Terry J. Dillon

Atmospheric oxidation capacity sustained by a tropical forest

Terrestrial vegetation, especially tropical rain forest, releases vast quantities of volatile organic compounds (VOCs) to the atmosphere, which are removed by oxidation reactions and deposition of reaction products. The oxidation is mainly initiated by hydroxyl radicals (OH), primarily formed through the photodissociation of ozone. Previously it was thought that, in unpolluted air, biogenic VOCs deplete OH and reduce the atmospheric oxidation capacity. Conversely, in polluted air VOC oxidation leads to noxious oxidant build-up by the catalytic action of nitrogen oxides (NOx = NO + NO2). Here we report aircraft measurements of atmospheric trace gases performed over the pristine Amazon forest. Our data reveal unexpectedly high OH concentrations. We propose that natural VOC oxidation, notably of isoprene, recycles OH efficiently in low-NOx air through reactions of organic peroxy radicals. Computations with an atmospheric chemistry model and the results of laboratory experiments suggest that an OH recycling efficiency of 40–80 per cent in isoprene oxidation may be able to explain the high OH levels we observed in the field. Although further laboratory studies are necessary to explore the chemical mechanism responsible for OH recycling in more detail, our results demonstrate that the biosphere maintains a remarkable balance with the atmospheric environment.

Kinetics of the reactions of HO with methanol (210-351 K) and with ethanol (216-368 K).

Absolute rate coefficients for the reaction of hydroxyl radicals (HO) with methanol, HO + CH3OH --> products (R1), and with ethanol, HO + C2H5OH --> products (R2) were measured over a range of temperatures using pulsed laser photolytic generation of HO coupled to its time resolved detection by pulsed laser induced fluorescence. The accuracy of the rate constants obtained was enhanced by on-line optical absorption measurements of the alcohol concentration. The temperature dependence of the rate coefficients is given by: k1(210-351 K) = 6.67 x 10(-18) T2 exp(140/T) cm3 molecule(-1) s(-1) with a rate coefficient at room temperature of (9.3 +/- 0.7) x 10(-13) cm3 molecule(-1) s(-1). For k2 we obtained: k2(216-368 K) = 4.0 x 10(-12) exp(-42/T) and a room temperature rate coefficient of (3.35 +/- 0.17) x 10(-12) cm3 molecule(-1) s(-1). The total error (at 95% confidence) associated with the rate coefficients derived from the expressions describing the temperature dependence is estimated as 7% at all temperatures. The present results, which extend the database on these reactions to cover temperatures relevant for the upper troposphere, are compared to previously published measurements, and values of k1 and k2 are recommended for atmospheric modelling.

Laser induced fluorescence studies of iodine oxide chemistry

The technique of pulsed laser photolysis was coupled to laser induced fluorescence detection of iodine oxide (IO) to measure rate coefficients, k for the reactions IO + CH(3)O(2)--> products (R1, 30-318 Torr N(2)), IO + CF(3)O(2)--> products (R2, 70-80 Torr N(2)), and IO + O(3)--> OIO + O(2) (R3a). Values of k(1) = (2 +/- 1) x 10(-12) cm(3) molecule(-1) s(-1), k(2) = (3.6 +/- 0.8) x 10(-11) cm(3) molecule(-1) s(-1), and k(3a) <5 x 10(-16) cm(3) molecule(-1) s(-1) were obtained at T = 298 K. In the course of this work, the product yield of IO from the reaction of CH(3)O(2) with I was determined to be close to zero, whereas CH(3)OOI was formed efficiently at 70 Torr N(2). Similarly, no evidence was found for IO formation in the CF(3)O(2) + I reaction. An estimate of the rate coefficients k(CH(3)O(2) + I) = 2 x 10(-11) cm(3) molecule(-1) s(-1) and k(CH(3)OOI + I) = 1.5 x 10(-10) cm(3) molecule(-1) s(-1) was also obtained. The results on k(1)-k(3) are compared to the limited number of previous investigations and the implications for the chemistry of the marine boundary layer are briefly discussed.

Direct Kinetic Study of OH and O3 Formation in the Reaction of CH3C(O)O2 with HO2

The reaction between HO2 and CH3C(O)O2 has three exothermic product channels, forming OH (R3a), peracetic acid (R3b), and acetic acid plus O3 (R3c). The branching ratios of the OH- and ozone-forming reaction channels were determined using a combination of laser-induced fluorescence (LIF, for time-resolved OH concentration measurement) and transient absorption spectroscopy (TAS, for time-resolved O3 concentration measurement) following pulsed laser generation of HO2 and CH3C(O)O2 from suitable precursors. TAS was also used to determine the initial concentration of the reactant peroxy radicals. The data were evaluated by numerical simulation using kinetic models of the measured concentration profiles; a Monte Carlo approach was used to estimate the uncertainties of the rate constants (k3) and branching ratios (α) thus obtained. The reaction channel forming OH (R3a) was found to be the most important with α3a = 0.61 ± 0.09 and α3c = 0.16 ± 0.08. The overall rate coefficient of the title reaction was found to be k3 = (2.1 ± 0.4) × 10(-11) cm(3) molecule(-1) s(-1) for both HO2 and DO2. Use of DO2 resulted in an increase in α3a to 0.80 ± 0.14. Comparison with former studies shows that OH formation via (R3) has been underestimated significantly to date. Possible reasons for these discrepancies and atmospheric implications are discussed.

LIF studies of iodine oxide chemistry. Part 3. Reactions IO + NO3 --> OIO + NO2, I + NO3 --> IO + NO2, and CH2I + O2 --> (products): implications for the chemistry of the marine atmosphere at night.

The technique of pulsed laser photolysis coupled to LIF detection of IO was used to study IO + NO(3) --> OIO + NO(2); I + NO(3) --> (products); CH(2)I + O(2) --> (products); and O((3)P) + CH(2)I(2) --> IO + CH(2)I, at ambient temperature. was observed for the first time in the laboratory and a rate coefficient of k(1 a) = (9 +/- 4) x 10(-12) cm(3) molecule(-1) s(-1) obtained. For , a value of k(2) (298 K) = (1.0 +/- 0.3) x 10(-10) cm(3) molecule(-1) s(-1) was obtained, and a IO product yield close to unity determined. IO was also formed in a close-to-unity yield in , whereas in an upper limit of alpha(3)(IO) < 0.12 was derived. The implications of these results for the nighttime chemistry of the atmosphere were discussed. Box model calculations showed that efficient OIO formation in was necessary to explain field observations of large OIO/IO ratios.

Reaction of HO with hydroxyacetone (HOCH2C(O)CH3): rate coefficients (233-363 K) and mechanism

Absolute rate coefficients for the title reaction, HO + HOCH(2)C(O)CH(3)--> products (R1) were measured over the temperature range 233-363 K using the technique of pulsed laser photolytic generation of the HO radical coupled to detection by pulsed laser induced fluorescence. The rate coefficient displays a slight negative temperature dependence, which is described by: k(1)(233-363 K) = (2.15 +/- 0.30) x 10(-12) exp{(305 +/- 10)/T} cm(3) molecule(-1) s(-1), with a value of (5.95 +/- 0.50) x 10(-12) cm(3) molecule(-1) s(-1) at room temperature. The effects of the hydroxy-substituent and hydrogen bonding on the rate coefficient are discussed based on theoretical calculations. The present results, which extend the database on the title reaction to a range of temperatures, indicate that R1 is the dominant loss process for hydroxyacetone throughout the troposphere, resulting in formation of methylglyoxal at all atmospheric temperatures. As part of this work, the rate coefficient for reaction of O((3)P) with HOCH(2)C(O)CH(3) (R4) was measured at 358 K: k(4)(358 K) = (6.4 +/- 1.0) x 10(-14) cm(3) molecule(-1) s(-1) and the absorption cross section of HOCH(2)C(O)CH(3) at 184.9 nm was determined to be (5.4 +/- 0.1) x 10(-18) cm(2) molecule(-1).

Absorption cross section and photolysis of OIO

Pulsed laser photolysis combined with transient absorption spectroscopy and resonance fluorescence was used to examine the photolysis of OIO at a number of wavelengths corresponding to absorption bands in its visible spectrum between approximately 530 and 570 nm. Photolysis at 532 nm was found to result in substantial depopulation of the absorbing ground state, enabling an estimate for the absorption cross section of OIO at 610.2 nm of (6 +/- 2) x 10(-18) cm2 molecule(-1) to be obtained. No evidence was found for I atom formation following photolysis of OIO at 532, 562.3, 567.9 and 573.8 nm, enabling an upper limit to the I atom quantum yield of < 0.05 (560-580 nm) and < 0.24 (532 nm) to be established.

Reaction between OH and HCHO: temperature dependent rate coefficients (202-399 K) and product pathways (298 K)

Absolute rate coefficients for the title reaction, OH + HCHO → products (R1) were measured over the temperature range 202–399 K using the technique of pulsed laser photolytic generation of OH radical coupled to detection by pulsed laser induced fluorescence. The accuracy of the rate constants obtained was enhanced by on-line optical absorption measurements of the formaldehyde concentration. The temperature dependence of the rate coefficient is given by: k1(202–399 K) = 9.52 × 10−18T2.03 exp{636/T} cm3 molecule−1 s−1 with the rate coefficient at room temperature of 8.46 × 10−12 cm3 molecule−1 s−1. The estimated total error (95% confidence) associated with the rate coefficient derived from this expression is estimated as 5% close to 300 K, increasing to 7% at the extremes of the temperature range covered. The present results, which extend the database on this reaction to cover temperatures relevant for the upper troposphere, are compared to previously published measurements, and values of k1 for atmospheric modelling are recommended. In accord with most previous studies, we find no evidence for H atom formation in (R1).

Reaction between OH and CH3CHO

Absolute rate coefficients for the reaction OH + CH3CHO → products (1) were measured over the temperature range 201–348 K using the technique of pulsed laser photolytic generation of OH coupled to detection by pulsed laser induced fluorescence. The experiments were carried out at total pressures of 50 or 100 Torr (He or Ar bath gas). The accuracy of the rate constants obtained was enhanced by on-line optical absorption measurements of the acetaldehyde concentration. The temperature dependence of the rate coefficient, expressed in Arrhenius form, is given by k1(201–348 K) = 4.38 × 10−12 exp{(366 ± 40)/T} cm3 s−1 with the rate coefficient at room temperature of 1.53 × 10−11 cm3 s−1. The estimated total error (95% confidence) associated with the rate coefficient derived from this expression is estimated as 5%, and is independent of temperature. The present results, which extend the database on this reaction to cover temperature relevant for the upper troposphere, are compared to previously published measurements, and values of k1 for atmospheric modelling are recommended.

Reaction of O(3P) with the alkyl iodides: CF3I, CH3I, CH2I2, C2H5I, 1-C3H7I and 2-C3H7I

The kinetics of the reactions between O(3P) and various alkyl iodides (RI) was investigated using the pulsed laser photolysis–resonance fluorescence (PLP–RF) technique at temperatures between 223 and 363 K. The reactions studied were O(3P)+CF3I (1), O(3P)+CH3I (2), O(3P)+CH2I2 (3), O(3P)+C2H5I (4), O(3P)+1-C3H7I (5), and O(3P)+2-C3H7I (6). The Arrhenius expressions (units of cm3 molecule−1 s−1) obtained were: k1(223–363 K)=(8.73±1.29)×10−12exp{−(216±38)/T}, k2(223–363 K)=(9.88±1.67)×10−12exp{(183±50)/T}, k3 (256–363 K)=(7.36±0.31)×10−11 cm3 molecule−1 s−1, k4(223–363 K)=(1.58±0.22)×10−11exp{(239±39)/T} k5(223–363 K)=(1.10±0.17)×10−11exp{(367±42)/T}, and k6(223–363 K)=(1.84±0.31)×10−11exp{(296±46)/T}. With the exception of k1 and k3, all the other rate coefficients display a negative temperature dependence, which highlights the importance of association complex formation in reactions of O(3P)+RI.