Environmental Engineering Reference
In-Depth Information
beaches, oil spills contaminate zooplankton and benthic
invertebrates (fish to a lesser extent) persist in anoxic
sediments, and reduce the abundance and diversity of
benthic communities.
mospheric O
2
during the course of human evolution.
Oxygen is replenished by photosynthesis (mining of
phosphates and their use as fertilizer is a minor source)
and consumed by oxidation of organic and inorganic
compounds, but its levels remain steady and globally uni-
form. Even complete combustion of all reserves of fossil
fuels would reduce atmospheric O
2
by less than 0.3%,
and the (theoretically possible) recovery of all liberally
estimated fossil fuel resources would lower it by no more
than 2%. In any case, levels of atmospheric oxygen have
shown no significant shift from 20.95% by volume since
the beginning of the twentieth century.
What has changed quite significantly are the atmo-
spheric concentrations of particulate matter (PM; solid
or liquid aerosols with diameter
<
500 mm) and trace
gases. PM is emitted as fly ash and black carbon (soot)
from coal and refined oil combustion, and the burning
of biomass fuels is yet another, regionally large, source
of aerosols (mainly organic carbon in smoke). Gaseous
emissions from fossil fuels are dominated by SO
x
(emis-
sion factors of 0.5-2 kg/GJ in coal combustion, 0.2-1
kg/GJ in oil burning), NO
x
(both from fuel and from
atmospheric N made available for oxidation by high-
temperature breakdown of N
2
; emission factors 0.1-1
kg/GJ) and volatile organic compounds (VOCs). SO
x
,
NO
x
, and VOCs also participate in complex photochem-
ical reactions whose products include both aerosols and
highly reactive gases (O
3
and peroxyacetyl nitrate).
Particulate matter is by far the most abundant global
air pollutant, and combustion of coal is a leading source
of anthropogenic particulates. Their irritating effects were
noted for the first time in densely inhabited medieval
London (Brimblecombe 1987), and during the nine-
teenth century these emissions reached objectionable
levels in just about every major city and industrial region
11.4 Energy and the Atmosphere
Human actions can have no global or lasting effect on
dinitrogen (N
2
), the atmosphere's dominant gas. Any
outflows from this immense reservoir, whether oxides
produced by combustion or NH
3
synthesized by the
Haber-Bosch process, will be balanced by denitrification.
Nor can fossil fuel combustion reduce atmospheric O
2
to
alarmingly low levels. As already noted (see section 8.3),
complete combustion of 1 t C requires 2.67 t of O
2
,
burning 1 t CH
4
needs 4 t of O
2
, and the average for
1 t of refined liquid fuels is about 3.5 t O
2
. Oxidation of
fuel sulfur (and sometimes a small amount of fuel N) and
production of NO
x
consume negligible amounts of O
2
compared to the generation of CO
2
. After the
subtraction of unburned fossil fuels that are used as lubri-
cants, paving materials, and feedstocks, the global com-
bustion of fossil fuels consumed about 25 Gt of O
2
in
the year 2000, and during the twentieth century cumula-
tive demand was at least 900 Gt of O
2
.
Oxygen's atmospheric mass amounts to 1.2 Pt, and
hence 25 Gt of O
2
is just 0.002%, and 900 Gt of O
2
just 0.075%, of the element's atmospheric presence.
Much larger fluctuations of O
2
took place in the distant
past. Modeling of the carbon and sulfur cycles indicates
that oxygen's atmospheric concentrations may have
varied between 15% and 30% over the past 550 Ma
(Berner 1999). Elevated O
2
levels, peaking during the
late Carboniferous period, could have been responsible
for gigantism in insects (dragonflies with 70-cm wing
spans), but there have been no appreciable changes of at-
