Heard D.E. (editor) Analytical Techniques for Atmospheric Measurement

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Figure 8.18 Sample chromatogram obtained during the PACIFIC 2001 ﬁeld study. Peak labels: 1. 2,4-dnph derivatizing agent; 2. formaldehyde-DNPH; 3.

acetaldehyde-DNPH; 4. acetone-DNPH; 5. propanal-DNPH; 6. glyoxal-DNPH; 7. methylglyoxal-DNPH; 8. pinonaldehyde-DNPH; 9. nonanal-DNPH. (Reproduced

with permission from Liggio & McLaren, 2003), Taylor and Francis Ltd., http://www.tandf.co.uk/journals)

Chromatographic Methods 399

100

NL: 1.47E7

TIC

100

NL: 8.53E6

/z = 185

Pinic acid

100

NL: 4.42E6

/z = 183 + 243

Pinonic acid

100

NL: 7.90E5

/z = 169 + 229

Pinalic acid, norpinonic acid

100

NL: 4.50E5

/z = 197 + 257

Keto-pinonic acids

Retention time (min)

NL: 2.51E6

/z = 199 + 259

10 12 14 16 18

100

OH-pinonic acids

642

Relative abundance

100

NL: 6.42E6

/z = 171 + 231

172

Norpinic acid

Figure 8.19 TIC and extracted ion chromatogram of a samples from -pinene-ozonolysis analysed in

the ESI(-) mode. (Reproduced with permission from Winterhalter et al., 2003.)

400 Analytical Techniques for Atmospheric Measurement

Figure 8.20 LC and GC chromatograms of extracted PAH from urban air particulates collected during

October 1994. 1. ﬂuoranthene; 2. pyrene; 3. methyl pyrenes; 4. benz[a]anthracene; 5. chrysene; 6. methyl

chrysenes; 7. benzo[b]ﬂuoranthene; 8. benzo[k]ﬂuoranthene; 9. benzo[e]pyrene; 10. benzo[a]pyrene;

11. indeno[1,2,3-cd]pyrene; 12. dibenzo[ah]anthracene; 13. benzo[ghi]perylene. (From Lewis et al.,

1995, reproduced with permission from Elsevier.)

Chromatographic Methods 401

equipment at the core of many atmospheric instruments, a spin-off from the enormous

industrial and commercial usage of chromatographic techniques.

Since chromatographic techniques are some of the most historically mature in

atmospheric chemistry, future developments are likely to be incremental rather than step-

change. The reliability and accuracy of GC methods is their key strength and this is only

likely to improve as commercially available instrumentation, and in particular GC-MS,

which forms the core of many field instruments, develops. Atmospheric measurements

may provide some very useful applications for miniaturised GC systems, an area which

is beginning to show promise in developmental laboratories. The chemical- and photo-

etching technology for production of microfabricated columns onto silica monolith

structures has been around for a number of years, but extending into real applications

has been limited by detectors. Semiconductor chip–sized thermal conductivity detectors

have been the first truly miniaturised detectors to emerge, but it seems only a matter of

time before microfabricated ionisation detectors such as PID and HID are produced with

the requisite levels of sensitivity for atmospheric applications. The enormous size and

electrical power savings that may result from miniaturisation may open up new areas of

measurements that have previously only been done using offline chemical methods.

Some of the developments in comprehensive chromatography and the ability to resolve

organic compounds in exceedingly high complexity mixtures have been an important

step forward. A valuable by-product of comprehensive methods has been the ability to

directly introduce heterogeneous samples such as aerosols to the analytical system without

sample preparation or clean-up. Saving labour time is an obvious benefit, but importantly

the reduction in amount of material required for analysis improves both measurement

resolution and detection limits. As high complexity methods reach maturity, however,

it is likely that it will be data interpretation that becomes the rate-determining step

in the analytical procedure. This may require us to consider adopting data-handling

technologies associated with image processing to find ways in which useful and timely

information can be extracted from chemical methods with such high levels of detail.

Liquid chromatography methods and LC coupled to mass spectrometry do offer real

potential for the future. State-of-the-art coupled LC-MS techniques (using TOF and

double or triple quadrupole MS) now offer similar or better detection limits to LC-

fluorescence techniques and have improved greatly in terms of reliability, sensitivity and

ease of use since the second half of 1990s. This improvement has been driven primarily

by the wide uptake of the technique by the bioscience and pharmaceutical communities.

Clearly MS as a detector has superior discrimination powers and it seems likely that it

may slowly replace non-selective detectors in a number of applications. The use of high

resolution LC for the analysis of polar and ionic compounds is perhaps underutilised

at present, and there are continuing detection and resolution problems with methods

built around industry standard, 4.6 mm i.d. columns. The use of capillary LC (cap-LC)

or microbore LC in atmospheric applications has been very limited indeed, but may

go some considerable way towards overcoming these limiting factors. The inherently

small sample capacity of cap-LC will always be a challenge and reinforces that sample

preparation cannot be underestimated in its importance. It is for sample capacity–related

reasons that there has yet to be any take-up of capillary electrochromatography (CEC) in

atmospheric applications and only very limited use of electrophoresis, but it is in using

electrically driven systems, however, that perhaps the most exciting possibilities for very

large volume sample pre-concentration may lie.

402 Analytical Techniques for Atmospheric Measurement

Further reading

Dewulf, J. & Van Langenhove, H. (2002) Analysis of volatile organic compounds using gas

chromatography, Trac-Trends in Analytical Chemistry, 21 (9–10), pp. 637–646.

Fifield, F.W. & Kealey, D. (1995) Principles and Practice of Analytical Chemistry, CRC Press.

Fowlis, I.A. (1995) Gas Chromatography, Second edition, John Wiley & Sons.

Helmig, D. (1999) Air analysis by gas chromatography, J. Chromatogr. A, 843 (1–2), pp. 129–146.

Phillips, J.B. & Beens, J. (1999) Comprehensive two-dimensional gas chromatography: A

hyphenated method with strong coupling between the two dimensions, J. Chromatogr. A, 856

(1–2), pp. 331–347.

Santos, F.J. & Galceran, M.T. (2002) The application of gas chromatography to environmental

analysis, Trac-Trends Anal. Chem., 21 (9–10), pp. 672–685.

Skoog, D.A., West, D.M. & Holler, F.J. (2003) Fundamentals of Analytical Chemistry, eighth edition,

Brooks Cole.

http://www.separationsnow.com/basehtml/SepH

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Chromatographic Methods 405

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Chapter 9

Measurement of Photolysis Frequencies

in the Atmosphere

Andreas Hofzumahaus

9.1 Introduction

9.1.1 Photolysis reactions in atmospheric chemistry

The absorption of ultraviolet (UV) and visible solar radiation by atmospheric molecules

followed by their dissociation plays a fundamental role in atmospheric chemistry. The

photolysis of gaseous compounds contributes to their removal from the atmosphere, but

probably the most important role played by these reactions is the generation of highly

reactive atoms and radicals. Some examples of relevant reactions are given in Table 9.1,

where reactions are listed in the order of their maximum wavelength for which photolysis

occurs.

Probably the most important photolysis reactions are the ones that result in the

formation of O

and OH radicals, the major atmospheric oxidants. In the stratosphere,

formation of ozone proceeds via the photodissociation of molecular oxygen into ground-

state oxygen atoms O

P, which combine with O

in a termolecular collision process

yielding O

molecules:

+h −→ O

P +O

P (R 9.1)

O

P +O

+M −→ O

+M (R 9.2)

Reaction R 9.1 can occur only at wavelengths less than 243 nm. The threshold corresponds

to the minimum photon energy h = hc/, which is required for the dissociation of O

into O

P atoms. Here  and  denote the wavelength and frequency of the photons,

respectively, h is Planck’s constant (6626×10

−34

J·s), and c is the speed of light (29979×

m ·s

−1

in vacuum).

Reaction R 9.1 followed by R 9.2 is the main source of stratospheric ozone which

efficiently absorbs all solar radiation below 290 nm when passing into the lower

atmosphere. The removal of short-wave UV prevents that ozone formation can take place

in the troposphere via R9.1. Rather, photochemical ozone formation in the troposphere

Measurement of Photolysis Frequencies in the Atmosphere 407

Table 9.1 Examples of photolysis reactions in the atmosphere

Photolysis reaction

Wavelengths

(nm) Photolysis frequency

s

−1



0km 25km

+h −→ O

P +O

P<243 43 ×10

−26

12 ×10

−11

O +h −→ O

D +N

<250 18 ×10

−23

25 ×10

−8

CCl

F +h −→ CCl

F +Cl <250 43 ×10

−22

51 ×10

−7

+h −→ O

D +O

a



<325

51 ×10

−5

11 ×10

−4

CHO +h −→ CH

+HCO <335 64 ×10

−6

13 ×10

−5

HCHO +h −→ H +HCO <340 33 ×10

−5

62 ×10

−5

OOH +h −→ CH

O +OH <350 57×10

−6

99 ×10

−6

+h −→ OH +OH <350 77 ×10

−6

12 ×10

−5

HNO

+h −→ OH +NO

<350 82 ×10

−7

56 ×10

−6

HCHO +h −→ H

+CO <360 44 ×10

−5

76 ×10

−5

HONO +h −→ OH +NO <400 19 ×10

−3

28 ×10

−3

+h −→ O

D +O

X



−

<411

––

+h −→ NO +O

P<420 88 ×10

−3

12 ×10

−2

HOCl +h −→ OH +Cl <420 26 ×10

−4

41 ×10

−4

+h −→ Cl +Cl <490 24 ×10

−3

35 ×10

−3

+h −→ NO

+O

P<640 28 ×10

−1

30 ×10

−1

+h −→ NO +O

580–640 25 ×10

−2

27 ×10

−2

+h −→ O

P +O

X



−

<1180 42 ×10

−4

50 ×10

−4

HNO

+h −→ HO

+NO

1450

––

Spectral and photochemical data can be found in, e.g., Atkinson et al. (2004); Sander et al. (2003); Finlayson-Pitts

and Pitts, Jr. (2000); Röth et al. (1997); Okabe (1978). If not otherwise indicated, photofragments are produced in their

electronic ground states.

Wavelengths at which photolysis occurs. Note that in the troposphere, photolysis is not possible below 290 nm, as

solar radiation at shorter wavelengths is entirely absorbed in the stratosphere.

From Table B.4 in Jacobson (1999). The photolysis frequencies at 0 and 25 km were modelled for a nonpolluted

atmosphere at a solar zenith angle of 0



and a surface albedo of 0.03.

The threshold wavelength for O

in the vibrational ground-state is 310 nm. At room temperature internal vibrational

energy shifts the threshold to 325 nm (Matsumi et al., 2002).

Spin-forbidden process. The O

D quantum yield for this process is small, about 008 ±004 for wavelength up to

340 nm (Matsumi et al., 2002).

Photodissociation by excitation of vibrational overtone transitions, of which the ﬁrst overtone 2

 at 14 m dominates

(Roehl et al., 2002; Donaldson et al., 1997).

is initiated by photolysis of nitrogen dioxide, which has an energetic threshold at a longer

wavelength (<420 nm):

+h −→ NO +O

P (R 9.3)

All O

P that is produced in the troposphere is converted into O

by reaction with O

(R9.2).

Ozone itself can undergo photolysis in the UV and visible range. Most important

for the chemistry of the atmosphere is the photolysis by UV radiation, which produces

electronically excited oxygen atoms O

D:

+h −→ O

D +O

a



 (R 9.4)