Question

Choose the incorrect statement

• A. Smog is formed near the ground in the troposphere.
• B. Nitrogen oxides impart yellow-brown colour to smog.
• C. Photochemical somg formation is maximum when sun is overhead in the day.
• D. Photochemical smog formation is maximum in winter compared to summer season.
• A. Photochemical smog
• B. Oxidising smog
• C. Los Angles smog
• D. Reducing smog
• A. Nitrogen oxides
• B. Sunlight
• C. Water Vapour
• D. Volatile hydrocarbons
• A. Ozone
• B. Aldehydes
• C. Peroxy acetyl nitrate
• D. Sulphuric Acid
• A. Ozone formed during photochemical smog protects us from UV radiations.
• B. Increase in tropospheric ozone can reduce food production.
• C. Photochemical smog formation is maximum in morning and evening due to heavy traffic hours.
• D. Vehicular exhaust does not contain any precursors of ozone formation in the troposphere.

Answer 1

The Correct Option is D

Concept:
Smog is a form of air pollution that develops in the lower troposphere.
There are two major categories: classical (London-type) smog and photochemical (Los Angeles-type) smog.
Photochemical smog is driven by solar ultraviolet radiation and elevated ambient temperatures.

Step 1: Assessing the Conditions for Photochemical Smog Formation:
Photochemical smog formation begins with the photolysis of nitrogen dioxide ($\text{NO}_2$) by solar UV radiation ($\lambda < 420\text{ nm}$):
\[ \text{NO}_2 + h\nu \rightarrow \text{NO} + \text{O}(^3P) \] The liberated oxygen radical combines with diatomic oxygen to produce ground-level ozone:
\[ \text{O} + \text{O}_2 + M \rightarrow \text{O}_3 + M \] Secondary reactions between hydroxyl radicals, volatile organic compounds (VOCs), and $\text{NO}_x$ yield peroxyacetyl nitrate (PAN) and aldehydes.
Because these photochemical chain reactions require intense solar insolation and warm temperatures, photochemical smog peaks during hot, sunny summer afternoons when the sun is directly overhead.

Step 2: Evaluating the Other Statements:
- Statement (A): Smog forms in the planetary boundary layer of the lower troposphere near the ground. (Correct)
- Statement (B): Nitrogen dioxide ($\text{NO}_2$) is a reddish-brown gas that imparts a characteristic yellow-brown haze to photochemical smog. (Correct)
- Statement (C): Smog production peaks around midday when solar insolation reaches its daily maximum. (Correct)
- Statement (D): Claiming that photochemical smog formation is higher in winter than in summer contradicts its reliance on solar insolation and warm temperatures. (Classical sulfurous smog occurs in cold, damp winter conditions, whereas photochemical smog develops in warm, sunny summer weather.)
Final Answer:
The incorrect statement is option (D).

Answer 2

Concept:
Smog types are classified based on their dominant chemical composition and overall oxidation-reduction behavior.
Atmospheric mixtures containing high concentrations of reducing agents behave differently from those dominated by strong chemical oxidants.

Step 1: Chemical Composition of Classical Smog:
As described in the passage, classical smog (exemplified by the Great London Smog of 1952) forms in cool, damp, foggy conditions where coal combustion releases large volumes of:
1. Sulphur dioxide ($\text{SO}_2$)
2. Soot and fly-ash particulates
3. Sulphur trioxide ($\text{SO}_3$)
4. Liquid droplets of sulphuric acid ($\text{H}_2\text{SO}_4$) formed by catalytic oxidation in fog droplets.

Step 2: Chemical Mechanism of Reducing Smog:
Because this smog contains elevated concentrations of sulphur dioxide, where sulfur exists in the intermediate $+4$ oxidation state ($\text{S}^{IV}$), it readily acts as a chemical reducing agent by undergoing oxidation to sulphate ($\text{S}^{VI}$):
\[ \text{SO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_3 \] \[ 2\text{H}_2\text{SO}_3 + \text{O}_2 \rightarrow 2\text{H}_2\text{SO}_4 \] Because reducing species dominate the mixture, classical sulfurous smog is chemically designated as reducing smog.
In contrast, photochemical smog contains high levels of ozone ($\text{O}_3$) and peroxides, making it an oxidising smog.
Final Answer:
Smog composed of sulphur dioxide, water droplets, and sulphuric acid is reducing smog, corresponding to option (D).

Answer 3

Concept:
Photochemical smog is a secondary atmospheric phenomenon formed through solar-driven reactions between primary vehicular emissions.
The essential chemical ingredients and physical conditions dictate whether photochemical smog can develop.

Step 1: The Essential Components of Photochemical Smog:
1. Nitrogen Oxides ($\text{NO}_x$): Primarily nitric oxide ($\text{NO}$) and nitrogen dioxide ($\text{NO}_2$) emitted by internal combustion engines. Photolysis of $\text{NO}_2$ provides the oxygen atoms needed to generate ground-level ozone.
2. Volatile Hydrocarbons (VOCs): Unburned hydrocarbons emitted in vehicular exhaust and evaporated from industrial solvents. They react with hydroxyl radicals to produce peroxy radicals ($\text{RO}_2^\bullet$), which oxidize $\text{NO}$ to $\text{NO}_2$ without consuming ozone, allowing ozone levels to build up.
3. Sunlight (Ultraviolet Radiation): Solar photon flux ($\lambda < 420\text{ nm}$) drives the dissociation of $\text{NO}_2$ and initiates the radical chain reactions.

Step 2: Role of Water Vapour:
While moisture is an essential component of classical London-type smog (where sulfur oxides dissolve into suspended fog droplets), it is not a prerequisite for photochemical smog.
Photochemical smog typically develops in arid, semi-arid, or dry Mediterranean basins (such as the Los Angeles basin, Mexico City, and Santiago) characterized by clear, sunny skies, low humidity, and stagnant atmospheric inversion layers.
High concentrations of water vapor are not required to drive the gas-phase photochemical chain reactions.
Final Answer:
Water vapour is not an essential requirement for photochemical smog formation, corresponding to option (C).

Answer 4

Concept:
Photochemical smog is characterized by the production of toxic secondary oxidants through chemical reactions between nitrogen oxides and volatile organic compounds under solar irradiation.
In contrast, sulfurous smog is characterized by sulfur oxidation chemistry.

Step 1: Products of Photochemical Smog Reactions:
As described in the passage, photochemical reactions between $\text{NO}_x$, hydrocarbons, and sunlight generate several characteristic secondary pollutants:
1. Ozone ($\text{O}_3$): Generated when atomic oxygen released from $\text{NO}_2$ photolysis combines with molecular oxygen.
2. Peroxyacetyl Nitrate (PAN): Formed by the reaction of peroxyacetyl radicals (derived from hydrocarbon oxidation) with nitrogen dioxide:
\[ \text{CH}_3\text{C(O)OO}^\bullet + \text{NO}_2 \rightarrow \text{CH}_3\text{C(O)OONO}_2 \text{ (PAN)} \] 3. Aldehydes and Ketones: Incomplete oxidation of volatile hydrocarbons produces formaldehyde ($\text{HCHO}$) and acrolein ($\text{CH}_2=\text{CHCHO}$), both of which cause respiratory and eye irritation.

Step 2: Source of Sulphuric Acid:
Sulphuric Acid ($\text{H}_2\text{SO}_4$): Produced through the gas-phase and aqueous oxidation of sulphur dioxide ($\text{SO}_2$) emitted by coal combustion or smelters.
It is a primary constituent of classical reducing smog and acid precipitation, but is not a characteristic secondary product of the $\text{NO}_x$-hydrocarbon photochemical smog cascade described in the text.
Final Answer:
Sulphuric acid is not a product of photochemical smog formation, corresponding to option (D).

Answer 5

Concept:
Ground-level tropospheric ozone ($\text{O}_3$) is a harmful secondary pollutant and phytotoxin that damages plant tissues, inhibits photosynthesis, and harms human health.
Evaluating its environmental impacts requires distinguishing its tropospheric effects from its stratospheric UV-shielding role.

Step 1: Evaluating the Impact on Crop Yields (Statement B):
Tropospheric ozone enters plant leaves through open stomata during normal gas exchange.
Inside the leaf apoplast, it degrades into reactive oxygen species (ROS, including hydrogen peroxide and hydroxyl radicals) that induce oxidative stress.
This damages chloroplast thylakoid membranes, degrades the primary carbon-fixing enzyme RuBisCO, accelerates foliar senescence, and inhibits photosynthesis.
Agronomic studies show that elevated tropospheric ozone causes yield reductions of $5-15\%$ in staple crops such as wheat, rice, and soybeans, directly reducing food production.
Hence, statement (B) is scientifically correct.

Step 2: Evaluating the Other Statements:
- Statement (A): While stratospheric ozone shields the biosphere from harmful solar UV-B radiation, ground-level ozone in photochemical smog is a toxic pollutant that does not provide useful UV shielding. Thus, statement (A) is incorrect.
- Statement (C): Morning and evening traffic emissions release precursor gases ($\text{NO}$ and hydrocarbons), but photochemical smog peaks in the early afternoon (12:00 PM to 3:00 PM) when solar UV intensity is highest. Thus, statement (C) is incorrect.
- Statement (D): Internal combustion engines are the primary urban source of $\text{NO}_x$ and VOC precursors that generate tropospheric ozone. Thus, statement (D) is incorrect.
Final Answer:
The correct statement is that an increase in tropospheric ozone can reduce food production, corresponding to option (B).