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TROPOSPHERIC CHEMISTRY AND COMPOSITION | Hydroxyl Radical
Hydroxyl Radical KC Clemitshaw, Imperial College of Science, Technology, and Medicine, Ascot, UK Ó 2003 Elsevier Ltd. All rights reserved. This article is reproduced from the previous edition, volume 6, pp 2403–2411, Ó 2003, Elsevier Ltd.
Introduction The key roles played by hydroxyl radicals (OH) within tropospheric chemistry, including the major sources, sinks, and chemical processes involving OH, are discussed. The major daytime sources of OH include the photodissociation of ozone (O3) and nitrous acid (HONO), the photo-oxidation of formaldehyde (CH2O) and acetone (CH3C(O)CH3), together with the ozonolysis of alkenes. Nighttime processes leading to OH production also include the ozonolysis of alkenes and, more importantly, nitrate radical (NO3)-facilitated decomposition of peroxyacyl nitrates (RC(O)O2NO2) and NO3-initiated oxidation of alkenes. The major sink processes for OH are to initiate the oxidation of carbon monoxide (CO), methane (CH4), and a wide range of reactive volatile organic compounds (VOCs), and to form nitric acid (HONO2) via reaction with NO2. In the presence of sufficient NO and NO2 (collectively termed NOx), these oxidation mechanisms are propagated by organic peroxy (RO2), alkoxy (RO), and hydroperoxy (HO2) radicals. They regenerate OH and proceed with the formation of oxidants such as O3, carbonyls (e.g., CH2O), peroxides (e.g., hydrogen peroxide, H2O2), and organic nitrates (e.g., peroxyacetylnitrate, PAN, CH3C(O) O2NO2). OH and HO2 chemistry may also lead to O3 depletion under conditions of low NOx. Finally, a brief description of several spectroscopic techniques that have been developed for quantitative ambient measurements of OH, together with their recent applications and intercomparisons in groundbased and airborne field studies of the tropospheric chemistry of OH, is also given.
remote environments and has relatively simple photochemistry. The rate of production of OH via O3 photodissociation is also influenced by quenching of O(1D) to O(3P) atoms via collision with tropospheric N2 and O2 (reaction [III]). Oð1 DÞ þ M / Oð3 PÞ þ M M ¼ N2 ; O2
[III]
The fractional conversion, f, of O( D) into OH is described approximately by eqn [1]), where PH2 O is the partial pressure of water vapor and P is the total pressure. 1
f ¼ PH2 O =½PH2 O þ 0:13ðP PH2 O Þ
[1]
Photodissociation of HONO and Photo-Oxidation of CH2O Clearly, the factor, f, must be taken into account when comparing the relative importance of OH produced directly upon UV photodissociation of other precursor molecules. For HONO, photodissociation occurs on a time scale of approximately 15 min (reaction [IV]). HONO þ hv / OH þ NO
l < 400 nm
[IV]
During the darkness of night, HONO concentrations may build up via heterogeneous hydrolysis of NO and NO2, and as a result of direct emissions from motor vehicles. At sunrise, the photodissociation of HONO provides an early morning pulse of OH at a time of day when j(O1D) is very low due to the long atmospheric path lengths and the low intensity of near-UV solar radiation. HO2 is an important reactive intermediate of the photo-oxidation of CH2O, and OH may be formed indirectly via the rapid reaction of HO2 with reactions [V], [VI], [VII], and [VIII].
Daytime Sources of OH in the Troposphere
CH2 O þ hv / H þ HCO
l < 340 nm
[V]
Photodissociation of O3
H þ O2 þ M / HO2 þ M M ¼ N2 ; O2
[VI]
HCO þ O2 / HO2 þ CO
[VII]
HO2 þ NO / OH þ NO2
[VIII]
The primary daytime source of OH radicals throughout much of the troposphere is the photodissociation of O3 in its nearultraviolet (UV) absorption band, followed by reaction of the O(1D) photoproduct with water vapor (reactions [I] and [II]). O3 þ hv / Oð1 DÞ þ O2 l < 340 nm
[I]
Oð1 DÞ þ H2 O / 2OH
[II]
However, both the O3 absorption cross sections and quantum yields of formation of O(1D) atoms decrease with increasing wavelength. Consequently, the rate of O3 photodissociation, j(O1D), and therefore the rate of OH production, varies strongly with changes in atmospheric path length that not only accompany spatial variations in altitude and latitude but also temporal variations on diurnal and seasonal timescales. Indeed, as illustrated in Figure 1, daytime measurements of j(O1D) and OH often display a high degree of positive correlation, especially in clean air that is characteristic of
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Urban environments often have high local emissions of CH2O, whereas O3 concentrations are generally suppressed due to rapid reaction with elevated levels of the primary pollutant, NO. Consequently, urban summertime rates of OH production via reactions [V], [VI], [VII], and [VIII] may exceed those via O3 photodissociation by a factor of 4 or so for typical photodissociation rates and precursor concentrations.
Photo-Oxidation of CH3C(O)CH3 By contrast, the photo-oxidation of acetone, CH3C(O)CH3, which produces CH2O as an intermediate species, has been suggested recently to be a significant source of HO2 and OH (collectively termed HOx) in the upper troposphere and
Encyclopedia of Atmospheric Sciences 2nd Edition, Volume 6
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Tropospheric Chemistry and Composition j Hydroxyl Radical 3.0 × 10−5 −5
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25 Jul
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29 Jul
31 Jul 2 Aug
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Figure 1 Simultaneous observations of OH (blue circles) and j(O1D) (black line) made during the PRIME’99 campaign at the Silwood Park Atmospheric Research Station, near Ascot, UK. Data kindly provided by Professor M.J. Pilling and Dr D.E. Heard, University of Leeds, UK.
lower stratosphere (reactions [IX], [X], [XI], [XII], [XIII], [XIV], and [XV]). CH3 CðOÞCH3 þ hv / CH3 CðOÞ þ CH3
[IX]
CH3 CðOÞ þ O2 þ M / CH3 CðOÞO2 þ M
[X]
CH3 CðOÞO2 þ NO / CH3 CðOÞO þ NO2
[XI]
CH3 CðOÞO þ M / CH3 þ CO2 þ M
[XII]
CH3 þ O2 þ M / CH3 O2 þ M
[XIII]
CH3 O2 þ NO / CH3 O þ NO2
[XIV]
CH3 O þ O2 / CH2 O þ HO2
[XV]
HO2 þ NO / OH þ NO2
[VIII]
Inclusion of this process in numerical models of atmospheric chemistry reduces considerably the discrepancies between calculated and measured HOx concentrations and O3 production rates.
Ozonolysis of Alkenes The ozonolysis of alkenes (R1R2C CR3R4) represents another important process that produces OH, and may even dominate the photodissociation of O3, HONO, and CH2O in polluted urban environments. The initial reactions of alkenes with O3 are generally slow, but, as shown in Table 1 may proceed at rates that are comparable to corresponding reactions of alkenes with OH occurring during daytime and with NO3 during nighttime. Although the details are not yet understood fully, kinetic and mechanistic laboratory studies indicate that the electrophylic addition of O3 across the double bond leads to the initial formation of an energy-rich primary ozonide ([R1R2COOOCR3R4]z), which decomposes rapidly to generate pairs of energy-rich, carbonyl-substituted Criegee biradicals ([R1R2COO]z and [R3R4COO]z), and primary carbonyl compounds (R1C(O)R2 and [R3C(O)R4]), according to reactions (reactions [XVI], [XVIIa], and [XVIIb]). O3 þ R 1 R 2 C ¼ CR 3 R 4 / ½R 1 R 2 COOOCR 3 R 4 z
[XVI]
½R 1 R 2 COOOCR 3 R 4 z / R 1 CðOÞR 2 þ ½R 3 R 4 COOz [XVIIa] / R 3 CðOÞR 4 þ ½R 1 R 2 COOz
[XVIIb]
The Criegee biradicals either collisionally stabilize, isomerize and/or decompose to form OH and other organic radicals, for example by reaction [XVIII] or reaction [XIX]. ½R 1 R 2 COOz / ½R 52 CH2 ¼ CðOOHÞR 2 z ½R 5 CH2 ¼ CðOOHÞR 2 / R 5 CHCðOÞR 2 þ OH
[XVIII] [XIX]
More recent laboratory studies have also shown that the fractional yields of OH produced upon ozonolysis of alkenes are significant under simulated tropospheric conditions, with approximate values ranging from 0.1 for ethene to 0.85 for a-pinene. The ozonolysis of alkenes therefore represents a potentially important daytime source of OH that may lead to the net production of O3 under certain conditions.
Nighttime Sources of OH in the Troposphere NO3-Facilitated Decomposition of RC(O)O2NO2 and Oxidation of Biogenic VOCs Clearly, OH may also be produced during nighttime upon ozonolysis of alkenes. However, of more significance at night are other non-photochemical thermal reactions that involve NO3. NO3 has a lifetime with respect to photodissociation of approximately 5 s during the day, and its daytime oxidation chemistry is much less important than that of the more reactive OH. However, under conditions of low NOx, nocturnal NO3 chemistry may lead to OH production by facilitating the decomposition of peroxyacyl nitrates (RC(O)O2NO2), as shown in reactions [XX] and [XXI]. RCðOÞO2 NO2 þ M 4 RCðOÞO2 þ NO2 þ M
[XX]
RCðOÞO2 þ NO3 / R þ CO2 þ NO2 þ O2
[XXI]
NO3 may also initiate the oxidation of VOCs such as alkane hydrocarbons (RH), although, as shown in Table 1, these are particularly slow reactions [XXII]. NO3 þ RH / HNO3 þ R
[XXII]
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Tropospheric Chemistry and Composition j Hydroxyl Radical
Table 1 Comparison of tropospheric lifetimes of a range of CO and VOCs at typical ambient rural concentrations with respect to reaction with 1.6106 molecule cm3 OH, 30 ppbv O3, and 10 pptv NO3 VOC
OH
O3
NO3
Carbon monoxide
30 days
Alkanes Methane Ethane Propane Butane 2-Methyl propane Pentane 2-Methyl butane
10 years 29 days 6.3 days 2.9 days 3.1 days 1.8 days 1.9 days
Alkenes Ethene Propene 1-Butene 2-Butene 3-Methyl propene 1-Pentene 2-Pentene 2-Methyl 1-butene 3-Methyl 1-butene 2-Methyl 2-butene 1,3-Butadiene Isoprene
20 h 6.6 h 5.5 h 2.9 h 3.4 h 5.5 h 2.6 h 2.8 h 5.5 h 2.0 h 2.6 h 1.7 h
Aromatics Benzene Toluene Ethyl benzene o-Xylene m-Xylene p-Xylene
5.7 days 1.2 days 23 h 12 h 7.1 h 12 h
4.1 months 6.6 months 3.4 months
Aldehydes Formaldehyde Acetaldehyde
18 h 11 h
2.7 months 17 days
Sulfur-containing Dimethyl sulfide Dimethyl disulfide
1.5 days 46 min
1.0 h 1.5 h
91 years 7.8 years 2.7 years 1.5 years 1.5 years 1.3 years 9.7 days 1.5 days 1.6 days 2.4 days 1.4 days 1.5 days 2.4 h 1.4 days 1.6 days 55 min 2.4 days 1.2 days
7.3 months 4.9 days 3.5 days 2.9 h 3.4 h 3.5 days 2.9 h 3.4 h 3.5 days 7.1 min 11 h 1.7 h
1.8 years
ppbv, parts per billion by volume; pptv, parts per trillion by volume. Adapted without permission from Jenkin, M.E., Clemitshaw, K.C., 2000. Ozone and other secondary photochemical pollutants: Chemical processes governing their formation in the planetary boundary layer. Atmospheric Environment 34 (16), 2499–2527.
Nevertheless, subsequent reactions ([XXIII], [XXIV], [XXIV], [XXV], and [XXVI]) lead to OH production. R þ O2 þ M / RO2 þ M
[XXIII]
RO2 þ NO3 / RO þ NO2 þ O2
[XXIV]
RO þ O2 / RO þ HO2
[XXV]
HO2 þ NO3 / OH þ NO2 þ O2
[XXVI]
For dimethyl sulfide (DMS), which is of marine phytoplankton origin, and biogenic alkenes such as isoprene and terpenes, which are emitted by certain plants and trees, the primary step is the rapid addition of NO3 rather than H atom abstraction. The subsequent reaction mechanisms are quite
complex, but in general, NO3 oxidation of lesser alkyl-substituted alkenes leads to higher yields of HOx (and bifunctional organic nitrates) and vice versa. Indeed, simultaneous observations of DMS, isoprene, terpenes, NO3, HO2, and RO2 in the marine boundary layer and forested regions have provided increasingly convincing evidence for nighttime oxidation chemistry involving these species and, by implication, the production of OH. Unfortunately, confirmatory nighttime measurements of OH are relatively scarce, but concentrations of the order of 2–3104 molecule cm3 have been recorded on occasion. These data are a factor of 102–104 less than typical maximum daytime OH values, and thus represent a significant observational achievement. Clearly, measurements of OH, NO3, HO2, and RO2, together with supporting data on O3, NOx, HONO, HONO2, and speciated VOCs and organic nitrates, would provide much improved understanding of the role of OH in nighttime oxidation chemistry.
Sinks and Reservoirs of OH in the Troposphere Oxidation of CO Figure 2 illustrates schematically the production of OH via O3 and HONO photodissociation, the ozonolysis of alkenes, and the photo-oxidation of CH2O. The mechansism of the oxidation of carbon monoxide (CO) in the troposphere is also shown. It is initiated solely by reaction with OH, propagated by HO2, which reacts with NO to regenerate OH and produce NO2, and proceeds via the photodissociation of NO2 with the formation of O3 (reactions [XXVII], [VI], [VIII], [XXVIII], and [XXIX]). OH þ CO / H þ CO2 H þ O2 þ M / HO2 þ M M ¼ N2 ; O2 HO2 þ NO / OH þ NO2
l < 420 nm
NO2 þ hv / NO þ O3 P Oð3 PÞ þ O2 þ M / O3 þ M
[XXVII] [VI] [VIII] [XXVIII] [XXIX]
Reaction [XXX] is the overall net reaction. CO þ 2O2 þ hv / CO2 þ O3
[XXX]
This represents a chemical process that rapidly interconverts HOx within seconds, NOx within a few minutes, and does so without consuming either HOx or NOx.
Oxidation of CH4 Similarly, as shown also in Figure 2, in the presence of sufficient NO, methylperoxy radicals (CH3O2), methoxy radicals (CH3O), and HO2 serve to propagate the OH-initiated oxidation of CH4. Once again OH is regenerated, in this case by reactions [XXXI], [XIII], [XIV], and [VIII], and O3 is formed via reactions [XXXVIII] and [XXIX]. OH þ CH4 / H2 O þ CH3
[XXXI]
CH3 þ O2 þ M / CH3 O2 þ M
[XIII]
CH3 O2 þ NO / CH3 O þ NO2
[XIV]
Tropospheric Chemistry and Composition j Hydroxyl Radical
O3
HO2NO2
h h , O2
HONO2
h , H2O
Alkenes
235
H2O2 NO2
NO
Δ
NO2 HO2
NO2 h HO2
OH
HONO NO
h , O2
CH2O
O2
CH4
CO2
CO
O2
O2 CH3O2 CH3O2
CH3O
NO HO2 CH3ONO2
NO2
Δ
NO
CH3OOH
NO2 h
h , O2 O3
CH3O2NO2
Figure 2 Daytime tropospheric chemistry illustrating sources of OH via photodissociation of O3 and HONO, ozonolysis of alkenes, and the photooxidation of CH2O. Reaction with OH initiates the oxidation of CO and CH4. In the presence of sufficient NO, OH is regenerated via reaction of NO with HO2, and O3 is formed in cyclic reaction mechanisms. Reactions that interrupt or terminate these cycles represent loss processes for OH and HO2 and include the production of HONO2, H2O2, CH3OOH, and CH3ONO2.
CH3 O þ O2 / CH2 O þ HO2
[XV]
HO2 þ NO / OH þ NO2
[VIII]
CH2O is also produced and acts on a time scale of 5 h as a temporary reservoir species or secondary photochemical source of HO2 (and thus OH) via reactions [V], [VI], [VII], and [VIII]. By contrast, in more remote environments with insufficient NO for reaction [VIII] to dominate the fate of HO2, O3 is removed via reaction reaction [XXXII] with HO2, which yields OH. HO2 þ O3 / OH þ 2O2
[XXXII]
Competition between reactions [VIII] and [XXXII] determines whether net production or loss of O3 occurs. The mutual reaction of OH and HO2, which is not shown in Figure 2, represents an important sink or termination process, which leads to a net loss of HOx (reaction [XXXIII]). OH þ HO2 / H2 O þ O2
[XXXIII]
By contrast, self- and analogous cross-reactions of HO2 and CH3O2 lead to the formation of the HOx reservoir species, hydrogen peroxide (H2O2), and methyl hydroperoxide (CH3O2H), via reactions [XXXIV] and [XXXV]. HO2 þ HO2 þ M / H2 O2 þ M
[XXXIV]
HO2 þ CH3 O2 þ M / CH3 O2 H þ M
[XXXV]
Figure 3 illustrates the percent contribution of the OH loss due to reaction with CO, CH4, and a wide range of biogenic and anthropogenic nonmethane hydrocarbons (NMHCs) as a diurnal cycle averaged over a field measurement campaign in
a forested region in Europe. In this case, isoprene was the dominant species reacting with OH during the day, whereas limonene and a- and b-pinene became more important during nighttime. These observations are consistent with their respective temperature- and light-dependent vegetative emission rates and OH reactivities.
Oxidation of NO2 and SO2 Apart from a wide range of VOCs and CO, the oxidation of inorganic compounds such as NO2 and SO2 is also initiated by reaction with OH, as shown by reactions [XXXVI], [XXXVII], [XXXVIII], and [XXIX]. OH þ NO2 þ M / HONO2 þ M
[XXXVI]
OH þ SO2 þ M / HSO3 þ M
[XXXVII]
HSO3 þ O2 / HO2 þ SO3
[XXXVIII]
SO3 þ H2 þ M / H2 SO4 þ M
[XXIX]
The production of nitric acid in reaction [XXXVI] is the dominant loss mechanism for HOx and NOx in polluted atmospheres. Dry deposition and heterogeneous hydrolysis of NOx are other important removal pathways. For SO2, however, reactions [XXXVII], [XXXVIII], and [XXIX] not only convert OH to HO2, but also lead to the formation of sulfuric acid (H2SO4), which rapidly condenses onto aerosol surfaces due to its low vapor pressure, thereby acting as a potential nucleus for new particle formation. SO2 possesses high aqueous solubility and
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Tropospheric Chemistry and Composition j Hydroxyl Radical
100 propene 90 ethene 80
i-butene
70
methane CO
60 % OH loss
terpinene 50
1,8-cineole
40
p-cymene carene
30
sabinene 20 camphene
23:00
22:00
20:00
21:00
18:00
19:00
16:00
17:00
15:00
14:00
12:00
13:00
11:00
9:00
10:00
7:00
8:00
5:00
6:00
3:00
4:00
-pinene 2:00
0 1:00
limonene
0:00
10
-pinene isoprene
Time (h)
Figure 3 The percent contribution of OH loss due to reaction with CO, CH4, and a wide range of biogenic and anthropogenic non-methane hydrocarbons (NMHCs) as a diurnal cycle averaged over a field measurement campaign in a forested region in Europe. Reproduced without permission from Carslaw N, Creasey DJ, Harrison D, et al. (2001) OH and HO2 radical chemistry in a forested region of north-western Greece. Atmospheric Environment 35: 4725–4737.
Table 2 Global emission rates of trace gases and their proportion that is removed by reaction with a mean global OH concentration of 1 106 molecule cm3 Trace gas
Global emission rate (Tg y1)
Removal by OH (%)
Carbon monoxide Methane Ethane Isoprene Terpenes Nitrogen dioxide Sulfur dioxide Dimethyl sulfide
2800 530 20 570 140 150 300 30
85 90 90 90 50 50 30 90
with many large sources of stationary and mobile anthropogenic pollutants, and forested regions that are characterized by a variety of biogenic emissions, the level of agreement between observations and calculations is typically less good. As described above, OH plays a central role in the tropospheric chemistry of most gas-phase organic and inorganic pollutants. Quantitative measurements of OH are invaluable in the validation of the chemical mechanisms and kinetic parameters incorporated within the models. Indeed, correct predictions of previously unidentified sources and sinks of OH, and of erroneous rate coefficient measurements, have resulted from the application of numerical chemical models to the interpretation of observational data sets from several field studies.
Reproduced without permission from Ehhalt DH (1999) Photooxidation of trace gases in the troposphere. Physical Chemistry Chemical Physics 1: 5401–5408.
Measurement Techniques for Field Studies of OH Radicals is therefore also oxidized in cloud droplets and precipitation, particularly via reaction with H2O2. To illustrate the wide range of compounds that OH reacts with, Table 2 lists estimated global emission rates of several important trace gases and the proportion that is removed via their reaction with an OH concentration of 1106 molecule cm3.
Comparison between Measured and Calculated OH Concentrations For remote, clean air environments, the high level of agreement typically observed between OH concentrations measured directly and those calculated from a hierarchy of numerical models of the chemical and physical sources and sinks of OH, indicate that these relatively simple systems are sufficiently well understood. However, perhaps it is not surprising that, in chemically more complex systems such as urban environments
High reactivity with a wide range of VOCs, rapid interconversion with HO2, lifetimes of the order of 0.1–1 s and highly variable, diurnal, seasonal, and spatial concentrations between 104 and 108 molecule cm3 mean that the quantitative detection of OH remains one of the most important yet difficult challenges in tropospheric chemistry. Nevertheless, several field-proven, spectroscopic measurement techniques have been developed over the last two decades. Each technique is described briefly below, together with recent applications and intercomparisons in ground-based and airborne field measurement studies of the tropospheric chemistry of OH.
DOAS, L-POAS, and MOAS DOAS (Differential Optical Absorption Spectroscopy) and L-POAS (Long-Path Optical Absorption Spectroscopy) are
Tropospheric Chemistry and Composition j Hydroxyl Radical similar to each other but distinct from MOAS (Multipass Optical Absorption Spectroscopy). The former are usually employed to measure over integrated optical path lengths of several kilometers, whereas the latter utilizes an open, wall-less White cell with a base length of 6 m to yield data that are more comparable with local, in situ measurements. Concentrations of OH are derived by applying least-squares fits of OH reference spectra to the recorded differential optical density using singular value decomposition routines. A considerable advantage of these optical spectroscopic methods for ambient measurements of OH is that calibration is achieved using data for absorption cross-sections predetermined in the laboratory. Unlike other techniques, sophisticated in situ calibration systems are not required. Detection limits of the order of 2 105 molecule cm3 are achievable for optical path lengths of 2–5 km, with measurement frequencies of approximately 1 to 5 min. There have been several recent reports of the application of DOAS/L-POAS and MOAS to measurements of OH in ground-based studies in remote, rural, and more polluted environments.
LIF and FAGE LIF (Laser-Induced Fluorescence) and FAGE (Fluorescence Assay by Gas Expansion) are essentially identical techniques. Ambient air is expanded as a continuous, supersonic free jet through a nominal 1 mm nozzle into a low-pressure detection chamber maintained at approximately 1 mbar. A laser beam at 308 nm is used to promote ground state X2P OH into the first P electronically excited state, A2 . At low pressure, the fluorescence lifetime of OH is increased and allows delayed-gated photon counting to capture the extremely weak OH LIF signal, while discriminating against the much more intense scattered light. Interferences arising from the photodissociation of O3 to O(1D) atoms followed by subsequent reaction of O(1D) with H2O to form OH radicals during the laser pulse is negligible using 308 nm excitation at low pressure. Note that HO2 may also be measured by reaction with NO in the lowpressure gas expansion chamber to form OH, which are subsequently detected as described above. Calibration requires not only measurement of the response to independently quantifiable sources of OH and HO2 coupled to the inlet system, but also determination of the efficiency of conversion of HO2 to OH. In addition to their increasing utility in photochemical reactor chambers, there have been several recent reports of the application of LIF/FAGE to measurements of OH (and HO2) in ground-based, shipboard, and airborne studies of tropospheric chemistry in a variety of remote, marine, rural, and urban environments.
CIMS and IMR-MS The development of CIMS (Chemical Ionization Mass Spectrometry) for the detection of OH in the atmosphere began in the late 1980s and 1990s. IMR-MS (Ion Molecule ReactionMass Spectrometry) is identical. In each case, OH radicals are reacted with 34SO2 in the presence of O2 to produce 34 SO3, which reacts with water vapor to form H34 2 SO4 according to reactions [XXXVII]–[XXXIX]. H34 SO is then 4 2 detected using quadrupole mass spectrometry as the highly
237
stable anion, H34 SO 4 , following H-atom abstraction by NO 3 ions (reaction [XL]). 34 H34 2 SO4 þ NO3 / HNO3 þ H SO4 5
[XL] 3
OH concentrations of less than 110 molecule cm can be measured in less than 1 min. These techniques have also found increasingly widespread application in recent groundbased and airborne studies of the sources and sinks of OH in the troposphere.
Other Methods Field measurements of ambient OH concentrations may also be made using a simple, portable and inexpensive method. This method utilizes the rapid reaction of OH with o-hydroxybenzoic acid in a buffered solution to produce 2,5-dihydroxybenzioc acid, which is quantified by reversephase high pressure liquid chromatography with fluorescence detection. In clean air, a detection limit of 3–6 105 molecule cm3 may be achieved for a sampling period of 45–90 min. Tropospheric OH concentrations may also be determined from measurements of the differential loss rates of many NMHCs, especially reactive alkenes, in well-defined, summertime urban plumes. A key assumption is that the observed hydrocarbon losses are due solely to OH radical reaction, and that plume dilution is correctly accounted for. Finally, globally averaged OH concentrations of approximately 1106 molecule cm3 may be estimated from measurements of the global budget of methyl chloroform (CH3CCl3), which is entirely of anthropogenic origin and almost exclusively removed by reaction with OH. Clearly, information about regional- and local-scale sources and sinks of OH is not provided from such estimates, but the important role of OH in tropospheric chemistry on a global scale may be deduced.
Intercomparisons of Ambient Measurements of OH Participation in intercomparison exercises represents an often undervalued but vitally important stage in the development and validation of new instrumentation and measurement techniques. Despite the undoubted importance of making ambient measurements of OH in order to understand further its central role in tropospheric chemistry, there have been relatively few intercomparison exercises carried out over the last two decades in order to compare measurement techniques. This may have been due partly to the difficult challenges posed, but this situation may change over the next decade with further evaluation of the more recently developed techniques in order to quantify their selectivity, accuracy, precision, detection limits, sensitivity, and reliability. However, those OH intercomparison exercises that have been carried out have been informative and instructive. For example, an intercomparison of airborne measurements of OH was recently carried out above the Pacific Ocean. LIF and CIMS data recorded on DC-8 and P3-B aircraft, respectively, during two brief close proximity flights in the marine boundary layer showed exceptionally good agreement. Flights at an altitude of 5.5 km resulted in an average concentration difference similar to the measurement uncertainties of each technique of 40%, but much less than the
Tropospheric Chemistry and Composition j Hydroxyl Radical
238
0.28 0.15 106 molecule cm3 was obtained from 137 data pairs. Earlier intercomparisons of ambient OH measurements using L-POAS and CIMS were carried out at Fritz Peak Observatory, Colorado, in 1991, and during the Tropospheric OH Photochemistry Experiment at Idaho Hill, Colorado, in 1993. In general, good agreement within 30% was obtained for similar chemical and meteorological conditions, which confirmed the absence of significant interferences or artefacts. A 1992 intercomparison of FAGE and an obsolete 14CO radiocarbon technique not only demonstrated good correlation between data sets, with an r value of 0.86, but also revealed a significant unresolved calibration discrepancy of a factor of 2.9. Sadly, the first OH measurement intercomparison exercise in 1983–84 demonstrated the then unreliable airborne operation and insufficient detection sensitivities of LIF and the radiocarbon technique.
12
[OH] (DOAS) (106 cm−3)
10
8
6
4
2
0 0
2
4
6
8
10
12
See also: Ozone Depletion and Related Topics: Photochemistry of Ozone; Stratospheric Ozone Recovery. Radiation Transfer in the Atmosphere: Ultraviolet Radiation. Stratospheric Chemistry Topics: HOx.
[OH] (LIF) (106 cm−3)
Figure 4 Bivariate plot of 137 OH data pairs measured by DOAS and LIF during the 1994 POPCORN campaign in a rural environment in northeast Germany. The correlation coefficient, r, is 0.90; the solid line represents a weighted linear fit with a gradient of 1.09 0.04 and an intercept of (0.28 0.15) 106 molecule cm3. Reproduced without permission from Hofzumahaus, A., Aschmutat, U., Brandenburger, U., et al., 1998. Intercomparison of tropospheric OH measurements by different laser techniques during the POPCORN campaign 1994. Journal of Atmospheric Chemistry 31 (1–2), 227–246.
combined uncertainties from all the contributing measurements. In each case, comparisons were made by normalizing the data with a photo-stationary state model. In addition, OH concentrations measured between 25 N to 25 S latitude over the entire longitude and altitude range of the study agreed to within 10%, although the ratio of DC-8:P-3B measurements increased at higher altitudes. These results illustrated no obvious measurement discrepancies and minimal common interferences and calibration errors for two quite distinct OH measurement techniques. An intercomparison of extensive LIF and DOAS measurements of OH radicals in a rural environment in north-east Germany during August 1994 showed excellent agreement for the same air mass, thereby demonstrating impressively high degrees of specificity, accuracy and reliability. As shown in Figure 4, a linear relationship with a correlation coefficient, r, of 0.90, a gradient of 1.09 0.04 and an intercept of
Further Reading Abram, J.P., Creasey, D.J., Heard, D.E., Lee, J.D., Pilling, M.J., 2000. Hydroxyl radical and ozone measurements in England during the solar eclipse of 11 August 1999. Geophysical Research Letters 27 (21), 3437–3440. Atkinson, R., 2000. Atmospheric chemistry of VOCs and NOx. Atmospheric Environment 34 (12–14), 2063–2101. Brauers, T., Hausmann, M., Bister, A., Kraus, A., Dorn, H.-P., 2001. OH radicals in the boundary layer of the Atlantic Ocean: 1. Measurements by long-path laser absorption spectroscopy. Journal of Geophysical Research 106 (D7), 7399–7414. Ehhalt, D.H., 1999. Photooxidation of trace gases in the troposphere. Physical Chemistry Chemical Physics 1 (24), 5401–5408. Eisele, F.L., Mount, G.H., Tanner, D., et al., 1997. Understanding the production and interconversion of the hydroxyl radical during the OH Photochemistry Experiment. Journal of Geophysical Research 102 (D5), 6457–6465. Eisele, F.L., Tanner, D.J., Cantrell, C.A., Calvert, J.G., 1996. Measurements and steady state calculations of OH concentrations at Mauna Loa observatory. Journal of Geophysical Research 101 (D9), 14 665–14 679. Finlayson-Pitts, B.J., Pitts Jr., J.N., 2000. Chemistry of the Upper and Lower Atmosphere. Theory, Experiments and Applications. Academic Press, San Diego, CA. Hofzumahaus, A., Aschmutat, U., Brandenburger, U., et al., 1998. Intercomparison of tropospheric OH measurements by different laser techniques during the POPCORN campaign 1994. Journal of Atmospheric Chemistry 31 (1–2), 227–246. Jaegle, L., Jacob, D.J., Brune, W.H., Wennberg, P.O., 2001. Chemistry of HOx radicals in the upper troposphere. Atmospheric Environment 35 (3), 469–489. Jenkin, M.E., Clemitshaw, K.C., 2000. Ozone and other secondary photochemical pollutants: chemical processes governing their formation in the planetary boundary layer. Atmospheric Environment 34 (16), 2499–2527.
Introduction The key roles played by hydroxyl radicals (OH) within tropospheric chemistry, including the major sources, sinks, and chemical processes involving OH, are discussed. The major daytime sources of OH include the photodissociation of ozone (O3) and nitrous acid (HONO), the photo-oxidation of formaldehyde (CH2O) and acetone (CH3C(O)CH3), together with the ozonolysis of alkenes. Nighttime processes leading to OH production also include the ozonolysis of alkenes and, more importantly, nitrate radical (NO3)-facilitated decomposition of peroxyacyl nitrates (RC(O)O2NO2) and NO3-initiated oxidation of alkenes. The major sink processes for OH are to initiate the oxidation of carbon monoxide (CO), methane (CH4), and a wide range of reactive volatile organic compounds (VOCs), and to form nitric acid (HONO2) via reaction with NO2. In the presence of sufficient NO and NO2 (collectively termed NOx), these oxidation mechanisms are propagated by organic peroxy (RO2), alkoxy (RO), and hydroperoxy (HO2) radicals. They regenerate OH and proceed with the formation of oxidants such as O3, carbonyls (e.g., CH2O), peroxides (e.g., hydrogen peroxide, H2O2), and organic nitrates (e.g., peroxyacetylnitrate, PAN, CH3C(O) O2NO2). OH and HO2 chemistry may also lead to O3 depletion under conditions of low NOx. Finally, a brief description of several spectroscopic techniques that have been developed for quantitative ambient measurements of OH, together with their recent applications and intercomparisons in groundbased and airborne field studies of the tropospheric chemistry of OH, is also given.
remote environments and has relatively simple photochemistry. The rate of production of OH via O3 photodissociation is also influenced by quenching of O(1D) to O(3P) atoms via collision with tropospheric N2 and O2 (reaction [III]). Oð1 DÞ þ M / Oð3 PÞ þ M M ¼ N2 ; O2
[III]
The fractional conversion, f, of O( D) into OH is described approximately by eqn [1]), where PH2 O is the partial pressure of water vapor and P is the total pressure. 1
f ¼ PH2 O =½PH2 O þ 0:13ðP PH2 O Þ
[1]
Photodissociation of HONO and Photo-Oxidation of CH2O Clearly, the factor, f, must be taken into account when comparing the relative importance of OH produced directly upon UV photodissociation of other precursor molecules. For HONO, photodissociation occurs on a time scale of approximately 15 min (reaction [IV]). HONO þ hv / OH þ NO
l < 400 nm
[IV]
During the darkness of night, HONO concentrations may build up via heterogeneous hydrolysis of NO and NO2, and as a result of direct emissions from motor vehicles. At sunrise, the photodissociation of HONO provides an early morning pulse of OH at a time of day when j(O1D) is very low due to the long atmospheric path lengths and the low intensity of near-UV solar radiation. HO2 is an important reactive intermediate of the photo-oxidation of CH2O, and OH may be formed indirectly via the rapid reaction of HO2 with reactions [V], [VI], [VII], and [VIII].
Daytime Sources of OH in the Troposphere
CH2 O þ hv / H þ HCO
l < 340 nm
[V]
Photodissociation of O3
H þ O2 þ M / HO2 þ M M ¼ N2 ; O2
[VI]
HCO þ O2 / HO2 þ CO
[VII]
HO2 þ NO / OH þ NO2
[VIII]
The primary daytime source of OH radicals throughout much of the troposphere is the photodissociation of O3 in its nearultraviolet (UV) absorption band, followed by reaction of the O(1D) photoproduct with water vapor (reactions [I] and [II]). O3 þ hv / Oð1 DÞ þ O2 l < 340 nm
[I]
Oð1 DÞ þ H2 O / 2OH
[II]
However, both the O3 absorption cross sections and quantum yields of formation of O(1D) atoms decrease with increasing wavelength. Consequently, the rate of O3 photodissociation, j(O1D), and therefore the rate of OH production, varies strongly with changes in atmospheric path length that not only accompany spatial variations in altitude and latitude but also temporal variations on diurnal and seasonal timescales. Indeed, as illustrated in Figure 1, daytime measurements of j(O1D) and OH often display a high degree of positive correlation, especially in clean air that is characteristic of
232
Urban environments often have high local emissions of CH2O, whereas O3 concentrations are generally suppressed due to rapid reaction with elevated levels of the primary pollutant, NO. Consequently, urban summertime rates of OH production via reactions [V], [VI], [VII], and [VIII] may exceed those via O3 photodissociation by a factor of 4 or so for typical photodissociation rates and precursor concentrations.
Photo-Oxidation of CH3C(O)CH3 By contrast, the photo-oxidation of acetone, CH3C(O)CH3, which produces CH2O as an intermediate species, has been suggested recently to be a significant source of HO2 and OH (collectively termed HOx) in the upper troposphere and
Encyclopedia of Atmospheric Sciences 2nd Edition, Volume 6
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Tropospheric Chemistry and Composition j Hydroxyl Radical 3.0 × 10−5 −5
2.5 × 10
8.0 × 106
2.0 × 10
6
1.5 × 10
−5
−5
6
4.0 × 10
1.0 × 10−5
6
2.0 × 10
5.0 × 10
0 21 Jul
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(OH) (molecule cm−3)
1.0 × 107
6.0 × 10
233
−6
0 23 Jul
25 Jul
27 Jul
29 Jul
31 Jul 2 Aug
4 Aug
6 Aug
8 Aug
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Figure 1 Simultaneous observations of OH (blue circles) and j(O1D) (black line) made during the PRIME’99 campaign at the Silwood Park Atmospheric Research Station, near Ascot, UK. Data kindly provided by Professor M.J. Pilling and Dr D.E. Heard, University of Leeds, UK.
lower stratosphere (reactions [IX], [X], [XI], [XII], [XIII], [XIV], and [XV]). CH3 CðOÞCH3 þ hv / CH3 CðOÞ þ CH3
[IX]
CH3 CðOÞ þ O2 þ M / CH3 CðOÞO2 þ M
[X]
CH3 CðOÞO2 þ NO / CH3 CðOÞO þ NO2
[XI]
CH3 CðOÞO þ M / CH3 þ CO2 þ M
[XII]
CH3 þ O2 þ M / CH3 O2 þ M
[XIII]
CH3 O2 þ NO / CH3 O þ NO2
[XIV]
CH3 O þ O2 / CH2 O þ HO2
[XV]
HO2 þ NO / OH þ NO2
[VIII]
Inclusion of this process in numerical models of atmospheric chemistry reduces considerably the discrepancies between calculated and measured HOx concentrations and O3 production rates.
Ozonolysis of Alkenes The ozonolysis of alkenes (R1R2C CR3R4) represents another important process that produces OH, and may even dominate the photodissociation of O3, HONO, and CH2O in polluted urban environments. The initial reactions of alkenes with O3 are generally slow, but, as shown in Table 1 may proceed at rates that are comparable to corresponding reactions of alkenes with OH occurring during daytime and with NO3 during nighttime. Although the details are not yet understood fully, kinetic and mechanistic laboratory studies indicate that the electrophylic addition of O3 across the double bond leads to the initial formation of an energy-rich primary ozonide ([R1R2COOOCR3R4]z), which decomposes rapidly to generate pairs of energy-rich, carbonyl-substituted Criegee biradicals ([R1R2COO]z and [R3R4COO]z), and primary carbonyl compounds (R1C(O)R2 and [R3C(O)R4]), according to reactions (reactions [XVI], [XVIIa], and [XVIIb]). O3 þ R 1 R 2 C ¼ CR 3 R 4 / ½R 1 R 2 COOOCR 3 R 4 z
[XVI]
½R 1 R 2 COOOCR 3 R 4 z / R 1 CðOÞR 2 þ ½R 3 R 4 COOz [XVIIa] / R 3 CðOÞR 4 þ ½R 1 R 2 COOz
[XVIIb]
The Criegee biradicals either collisionally stabilize, isomerize and/or decompose to form OH and other organic radicals, for example by reaction [XVIII] or reaction [XIX]. ½R 1 R 2 COOz / ½R 52 CH2 ¼ CðOOHÞR 2 z ½R 5 CH2 ¼ CðOOHÞR 2 / R 5 CHCðOÞR 2 þ OH
[XVIII] [XIX]
More recent laboratory studies have also shown that the fractional yields of OH produced upon ozonolysis of alkenes are significant under simulated tropospheric conditions, with approximate values ranging from 0.1 for ethene to 0.85 for a-pinene. The ozonolysis of alkenes therefore represents a potentially important daytime source of OH that may lead to the net production of O3 under certain conditions.
Nighttime Sources of OH in the Troposphere NO3-Facilitated Decomposition of RC(O)O2NO2 and Oxidation of Biogenic VOCs Clearly, OH may also be produced during nighttime upon ozonolysis of alkenes. However, of more significance at night are other non-photochemical thermal reactions that involve NO3. NO3 has a lifetime with respect to photodissociation of approximately 5 s during the day, and its daytime oxidation chemistry is much less important than that of the more reactive OH. However, under conditions of low NOx, nocturnal NO3 chemistry may lead to OH production by facilitating the decomposition of peroxyacyl nitrates (RC(O)O2NO2), as shown in reactions [XX] and [XXI]. RCðOÞO2 NO2 þ M 4 RCðOÞO2 þ NO2 þ M
[XX]
RCðOÞO2 þ NO3 / R þ CO2 þ NO2 þ O2
[XXI]
NO3 may also initiate the oxidation of VOCs such as alkane hydrocarbons (RH), although, as shown in Table 1, these are particularly slow reactions [XXII]. NO3 þ RH / HNO3 þ R
[XXII]
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Tropospheric Chemistry and Composition j Hydroxyl Radical
Table 1 Comparison of tropospheric lifetimes of a range of CO and VOCs at typical ambient rural concentrations with respect to reaction with 1.6106 molecule cm3 OH, 30 ppbv O3, and 10 pptv NO3 VOC
OH
O3
NO3
Carbon monoxide
30 days
Alkanes Methane Ethane Propane Butane 2-Methyl propane Pentane 2-Methyl butane
10 years 29 days 6.3 days 2.9 days 3.1 days 1.8 days 1.9 days
Alkenes Ethene Propene 1-Butene 2-Butene 3-Methyl propene 1-Pentene 2-Pentene 2-Methyl 1-butene 3-Methyl 1-butene 2-Methyl 2-butene 1,3-Butadiene Isoprene
20 h 6.6 h 5.5 h 2.9 h 3.4 h 5.5 h 2.6 h 2.8 h 5.5 h 2.0 h 2.6 h 1.7 h
Aromatics Benzene Toluene Ethyl benzene o-Xylene m-Xylene p-Xylene
5.7 days 1.2 days 23 h 12 h 7.1 h 12 h
4.1 months 6.6 months 3.4 months
Aldehydes Formaldehyde Acetaldehyde
18 h 11 h
2.7 months 17 days
Sulfur-containing Dimethyl sulfide Dimethyl disulfide
1.5 days 46 min
1.0 h 1.5 h
91 years 7.8 years 2.7 years 1.5 years 1.5 years 1.3 years 9.7 days 1.5 days 1.6 days 2.4 days 1.4 days 1.5 days 2.4 h 1.4 days 1.6 days 55 min 2.4 days 1.2 days
7.3 months 4.9 days 3.5 days 2.9 h 3.4 h 3.5 days 2.9 h 3.4 h 3.5 days 7.1 min 11 h 1.7 h
1.8 years
ppbv, parts per billion by volume; pptv, parts per trillion by volume. Adapted without permission from Jenkin, M.E., Clemitshaw, K.C., 2000. Ozone and other secondary photochemical pollutants: Chemical processes governing their formation in the planetary boundary layer. Atmospheric Environment 34 (16), 2499–2527.
Nevertheless, subsequent reactions ([XXIII], [XXIV], [XXIV], [XXV], and [XXVI]) lead to OH production. R þ O2 þ M / RO2 þ M
[XXIII]
RO2 þ NO3 / RO þ NO2 þ O2
[XXIV]
RO þ O2 / RO þ HO2
[XXV]
HO2 þ NO3 / OH þ NO2 þ O2
[XXVI]
For dimethyl sulfide (DMS), which is of marine phytoplankton origin, and biogenic alkenes such as isoprene and terpenes, which are emitted by certain plants and trees, the primary step is the rapid addition of NO3 rather than H atom abstraction. The subsequent reaction mechanisms are quite
complex, but in general, NO3 oxidation of lesser alkyl-substituted alkenes leads to higher yields of HOx (and bifunctional organic nitrates) and vice versa. Indeed, simultaneous observations of DMS, isoprene, terpenes, NO3, HO2, and RO2 in the marine boundary layer and forested regions have provided increasingly convincing evidence for nighttime oxidation chemistry involving these species and, by implication, the production of OH. Unfortunately, confirmatory nighttime measurements of OH are relatively scarce, but concentrations of the order of 2–3104 molecule cm3 have been recorded on occasion. These data are a factor of 102–104 less than typical maximum daytime OH values, and thus represent a significant observational achievement. Clearly, measurements of OH, NO3, HO2, and RO2, together with supporting data on O3, NOx, HONO, HONO2, and speciated VOCs and organic nitrates, would provide much improved understanding of the role of OH in nighttime oxidation chemistry.
Sinks and Reservoirs of OH in the Troposphere Oxidation of CO Figure 2 illustrates schematically the production of OH via O3 and HONO photodissociation, the ozonolysis of alkenes, and the photo-oxidation of CH2O. The mechansism of the oxidation of carbon monoxide (CO) in the troposphere is also shown. It is initiated solely by reaction with OH, propagated by HO2, which reacts with NO to regenerate OH and produce NO2, and proceeds via the photodissociation of NO2 with the formation of O3 (reactions [XXVII], [VI], [VIII], [XXVIII], and [XXIX]). OH þ CO / H þ CO2 H þ O2 þ M / HO2 þ M M ¼ N2 ; O2 HO2 þ NO / OH þ NO2
l < 420 nm
NO2 þ hv / NO þ O3 P Oð3 PÞ þ O2 þ M / O3 þ M
[XXVII] [VI] [VIII] [XXVIII] [XXIX]
Reaction [XXX] is the overall net reaction. CO þ 2O2 þ hv / CO2 þ O3
[XXX]
This represents a chemical process that rapidly interconverts HOx within seconds, NOx within a few minutes, and does so without consuming either HOx or NOx.
Oxidation of CH4 Similarly, as shown also in Figure 2, in the presence of sufficient NO, methylperoxy radicals (CH3O2), methoxy radicals (CH3O), and HO2 serve to propagate the OH-initiated oxidation of CH4. Once again OH is regenerated, in this case by reactions [XXXI], [XIII], [XIV], and [VIII], and O3 is formed via reactions [XXXVIII] and [XXIX]. OH þ CH4 / H2 O þ CH3
[XXXI]
CH3 þ O2 þ M / CH3 O2 þ M
[XIII]
CH3 O2 þ NO / CH3 O þ NO2
[XIV]
Tropospheric Chemistry and Composition j Hydroxyl Radical
O3
HO2NO2
h h , O2
HONO2
h , H2O
Alkenes
235
H2O2 NO2
NO
Δ
NO2 HO2
NO2 h HO2
OH
HONO NO
h , O2
CH2O
O2
CH4
CO2
CO
O2
O2 CH3O2 CH3O2
CH3O
NO HO2 CH3ONO2
NO2
Δ
NO
CH3OOH
NO2 h
h , O2 O3
CH3O2NO2
Figure 2 Daytime tropospheric chemistry illustrating sources of OH via photodissociation of O3 and HONO, ozonolysis of alkenes, and the photooxidation of CH2O. Reaction with OH initiates the oxidation of CO and CH4. In the presence of sufficient NO, OH is regenerated via reaction of NO with HO2, and O3 is formed in cyclic reaction mechanisms. Reactions that interrupt or terminate these cycles represent loss processes for OH and HO2 and include the production of HONO2, H2O2, CH3OOH, and CH3ONO2.
CH3 O þ O2 / CH2 O þ HO2
[XV]
HO2 þ NO / OH þ NO2
[VIII]
CH2O is also produced and acts on a time scale of 5 h as a temporary reservoir species or secondary photochemical source of HO2 (and thus OH) via reactions [V], [VI], [VII], and [VIII]. By contrast, in more remote environments with insufficient NO for reaction [VIII] to dominate the fate of HO2, O3 is removed via reaction reaction [XXXII] with HO2, which yields OH. HO2 þ O3 / OH þ 2O2
[XXXII]
Competition between reactions [VIII] and [XXXII] determines whether net production or loss of O3 occurs. The mutual reaction of OH and HO2, which is not shown in Figure 2, represents an important sink or termination process, which leads to a net loss of HOx (reaction [XXXIII]). OH þ HO2 / H2 O þ O2
[XXXIII]
By contrast, self- and analogous cross-reactions of HO2 and CH3O2 lead to the formation of the HOx reservoir species, hydrogen peroxide (H2O2), and methyl hydroperoxide (CH3O2H), via reactions [XXXIV] and [XXXV]. HO2 þ HO2 þ M / H2 O2 þ M
[XXXIV]
HO2 þ CH3 O2 þ M / CH3 O2 H þ M
[XXXV]
Figure 3 illustrates the percent contribution of the OH loss due to reaction with CO, CH4, and a wide range of biogenic and anthropogenic nonmethane hydrocarbons (NMHCs) as a diurnal cycle averaged over a field measurement campaign in
a forested region in Europe. In this case, isoprene was the dominant species reacting with OH during the day, whereas limonene and a- and b-pinene became more important during nighttime. These observations are consistent with their respective temperature- and light-dependent vegetative emission rates and OH reactivities.
Oxidation of NO2 and SO2 Apart from a wide range of VOCs and CO, the oxidation of inorganic compounds such as NO2 and SO2 is also initiated by reaction with OH, as shown by reactions [XXXVI], [XXXVII], [XXXVIII], and [XXIX]. OH þ NO2 þ M / HONO2 þ M
[XXXVI]
OH þ SO2 þ M / HSO3 þ M
[XXXVII]
HSO3 þ O2 / HO2 þ SO3
[XXXVIII]
SO3 þ H2 þ M / H2 SO4 þ M
[XXIX]
The production of nitric acid in reaction [XXXVI] is the dominant loss mechanism for HOx and NOx in polluted atmospheres. Dry deposition and heterogeneous hydrolysis of NOx are other important removal pathways. For SO2, however, reactions [XXXVII], [XXXVIII], and [XXIX] not only convert OH to HO2, but also lead to the formation of sulfuric acid (H2SO4), which rapidly condenses onto aerosol surfaces due to its low vapor pressure, thereby acting as a potential nucleus for new particle formation. SO2 possesses high aqueous solubility and
236
Tropospheric Chemistry and Composition j Hydroxyl Radical
100 propene 90 ethene 80
i-butene
70
methane CO
60 % OH loss
terpinene 50
1,8-cineole
40
p-cymene carene
30
sabinene 20 camphene
23:00
22:00
20:00
21:00
18:00
19:00
16:00
17:00
15:00
14:00
12:00
13:00
11:00
9:00
10:00
7:00
8:00
5:00
6:00
3:00
4:00
-pinene 2:00
0 1:00
limonene
0:00
10
-pinene isoprene
Time (h)
Figure 3 The percent contribution of OH loss due to reaction with CO, CH4, and a wide range of biogenic and anthropogenic non-methane hydrocarbons (NMHCs) as a diurnal cycle averaged over a field measurement campaign in a forested region in Europe. Reproduced without permission from Carslaw N, Creasey DJ, Harrison D, et al. (2001) OH and HO2 radical chemistry in a forested region of north-western Greece. Atmospheric Environment 35: 4725–4737.
Table 2 Global emission rates of trace gases and their proportion that is removed by reaction with a mean global OH concentration of 1 106 molecule cm3 Trace gas
Global emission rate (Tg y1)
Removal by OH (%)
Carbon monoxide Methane Ethane Isoprene Terpenes Nitrogen dioxide Sulfur dioxide Dimethyl sulfide
2800 530 20 570 140 150 300 30
85 90 90 90 50 50 30 90
with many large sources of stationary and mobile anthropogenic pollutants, and forested regions that are characterized by a variety of biogenic emissions, the level of agreement between observations and calculations is typically less good. As described above, OH plays a central role in the tropospheric chemistry of most gas-phase organic and inorganic pollutants. Quantitative measurements of OH are invaluable in the validation of the chemical mechanisms and kinetic parameters incorporated within the models. Indeed, correct predictions of previously unidentified sources and sinks of OH, and of erroneous rate coefficient measurements, have resulted from the application of numerical chemical models to the interpretation of observational data sets from several field studies.
Reproduced without permission from Ehhalt DH (1999) Photooxidation of trace gases in the troposphere. Physical Chemistry Chemical Physics 1: 5401–5408.
Measurement Techniques for Field Studies of OH Radicals is therefore also oxidized in cloud droplets and precipitation, particularly via reaction with H2O2. To illustrate the wide range of compounds that OH reacts with, Table 2 lists estimated global emission rates of several important trace gases and the proportion that is removed via their reaction with an OH concentration of 1106 molecule cm3.
Comparison between Measured and Calculated OH Concentrations For remote, clean air environments, the high level of agreement typically observed between OH concentrations measured directly and those calculated from a hierarchy of numerical models of the chemical and physical sources and sinks of OH, indicate that these relatively simple systems are sufficiently well understood. However, perhaps it is not surprising that, in chemically more complex systems such as urban environments
High reactivity with a wide range of VOCs, rapid interconversion with HO2, lifetimes of the order of 0.1–1 s and highly variable, diurnal, seasonal, and spatial concentrations between 104 and 108 molecule cm3 mean that the quantitative detection of OH remains one of the most important yet difficult challenges in tropospheric chemistry. Nevertheless, several field-proven, spectroscopic measurement techniques have been developed over the last two decades. Each technique is described briefly below, together with recent applications and intercomparisons in ground-based and airborne field measurement studies of the tropospheric chemistry of OH.
DOAS, L-POAS, and MOAS DOAS (Differential Optical Absorption Spectroscopy) and L-POAS (Long-Path Optical Absorption Spectroscopy) are
Tropospheric Chemistry and Composition j Hydroxyl Radical similar to each other but distinct from MOAS (Multipass Optical Absorption Spectroscopy). The former are usually employed to measure over integrated optical path lengths of several kilometers, whereas the latter utilizes an open, wall-less White cell with a base length of 6 m to yield data that are more comparable with local, in situ measurements. Concentrations of OH are derived by applying least-squares fits of OH reference spectra to the recorded differential optical density using singular value decomposition routines. A considerable advantage of these optical spectroscopic methods for ambient measurements of OH is that calibration is achieved using data for absorption cross-sections predetermined in the laboratory. Unlike other techniques, sophisticated in situ calibration systems are not required. Detection limits of the order of 2 105 molecule cm3 are achievable for optical path lengths of 2–5 km, with measurement frequencies of approximately 1 to 5 min. There have been several recent reports of the application of DOAS/L-POAS and MOAS to measurements of OH in ground-based studies in remote, rural, and more polluted environments.
LIF and FAGE LIF (Laser-Induced Fluorescence) and FAGE (Fluorescence Assay by Gas Expansion) are essentially identical techniques. Ambient air is expanded as a continuous, supersonic free jet through a nominal 1 mm nozzle into a low-pressure detection chamber maintained at approximately 1 mbar. A laser beam at 308 nm is used to promote ground state X2P OH into the first P electronically excited state, A2 . At low pressure, the fluorescence lifetime of OH is increased and allows delayed-gated photon counting to capture the extremely weak OH LIF signal, while discriminating against the much more intense scattered light. Interferences arising from the photodissociation of O3 to O(1D) atoms followed by subsequent reaction of O(1D) with H2O to form OH radicals during the laser pulse is negligible using 308 nm excitation at low pressure. Note that HO2 may also be measured by reaction with NO in the lowpressure gas expansion chamber to form OH, which are subsequently detected as described above. Calibration requires not only measurement of the response to independently quantifiable sources of OH and HO2 coupled to the inlet system, but also determination of the efficiency of conversion of HO2 to OH. In addition to their increasing utility in photochemical reactor chambers, there have been several recent reports of the application of LIF/FAGE to measurements of OH (and HO2) in ground-based, shipboard, and airborne studies of tropospheric chemistry in a variety of remote, marine, rural, and urban environments.
CIMS and IMR-MS The development of CIMS (Chemical Ionization Mass Spectrometry) for the detection of OH in the atmosphere began in the late 1980s and 1990s. IMR-MS (Ion Molecule ReactionMass Spectrometry) is identical. In each case, OH radicals are reacted with 34SO2 in the presence of O2 to produce 34 SO3, which reacts with water vapor to form H34 2 SO4 according to reactions [XXXVII]–[XXXIX]. H34 SO is then 4 2 detected using quadrupole mass spectrometry as the highly
237
stable anion, H34 SO 4 , following H-atom abstraction by NO 3 ions (reaction [XL]). 34 H34 2 SO4 þ NO3 / HNO3 þ H SO4 5
[XL] 3
OH concentrations of less than 110 molecule cm can be measured in less than 1 min. These techniques have also found increasingly widespread application in recent groundbased and airborne studies of the sources and sinks of OH in the troposphere.
Other Methods Field measurements of ambient OH concentrations may also be made using a simple, portable and inexpensive method. This method utilizes the rapid reaction of OH with o-hydroxybenzoic acid in a buffered solution to produce 2,5-dihydroxybenzioc acid, which is quantified by reversephase high pressure liquid chromatography with fluorescence detection. In clean air, a detection limit of 3–6 105 molecule cm3 may be achieved for a sampling period of 45–90 min. Tropospheric OH concentrations may also be determined from measurements of the differential loss rates of many NMHCs, especially reactive alkenes, in well-defined, summertime urban plumes. A key assumption is that the observed hydrocarbon losses are due solely to OH radical reaction, and that plume dilution is correctly accounted for. Finally, globally averaged OH concentrations of approximately 1106 molecule cm3 may be estimated from measurements of the global budget of methyl chloroform (CH3CCl3), which is entirely of anthropogenic origin and almost exclusively removed by reaction with OH. Clearly, information about regional- and local-scale sources and sinks of OH is not provided from such estimates, but the important role of OH in tropospheric chemistry on a global scale may be deduced.
Intercomparisons of Ambient Measurements of OH Participation in intercomparison exercises represents an often undervalued but vitally important stage in the development and validation of new instrumentation and measurement techniques. Despite the undoubted importance of making ambient measurements of OH in order to understand further its central role in tropospheric chemistry, there have been relatively few intercomparison exercises carried out over the last two decades in order to compare measurement techniques. This may have been due partly to the difficult challenges posed, but this situation may change over the next decade with further evaluation of the more recently developed techniques in order to quantify their selectivity, accuracy, precision, detection limits, sensitivity, and reliability. However, those OH intercomparison exercises that have been carried out have been informative and instructive. For example, an intercomparison of airborne measurements of OH was recently carried out above the Pacific Ocean. LIF and CIMS data recorded on DC-8 and P3-B aircraft, respectively, during two brief close proximity flights in the marine boundary layer showed exceptionally good agreement. Flights at an altitude of 5.5 km resulted in an average concentration difference similar to the measurement uncertainties of each technique of 40%, but much less than the
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0.28 0.15 106 molecule cm3 was obtained from 137 data pairs. Earlier intercomparisons of ambient OH measurements using L-POAS and CIMS were carried out at Fritz Peak Observatory, Colorado, in 1991, and during the Tropospheric OH Photochemistry Experiment at Idaho Hill, Colorado, in 1993. In general, good agreement within 30% was obtained for similar chemical and meteorological conditions, which confirmed the absence of significant interferences or artefacts. A 1992 intercomparison of FAGE and an obsolete 14CO radiocarbon technique not only demonstrated good correlation between data sets, with an r value of 0.86, but also revealed a significant unresolved calibration discrepancy of a factor of 2.9. Sadly, the first OH measurement intercomparison exercise in 1983–84 demonstrated the then unreliable airborne operation and insufficient detection sensitivities of LIF and the radiocarbon technique.
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See also: Ozone Depletion and Related Topics: Photochemistry of Ozone; Stratospheric Ozone Recovery. Radiation Transfer in the Atmosphere: Ultraviolet Radiation. Stratospheric Chemistry Topics: HOx.
[OH] (LIF) (106 cm−3)
Figure 4 Bivariate plot of 137 OH data pairs measured by DOAS and LIF during the 1994 POPCORN campaign in a rural environment in northeast Germany. The correlation coefficient, r, is 0.90; the solid line represents a weighted linear fit with a gradient of 1.09 0.04 and an intercept of (0.28 0.15) 106 molecule cm3. Reproduced without permission from Hofzumahaus, A., Aschmutat, U., Brandenburger, U., et al., 1998. Intercomparison of tropospheric OH measurements by different laser techniques during the POPCORN campaign 1994. Journal of Atmospheric Chemistry 31 (1–2), 227–246.
combined uncertainties from all the contributing measurements. In each case, comparisons were made by normalizing the data with a photo-stationary state model. In addition, OH concentrations measured between 25 N to 25 S latitude over the entire longitude and altitude range of the study agreed to within 10%, although the ratio of DC-8:P-3B measurements increased at higher altitudes. These results illustrated no obvious measurement discrepancies and minimal common interferences and calibration errors for two quite distinct OH measurement techniques. An intercomparison of extensive LIF and DOAS measurements of OH radicals in a rural environment in north-east Germany during August 1994 showed excellent agreement for the same air mass, thereby demonstrating impressively high degrees of specificity, accuracy and reliability. As shown in Figure 4, a linear relationship with a correlation coefficient, r, of 0.90, a gradient of 1.09 0.04 and an intercept of
Further Reading Abram, J.P., Creasey, D.J., Heard, D.E., Lee, J.D., Pilling, M.J., 2000. Hydroxyl radical and ozone measurements in England during the solar eclipse of 11 August 1999. Geophysical Research Letters 27 (21), 3437–3440. Atkinson, R., 2000. Atmospheric chemistry of VOCs and NOx. Atmospheric Environment 34 (12–14), 2063–2101. Brauers, T., Hausmann, M., Bister, A., Kraus, A., Dorn, H.-P., 2001. OH radicals in the boundary layer of the Atlantic Ocean: 1. Measurements by long-path laser absorption spectroscopy. Journal of Geophysical Research 106 (D7), 7399–7414. Ehhalt, D.H., 1999. Photooxidation of trace gases in the troposphere. Physical Chemistry Chemical Physics 1 (24), 5401–5408. Eisele, F.L., Mount, G.H., Tanner, D., et al., 1997. Understanding the production and interconversion of the hydroxyl radical during the OH Photochemistry Experiment. Journal of Geophysical Research 102 (D5), 6457–6465. Eisele, F.L., Tanner, D.J., Cantrell, C.A., Calvert, J.G., 1996. Measurements and steady state calculations of OH concentrations at Mauna Loa observatory. Journal of Geophysical Research 101 (D9), 14 665–14 679. Finlayson-Pitts, B.J., Pitts Jr., J.N., 2000. Chemistry of the Upper and Lower Atmosphere. Theory, Experiments and Applications. Academic Press, San Diego, CA. Hofzumahaus, A., Aschmutat, U., Brandenburger, U., et al., 1998. Intercomparison of tropospheric OH measurements by different laser techniques during the POPCORN campaign 1994. Journal of Atmospheric Chemistry 31 (1–2), 227–246. Jaegle, L., Jacob, D.J., Brune, W.H., Wennberg, P.O., 2001. Chemistry of HOx radicals in the upper troposphere. Atmospheric Environment 35 (3), 469–489. Jenkin, M.E., Clemitshaw, K.C., 2000. Ozone and other secondary photochemical pollutants: chemical processes governing their formation in the planetary boundary layer. Atmospheric Environment 34 (16), 2499–2527.