ATM 507 Lecture 2 Fall 2012 - University at Albany,...

ATM 507 – Lecture 2

Fall 2012

• Reading assignment – Seinfeld and

Pandis, Chapter 2

• Other references

• Problem Set #1 – due Sept. 11

• Topics – Various concepts and definitions

are introduced for later use and reference

Outline

• In this lecture we will jump around a lot –

introducing a number of concepts useful to

the study of atmospheric chemistry. They

may not seem to “fit together” today, but

the ideas will return as the semester

progresses. It is essentially a set of

definitions to use as the course unfolds.

You may want to refer back to these

charts and tables later in the semester.

Ideal Gas Law

• Chemists write it as pV = nRT

– p = pressure in atmosphere

– V = volume in liters

– n = number of moles

– R = Gas Constant

– T = temperature in degrees Kelvin

• R has the value 0.08206 L•atm•mole-1•K-1

• Concentration has units moles/L or M

Ideal Gas Law (cont.)

• For a meteorologist – P = RT

– P = pressure in Pascal

– = concentration in moles/L

– R = Gas Constant = 8.31 J•mole-1•K-1

• For a physicist – p = nkT

– p = pressure in dyne•cm-2 (1 dyne•cm-2 = 0.001

millibar)

– n = concentration in molecules*cm-3

– k = Boltzmann constant = 1.38x10-16

erg•K-1 •molecule-1

Pressure Conversions

1 atm = 1013 mbar

101,300 Pascal (Newton m-2)

1,013,000 dyne cm-2

760 torr (mm Hg)

14.69 psi (pounds per sq. inch)

29.92 inches Hg

Concentration Units

• Absolute units:

– Moles per unit volume (i.e. moles L-1)

– Number per unit volume (i.e. molecules cm-3)

• Relative units (fraction of total):

– %, ppm, ppb, ppt (i.e., ppmv, ppbv, pptv)

– Mole fraction

– Mass Fraction, esp. ppmm

• Mass units:

– g L-1, µg m-3

Why so many units?

• Historical reasons

• Absolute units are preferred for calculations, but watch the T and p dependence (next slide).

• Relative units are more convenient when looking at concentration profiles. They scale with pressure (& T) and give a truer sense of the change in concentration as a function of height.

Conversion of Concentration Units

• Conversion from absolute to relative units (or vice versa) requires knowledge of the temperature and pressure. This can be confusing and requires attention to detail!

• 1 mole = 6.02x1023 molecules (Avogadro’s number)

• 1 m3 = 1000 L = 106 cm3

• Use the Ideal Gas Law (p/RT = n/V = C) to find concentration if pressure and temperature are known.

• At 1 atm, 273 K, CM = 4.46x10-2 mole/L. (If this is inverted, the STP molar volume is 22.42 L/mole.)

• To obtain number concentration from molar concentration, multiply by Avogadro’s number and divide by 1000:

• Cn = 2.69x1019 molecules cm-3 (Loschmidt’s number at STP)

Example

• How does a number concentration at one

pressure and temperature convert to

another temperature and pressure?

• pV/(nRT) = p/(CRT) = constant

• p1/(C1RT1) = p2/(C2RT2), rearrange

• C2 = C1(T1/T2)(p2/p1)

Conversion of Concentrations at 1

atm and 298 K

1 atm, 298 K 4.09x10-2 mole L-1

2.46x1019 molecule cm-3

106 ppm, 109 ppb, 1012 ppt

1 ppb 4.09x10-11 mole L-1


1 ppt 4.09x10-14 mole L-1


1 torr 133.3 Pa or 1.333 mbar

3.24x1016 molecules cm-3

1.32x106 ppb

Chemical Families

• Oxygen

• Hydrogen

• Nitrogen

• Sulfur

• Halogens (Chlorine, Bromine, etc.)

• Carbon and Hydrocarbons

“Simplest” Chemical Families

• Oxygen – O2(almost all), O3, O atom, O2 dimer (O2*O2 or

O4)

– Text and other sources don’t even list the oxygen “family”

• Hydrogen – H2 and H2O (stable molecules), and OH (HO) and

HO2 (free radicals), H2O2

– Hydrogen bonds readily with many other species – most notably carbon, to form, in this case, hydrocarbons

Nitrogen Family (Major Species)

N2 Nitrogen NO3 Nitrogen trioxide or

Nitrate radical

N2O Nitrous oxide N2O5 Dinitrogen

pentoxide

NO Nitric oxide HONO Nitrous acid

NO2 Nitrogen dioxide HNO3 Nitric acid

NOX = NO + NO2 HO2NO2 Pernitric acid

NOY = NOX + all

oxidation prod.

CH3C(O)

O2NO2

Peroxyacetyl

nitrate or PAN

NOz = NOY - NOX NH3 Ammonia

Sulfur Family

Reduced Sulfur compounds Oxidized Sulfur compounds

Halogens (Esp. Chlorine & Bromine)

• “Non-reactive” forms: fully halogenated hydrocarbons like Freons (CFCl3 or CFC-11; CF2Cl2 or CFC-12), or CCl4; SF6

• Slow reacting and natural forms: CH3Cl, CH3Br, CH3CCl3, CHCl3

• More reactive forms: Cl2, HOCl, HOBr, BrCl, ClONO2, BrONO2, HCl

• Free radicals (very reactive forms): Cl, ClO, Br, BrO

Carbon Compounds

• Here’s where things can get really out of hand!

• Reduced forms of carbon (next slides) include fossil fuels and natural and man-made emissions

• NMHC = Non-methane hydrocarbons

• These reduced forms are quickly oxidized in combustion and slowly oxidized in the atmosphere

• Partially oxidized forms include carbon monoxide (CO), formaldehyde (HCHO) and other oxygenated hydrocarbons

• Fully oxidized carbon is carbon dioxide (CO2)

Hydrocarbons

Hydrocarbons (cont.)

Oxygenated Hydrocarbons Carbonyls (-C=O), Alcohols (-C-OH),

and Organic Acids (-C=O-OH)

Particulate Matter

• Complex mixture of phases, sizes and

composition

• Table 2.18 lists terminology used for

atmospheric particles

• Often measured in mass concentration units of

µg m-3

• Different size fractions behave rather differently

• Topic of intense research

• More (much more) later!

Atmospheric Structure (review)

Time and space scales for

atmospheric processes • Domains of influence (overlapping spatial

scales) – Microscale

– (Urban or Local scale)

– Mesoscale (10’s to 100’s km)

– Synoptic scale (100’s to few 1000’s km)

– Global scale

• Transport or mixing times in the troposphere – Zonal (E-W) – weeks

– Hemispheric (N-S cross equatorial) – 6-12 months

– Vertical – days (BL) to months (FT) to years (cross tropopause)

• Chemical lifetimes – seconds to centuries

Spatial and Temporal Scales

Spatial Scales (Section 1.7)

Chemical Lifetimes

• Chemical Reaction A + B → C + D;

rate constant k

• [A] = concentration of species A, etc.

• -d[A]/dt = k [A] [B]

• Assume [B] = constant (or nearly so) and that other

reactions involving A are negligible, solve for [A]

• -d[A]/[A] = k [B] dt; integrate to obtain

• ln [A] (evaluated at [A] and [A]0) = k [B] t

• [A] = [A]0 e-k [B] t or [A]/[A]0 = e-k [B] t

Chemical Lifetime (cont)

• By definition, the lifetime of a species is the time it takes for its concentration to decrease to 1/e of its initial concentration

• [A]/[A]0 = e -1 or k [B] t = 1, t = τCHEM

• τCHEM = 1/(k [B])

• This is the lifetime of A wrt constant [B]

• If τCHEM << τTRANS (τMIX), then the species is “chemically controlled”

• If τCHEM >> τTRANS, then the species is “dynamically (or transport) controlled”

Chemical Composition of the

Atmosphere

• Major constituents are

– Outgassed from the mantle

– Radioactive decay products

– Photochemically produced

– Biologically produced

• Trace gases are generally most reactive –

we will focus on these

Table is from ~1986 - CO2 is now ~390 ppm, and CH4 is now ~1.9 ppm, etc

Clean and Polluted Concentration Levels of

Chemically Active Trace Gases

Species Remote Rural Polluted

SO2 0.020 0.2 – 2 5-50

CO 120(N), 60(S) 120-400 1000- 10,000

NO 0.02 – 0.04 0.2 – 2 10-100

NO2 0.02 – 0.04 0.2 -10 10 – 500

O3 5-10 (or less) 20 – 80 0 – 300

HNO3 0 – 0.1 0 – 5 1- 30

NH3 0 – 0.5 0 – 5 1 – 10

HCHO 0 - .5 0.1 – 5 20 - 50

NMHC 2 – 20 50 – 100 100 - 1000

Boundary layer concentrations in ppbv

ATM 507 Lecture 2 Fall 2012 - University at Albany,...

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