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This paper explores a new approach to estimating atmospheric hydroxyl radical concentrations from regional measurements of a suite of hydrocarbons. The approach is guided by the study of a suite of synthetic tracers, i, with uniform continental sources and constant but different lifetimes of 1, 2, 5, 20, 50, and 100 days, whose global distributions are calculated from a three-dimensional chemical tracer model. With the help of the model we show that in a grid box the standard deviation σi divided by the average concentration  is a unique function of the chemical lifetime τi. In favorable cases, for instance, in surface air within a specific region sampled by the Pacific Exploratory Mission (PEM) West B campaign, that function takes a simple form: , with α = 0.48, very close to 1/2. An analogous relation is found for the alkanes, ethane through n-hexane, measured during the PEM West B campaign in the same domain, with their reaction rate constant with OH, kOH. That relation has the form , with α′ = 0.49. Using the alkenes, for example propene, which also react with O3, the dependence on k OH,i can be related to a dependence on τi. This allows us to estimate the OH concentration, 6×105cm−3, with an error of roughly a factor of 2 for this region (boundary layer, 30°N-40°N latitude, and 135°E-155°E longitude in March). This estimate is essentially based on empirical relations only and the assumption that the considered hydrocarbons have the same source distribution. As a by-product, we show that the α defined above is related to the slope in the (logarithmic) correlation plot between the mixing ratios of two trace gases with different lifetimes. We also show that the global distribution of a appears to be a useful tool to diagnose fast regional transport.

With the help of a chemical transport model we calculate the global distributions of three synthetic tracers with uniform emission rates over the continents, zero emissions over the ocean, and constant lifetimes of 5, 20, and 100 days. These distributions serve as a first approximation to those of the lower alkanes and are analyzed in detail for the region between 115°E and 155°E longitude, 0°N and 56°N latitude, within which the Pacific Exploratory Mission-West (PEM-West) A and B campaigns took place. We show that the longitudinally averaged latitude by altitude distributions of the tracers over that domain obtained from sampling the model data along the flight tracks closely represent those obtained from complete averages over the full data sets for the months March and September when the campaigns took place. We further show that for each campaign the modeled distributions closely resemble those measured for ethane and propane, although these distributions change significantly between the two seasons. Taken together, this demonstrates that the average distributions of ethane and propane measured during PEM-West A and B describe reasonably well the true pattern of these gases between 0°N and 50°N latitude and 115°E and 155°E longitude in both seasons, despite incomplete and irregular sampling of that domain. In turn, the measured distributions can be used to test the model, mainly for correct parameterization of transport.

This article reports on the increasing amount of methane in the atmosphere and its possible impact on the climate as of December 1, 1985. Methane is a colorless, odorless trace gas found throughout the Earth's atmosphere. Even at low concentrations, methane has a significant role in the overall chemistry of the atmosphere. Recently recognized increases in methane levels carry potentially important consequences that are only beginning to be understood. Research findings did not raise much interest within the scientific community because methane is not a noxious gas and was considered to be chemically inert in the troposphere. Not until the 1970s did methane attract attention when new interest arose from research on the impact of trace gases on stratospheric ozone. It was recognized that methane accounts for a very substantial portion of stratospheric hydrogen. In fact, it contributes about 50 percent to the budget of hydrogen compounds in the stratosphere. But particularly intriguing is the most recent finding of an increase in atmospheric methane of about 1 to 2 percent per year. Because methane is a gas that absorbs infrared radiation, contributing to the greenhouse effect, its increase has climatic implications. INSET: Methane: The Microbial Source.

Ambient mixing ratios of NO, NO2, and O3 were determined together with the photolysis frequency of NO2, JNO2, at a rural, agricultural site in Germany. The data were collected during the POPCORN-campaign from August 1 to August 24,1994, in a maize field 6 m above ground. The medians of the NO, NO2, and O3 mixing ratios between 10:00 and 14:00 UT were 0.25, 1.09, and 45 ppbv, respectively. The corresponding median of JNO2 was 6.0 . 10-3 s-1. NOx = NO + NO2 showed a strong diurnal variation withmaximum mixing ratios at night, suggestive of a strong local surfacesource of NO, probably by microbial activity in the soil. The estimated average emission rate was 40 ng(N) m-2 s-1 of NOx, the major part of it probably in the form of NO. The available measurements allowed the estimation of the local NOx budget. At night the budget is almost closed and the measuredNOx mixing ratios can be explained by the local source, local dry deposition of NO2, formation of NO3 and N2O5, and vertical exchange of air across thenocturnal inversion. During day-time, the local surface source of NOis not sufficient to explain the measured mixing ratios, and horizontal advection of NOx to the site must be included. The NO2/NO ratio during the morning and late afternoon is lower than predicted from the photostationary state owing to the local NO surface source, but is regularly higher during the hours around noon. For noon, August 10, 1994, the NO2/NO ratio was used to derive the momentary lower limit for the concentration of the peroxy-radicals of 2.2 109 cm-3 (86 pptv). [ABSTRACT FROM AUTHOR]

Tropospheric hydroxyl radical (OH) concentrations were measured by laser-induced fluorescence (LIF) during the POPCORN field campaign in August 1994 at a rural site in the north east of Germany. Ambient airspectra were recorded by tuning the laser wavelength over a spectralregion covering the Q11(3), Q21(3), and P11(1) rotational transitions of the (0-0) band in the A-X system of OH around 308 nm. The observed spectra clearly identify the OH radical in the atmosphere. Besides the OH absorption lines there was no sign of any other narrow-band spectral structure nearby demonstrating the high specificity of the method. For OH measurements with a typical time resolution of 60-100 seconds per data point the laser wavelength was tuned repetitively over small spectral intervals covering the peak position of the P11(1) OH-line and background positions. A total of 2300 measurements were recorded including diurnal cycles of OH with more than 300 data points. The OH as well as the LIFbackground signal data will be presented. In a first analysis the background signal will be characterized and the correlation between OH and the ozone photolysis frequency will be derived. [ABSTRACT FROM AUTHOR]