A model for the photochemistry of the global troposphere constrained by observed concentrations of H2O, O3, CO, CH4, NO, NO2, and HNO3 is presented. Data for NO and NO2 are insufficient to define the global distribution of these gases but are nonetheless useful in limiting several of the more uncertain parameters of the model. Concentrations of OH, HO2, H2O2, NO, NO2, NO3, N2O5, HNO2, HO2NO2, CH3O2, CH3OOH, CH2O, and CH3CCl3 are calculated as functions of altitude, latitude, and season. Results imply that the source for nitrogen oxides in the remote troposphere is geographically dispersed and surprisingly small, less than 107 tons N yr−1. Global sources for CO and CH4 are 1.5 × 109 tons C yr−1 and 4.5 × 108 tons C yr−1, respectively. Carbon monoxide is derived from combustion of fossil fuel (15%) and oxidation of atmospheric CH4 (25%), with the balance from burning of vegetation and oxidation of biospheric hydrocarbons. Production of CO in the northern hemisphere exceeds that in the southern hemisphere by about a factor of 2. Industrial and agricultural activities provide approximately half the global source of CO. Oxidation of CO and CH4 provides sources of tropospheric O3 similar in magnitude to loss by in situ photochemistry. Observations of CH3CCl3 could offer an important check of the tropospheric model and results shown here suggest that computed concentrations of OH should be reliable within a factor of 2. A more definitive test requires better definition of release rates for CH3CCl3 and improved measurements for its distribution in the atmosphere.

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Tropospheric chemistry: A global perspective

A detailed study of the photochemistry of iodine and its oxides indicates that iodine species may play an important role in the tropospheric photochemical system. Methyl iodide, often observed in the marine troposphere with an average concentration of 5-10 ppt, is photolyzed and thereby produces I atoms. Chemical interactions with O3, HxOy, and NOx cause I to be converted to other inorganic compounds such as IO, HOI, IONO2, and I2. The production of these species and their subsequent recycling back to I can lead to the catalytic removal of tropospheric O3, the enhancement of the NO2/NO ratio, the destruction of HxOy free radicals, and the conversion of HO2 to OH. Ultimately, tropospheric inorganic iodine is removed by heterogeneous processes. Calculations using a numerical model to simulate tropospheric photochemistry indicate that iodine may have a strong impact upon the atmospheric O3-NOx-HxOy system. The magnitude of these effects is dependent upon the value of several uncertain rate constants and the primary source distributions of CH3I and other organic and inorganic iodine compounds.

Iodine: Its possible role in tropospheric photochemistry

Measurement of OH and HO2 in the Troposphere

Extrapolating from extensive field measurements on foliar emissions in the U.S. approximate global inputs of isoprene and terpenes of 3.5 times 10 to the 14th power and 4.8 times 10 to the 14th power g(C)/yr, respectively, are obtained. The oxidation of these hydrocarbons could contribute in an important way to the atmospheric sources of CO (4.2-13.3 times 10 to the 14th power g/yr) and H2 (10-35 times 10 to the 12th power g/yr), and to organic species soluble in rainwater

Estimates on the production of CO and H2 from the oxidation of hydrocarbon emissions from vegetation

The concentrations of the hydrogen radicals OH and HO2in the middle and upper troposphere were measured simultaneously with those of NO, O_3, CO, H_(2)O, CH_4, non-methane hydrocarbons, and with the ultraviolet and visible radiation field. The data allow a direct examination of the processes that produce O_3 in this region of the atmosphere. Comparison of the measured concentrations of OH and HO_2 with calculations based on their production from water vapor, ozone, and methane demonstrate that these sources are insufficient to explain the observed radical concentrations in the upper troposphere. The photolysis of carbonyl and peroxide compounds transported to this region from the lower troposphere may provide the source of HO_x required to sustain the measured abundances of these radical species. The mechanism by which NO affects the production of O_3 is also illustrated by the measurements. In the upper tropospheric air masses sampled, the production rate for ozone (determined from the measured concentrations of HO_2 and NO) is calculated to be about 1 part per billion by volume each day. This production rate is faster than previously thought and implies that anthropogenic activities that add NO to the upper troposphere, such as biomass burning and aviation, will lead to production of more O_3 than expected.

https://science.sciencemag.org/content/sci/279/5347/49.full.pdf

Hydrogen Radicals, Nitrogen Radicals, and the Production of O3 in the Upper Troposphere

Using an airborne mounted UV tunable laser system, natural tropospheric OH radical concentrations at 7 and 11.5 km are here reported for the first time. The technique employed was that of Laser Induced Fluorescence where the OH radicals were excited at 2820.6A with the fluorescence being monitored at ∼3095A. Sampling of the OH was carried out continuously using a laminar flow intake manifold in which the UV laser beam (7mm diameter) was passed through the center core of the gas stream. Measurements at 32° north latitude and 21° north latitude at 7 and 11.5 km revealed significant latitude and altitude variations of the OH radical during the month of October 1975. Details on the importance of these measurements to tropospheric chemistry are discussed.

Direct measurements of natural tropospheric levels of OH via an aircraft borne tunable dye laser

Boundary layer measurements of the OH radical in the vicinity of an isolated power plant plume: SO2 and NO2 chemical conversion times

Described in detail is a laser induced fluorescence system which has been successfully interfaced with two aircraft sampling platforms (i.e., Sabreliner jet and an L-188C Electra). This system, which has been under development for four years, presently consists of the following major components: (1) a Nd-Yag laser driven oscillator-amplifier dye laser; (2) a sampling manifold with associated fluorescence detection optics; (3) an OH calibration chamber; (4) a laser beam steering assembly; and (5) sampling electronics and data processing hardware. During the last three years, this system has been flown some 50,000 air miles making tropospheric OH radical measurements over the latitude range of 70 N to 57 S. OH concentrations measured during these flights have ranged from 30 parts-per-quadrillion (3.7 x 10 to the 5th molecules/sq cm) at altitudes of 6 km to 0.8 parts-per-trillion (2.0 x 10 to the 7th molecules/sq cm) at 0.5 km. Computations have been completed which indicate that the existing aircraft system with modest modifications should also be capable of detecting natural tropospheric levels of NO, SO2, CH2O, NO2, HNO2, NO3, H2O2 and CS2 by using both conventional laser-induced fluorescence methodology and multiphoton techniques.

Air‐borne laser induced fluorescence system for measuring OH and other trace gases in the parts‐per‐quadrillion to parts‐per‐trillion range

Theoretical calculations are presented which estimate the possible magnitude of the O3/H2O derived OH interference signal resulting from the use of the laser-induced fluorescence technique in measuring natural levels of tropospheric OH. Critical to this new assessment has been the measurement of the nascent OH quantum state distribution resulting from the reaction O(1D) + H2O yields 2OH, and an assessment of the subsequent rotational relaxation of the OH species when formed in high k levels.

W. Heaps

Papers

Direct measurements of natural tropospheric levels of OH via an aircraft borne tunable dye laser

Boundary layer measurements of the OH radical in the vicinity of an isolated power plant plume: SO2 and NO2 chemical conversion times

Air‐borne laser induced fluorescence system for measuring OH and other trace gases in the parts‐per‐quadrillion to parts‐per‐trillion range

A theoretical assessment of the O3/H2O interference problem in the detection of natural levels of OH via laser induced fluorescence