Atmospheric Composition
Composition of the atmosphere
Gas % by volume
N2 78.08
O2 20.95
Ar 0.93
CO2 0.03
All other gases (Ne,He,
Kr,H,etc)
0.01
Water Variable
Dry air
Water vapor in the air
? The % volume of Water vapor is variable,
depending on temperature,precipitation,rate of
evaporation and other factors at a particular
location.
? The percentage of water vapor ranges from 0.1-
5%,Generally it is 1-3% (the 3rd most abundant
constituents in the air).
Expressing the amount of substances in the
atmosphere
? Concentration
– the amount (mass,moles,molecules,etc) of a
substance in a given volume divided by that
volume,
– The example concentration units are mg/m3,
mol/m3,molecules/cc,and etc,
? Mixing ratio
– the ratio of the amount of the substance in a
given volume to the total amount of all
constituents in that volume.
Mixing Ratio
ni is the molar concentration of i and ntotal is the total
molar concentration of air.
parts per million (ppm) 10-6 mmol mol-1
parts per billion (ppb) 10-9 nmol mol-1
parts per trillion (ppt) 10-12 pmol mol-1
to ta l
i
i n
n
??
t o t a l
i
i n
n
xppm 610)( ??
Conversion between ppm and mg/m3
RT
P
V
Nn
to ta l ??
i
i
i M
mn 10 6??
ni,mol/m3
mi,mg/m3
Mi,g/mol
t o t a l
i
n
np p minr a tiom ix in g 610___ ?
i
i
i
i
m
pM
RT
RT
p
M
m
??
? 6
6
10
10
3/_____ mgintio nx C o n c e n tr a
pM
RTp p minr a tiom ix in g
i
m?
3/_____ mgintio nx C o n c e n tr a
pM
RTp p minr a tiom ix in g
i
m?
Pressure unit and R Constant:
P= 1.01325x10^5 pascal
R= 8.314 J/k.mol for P in Pa and volume in m3
Conversion between ppm and mg/m3
Example:
The Hong Kong Air Quality Objective for ozone is
240 mg/m3,The U.S,National Ambient Air Quality
Standard for ozone is 120 ppb,Which standard is
stricter at the same temperature (25oC) and the
pressure (1atm)?
pp bpp mmgxxx xpp minr at iom ix i ng 12 212 2.0/24 0481001 32 5.1 29 831 4.8___ 35 ??? m
Typical mixing ratios for some compounds of
environmental importance
C a r b o n d i o x i d e 3 5 5 p p m
C a r b o n m o n o x i d e 1 0 0 p p b t o 2 0 p p m
O z o n e 1 t o 1 0 0 p p b
M e t h a n e 1, 7 2 p p m
N o n m e t h a n e h y d r o c a r b o n 1 p p t t o < 1 p p b
N i t r i c o x i d e ( N O ) 5 p p t t o 1 p p b
N i t r o g e n d i o x i d e ( N O 2 ) 1 t o 1 5 0 p p b
N i t r o u s o x i d e ( N 2 O ) 3 1 0 p p b
S u l f u r d i o x i d e 1 t o 1 0 0 p p b
C F C l 3 ( F r e o n 1 1 ) 2 0 0 p p t
C F 2 C l 2 ( F r e o n 1 2 ) 3 5 0 p p t
ppm in solution vs,ppm as mixing ratio for
airborne substances
ppm,Parts per million
solution
610
__
___)( x
s o lu t io nofw e ig h t
iS p e c i e sofw e ig h tp p mC
i ?
air
m o l e c u le sa irofm o l e
is p e c ie sofm o l e
i ___
___??
Lmgl i te rgmlg
w a t e rofd e n s i t ys o l u t i o na q u e o u sd i lu t eofd e n s i t y
/10/10/1
______
63 ???
?
Lmg
s o l u t i o nl i te r
mg
s o l u t i o nmg
is p e c i e smg
ppm
/1
__1
1
_10
__1
_1
6
??
?
Relative Humidity
Definition,ratio of the partial pressure of water to its saturation
vapor pressure at the same temperature.
o
OH
OH
P
P
2
2100RH ?
sy
y100RH ?
y,mole fraction of water vapor at temperature T
ys,mole fraction of saturation water vapor at Temperature T
Relative Humidity
Calculation of H2O mixing ratio in ppm from RH
= p exp[13.3185a - 1.976 a2 - 0.6445 a3 -0.1299 a4]
where p = 1 atm and a = 1-(373.15/T).
sy
y100RH ?
)(1010][ 462 p p mR H yyOH s??
)(10][ 242 ppmpPRHOH
o
OH?
o OHp
2
Hong Kong Air Quality Objectives (HKAQO) μg/m3
A i r P o l l u t a n t s 1 - h o u r 8 - hour 24 - h o u r 3 - m o n t h 1 - y e a r
S u l p h u r d i o x i d e 800 -- 350 -- 80
N i t r o g e n d i o x i d e 300 -- 150 -- 80
C a r b o n m o n o x i d e 30000 10000 -- -- --
O z o n e 240 -- -- -- --
T o t a l s u s p e n d e d
p a r t i c u l a t e
-- -- 260 -- 80
R e s p i r a b l e s u s p e n d e d
p a r t i c u l a t e s
-- -- 180 -- 55
L e a d -- -- -- 1, 5 --