100 Years of Progress in Gas-Phase Atmospheric Chemistry Research

Remarkable progress has occurred over the last 100 years in our understanding of atmospheric chemical composition, stratospheric and tropospheric chemistry, urban air pollution, acid rain, and the formation of airborne particles from gas-phase chemistry. Much of this progress was associated with the developing understanding of the formation and role of ozone and of the oxides of nitrogen, NO and NO2, in the stratosphere and troposphere. The chemistry of the stratosphere, emerging from the pioneering work of Chapman in 1931, was followed by the discovery of catalytic ozone cycles, ozone destruction by chlorofluorocarbons, and the polar ozone holes, work honored by the 1995 Nobel Prize in Chemistry awarded to Crutzen, Rowland, and Molina. Foundations for the modern understanding of tropospheric chemistry were laid in the 1950s and 1960s, stimulated by the eye-stinging smog in Los Angeles. The importance of the hydroxyl (OH) radical and its relationship to the oxides of nitrogen (NO and NO2) emerged. The chemical processes leading to acid rain were elucidated. The atmosphere contains an immense number of gas-phase organic compounds, a result of emissions from plants and animals, natural and anthropogenic combustion processes, emissions from oceans, and from the atmospheric oxidation of organics emitted into the atmosphere. Organic atmospheric particulate matter arises largely as gas-phase organic compounds undergo oxidation to yield low-volatility products that condense into the particle phase. A hundred years ago, quantitative theories of chemical reaction rates were nonexistent. Today, comprehensive computer codes are available for performing detailed calculations of chemical reaction rates and mechanisms for atmospheric reactions. Understanding the future role of atmospheric chemistry in climate change and, in turn, the impact of climate change on atmospheric chemistry, will be critical to developing effective policies to protect the planet.

Free Radicals Within the Antarctic Vortex: The Role of CFCs in Antarctic Ozone Loss

How strong is the case linking global release of chlorofluorocarbons to episodic disappearance of ozone from the Antarctic stratosphere each austral spring? Three lines of evidence defining a link are (i) observed containment in the vortex of ClO concentrations two orders of magnitude greater than normal levels; (ii) in situ observations obtained during ten high-altitude aircraft flights into the vortex as the ozone hole was forming that show a decrease in ozone concentrations as ClO concentrations increased; and (iii) a comparison between observed ozone loss rates and those predicted with the use of absolute concentrations of ClO and BrO, the rate-limiting radicals in an array of proposed catalytic cycles. Recent advances in our understanding of the kinetics, photochemistry, and structural details of key intermediates in these catalytic cycles as well as an improved absolute calibration for ClO and BrO concentrations at the temperatures and pressures encountered in the lower antarctic stratosphere have been essential for defining the link.

Changes in stratospheric ozone

The ozone layer in the upper atmosphere is a natural feature of the earth's environment. It performs several important functions, including shielding the earth from damaging solar ultraviolet radiation. Far from being static, ozone concentrations rise and fall under the forces of photochemical production, catalytic chemical destruction, and fluid dynamical transport. Human activities are projected to deplete substantially stratospheric ozone through anthropogenic increases in the global concentrations of key atmospheric chemicals. Human-induced perturbations may be occurring already.

SAOZ measurements of NO2 at Aberystwyth

We present in this paper fifteen years' measurements, from March 1991 to September 2005, of stratospheric NO2 vertical columns measured by a SAOZ zenith-sky visible spectrometer. The instrument spent most of its time at Aberystwyth, Wales, with occasional excursions to other locations. The data have been analysed with the WinDOAS analysis program with low-temperature high-resolution NO2 cross-sections and fitting a slit function to each spectrum. Because of a change in detector in May 1998 there is some uncertainty about the relative changes before and after this date, which are partially constrained by the results of an intercomparison exercise. However, the effect of the Mt Pinatubo aerosol cloud is very evident in the data from 1991-94, with a decrease of 10% in NO2 in the summer of 1992 (the SAOZ was located in Lerwick, Scotland during the winter of 1991-92 and observed very low NO2 values but these cannot be directly compared to the Aberystwyth data). To focus more on interannual and long-term variations in NO2, a seasonal variation comprising an annual and semi-annual component was fitted to the morning and evening twilight separately from 1995 to the present. This fit yielded average NO2 columns of 4.08 x 10(15) cm(-2) and 2.68 x 10(15) cm(-2) for the evening and morning twilight, respectively, with a corresponding annual amplitude of +/-2.08 x 10(15) cm(-2) and +/-1.50 x 10(15) cm(-2). Departures from the fitted curve show a trend of 6% per decade, consistent with that reported elsewhere, for the period 1998-2003, but in the past two years a distinct interannual variation of amplitude of approximately 8% has emerged.

Absolute cross sections of ozone at atmospheric temperatures for the Wulf and the Chappuis bands

Ozone absorption constitutes a variable background against which measurement of pollution is observed from satellites by the solar occultation technique. The temperature-altitude gradient of ozone spans the range 180-340 K. Laboratory measurements of ozone absorption at temperatures across this range are needed to calibrate spectroscopic remote sensing, but have yielded results in substantial disagreement with each other. This paper presents the first measurements of the variation of absorption cross-sections of ozone at temperatures from 144 to 300 K over the 9250-18 500 cm(-1) spectral region.

Interpolation errors in UV-visible spectroscopy for stratospheric sensing: implications for sensitivity, spectral resolution, and spectral range

UV-visible measurements of stratospheric constituents require the ratio of a pair of spectra to be determined. If their wavelength calibrations differ and if an array detector is used, at least one spectrum must be interpolated. This introduces error if the spectrum is undersampled; the error is smaller if wavelength stability is good. Increasing the sampling ratio by making the spectral resolution poorer reduces the optical depths of absorption by constituents. Exact values of interpolation errors from real spectra are a difficult topic, but with a theoretical study with a simulated spectrum we show that the sampling ratio should exceed ~4.5 pixels/FWHM but need not exceed 6.5 pixels/FWHM. To avoid significant reduction in the optical depth of NO(2), the resolution should be smaller than ~1.0 nm FWHM. Hence a spectrometer system that measures both OClO and NO(3) by observing one order from one stationary grating should have more than ~1500 pixels, more than many currently available array detectors.

Differential optical absorption spectrometer for measurement of tropospheric pollutants

Our institute has recently developed a differential optical absorption spectrometry system called the gas analyzer spectrometer correlating optical absorption differences (GASCOAD), which features as a detector a linear image sensor that uses an artificial light source for long-path tropospheric-pollution monitoring. The GASCOAD, its method of eliminating interference from background sky light, and subsequent spectral analysis are reported and discussed. The spectrometer was used from 7 to 22 February 1993 in Milan, a heavily polluted metropolitan area, to measure the concentrations of SO(2), NO(2), O(3), and HNO(2) averaged over a 1.7-km horizontal light path. The findings are reported and briefly discussed.

Using stars for remote sensing of the Earth's stratosphere

A new UV-visible spectrometer system that measures the absorption of light from stars and planets by constituents in the Earth's atmosphere is described. Because it can be used to make measurements at night, the system has a significant advantage for measuring polar constituents in winter, when conditions that might give rise to ozone loss are initiated. Other advantages arise from the use of a cooled two-dimensional CCD array as the detector: an array detector avoids spectral noise resulting from scintillation of stars or from clouds passing overhead and allows for the possibility of measuring several constituents simultaneously; its second dimension permits auroral light from the atmosphere adjacent to the star to be measured simultaneously and subtracted from the stellar light, and a modern low-noise CCD allows us to use a telescope of modest diameter. The few previous measurements of constituents made by the use of stellar absorption did not have these advantages. The instrument was configured for simplicity and ease of use in field measurements and was deployed outside in winter in Northern Sweden in 1991. Examples of ozone measurements are shown.

Interference effects in reticon photodiode array detectors

A detector system incorporating the Reticon RL1024S photodiode array has been constructed at the National Oceanic and Atmospheric Administration Aeronomy Laboratory as part of a double spectrograph to be used to study the Earth's atmosphere from ground-based and aircraft-based platforms. To determine accurately the abundances of atmospheric trace gases, this new system must be able to measure spectral absorptions as small as 0.02%. The detector, manufactured by EG&G Reticon, exhibits superior signal-to-noise characteristics at the light levels characteristic of scattered skylights, but interference in the passivating layer (a thin layer of SiO(2) that is deposited during the manufacture to protect the silicon active area from water vapor) causes major problems in achieving the required precision. The mechanism of the problems and the solution we have implemented are described in detail.

Instrumental Requirements for Global Atmospheric Chemistry

The field of atmospheric chemistry is data-limited, primarily because of the challenge of measuring the key chemical constituents in the global environment. Several recent advances, however, in rugged, portable, remotesensing, ground-based instrumentation and accurate, fast-response airborne instrumentation have provided powerful tools for the understanding of stratospheric ozone, particularly in polar regions. Current discoveries of the role of heterogeneous chemical processes point to the need for better techniques for characterization of stratospheric aerosols. In the troposphere, advances in in situ, sensitive methods for detecting reactive nitrogen compounds have demonstrated the role that these compounds have in controlling global oxidation processes, but better measurements of the reservoir species by which the long-ranged transport of pollutant-reactive nitrogen compounds is thought to occur are urgently needed. The role of hydrocarbons, particularly those of natural origin, in ozone formation in rural areas has focused attention on the requirement for better speciation of these ubiquitous compounds. Lastly, rigorous instrument intercomparison experiments have provided unbiased estimates of measurement capabilities.