15 Tropospheric Photochemistry – Formation of Reactive Radicals and Molecules

Prof. K.S. Gupta

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Contents

  1. Introduction
  2. Photodissociation of Nitrogen dioxide and Production of Ozone
  3. Photodissociation of Ozone
  4. Production of OH and HO2 radicals
  5. Tropospheric OH concentration
  6. Factors governing tropospheric ozone formation
  7. Reactions mechanism of CO oxidation by OH: sinks for OH
  8. Mechanism of removal of methane from atmosphere
  9. Reactions of methylperoxide radical
  10. The reaction of peroxy radicals
  11. Recycling between HO2 and OH
  12. Thermal reaction of ozone
  13. Formation of hydrogen peroxide
  14. Formation of nitrous acid
  15. Damaging effect of O3
  16. Suggested reading

Introduction

The atmospheric chemistry is driven primarily by photochemical reactions, which are initiated by solar radiation, which are a constant and inexhaustible source of energy. The gas phase tropospheric chemistry of pollutants like NO, NO 2 , NO2 , SO2, volatile organic compounds (VOCs) and others involves their reactions with a variety of reactive transient or short lived species such as excited molecules, which are formed as a result of photochemical and thermal chemical reactions initiated by sun-light absorption by trace gases in atmosphere. These reactions are the source of several secondary pollutants like ground level ozone, etc.

The fundamental chemical nature of the atmosphere is oxidizing, the main actors of which are molecules like oxygen and ozone, and radicals like hydroxyl, OH, hydroperoxy, HO2, nitrate, NO3 and a variety of organic ones. Although oxygen is 21% of air, its reactions are very slow. However, the radicals are very reactive. So, most of the pollutants, released in the atmosphere, are removed through oxidation by radicals. The development of air pollution abatement and control strategies require the quantitative inputs from several areas: emission inventory of pollutants, atmospheric chemistry of the pollutants, sinks and removal processes and transport and diffusion processes. We shall limit ourselves to the discussion of chemical processes only.

 

Photodissociation of Nitrogen dioxide and Production of Ozone

Tropospheric ozone is precursor of OH radicals and therefore it plays the most important role in maintaining the oxidizing power of the atmosphere. The tropospheric production of ozone is almost entirely due to the photodissociation of NO2 by solar radiation. Nitrogen dioxide, NO2, is dissociated by absorption of visible radiation in the wavelength region 280 – 430 nm to form reactive ground state oxygen atom, O(3P), and NO as in reaction (1).

The reactions (1 – 3) are responsible for giving photostationary concentration of these species in troposphere. The relationship among the concentration of O 3 , NO and NO2 may be obtained as follows.

Applying steady state treatment on the concentrations of both O atom and O3 from Eqs. (1-3), we get Eqs. (4– 5).

where Jp is a composite rate constant including the dependence on the intensity of radiations. k2 and k3 are the rate constant2 of the reactions(2) and (3).

Thus, in atmosphere containing NO and NO2 fossil fuel combustion, the concentration of O3 which are common man-made pollutants generated by will be determined by [NO 2 ] / NO] ratio.

 

Photodissociation of Ozone

Corresponding to a wavelength of 1180 nm, only 1.1 eV energy is required for the dissociation of ozone. Hence, the photodissociation of ozone can take place in entire UV-visible region by solar radiation. However, the cross-section for photodissociation of ozone varies by several orders of magnitude with wavelength. The most important wavelength regions are in the Hartley band and the continuum (280 – 300 nm). The photodissociation of ozone generates electronically excited oxygen atoms, O(1D), known as singlet oxygen atom.

 

Production of OH and HO2 Radicals

 

Nitrogen oxides( NOX ) and hydrogen oxides(HOx, hydroxyl radical, OH and hydroperoxy radical, HO2 ) are basic components of atmospheric chemistry, which determine the atmosphere’s oxidizing or cleansing power and production of O3. The photodissociation of O3, produces the excited oxygen atom, O(1 D) , as in reaction(9). It is actually excited state oxygen atom without any unpaired electron. O(1 D) is highly reactive and reacts with water vapor to generate OH radical as in reaction (10).

The largest source of OH radicals in the lower troposphere is reaction (10) . In upper troposphere and in some continental regions, the pollutants generated by the reactions of OH with hydrocarbons, which are lifted convectively, become more important.

 

Tropospheric OH Concentration

OH concentration depends on the solar UV flux and therefore there is a diurnal variation

 

HOx and NOx catalyze the formation of O3. Carbon monoxide, CO, and volatile organic compounds(VOCs) are known as ozone precursors to describe their O3 forming potential.

In a similar way, the reactions of hydrocarbons also produce ozone.

Reactions Mechanism of CO Oxidation by OH: Sinks for OH

The OH radical is unreactive towards oxygen. The reaction of OH with CO and other pollutants are very important. Indeed, in troposphere, CO and CH4 and other volatile organic compounds are major sinks for OH radicals. These trace gases are responsible for controlling the OH concentration in troposphere. Reaction of OH with CO produces another reactive radical, HO2 , as in the reactions( 24- 25).

Mechanism of Removal of Methane from Atmosphere

The loss of methane in atmosphere is dominated by the reaction with OH radical. The reaction of OH with methane through a series of reactions, involving NO, eventually produces water vapor and CO2 together with some other  products

CH 3 O2 is methylperoxy radical and its reactions are similar to HO2. The reactions of this radical are as follows.

The methxy radical reacts rapidly with O2 to generate Formaldehyde and HO2 radical.

 

Formation of Formaldehyde

The most common fate of smaller alkoxy radicals is reaction with radical, CH3O, reacts with O2 to yield HCHO.

The Reaction of Peroxy Radicals

In case of larger non-methane hydrocarbons, as are found in urban area and forests, the oxidation pathways are very complex than for those of methane. The reaction of OH with volatile organic compounds (VOCs) results in the formation of RO 2 , where R is a hydrocarbon radical. Formation of RO2 after leads to formation of HO2 . The amount of NO present determines whether the oxidation of a particular hydrocarbon is a net source or sink for HOx.

Since HOx production, is generally driven by sunlight, it is expected that OH and HO2 would exist only in daylight. However, HO2 has been observed to persist through the night at a level of few pptv. This necessitates the presence of nighttime HOx sources. Paulson and Orlando (1996) have

 

Reactions of OH Radicals

Hydroxyl radical is one of the strongest oxidant. It oxidation reactions wit large number of compounds are known. The hydroxyl radical is often referred to as the “detergent” or ‘scavenger’ of the troposphere owing to the removal of many pollutants by oxidation. The lifetime of a pollutant is determined by the value of its rate constant with OH radical. The large number of biogenic organic gases such as isoprene are removed by OH radicals. In general, OH reacts by abstracting a hydrogen atom from the organic molecule forming a radical and water. Its hydrogen – abstraction reaction is non-specific. The organic radical then reacts with O2 to form organic peroxides, RO2, as discussed earlier. These compounds are an essential part of the ozone formation cycle.

OH  reacts  with  aldehydes  to  yield  oxidation  products  by  hydrogen  abstraction.     Acetaldehyde,

CH 3 CO is acetyl radical and CH3COO2 is acetylperoxy radical.

In the reaction with isoprene, CH2=C(CH2)CH=CH2, Oh can abstract H atom from any of the four carbon atoms generation four radicals which subsequently react with O2 followed by degradation to yield other compounds.

 

Oxidation of NO and NO2

Reactions with Alkenes

The oxidation of hydrocarbons containing π-bonds results in the formation of several reactive intermediates and other organic compounds. For example, the reaction of O3 with ethylene is a follows:

The reactions with larger hydrocarbons are very complex and yield a variety of products.

 

Formation of Hydrogen Peroxide

Hydrogen peroxide is a strong oxidant and it plays an important role in the atmospheric oxidation of SO2 as discussed in another module. The following reactions yield H2O2.

Formation of Nitrous Acid

Damaging Effect of  

 

you can view video on Tropospheric Photochemistry – Formation of Reactive Radicals and Molecules

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