published in pollution maps. Ozone attacks the respiratory organs and damages plants and animals. Since even low concentrations of nitrogen oxides are sufficient, only extremely clean air areas are not affected. The global increase in ground-level ozone is contributing to climate change - directly as a greenhouse gas and indirectly through the reduced photosynthesis capacity of plants.

## properties

The main component of photochemical smog is ozone, one of the most powerful oxidizing agents of all. It accounts for up to 90 percent of photooxidants. In addition, there is a complex mixture of various irritants, including peroxiacetyl nitrate , peroxibenzoyl nitrate , acrolein and formaldehyde . In addition to short-lived, highly reactive radicals, there are also stable products of the oxidative degradation of organic compounds. However, these are in significantly lower concentrations than ozone and are therefore of less importance.

## Ozone generation

Both in the stratospheric ozone layer and in summer smog, ozone (O 3 ) is created by the addition of an oxygen atom (O) to an oxygen molecule (O 2 ), whereby a third collision partner (M) must be present to dissipate binding energy:

O + O 2 + M → O 3 + M (1)

However, the O atoms come from different sources. In the stratosphere, it is the photolysis of O 2 by UV-C radiation, but it does not reach the troposphere . In summer smog it is (2) the photolysis of ozone by UV-B ( , beyond about 308 nm) or (3) of nitrogen dioxide (NO 2 ) by violet light ( <420 nm): ${\ displaystyle h \ nu}$${\ displaystyle \ lambda}$

O 3 + → O 2 + O ( 1 D) (2)${\ displaystyle h \ nu}$
NO 2 + → NO + O (3)${\ displaystyle h \ nu}$

At first glance (2) and (3) do not seem to change the O 3 balance, because in (2) one O 3 is lost, as is the reaction (4) that usually follows (3):

NO + O 3 → NO 2 + O 2   (4)

However, besides (4) there are other sources of NO 2 , in particular

ROO + NO → RO + NO 2   (5)

In this, R stands either for a hydrogen atom (H) or for an organic radical, such as CH 3 .

The formation of ROO begins with reaction (2). The product O ( 1 D), an electronically excited oxygen atom, is usually already de-excited to the ground state O ( 3 P) at the next collision , followed by (1), but O ( 1 D) can be an alternative , especially in humid and warm air encounter a water molecule (H 2 O):

O ( 1 D) + H 2 O → 2 OH

The reactive hydroxyl radical (OH, also written HO) is considered to be a detergent in the atmosphere, as the reaction with OH is the dominant sink for many trace substances (predominantly volatile organic compounds ). Here, for example, the reaction chain for the oxidation of methane (CH 4 ) via the intermediate products formaldehyde (HCHO) and carbon monoxide (CO):

OH + CH 4 → CH 3 + H 2 O
CH 3 + O 2 → CH 3 OO
CH 3 OO + NO → CH 3 O + NO 2   (5.1)
CH 3 O + O 2 → HCHO + HO 2 ,
HO 2 + NO → OH + NO 2   (5.2)
OH + HCHO → HCO + H 2 O
HCO + O 2 → HO 2 + CO
HO 2 + NO → OH + NO 2   (5.3)
OH + CO, + O 2 → HO 2 + CO 2
HO 2 + NO → OH + NO 2   (5.4)

The three OH radicals used were regenerated (5.2 to 5.4). A total of four NOs were oxidized. After photolysis (3), NO is available again. (3) is followed by ozone formation (1). Increased ozone concentration accelerates the formation of further OH radicals. The chain reaction does not lead to an explosion, because the VOC oxidation cannot become much faster than NO is reproduced by (3), otherwise competing reactions of the type that consume ozone take place with (5)

ROO + R'OO → ROOR '+ O 2

overhand. Since nitrogen oxides are predominantly of anthropogenic origin in metropolitan areas, the nitrogen oxide concentration is the appropriate lever to limit summer smog - one of the reasons for much lower nitrogen oxide limit values ​​in the environment than in exposed workplaces ( MAK ). As a result, the oxidation of the VOC is delayed, the ozone pollution decreases in the conurbation and increases in the surrounding area. For large-scale horizontal distribution see. Only in extremely clean air areas does convection from the stratosphere make a significant contribution to the ozone concentration in the lower troposphere. For vertical profiles see.

## Ozone formation potential

The main sources of ozone-forming nitrogen oxides and hydrocarbons are traffic (internal combustion engines), industry (power plants), households (heating systems) and products containing solvents (paints).

The exhaust gases emitted by a vehicle with a combustion engine, for example, contribute to the formation of ozone in the troposphere close to the ground with the different reactivities of their components ( VOC ). Uncombusted hydrocarbons, in particular, are highly reactive with HO radicals and accordingly have a high ozone formation potential. The Federal Immission Control Act calls such substances "ozone precursors" and recommends monitoring 27 chemical compounds, including alkanes, alkenes, substituted benzene compounds and formaldehyde (39th BImschV, Part 8, Annex 10, (B)). The yardstick for evaluating the ozone formation potential is a method developed in the USA in the early 1990s, in which the exhaust gas components are individually recorded and evaluated as part of the legally prescribed driving cycle test (e.g. US FTP). In California, this procedure is now used for the certification of new vehicles registered on the market - especially those with reformulated and alternative fuels. This is done with the help of the MIR (maximum incremental reactivity) scale, which enables the consideration of relative ozone formation potentials under certain atmospheric conditions. MIR factors have meanwhile been determined empirically for around 200 exhaust gas components. Components with the highest reactivities are some olefins (MIR = 8–11 gO 3 / g VOC ), some aromatics (7–9 gO 3 / g VOC ) and some oxygenates (aldehydes with 5–7 gO 3 / g VOC ); Methane has the lowest reactivity with 0.015 gO 3 / g VOC .

Vehicle tests have shown the following:

• The ozone formation potential is reduced by 80-95% compared to a vehicle without a catalyst
• The balance for a vehicle powered by natural gas (CNG) is very favorable
• In contrast, there are only minor differences between different fuels (gasoline, diesel, alcohol fuels M85 / E85)
• In petrol operation, 50% of the ozone formation potential is created by just four exhaust gas components, while another 40% is created by 16 other components
• In gas (CNG) and diesel operation, 50% of the ozone formation potential is formed by only two components
• Likewise, in alcohol operation (M85 / E85) 50% of the ozone formation potential is formed by only two components (formaldehyde or acetaldehyde, unburned alcohol), while another 40% is formed by 18 other components.

## Effects in humans

Ozone penetrates deep into the lungs as an irritant gas and can cause inflammation . Depending on the duration of the exposure and the concentration, there are health effects such as coughing, eye irritation, headaches or lung dysfunction. According to the recommendations of doctors, physical exertion should be avoided when ozone levels are high.

## Legal limit values

According to the 3rd EU Directive 2002/3 / EC for "Limit values ​​for the protection of health" (replaced by the new Air Quality Directive 2008/50 / EC on June 11, 2010 ):

• 1-hour concentration> 180 μg / m 3 : Information for the population
• 1-hour concentration> 240 μg / m 3 : warning the public
• Max. Daily exposure (8-hour value): 120 μg / m 3

## Possible countermeasures

Individual (short-term) avoidance is to go to closed rooms or leave polluted areas. A long-term reduction can only be achieved on a collective level. Since the weather is seen as one of the triggers that cannot be influenced to a large extent, the measures to reduce summer smog are aimed at reducing nitrogen oxides and volatile hydrocarbons. Since these are caused by traffic, private firing systems and industry / trade, shutdowns and traffic avoidance help (short-term). In the long term, it is necessary to retrofit and / or replace systems and vehicles.

After sunset, the formation of new ozone comes to a standstill. In regions with heavy traffic, the existing ozone reacts with nitrogen monoxide and the ozone concentration drops rapidly. In rural areas, the ozone content of the air drops only slightly at night. In general, ozone concentrations are lowest in the morning.

## literature

• Ian Barnes, Karl-Heinz Becker, Peter Wiesen: Organic compounds and photosmog . In: Chemistry in Our Time . tape 41 , no. 3 , June 2007, p. 200 , doi : 10.1002 / ciuz.200700415 .
• Johannes Staehelin, Christoph Hüglin, Stefan Brönnimann, Nino Künzli: Ozon and summer smog , Swiss Academies of Arts and Sciences , 2016.
• Chemistry today - upper secondary level, Schrödel Verlag
• CE Mortimer, U. Müller: Chemie, 8th edition, Thieme, Stuttgart, 2003
• B. Höhlein, P. Biedermann et al .: Traffic emissions and summer smog , Monographs from Forschungszentrum Jülich, Volume 26/1996, ISBN 3-89336-188-X
• G. Decker, J. Beyersdorf et al .: The ozone formation potential of different vehicle and fuel concepts, ATZ Automobiltechnische Zeitschrift 98 (1996) 5

## Individual evidence

1. ^ Arie J. Haagen-Smit: Chemistry and Physiology of Los Angeles Smog. Industrial and Engineering Chemistry 44, 1952, doi: 10.1021 / ie50510a045 ( free full text ).
2. ^ John M. Wallace, Peter V. Hobbs: Atmospheric Science: An Introductory Survey. Elsevier, 2006, ISBN 9780080499536 , limited preview in Google Book Search.
3. ^ History of Smog. LA Weekly, September 22, 2005.
4. ^ California Military Department: California and the Second World War - Los Angeles Metropolitan Area during World War II. California Military History Online, 2019.
5. ^ Elizabeth A. Ainsworth et al .: The Effects of Tropospheric Ozone on Net Primary Productivity and Implications for Climate Change. Annual Review of Plant Biology 63, 2012, doi: 10.1146 / annurev-arplant-042110-103829 ( free full text ).
6. a b Ground-level ozone - information brochure of the Bavarian State Environment Agency
7. Yutaka Matsumi, Masahiro Kawasaki: photolysis of Atmospheric Ozone in the Ultraviolet Region. Chem. Rev. 103, 2003, doi: 10.1021 / cr0205255 ( free full text ).
8. This reaction is not elementary, but takes place depending on the pressure via HOCO * or H, see David C. McCabe, Tomasz Gierczak, Ranajit K. Talukdar and AR Ravishankara : Kinetics of the Reaction OH + CO Under Atmospheric Conditions. Geophysical Research Letters 28, 2001, doi: 10.1029 / 2000GL012719 (free full text).
9. ^ KB Moiseenko et al .: Regional Photochemical Surface-Ozone Sources in Europe and Western Siberia. Izvestiya, Atmospheric and Oceanic Physics 54, 2018, doi: 10.1134 / S0001433818060105 ( free full text ).
10. LL Pan et al .: Bimodal distribution of free tropospheric ozone over the tropical western Pacific revealed by airborne observations. Geophysical Research Letters 42, 2015, doi: 10.1002 / 2015GL065562 (free full text).
11. ^ Information about ozone - information brochure of the Bavarian State Environment Agency
12. 39. BImschV, Part 8, Annex 10
13. NRW Landtag 1985
14. ^ Ozone - Measures against summer smog ( Memento of November 3, 2012 in the Internet Archive )