Tropospheric Gas-phase Chemistry Research Articles

Influence of updated gas-phase rate constants on modeled tropospheric OH concentrations

The impact of updated rate constants for several key reactions of tropospheric gas-phase chemistry, namely the reactions of OH with NO 2 (reduced by 30%) and the reaction of HO 2 with NO (increased by 20%) are studied using the gas-phase chemical scheme RADM2. Furthermore, the earlier value of the rate constant of CH 4 with OH used in RADM2 is reduced by 30% due to recent literature. The updated rate constants influence in particular the calculated budget of HO x and O 3. For trace gas mixes in a rural environment the average calculated OH concentration increases by 20%, while HO 2 exhibits only a small decrease of 2%. The net formation of ozone is about 12% larger. The long-term impact on the globally averaged calculated OH concentration is estimated in a simple box model to be 48%, i.e. much larger than the short-term effect due to the feedback on the mixing ratios of long-lived precursors and reaction partners. The predicted global methane and ozone mixing ratios are unaffected because of compensating effects of the simultaneous change of all three rate parameters.

Time constant analysis of tropospheric gas-phase chemistry

Time constants are often used to describe and categorize the time dependence of concentrations in chemically reactive systems. Though originally deduced for linear systems (defined as those that have chemical balance equations that are linear in the concentrations), they are also useful for non-linear problems. Here, the concept is reviewed for tropospheric gas-phase chemistry which is substantially non-linear due to the majority of bimolecular reactions introducing quadratic terms into the balance equations. Application to a simplified reaction scheme reveals that, in many cases, the time constants for the locally linearized equations provide a reasonable approximation to the global (as opposed to local in the above sense) temporal behaviour. For a simple CO, NOx, O3 and HOx chemistry, the range of time constants is characterized by one group of small values below 10 min separated by a substantial gap from longer term processes with time constants above 1 day. For NOx 1 ppb, in particular around the NOx mixing ratio that maximizes the OH concentration, the analysis indicates linearly unstable motion with an exponential increase on a time scale of weeks which is, however, unimportant under real atmospheric conditions. In this region, an assignment of time constants and compounds is not possible indicating the strong coupling between all compounds.

Measurement Techniques in Gas-Phase Tropospheric Chemistry: A Selective View of the Past, Present, and Future

Measurements of trace gases and photolysis rates in the troposphere are essential for understanding photochemical smog and global environmental change. Chemical measurement techniques have progressed enormously since the first regular observations of tropospheric ozone in the 19th century. In contrast, by the 1940s spectroscopic measurements were already of a quality that would have allowed the use of modern analysis techniques to reduce interference between gases, although such techniques were not applied at the time. Today, chemical and spectroscopic techniques complement each other on a wide range of platforms. The boundaries between spectroscopic techniques will retreat as more Fourier transform spectrometers are used at visible wavelengths and as wide-band lidars are extended, and combining chemical techniques will allow detection of more trace gases with better sensitivity. Other future developments will focus on smaller, lighter instruments to take advantage of new platforms such as unmanned aircraft and to improve the effectiveness of urban sampling.

Gas-Phase Tropospheric Chemistry of Volatile Organic Compounds: 1. Alkanes and Alkenes

Literature data (through mid-1996) concerning the gas-phase reactions of alkanes and alkenes (including isoprene and monoterpenes) leading to their first generation products are reviewed and evaluated for tropospheric conditions. The recommendations of the most recent IUPAC evaluation [J. Phys. Chem. Ref. Data, 26, No. 3 (1997)] are used for the ⩽C3 organic compounds, unless more recent data necessitates reevaluation. The most recent review and evaluation of Atkinson [J. Phys. Chem. Ref. Data, Monograph 2, 1 (1994)] concerning the kinetics of the reactions of OH radicals, NO3 radicals, and O3 is also updated for these two classes of volatile organic compounds.

NASA global tropospheric experiment

A major research effort, the Global Tropospheric Experiment (GTE), has been initiated by the National Aeronautics and Space Administration (NASA) to study the chemistry of the global troposphere and its interaction with the stratosphere, land, and oceans. The project currently involves approximately 20 principal investigators from 16 different institutions and is expected to expand over the next decade. The first phase of the project is aimed at developing and validating measurement techniques for trace species in tropospheric chemical cycles. It is designed to lead toward development and implementation of a cooperative research program involving NASA, scientists sponsored by the National Science Foundation, the National Oceanic and Atmospheric Administration, other government agencies, and research institutions abroad. The goal of the GTE is to understand the chemical cyles that control the composition of the global troposphere and its changes.

Acid generation in the troposphere by gas-phase chemistry

Some of the important aspects of the problem of acid rain are presented. They are: identifying the important chemical reactions that form acids in the troposphere, the related reactions that generate and remove the reactive precursors required for these reactions, and the theoretical estimation of the rates at which acid generation is expected to occur in these reactions. Computer simulations of summertime conditions are used to evaluate the reactions. Major emphasis is on the role of gas-phase chemistry in acid development in the troposphere. The reaction sequence involves 130 chemical reactions that show the inorganic and organic chemistry of the polluted troposphere. The important gas-phase tropospheric chemistry of SO/sub 2/ and NO-NO/sub 2/ leading to acid generation are presented. Results indicate that the average rate of gas-phase oxidation of SO/sub 2/ to H/sub 2/SO/sub 4/ is in the range of 13 - 24% per 24-h period (relative humidity 50%). The theoretical conversion rates of NO/sub x/ oxidation to HONO/sub 2/ were found to be much higher than those for SO/sub 2/. The theoretical influence of the gas-phase tropospheric chemistry on the liquid-phase processes leading to acid rain is discussed. When clouds are abundant in the troposphere, the very fast in-cloudmore » conversion of SO/sub 2/ to H/sub 2/SO/sub 4/ through O/sub 3/ and H/sub 2/O/sub 2/ dominate the mechanisms of acid generation. The model calculations suggest that from 5% to 20% of the total acids in the troposphere may be composed of organic acids.« less

Measurements of extreme concentrations of tropospheric hydrogen sulfide

The ambient concentration of hydrogen sulfide, a species of significance to both gas phase tropospheric chemistry and the interior chemistry of atmospheric aerosols, has been measured over a 13-month period in a location representative of an extreme H2S environment. Peak concentrations exceeding 1 ppm (20-min average) and long-term average values of 24 and 12 ppb (for two different source configurations) were measured at the Artesia Junction, New Mexico, site: such levels exceed nearly all known ambient values by factors of 10–20. The concentration data are found to be represented (in different cases) by either Weibull or gamma distributions or by beta or log normal distributions, this finding thus contradicting previous suggestions of universal log normality in air pollution data and rendering inappropriate the attempts to predict maximum concentrations by the imposition of unverified concentration distributions on limited quantities of data. Nearly all extremely high concentrations occurred between 1800 and 600 hours and were strongly associated with minimums in the local wind turbulence spectrum. The ‘contaminant exposure diagrams’ and the concentration distributions that are presented constitute the first detailed determinations of ambient hydrogen sulfide concentrations.