354
S. Kalita and H. Talukdar
(restriction on aeroplanes, trains, motor vehicles and ships) activities on the current
air quality status.
Air pollutants generated from natural and human sources fall under two broad categories, namely primary pollutant and secondary pollutant. Pollutants such as carbon
monoxide (CO), carbon dioxide (CO 2 ), methane (CH 4 ), nitric oxide (NO), nitrogen
dioxide (NO 2 ), nitrous oxide (N 2 O), sulphur dioxide (SO 2 ), ammonia (NH 3 ), most
hydrocarbons, and most suspended particles matter which are directly emitted into
the atmosphere from their sources are called primary pollutants; whereas, the chemicals such as ozone (O 3 ), peroxy acetyl nitrate (PAN), hydrogen peroxide (H 2 O 2 ),
sulphuric acid (H 2 SO 4 ), nitric acid (HNO 3 ) which formed by the reaction of primary
pollutants with one another or other natural constituents of air are called secondary
pollutants. Globally, most of these pollutants are regularly monitored and reported
as they are extremely hazardous to living organisms. Apart from quantifying the
concentration of pollutants and then comparing them to ambient air standards, the
quality of air is also evaluated by application of different types of air quality indices
such as Air Quality Index [13], Aggregate Air Quality Index [14], Common Air
Quality Index [15], New Air Quality Index [16], Pollution Index [17], Aggregate
Risk Index [18] and National Air quality Index [19]. Among these indices, the Air
Quality Index (AQI) (US EPA) is the most commonly used index to denote the
magnitude of air pollution of an area. AQI is based on the aggregation method where
the AQI value is calculated using PM 10 , PM 2.5 , SO 2 , NO 2 and CO [20]. Based on
AQI values, the quality of air is defined from good to hazardous/severe for human
health conditions. Thus, lower AQI values (i.e., <100 is considered to be desirable
for breathing) while higher AQI values > 100 denotes undesirable air conditions
for breathing [21]. Studies by [8, 20–25] showed a reduction in the AQI values for
northern China (8%), 366 areas of China’s mainland (20%), Wuhan (48%), Delhi
(61%), Gujarat (32–75%), 22 cities of India (15–44%), and Lucknow (50–60%),
respectively. A decline in the AQI values indicates an overall improvement in the
quality of air during the COVID-19 lockdown period. The pollutants, which were
mostly assessed and reported as indicators of quarantine lockdown impact on air
quality, are in the order of their popularity in air quality studies oxides of nitrogen,
particulate matter, oxides of carbon and SO 2 .
15.2.1.1 Nitrogen Containing Species
Nitrogen is the most abundant element of the Earth’s atmosphere. It is also present
in other forms such as N 2 O, NO, NO 2 and NH 3 . The oxides of nitrogen influences
the Earth’s atmospheric oxidising capacity; while, NH 3 plays a role in neutralization of acid aerosols. N 2 O, an important greenhouse gas, is about 200 times more
efficient than CO 2 in absorbing IR radiation. This gas is emitted from undisturbed
soils, fertilised cultivated soils, oceans, animal wastes, and burning of biomass and
fossil fuel. N 2 O is virtually inert in the troposphere and is depleted by the photochemical reaction in the stratosphere [3]. NO and NO 2 collectively designated as
NO x enter the atmosphere from natural sources (lightning and biological processes),
S. Kalita and H. Talukdar
(restriction on aeroplanes, trains, motor vehicles and ships) activities on the current
air quality status.
Air pollutants generated from natural and human sources fall under two broad categories, namely primary pollutant and secondary pollutant. Pollutants such as carbon
monoxide (CO), carbon dioxide (CO 2 ), methane (CH 4 ), nitric oxide (NO), nitrogen
dioxide (NO 2 ), nitrous oxide (N 2 O), sulphur dioxide (SO 2 ), ammonia (NH 3 ), most
hydrocarbons, and most suspended particles matter which are directly emitted into
the atmosphere from their sources are called primary pollutants; whereas, the chemicals such as ozone (O 3 ), peroxy acetyl nitrate (PAN), hydrogen peroxide (H 2 O 2 ),
sulphuric acid (H 2 SO 4 ), nitric acid (HNO 3 ) which formed by the reaction of primary
pollutants with one another or other natural constituents of air are called secondary
pollutants. Globally, most of these pollutants are regularly monitored and reported
as they are extremely hazardous to living organisms. Apart from quantifying the
concentration of pollutants and then comparing them to ambient air standards, the
quality of air is also evaluated by application of different types of air quality indices
such as Air Quality Index [13], Aggregate Air Quality Index [14], Common Air
Quality Index [15], New Air Quality Index [16], Pollution Index [17], Aggregate
Risk Index [18] and National Air quality Index [19]. Among these indices, the Air
Quality Index (AQI) (US EPA) is the most commonly used index to denote the
magnitude of air pollution of an area. AQI is based on the aggregation method where
the AQI value is calculated using PM 10 , PM 2.5 , SO 2 , NO 2 and CO [20]. Based on
AQI values, the quality of air is defined from good to hazardous/severe for human
health conditions. Thus, lower AQI values (i.e., <100 is considered to be desirable
for breathing) while higher AQI values > 100 denotes undesirable air conditions
for breathing [21]. Studies by [8, 20–25] showed a reduction in the AQI values for
northern China (8%), 366 areas of China’s mainland (20%), Wuhan (48%), Delhi
(61%), Gujarat (32–75%), 22 cities of India (15–44%), and Lucknow (50–60%),
respectively. A decline in the AQI values indicates an overall improvement in the
quality of air during the COVID-19 lockdown period. The pollutants, which were
mostly assessed and reported as indicators of quarantine lockdown impact on air
quality, are in the order of their popularity in air quality studies oxides of nitrogen,
particulate matter, oxides of carbon and SO 2 .
15.2.1.1 Nitrogen Containing Species
Nitrogen is the most abundant element of the Earth’s atmosphere. It is also present
in other forms such as N 2 O, NO, NO 2 and NH 3 . The oxides of nitrogen influences
the Earth’s atmospheric oxidising capacity; while, NH 3 plays a role in neutralization of acid aerosols. N 2 O, an important greenhouse gas, is about 200 times more
efficient than CO 2 in absorbing IR radiation. This gas is emitted from undisturbed
soils, fertilised cultivated soils, oceans, animal wastes, and burning of biomass and
fossil fuel. N 2 O is virtually inert in the troposphere and is depleted by the photochemical reaction in the stratosphere [3]. NO and NO 2 collectively designated as
NO x enter the atmosphere from natural sources (lightning and biological processes),