Q

Process 1, for example, could be dry deposition at the Earth's surface and process 2 cloud scavenging. We can actually associate time constants with the two individual removal processes,

where xi can be thought of as the lifetime of the species against removal process 1, and x2 the lifetime against process 2. From (2.8) and (2.9) we can express the overall lifetime x in terms of the two individual removal times xi and x2 by

Equation (2.10) shows that separate removal paths add together to give a total lifetime, like electrical resistances in parallel add to give a total resistance that is even smaller than the smallest resistance. From (2.10)

If X] X2, the lifetime associated with removal by process 1 is much longer than that associated with process 2, process 2 is the more effective removal mechanism, and t — t2. Thus, when there are several competing removal paths, in order to estimate the overall lifetime of a species, focus should always be on improving estimates for the fastest removal rate.

Removal in the troposphere of those compounds that react with the hydroxyl radical occurs according to the chemical reaction

OH + A products

The parameter k is the rate constant for the reaction, such that the rate of the reaction is ¿[OH][A], where the brackets denote the concentration of the species contained therein.

From the analysis of atmospheric residence times, if Q is the total quantity of species A in the troposphere and its rate of removal is R = fc[OH]g, then the compound's mean lifetime is, from (2.6)

where [OH] is an appropriate globally averaged tropospheric concentration of OH radicals.

Let us develop the equations governing the total moles of a species i in the troposphere Qi. The dynamic material balance can be written as f = (2-13)

where P, and Rt represent the source and loss rates. The terms P, and /?, consist of the following contributions:

P"

natural emissions

H

pfl

anthropogenic emissions

chemical reactions

[Ri

dry deposition

R,<

RJ

wet deposition

%

chemical reactions

transport to the stratosphere

The loss processes are usually represented as first order; for example, Rf = kffQt, where the first-order rate constants, which we will denote by k's, must be specified. Thus (2.13) becomes

If the concentration of the species is not changing, then a steady state may be presumed in which

The lifetime of species i can be calculated by either t i=-r--—------(2.16)

To use (2.16) the individual first-order rate constants for removal must be estimated, whereas in (2.57), estimates for the total number of moles in the troposphere, which can be derived from a concentration measurement, and for the source strength terms are needed. Ef the kj values are difficult to specify, mean residence times are often estimated from (2,17).

2,2 SULFUR-CONTAINING COMPOUNDS

Sulfur is present in the Earth's crust at a mixing ratio of less than 500 parts per million by mass and in the Earth's atmosphere at a total voiume mixing ratio of less than 1 ppm. Yet, sulfur-containing compounds exert a profound influence on the chemistry of the atmosphere and on climate.

Table 2.1 lists atmospheric sulfur compounds. The principal sulfur compounds in the atmosphere are H;S, CH3SCH3, CS?, OCS, and S02, Sulfur occurs in five oxidation states in the atmosphere. (See Box) Chemical reactivity of atmospheric sulfur compounds is inversely related to their suifur oxidation state. Reduced sulfur compounds, those with oxidation state —2 or —1, are rapidly oxidized by the hydroxy! radical and, to a lesser extent, by other species, with resulting atmospheric lifetimes of a few days. The water solubility of sulfur species increases with oxidation state; reduced sulfur species occur preferentially in the gas phase, whereas the S(+6) compounds often tend to be found in particles or droplets. Once converted to compounds in the S(+6) state, sulfur species residence times are determined by removal by wet and dry deposition.

Oxidation State The oxidation states of atoms in covaient compounds are obtained by arbitrarily assigning the electrons to particular atoms. For a covaient bond between two identical atoms, the electrons are split equally between the two. When two different atoms are involved, the shared electrons are assigned completely to the atom that has the stronger attraction for the electrons. In the water molecule, for example, oxygen has a greater attraction for electrons than hydrogen, so in assigning the oxidation states of oxygen and hydrogen in H?0, it is assumed that the oxygen atom possesses all the electrons. This gives the oxygen an excess of two electrons, and its oxidation state is —2. Each hydrogen has no electrons, and the oxidation state of each hydrogen is +1.

28 ATMOSPHERIC TRACE CONSTITUENTS TABLE 2.1 Atmospheric Sulfur Compounds

Compound

Oxidation - Chemical Usual

State Name Formula Structure Atmospheric State

Compound

Oxidation - Chemical Usual

State Name Formula Structure Atmospheric State

-2

Hydrogen sulfide

H2S

H-S-H

Gas

Dimethyl sulfide (DMS)

CH3SCH3

CH3-S-CH3

Gas

Carbon disulfide

CS2

s=c=s

Gas

Carbonyl sulfide

OCS

0 = C=S

Gas

Methyl mercaptan

CH3SH

CH3-S-H

Gas

- 1

Dimethyl disulfide

CH3SSCH3

CH3-S-S-CH3

Gas

0

Dimethyl sulfoxide

CH3-S-CH3

Gas

Bisulfite ion Sulfite ion

HSOJ

SO2-

0 = S =0

Aqueous Aqueous Aqueous

6

Sulfuric acid

II 0

Gas aqueous/aerosol

Bisulfate ion

hso4-

HO-S-O" 11 0

Aqueous/aerosol

Sulfate ion

II 0

Methane sulfonic acid (MSA)

CH3S03H

CH3-S-OH II 0

Gas/aqueous

Dimethyl sulfone

CH3-S-CH3 II

0

Gas

Hydroxymethane sulfonic acid (HMSA)

0

Aqueous

Rules for assigning oxidation states are

1. The oxidation state of an atom in an element is 0.

2. The oxidation state of a monatomic ion is the same as its charge.

3. Oxygen is assigned an oxidation state of —2 in its covalent compounds, such as CO, C02, SO?, and so3. An exception Lo this rule occurs in peroxides, where each oxygen is assigned an oxidation state of —I.

4. In its covalent compounds with nonmetals, hydrogen is assigned an oxidation state of +1. Examples include HC1, H20, NH-,, and CH4.

5. In its compounds fluorine is always assigned an oxidation stale of -1.

6. The sum of the oxidation states must be zero for an electrically neutral compound. For an ion, the sum must equal the charge of the ion. For example, the sum of oxidation slates for the nitrogen and hydrogen atoms in NHJ" is +1, and the oxidation state of nitrogen is —3. For N03, the sum of oxidation states is —1. Since oxygen has an oxidation slate of —2, nitrogen must have an oxidation state of +5. Sometimes, oxidation states are indicated with roman numerals, for example, the sulfur atom in SO^- is +VI.

Oxidation states of sulfur in various compounds of atmospheric importance are as follows:

Oxidation stales of nitrogen in atmospheric species are as follows:

NH3, RNH2. R2NH, R3N = -3 N2 = 0 N:0 = +1 NO = +2 HN02 = +3 N02 = +4 HNO3. NO3 , N2Os = +5 NO3 = +6

Table 2.2 presents estimates of total sulfur emissions to the atmosphere, both anthropogenic and natural, including estimated division between Northern and Southern Hemispheres. Current estimates place total global emissions (excluding seasalt) in the range of 98-120 Tg(S)yr_1. At present, anthropogenic emissions account for about 75% of total sulfur emissions, and 90% of the anthropogenic emissions occur in the Northern

TABLE 2.2 Global Sulfur Emissions Estimates, Tg(S) yr 1

Source

Total"

Fossil fuel combustion + industry Biomass burning Oceans Wetlands Plants + soils Volcanoes

Anthropogenic (total) Natural (total, without sea salt and soil dust) Total

Total reduced S: 2.2

15-25

0.003-0.68

Was this article helpful?

0 0
How to Improve Your Memory

How to Improve Your Memory

Stop Forgetting and Start Remembering...Improve Your Memory In No Time! Don't waste your time and money on fancy tactics and overpriced

Get My Free Ebook


Post a comment