The amount of organic chlorine and bromine entering the stratosphere have a direct influence on the magnitude of chlorine and bromine catalyzed ozone losses. Twelve organic chlorine species and five organic bromine species were measured from 12 samples collected near the tropopause between 23.8°N and 25.3°N during AASE II. The average mixing ratios of total organic chlorine and total organic bromine were 3.50 ± 0.06 ppbv and 21.1 ± 0.8 pptv, respectively. CH3Cl represented 15.1% of the total organic chlorine, with CFC 11 (CCl3F) and CFC 12 (CCl2F2) accounting for 22.6% and 28.2%, respectively, with the remaining 34.1% primarily from CCl4, CH3CCl3, and CFC 113 (CCl2FCClF2). CH3Br represented 54% of the total organic bromine. The 95% confidence intervals of the mixing ratios of all but four of the individual compounds were within the range observed in low and mid‐latitude mid‐troposphere samples. The four compounds with significantly lower mixing ratios at the tropopause were CHCl3, CH2Cl2, CH2Br2, and CH3CCl3. The lower mixing ratios may be due to entrainment of southern hemisphere air during vertical transport in the tropical region and/or to exchange of air across the tropopause between the lower stratosphere and upper troposphere.

Abstract. The Nonmethane Hydrocarbon Intercomparison Experiment (NOMHICE) was designed to evaluate current analytical methods used to determine mixing ratios of atmospheric nonmethane hydrocarbons (NMHCs). A series of planned experiments, or tasks, were implemented to test the analytical methods in a graduated fashion. Tasks 1 and 2 involved relatively simple standard gas mixtures prepared by the National Institute for Standards and Technology (NIST). Results are presented here for task 3 in which a complex mixture containing 60 commonly observed NMHCs at concentrations of 1-30 parts per billion by volume (ppbv) in nitrogen diluent gas was prepared and distributed for analysis to 29 participating laboratories throughout the world. Reference mixing ratios were determined jointly by scientists from the National Center for Atmospheric Research (NCAR) and from the U.S. Environmental Protection Agency (EPA). Participants were asked to identify and quantify the hydrocarbons present in the •nixture and submit their results to NCAR-NOMHICE scientists. The results were encouraging overall. Some laboratories performed extremely well during this exercise whereas other laboratories experienced problems in either identification or quantification or both. It is e,,ident from the comparison of the NCAR-NOMHICE results with both the EPA analysis and the top 11 analyses in the study that very good agreement is achievable between laboratories for mixtures in this concentration range. Some of the largest analytical discrepancies were from laboratories that used in-house standards for their calibration and/or syringe sample injection techniques. A major conclusion from this study is that the use of high-quality gas phase standards, introduced into the measurement instrument in a similar manner to air samples, is an important prerequisite for an accurate analysis.

Aircraft sampling has provided extensive in situ and flask measurements of organic chlorine species in the lower stratosphere. The recent Airborne Arctic Stratospheric Expedition II (AASE II) included two independent measurements of organic chlorine species using whole air sample and real‐time techniques. From the whole air sample measurements we derive directly the burden of total organic chlorine (CCly) in the lower stratosphere. From the more limited real‐time measurements we estimate the CCly burden using mixing ratios and growth rates of the principal CCly species in the troposphere in conjunction with results from a two‐dimensional photochemical model. Since stratospheric chlorine is tropospheric in origin and tropospheric mixing ratios are increasing, it is necessary to establish the average age of a stratospheric air parcel to assess its total chlorine (ClTotal) abundance. Total inorganic chlorine (Cly) in the parcel is then estimated by the simple difference, Cly = ClTotal ‐ CCly. The consistency of the results from these two quite different techniques suggests that we can determine the CCly and Cly in the lower stratosphere with confidence. Such estimates of organic and inorganic chlorine are crucial in evaluating the photochemistry controlling chlorine partitioning and hence ozone loss processes in the lower stratosphere.

Measurements of peroxy radical concentrations ([HO2] + [RO2] ) were made by the "chemical amplifier" technique during the four intensives of the Mauna Loa Observatory Photochemistry Experiment 2 (MLOPEX 2) at the Mauna Loa Observatory in 1991-1992. In this study these data are compared with the theoretical values of the peroxy radical concentrations obtained from steady state analysis of the complete suite of trace gas measurements and other relevant parameters also measured during the experiment. The data from 33 days of the study contain time overlap of the concentration and physical data which allow a meaningful theoretical treatment. The experimental results for [HO2] + [RO2] agree well with theory for many of the days, but are significantly suppressed from the theoretical expectations on other days. Two hypotheses are presented and tested to explain the observed suppression. The first involves the reaction of the peroxy radicals at aerosol surfaces. The second proposes the loss of [OH] through its reaction with unknown and undetected species to develop peroxy radicals subsequently to which the "chemical amplifier" is insensitive. Evidence at hand does not allow a clear choice between these or possible alternative explanations. The data suggest that the net rate of 0 3 generation in the 1 free troposphere is about-1.5 parts per billion by volume per day (ppbv d-; 24-hour average). CH30 2. At sites in highly forested areas and in polluted regions, a large variety of organic peroxy radicals is expected since many reactive hydrocarbons in addition to CH 4 react with the OH radical to form different RO 2 species. One of the important aspects of the peroxy radicals in atmospheric chemistry is their ability to oxidize NO to NO 2 with subsequent ozone generation: HO 2 + NO --> HO + NO 2 RO 2 + NO --> RO + NO 2 NO 2 + h v --> O(3p) + NO O(3p) + 02 (+M) --> 03 (+M) Paper number 95JD03613. 0148-0227/96/95 JD-03613509.00 HO 2 + 03 --> HO + 202 HO + 03 --> HO 2 + 02 The relative importance of the 03 formation, stratospherictropospheric transport, and chemical and physical removal pathways determines whether 03 builds or is depleted in the troposphere. , The HO 2 and RO 2 radicals are also important as major sources of peroxides in the atmosphere (H202, CH302H, etc.). Studies made during the Mauna Loa Observatory Photochemistry Experiment 1 (MLOPEX 1) have been used to estimate the rate of ozone production in the free troposphere as about-0.5 parts per billion by volume per day (ppbv d-i; 24hour average) for upslope air and -0.9 ppbv d -1 for free tropospheric air [Ridley et al., 1992]. Although most of the important trace gases were measured during these studies, several key elements for the calculation had to be estimated from theory or inferred from indirect estimates. Thus the most important loss rate for ozone is ozone photodissociation into O(1D); this was not measured but was estimated from theory. In addition, the concentrations of HO 2, CH302 , and OH could not be measured but were estimated from photostationary ...

The in situ photolysis rate coefficient of 0 3 to O(1D) has been measured at Mauna Loa Observatory using a new actinometric instrument based on the reaction of O(1D) with N20 and with a hemispherical radiometer. One minute averaged photolysis rate coefficients were determined with an overall uncertainty of approximately + 11% at the 1 c• level for the actinometer and + 15% at the 1 c• level for the radiometer. Over 120 days of data were collected with varying cloud cover, aerosol loadings, and overhead ozone representing the first set of long term measurements. Clear sky solar noon values vary between approximately 3.0 x 10 -5 and 4.5 x 10 -5 sec-1. Modeling of the photolysis rate coefficients was done using a discrete ordinate radiative transfer scheme and results were compared with the actinometric measurements. The quantum yields for O(1D) production are reevaluated from existing data and reported here. The comparisons were done using the quantum yields for the photolysis of ozone recommended by DeMore et al. [ 1994], the newer evaluation of Michelsen et al. [ 1994], and also with reevaluated values in this paper. An analysis of the measured photolysis rate coefficient of 0 3 to O(1D) and model simulations of the photolysis rate coefficient data from clear days during the study provides added insight into the choice of quantum yield data for use in photochemical models of the troposphere. where jO3 is the photolysis rate coefficient of 03 --> O(•D), F is the zenith angle dependent solar actinic flux, 0 is the Copyright 1996 by the American Geophysical Union. Paper number 96JD00211. 0148-0227/96/96JD-00211 $09.00 absorption cross section of ozone as a function of wavelength and temperature, and q• is the photodissociation quantum yield of O(1D) as a function of wavelength and temperature. Theoretical calculations of jO3 are subject to uncertainties in the absorption cross section and quantum yield. In addition, atmospheric aerosols and clouds can diffuse, reflect, and attenuate actinic radiation adding uncertainty to the calculations of the radiation field. Madronich [ 1987] concluded that because of the intricate and relatively untested nature of radiative transfer calculations, the total uncertainties in theoretical fiqO2 values could be substantially higher than the errors introduced by the cross section and quantum yield data alone. Calculations for jO3 could be subject to large errors in particular due to recent conflicting recommendations in the quantum yield data. Measurements of the photolysis rate coefficient of ozone to O(•D) have been made by a number of investigators using a variety of techniques [Miiller et al., 1995; H. Cotte et al., A spectroradiometric method for the determination of the photodissociation rates of NO 2, 0 3, and other atmospheric molecules and comparison with a chemical actinometer, Journal of Atmospheric Chemistry, submitted, 1995; Hofzumahaus et al., 1992; Blackburn et al., 1992; Bairai and Stedman, 1992; Junkermann et al., 1989; Dickerson et al., 1982; Dickerson et al., 1979;...