Ozone plays a very important role in our atmosphere because it protects any living organisms at the Earth's surface against the harmful solar UVB and UVC radiation. In the stratosphere, ozone plays a critical role in the energy budget because it absorbs both solar UV and terrestrial IR radiation. Further, ozone in the tropopause acts as a strong greenhouse gas, and increasing ozone trends at these altitudes contribute to climate change. This review contains a short description of the various techniques that provided atmospheric ozone measurements valuable for long-term trend analysis. The anthropogenic emissions of substances that deplete ozone (chlorine- and bromine-containing volatile gases) have increased from the 1950s until the second half of the 1980s. The most severe consequence of the anthropogenic release of ozone-depleting substances is the "Antarctic ozone hole." Long-term observations indicate that stratospheric ozone depletion in the southern winter-spring season over Antarctica started in the late 1970s, leading to a strong decrease in October total ozone means. Present values are only approximately half of those observed prior to 1970. In the Arctic, large ozone depletion was observed in winter and spring in some recent years. Satellite and ground-based measurements show no significant trends in the tropics but significant long-term decreasing trends in the northern and southern midlatitudes (of the order of 2-4% per decade in the period from 1970 to 1996 and an acceleration in trends in the 1980s). Ozone at northern midlatitudes decreased by -7.4 +/- 2% per decade at 40 km above mean sea level, while ozone loss was small at 30 km. Large trends were found in the lower stratosphere, -5.1 +/- 1.8% at 20 km and -7.3 +/- 4.6% at 15 km, where the bulk of the ozone resides. The possibility of a reduction in the observed trends has been discussed recently, but it is very hard to distinguish this from the natural variability. As a consequence of the Montreal Protocol process, the emissions of ozone-depleting substances have decreased since the late 1980s. Chlorine is no longer increasing in the stratosphere, although the total bromine amount is still increasing. Considering anthropogenic emissions of substances that deplete ozone, the turnaround in stratospheric ozone trends is expected to take place in the coming years. However, anthropogenic climate change could have a large influence on the future evolution of the Earth's ozone shield.

A discharge flow-photoionization mass spectrometric system coupled to a synchrotron is employed to study intermediates and products of sulfur radical reactions related to atmospheric chemistry. Sulfur radicals are generated from reactions of oxygen or chlorine atoms with sulfur compounds in a flow tube. The gaseous reaction products are sampled into the ionization region via a three-stage differential pumping scheme. Photoionization spectra and ionization energies are measured by dispersing synchrotron radiation to ionize the samples. Using this technique, photoionization spectra and ionization energies of HSO, CH(3)SO, C(2)H(5)SO, HSCl, and some secondary reaction products, SSCl, HSSCl, HSSSH, CH(3)SOH and CH(3)SS(O)CH(3), were measured for the first time.

Photoionization-efficiency (PIE) spectra in the wavelength range 110–140 nm were measured for products of the reaction system Cl/Cl2/H2S in a discharge-flow reactor coupled to a photoionization mass spectrometer employing a synchrotron as source of radiation. According to PIE spectra of HSCl, HSSSH, SSCl, and HSSCl, obtained for the first time, the ionization energies (IE) derived are (9.887±0.016), ⩽9.09, (9.04±0.03), and (9.266±0.014) eV, respectively. Ab initio calculations of these IE with the GAUSSIAN-2 method agree well with experimental results. Other products observed in the system include S2, HSSH, S3, and SCl2. Their PIE spectra and IE were also measured; in some cases discrepancies with previous reports are found. The formation mechanism of the observed products is discussed.

The rate coefficients for the gas-phase reactions of allyl-, tert-butyl-, cyclopentyl-, and 2-pentylperoxy radicals with NO have been measured at 297 +/- 2 K in a flow tube reactor using chemical ionization mass spectrometric (CIMS) detection of the peroxy radical. The hydrocarbon radicals were produced through the dissociation of the parent alkyl iodide in a low-power radio frequency (rf) discharge. The unimolecular decomposition of the c-pentyl radicals in the rf discharge yielded allyl radicals, The peroxy radicals were generated by reacting the hydrocarbon radicals with O-2. The rate coefficients were found to be, in units of 10(-12) cm(3) molecule(-1) s(-1), 10.5 +/- 1.8, 7.9 +/- 1.3, 10.9 +/- 1.9, and 8.0 +/- 1.4 for the reactions of NO with CH2=CHCH2O2, t-C4H9O2, C-C5H9O2, and 2-C5H11O2 radicals, respectively. The results of this study together with our previous results for nonsubstituted C-1-C-3 alkyl peroxy radicals suggest no significant trend in the rate coefficients with size and branching of the radicals, This is in contradiction to some previous studies, which found that the rate coefficients decrease with increasing radical size and complexity. Some implications of this finding for atmospheric chemistry are briefly discussed.

The photoionization efficiency (PIE) spectrum of HSO was measured in the spectral range (107–130) nm by means of a discharge flow and a photoionization mass spectrometer coupled to a synchrotron as the radiation source. HSO radicals were generated by reacting O atoms with various organothiol compounds, C2H5SH, 2-C3H7SH, or HSC2H4SH, in the flow tube. The ionization energy of HSO was determined for the first time and found to be (9.918±0.016) eV. GAUSSIAN-2 calculations predict 9.897 eV for ionization to HSO+, the singlet ground state of the molecular ion, in satisfactory agreement with the experimental result. The onset to triplet HSO+ may occur at (11.15±0.04) eV. A vibrational frequency of HSO+ of (1150±160) cm−1 was derived from the separation of steps in the PIE spectrum. The heat of formation of HSO+ was also derived and calculated to be ΔfH298∘(HSO+)=(228±5) kcal mol−1.

The photoionization efficiency (PIE) spectrum of HSSH was measured in the spectral range (110–140) nm by means of a discharge flow and a photoionization mass spectrometer coupled to a synchrotron radiation source. HSSH was generated from the self-reaction of HS radicals in the flow tube. HS radicals were produced by reacting Cl atoms with H2S. The adiabatic ionization energy of HSSH was determined to be (9.06±0.02) eV. GAUSSIAN-2 ab initio calculations predict 9.086 eV for ionization to the ground state of trans-HSSH+, in satisfactory agreement with the experimental result. The heat of formation of HSSH+, ΔfH°298(HSSH+), was derived to be (212.7±0.5) kcal mol−1.

A system to measure time-resolved Fourier-transform infrared absorption spectra of gaseous samples using a commercial step-scan spectrometer is described. To increase the signal intensity, the incident infrared light is multipassed within a White cell. Light from a photolysis laser passes through the reaction cell to initiate the reaction in the flowing gaseous sample. The variation of absorbance is obtained from the ac-coupled signal whereas phase information and a reference spectrum are from the dc-coupled signal. The system is tested by probing the temporal evolution of HCl(v) in the chain reaction of H2 and Cl2 initiated by photolysis at 355 nm. Time-resolved absorption spectra of HCl(v=0–2) were obtained with spectral resolution 0.75 cm−1 and intervals down to 5 μs. Kinetic modeling of deduced temporal profiles of HCl(v=0–2) yields rate coefficients of (1.38±0.04)×10−14 and (5.8±0.4)×10−15 cm3 molecule−1 s−1 (in which error limits represent only the uncertainty of the fit) for reactions Cl+H2→HCl(v=0)+H and Cl+H2→HCl(v=1)+H, respectively; the total rate coefficient is in agreement with previous kinetic measurements.

The rate coefficients for the gas-phase reactions of C2H5O2 and n-C3H7O2 radicals with NO have been measured over the temperature range of (201-403) K using chemical ionization mass spectrometric detection of the peroxy radical, The alkyl peroxy radicals were generated by reacting alkyl radicals with O-2, where the alkyl radicals were produced through the pyrolysis of a larger alkyl nitrite. In some cases C2H5 radicals were generated through the dissociation of iodoethane in a low-power radio Frequency discharge. The discharge source was also tested for the i-C3H7O2 + NO reaction, yielding k(298 K) = (9.1 +/- 1.5) x 10(-12) cm(3) molecule(-1) s(-1), in excellent agreement with our previous determination. The temperature dependent rate s coefficients were found to be k(T) = (2.6 +/- 0.4) x 10(-12) exp{(380 +/- 70)/T} cm(3) molecule(-1) s(-1) and = 12.9 +/- 0.5) x 10(-12) exp{(350 +/- 60)T} cm(3) molecule(-1) s(-1) for the reactions of C2H5O2 and n-C3H7O2 radicals with NO, respectively. The rate coefficients at 298 K derived from these Arrhenius expressions are k = (9.3 +/- 1.6) x 10(-12) cm(3) molecule(-1) s(-1) for C2H5O2 radicals and k = (9.4 +/- 1.6) x 10(-12) cm(3) molecule(-1) s(-1) for n-C2H5O2 radicals. (C) 1996 John Wiley & Sons, Inc.

The rate constant for the gas-phase reaction of isopropyl peroxy radicals with NO has been measured over the temperature range of 201−401 K using chemical ionization mass spectrometric detection of the peroxy radical. The temperature dependent expression for the rate constant was found to be k(T) = (2.7 ± 0.5) × 10-12 exp{(360 ± 60)/T} cm3 molecule-1 s-1 which gives a rate constant of k = (9.0 ± 1.5) × 10-12 cm3 molecule-1 s-1 at 298 K. This value is a factor 1.8−2.6 higher than previous measurements.

The reactions of the 2-hexoxy radical and the 3-hexoxy radical have been studied in a collapsible Teflon bag reactor under conditions relevant to the atmosphere. The alkoxy radicals were generated either by the photolysis of the corresponding hexyl nitrite or by the OH radical initiated photooxidation of hexane. The hexoxy radicals were chosen as model species to examine the importance of alkoxy radical isomerization versus unimolecular decomposition or reaction with oxygen. The fraction of 2-hexoxy radicals undergoing isomerization was determined directly from the analysis of the 5-hydroxyhexan-2-one product. The fraction of 3-hexoxy radicals undergoing isomerization could only be determined indirectly. The formation of products proposed to be generated from the isomerization reaction channels was observed for the first time, including 5-nitrooxyhexan-2-ol from the reaction of isomerized radicals with NO. The approximate quantitative results confirm the predicted dominance of isomerization (greater than or equal to 68%) over other reaction pathways for longer chain alkoxy radicals. The results are compared with predictions based on kinetic estimations.