Box 5.2: What are Trace Gases?
The Earth’s atmosphere consists of large amounts of
nitrogen (78% by volume) and oxygen (21% by volume). The remaining 1% of the atmospheric gases are
known as trace gases because they are present in small
concentrations, mostly one part per billion (ppb) or
lower. The sources of trace gases can be natural or
anthropogenic. The natural sources are biogenic, volcanoes, lightning and forest fires, and emission from
the Oceans. The global ocean is a source of several
trace gases, including sulfur-containing gases. The
trace gases are also formed in the atmosphere through
chemical reactions in the gas phase. The anthropogenic sources of trace gases are fossil fuel combustion, fossil fuel mining, biomass burning, and
industrial activity, etc. (Brasseur et al. 1999).
The most important trace gases found in the
atmosphere are greenhouse gases. These trace gases
are called greenhouse gases because they help to keep
Earth warm by absorbing sunlight. In the troposphere,
water vapor, ozone (O 3 ), carbon dioxide (CO 2 ),
methane (CH 4 ), sulfur dioxide (SO 2 ), and nitrous
oxide (N 2 O) are the important greenhouse (trace)
gases. The two most abundant greenhouse gases by
volume are water vapor and CO 2 (Brasseur et al.
1999).
Ozone acts as a greenhouse gas in the troposphere,
while in the stratosphere, filter out the incoming ultraviolet radiation coming from the Sun. Thus, it helps in
protecting life on the Earth. The human-made processes
have injected new trace gases into the atmosphere, for
example, chlorofluorocarbons (CFCs), which damage
the ozone layer in the stratosphere.
The increased burden of trace gases in the atmosphere leads to global warming and climate change.
The focus of the current chapter is ozone and related
trace gases, while other trace gases, e.g., CO 2 , CH 4 ,
and, N 2 O, etc., are discussed in Chap. 4.
5.2 Aerosols
5.2.1 Emissions of Different Aerosol Species
Atmospheric aerosols originate from two distinct pathways—
either by direct emission of primary aerosols into the atmosphere (e.g., dust, sea salt, OC, BC) or by the formation of
secondary aerosols via atmospheric chemical reactions (e.g.,
sulfate, nitrate, ammonium, and SOA). While BC, along with
sulfate, nitrate, and ammonium, has anthropogenic sources
like incomplete combustion of biomass and fossil fuel, sea
salt, dust, and primary biological aerosols are naturally produced in the atmosphere. Over the Indian subcontinent, the
aerosol emission rates are 8.9 Tg year
−1 for NMVOCs,
0.7 Tg year
−1 for BC, 1.9 Tg year
−1 for primary organic
aerosol, 2.9 TgS year
−1 for SO 2 , 5.8 Tg year
−1 for NH 3 , and,
0.5 Tg year
−1 for biomass burning aerosols (IPCC 2013).
The major source of BC emissions in India is the combustion of solid biofuel (172–340 Gg year
−1 ), while other
sources include wood fuel (143 Gg year
−1 ), dried cattle
manure (8 Gg year
−1 ), and crop waste (21 Gg year
−1 ). The
relative contributions of fossil fuel, open burning, and biofuel consumption to total BC emissions over the Indian
subcontinent are 25%, 35%, and 42%, respectively. Biofuel
consumption also results in the emission of OC (583–
1683 Gg year
−1 ). The relative contributions from fossil fuel,
open burning, and biofuel consumption to the total OC
emissions are estimated to be 13%, 43%, and 44%, respectively (Venkataraman et al. 2005).
The transport sector, the second-largest contributor to
organic aerosols over India, shows an emission rate of 0.14
(0.1–0.3) Tg year
−1 for BC and 0.07 (0.02–0.2) Tg year
−1
for OC. For both the emissions, diesel vehicles were found
to be the primary cause (92% to BC and 78% to OC).
However, the combined emission rate of BC and OC from
both the industry and transport sectors is 0.23 Tg year
−1 and
0.15 Tg year
−1 , respectively (Sadavarte and Venkataraman
2014). In the Indian rural sector, the emission estimate of
organic and elemental carbon from biomass fuels over the
IGP is 361.96 ± 170.18 Gg and 56.44 ± 29.06 Gg,
respectively (Saud et al. 2012). The estimates of aerosols
emissions from open burning (forest and crop waste) are
102–409 Gg year
−1 for BC, 399–1529 Gg year
−1 for OC,
and 663–2303 Gg year
−1 for organic matter. This overall
contributes to about 25% of the total BC, OC/OM emissions
(Venkataraman et al. 2006).
Residential sector SO 2 emissions have been estimated at
0.2 (0.08–0.4) Tg year
−1 with major contribution from
dung-based (56%) and coal-based stoves (19%). Emissions
of SO 2 from agriculture were estimated at 0.09 (0.02–0.2)
Tg year
−1 . From the industry sector and transport sector,
emission estimates of SO 2 were reported to be 7 (6.0–
9.6) Tg year
−1 and 0.08 (0.04–0.3) Tg year
−1 , respectively,
for the year 2015 (Pandey et al. 2014).
From 1996 to 2015, there has been a 30% increase in BC
emissions due to increased emissions from informal industries, while OC emissions increased by only 4% (Pandey
et al. 2014). Vehicular emissions of BC have increased by
112% during 1991–2001. From all the sources, the estimated
BC emission for India is around 835.50 Gg for 1991 and
1343.78 Gg in 2001, indicating a growth of about 61%
during the 1990s. During the same period, trends in SO 2
emissions increased by 32% (Pandey et al. 2014).
96
S. Fadnavis et al.
The Earth’s atmosphere consists of large amounts of
nitrogen (78% by volume) and oxygen (21% by volume). The remaining 1% of the atmospheric gases are
known as trace gases because they are present in small
concentrations, mostly one part per billion (ppb) or
lower. The sources of trace gases can be natural or
anthropogenic. The natural sources are biogenic, volcanoes, lightning and forest fires, and emission from
the Oceans. The global ocean is a source of several
trace gases, including sulfur-containing gases. The
trace gases are also formed in the atmosphere through
chemical reactions in the gas phase. The anthropogenic sources of trace gases are fossil fuel combustion, fossil fuel mining, biomass burning, and
industrial activity, etc. (Brasseur et al. 1999).
The most important trace gases found in the
atmosphere are greenhouse gases. These trace gases
are called greenhouse gases because they help to keep
Earth warm by absorbing sunlight. In the troposphere,
water vapor, ozone (O 3 ), carbon dioxide (CO 2 ),
methane (CH 4 ), sulfur dioxide (SO 2 ), and nitrous
oxide (N 2 O) are the important greenhouse (trace)
gases. The two most abundant greenhouse gases by
volume are water vapor and CO 2 (Brasseur et al.
1999).
Ozone acts as a greenhouse gas in the troposphere,
while in the stratosphere, filter out the incoming ultraviolet radiation coming from the Sun. Thus, it helps in
protecting life on the Earth. The human-made processes
have injected new trace gases into the atmosphere, for
example, chlorofluorocarbons (CFCs), which damage
the ozone layer in the stratosphere.
The increased burden of trace gases in the atmosphere leads to global warming and climate change.
The focus of the current chapter is ozone and related
trace gases, while other trace gases, e.g., CO 2 , CH 4 ,
and, N 2 O, etc., are discussed in Chap. 4.
5.2 Aerosols
5.2.1 Emissions of Different Aerosol Species
Atmospheric aerosols originate from two distinct pathways—
either by direct emission of primary aerosols into the atmosphere (e.g., dust, sea salt, OC, BC) or by the formation of
secondary aerosols via atmospheric chemical reactions (e.g.,
sulfate, nitrate, ammonium, and SOA). While BC, along with
sulfate, nitrate, and ammonium, has anthropogenic sources
like incomplete combustion of biomass and fossil fuel, sea
salt, dust, and primary biological aerosols are naturally produced in the atmosphere. Over the Indian subcontinent, the
aerosol emission rates are 8.9 Tg year
−1 for NMVOCs,
0.7 Tg year
−1 for BC, 1.9 Tg year
−1 for primary organic
aerosol, 2.9 TgS year
−1 for SO 2 , 5.8 Tg year
−1 for NH 3 , and,
0.5 Tg year
−1 for biomass burning aerosols (IPCC 2013).
The major source of BC emissions in India is the combustion of solid biofuel (172–340 Gg year
−1 ), while other
sources include wood fuel (143 Gg year
−1 ), dried cattle
manure (8 Gg year
−1 ), and crop waste (21 Gg year
−1 ). The
relative contributions of fossil fuel, open burning, and biofuel consumption to total BC emissions over the Indian
subcontinent are 25%, 35%, and 42%, respectively. Biofuel
consumption also results in the emission of OC (583–
1683 Gg year
−1 ). The relative contributions from fossil fuel,
open burning, and biofuel consumption to the total OC
emissions are estimated to be 13%, 43%, and 44%, respectively (Venkataraman et al. 2005).
The transport sector, the second-largest contributor to
organic aerosols over India, shows an emission rate of 0.14
(0.1–0.3) Tg year
−1 for BC and 0.07 (0.02–0.2) Tg year
−1
for OC. For both the emissions, diesel vehicles were found
to be the primary cause (92% to BC and 78% to OC).
However, the combined emission rate of BC and OC from
both the industry and transport sectors is 0.23 Tg year
−1 and
0.15 Tg year
−1 , respectively (Sadavarte and Venkataraman
2014). In the Indian rural sector, the emission estimate of
organic and elemental carbon from biomass fuels over the
IGP is 361.96 ± 170.18 Gg and 56.44 ± 29.06 Gg,
respectively (Saud et al. 2012). The estimates of aerosols
emissions from open burning (forest and crop waste) are
102–409 Gg year
−1 for BC, 399–1529 Gg year
−1 for OC,
and 663–2303 Gg year
−1 for organic matter. This overall
contributes to about 25% of the total BC, OC/OM emissions
(Venkataraman et al. 2006).
Residential sector SO 2 emissions have been estimated at
0.2 (0.08–0.4) Tg year
−1 with major contribution from
dung-based (56%) and coal-based stoves (19%). Emissions
of SO 2 from agriculture were estimated at 0.09 (0.02–0.2)
Tg year
−1 . From the industry sector and transport sector,
emission estimates of SO 2 were reported to be 7 (6.0–
9.6) Tg year
−1 and 0.08 (0.04–0.3) Tg year
−1 , respectively,
for the year 2015 (Pandey et al. 2014).
From 1996 to 2015, there has been a 30% increase in BC
emissions due to increased emissions from informal industries, while OC emissions increased by only 4% (Pandey
et al. 2014). Vehicular emissions of BC have increased by
112% during 1991–2001. From all the sources, the estimated
BC emission for India is around 835.50 Gg for 1991 and
1343.78 Gg in 2001, indicating a growth of about 61%
during the 1990s. During the same period, trends in SO 2
emissions increased by 32% (Pandey et al. 2014).
96
S. Fadnavis et al.