Average OH Concentration Research Articles

Measurements of C6-C8 hydrocarbons at a UK rural site during January 1999. Site evaluation and correlations between species.

Ambient concentrations of C6-C8 aromatic hydrocarbons and n-heptane, determined by gas chromatography with flame ionisation detection, are presented from a winter campaign during January 1999 at a rural site near Leeds. Absolute concentrations are significantly lower than those obtained from the only designated UK rural site (Harwell) in the automated UK hydrocarbon network. Both absolute and relative concentrations of hydrocarbons measured at the site have been interpreted in terms of the arriving back-trajectories. The site is subject to two main airflows during the winter months; relatively polluted air from the southwest and much cleaner air from the northwest. Ratios of hydrocarbon concentrations show evidence of significant chemical processing consistent with chemical removal by OH. Uncertainties in the ages of the trajectories prevent a reliable estimation of the average OH concentration over the trajectory. The dependence of the variance of the hydrocarbon concentrations with their lifetime with respect to removal by OH does not show the expected behaviour.

Chemical evolution of the Sacramento urban plume: Transport and oxidation

Measurements of anthropogenic hydrocarbons, ozone, and meteorological variables were obtained during the summer of 1997 near the University of California Blodgett Forest Research Station on the western slope of the Sierra Nevada Mountains. These measurements and related observations obtained upwind in Sacramento and Folsom, California, by the California Air Resources Board and the National Weather Service demonstrate that the transport of the Sacramento plume is controlled by extremely consistent, terrain‐driven winds that draw polluted air into the Sierra Nevada by day and flush the mountains at night with clean, continental background air. In effect the plume serves as a mesoscale (100 km) daytime flow reactor where the daily evolution of the Sacramento urban plume can be characterized as a Lagrangian air parcel transported from the urban core into the sparsely populated Sierra Nevada mountains. Using observations of slowly reacting anthropogenic hydrocarbons, we demonstrate that at the peak impact of the Sacramento plume the air at Blodgett Forest can be characterized as a mixture of 32% air from the urban core and 68% from the relatively clean background. From measurements of more reactive hydrocarbons we infer an average daytime OH concentration of 1.1 × 107 molecules cm−3 during the transit of the urban plume.

Behavior of OH and HO2 radicals during the Observations at a Remote Island of Okinawa (ORION99) field campaign: 1. Observation using a laser‐induced fluorescence instrument

The hydroxyl (OH) and hydroperoxyl (HO2) radicals were measured by a laser‐induced fluorescence instrument at Cape Hedo, Okinawa Island, Japan, in summer 1999 during the Observations at a Remote Island of Okinawa (ORION99) field campaign. The field deployment of the instrument and its calibrations are described in detail. From the frequent calibrations during the field campaign, it was shown that the instrument, utilizing an optical fiber to transmit the laser light to the detection cell located on a tower, had a sufficiently stable sensitivity to OH and HO2 in order to trace their diurnal and day‐to‐day variations. The detection limit of the instrument was typically around 4 × 106 radicals cm−3 with an integration time of 1 min. We could not examine fast OH variations. However, hourly averaged OH concentrations during daytime were statistically significant. The HO2 concentrations were higher and the detailed variations were detected. On average, OH and HO2 showed daytime maxima of around 4 × 106 radicals cm−3 and of around 17 pptv, respectively. The median, tenth, and ninetieth percentiles of the measured daytime HO2/OH concentration ratio were 76, 32, and 143, respectively. The power law dependence of HO2 on J(O1D) was about 0.5 when the NO concentration was lower than 300 pptv and was about 1 when the NO concentration was higher than 1 ppbv, which was consistent with the known radical chemistry in the lower troposphere.

The hydroxyl radical reaction rate constant and products of cyclohexanol

The gas phase reaction of the hydroxyl radical (OH) with cyclohexanol (COL) has been studied. The rate coefficient was determined to be (19.0 ± 4.8) × 10−12 cm3 molecule−1 s−1 (at 297 ± 3 K and 1 atmosphere total pressure) using the relative rate technique with pentanal, decane, and tridecane as the reference compounds. Assuming an average OH concentration of 1 × 106 molecules cm−3, an atmospheric lifetime of 15 h is calculated for cyclohexanol. Products of the OH + COL reaction were determined to more clearly define COL's atmospheric degradation mechanism. The observed products and their formation yields were: cyclohexanone (0.55 ± 0.06), hexanedial (0.32 ± 0.15), 3-hydroxycyclohexanone (0.31 ± 0.14), and 4-hydroxycyclohexanone (0.08 ± 0.04). Consideration of the potential reaction pathways suggests that each of these products is formed via hydrogen abstraction at a different site on the COL ring. The products and their relative amounts are discussed in light of the predicted yields for each reaction channel. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 108–117, 2001

The hydroxyl radical reaction rate constant and products of cyclohexanol

The gas phase reaction of the hydroxyl radical (OH) with cyclohexanol (COL) has been studied. The rate coefficient was determined to be (19.0 ± 4.8) × 10−12 cm3 molecule−1 s−1 (at 297 ± 3 K and 1 atmosphere total pressure) using the relative rate technique with pentanal, decane, and tridecane as the reference compounds. Assuming an average OH concentration of 1 × 106 molecules cm−3, an atmospheric lifetime of 15 h is calculated for cyclohexanol. Products of the OH + COL reaction were determined to more clearly define COL's atmospheric degradation mechanism. The observed products and their formation yields were: cyclohexanone (0.55 ± 0.06), hexanedial (0.32 ± 0.15), 3-hydroxycyclohexanone (0.31 ± 0.14), and 4-hydroxycyclohexanone (0.08 ± 0.04). Consideration of the potential reaction pathways suggests that each of these products is formed via hydrogen abstraction at a different site on the COL ring. The products and their relative amounts are discussed in light of the predicted yields for each reaction channel. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 108–117, 2001

Use of a bench‐top photochemical reactor and solid‐phase microextraction to measure semivolatile organic compound‐hydroxyl radical rate constants

AbstractIt is increasingly important to be able to measure semivolatile organic compound‐hydroxyl (SOC‐OH) radical rate constants and estimate semivolatile organic compounds' (SOCs) atmospheric half‐lives because of potential for atmospheric long‐range transport. We have used a bench‐top photochemical reactor, along with solid‐phase microextraction (SPME) and ethyl nitrite, to successfully measure the rate constants of naphthalene, linalool, biphenyl, and phenanthrene with hydroxyl (OH) radical. Biphenyl and phenanthrene underwent wall loss in the reactor. The wall loss rates were determined and were used to correct the measured gas‐phase rate constants. The reaction rate constants for naphthalene, linalool, biphenyl, and phenanthrene with OH radical, in our bench‐top system at 295 ± 3 K, were determined to be 2.73 ± 0.37 × 10−11, 1.93 ± 0.24 × 10−10, 7.44 ± 1.9 × 10−12, 1.73 ± 0.21 × 10−11 (cm3/molecule/s), respectively, and were in excellent agreement with previous studies and model predictions. Based on the range of experimental and predicted rate constants for these reactants and an estimated average OH concentration in the atmosphere, the atmospheric half‐lives of these SOCs are significantly less than 2 d. This indicates that the global presence of these compounds in the atmosphere is primarily due to regional sources and not to atmospheric long‐range transport. This study shows that bench‐top reactors, combined with corrections for reactant wall loss and simplified analytical tools (such as solid‐phase microextraction), can be used to measure SOC‐OH rate constants.

Atmospheric 14CO: A tracer of OH concentration and mixing rates

Time series measurements of the ground level 14CO concentration were made at Olympic Peninsula, Washington (48°N), and Point Barrow, Alaska (71°N), between 1991 and 1997. Measurements of the meridional gradients of the 14CO concentration at sea level were made during five oceanographic cruises in the Pacific Ocean between 55°N and 65°S during 1991–1995. These measurements were combined with earlier time series measurements of atmospheric 14CO at 41°S and 77°S [Brenninkmeijer, 1993] and at 13°N [Mak and Southon, 1998] and meridional transects of 14CO at 6–8 km [Mak et al., 1994]. These 14CO data sets were analyzed using a two‐dimensional atmospheric circulation and chemistry model in order to determine the tropospheric OH concentration that could explain the temporal and spatial trends in 14CO. Additionally, the interannual trend in tropospheric methyl chloroform concentration and the stratospheric time history of bomb 14CO2 were simulated by the model. The results of this analysis indicate that an average tropospheric OH concentration of ∼l0×l05 radicals cm−3 explains both the 14CO and methyl chloroform trends. The model‐predicted 14CO concentrations, however, are sensitive to the rate of stratosphere‐troposphere exchange and horizontal mixing in the troposphere. Model predictions of tropospheric 14CO at high latitudes improved when the stratosphere‐troposphere exchange rate was slowed, based on the results of the stratospheric bomb 14CO2 model simulation. Substantial improvement in the model 14CO simulations occurred with increased horizontal diffusion rates in the troposphere and lower cosmogenic 14CO production rates. Significantly lower 14CO concentrations (∼50%) are observed in the Southern versus Northern Hemisphere. Model simulations indicate that either higher tropospheric horizontal mixing or higher OH concentrations in the Southern Hemisphere can explain the hemispheric asymmetry in 14CO.

Changing methane lifetime: Possible cause for reduced growth

We present a 3‐D model study, where the focus is on CH4 lifetime and variations related to changes in the global distribution of OH. The observed CH4 concentration and estimates of the anthropogenic emissions of CO, NOx and NMHC's, for the period 1980–96 is used to calculate changes in OH and in CH4 lifetime. The model suggests that CH4 lifetime has decreased by 0.49%/yr, while the annual global average OH concentration has increased by 0.43%/yr. Combining the calculated change in CH4 lifetime and the observed CH4 concentration the yearly average CH4 emission from 1980–96 is calculated to increase by 0.67%/yr. A moderate decrease in CH4 emissions is obtained towards the end of the period, 0.53%/yr from 1993–96 compared to 0.69%/yr from 1980–90. The key to the changing oxidation of CH4 is the increasing anthropogenic emissions in SE‐Asia, which more than compensate for the declining emissions seen over Europe and N‐America.

Past and future changes in global tropospheric ozone: Impact on radiative forcing

Calculations by a global three‐dimensional chemical transport model of the atmosphere suggest that increased surface emissions of chemical compounds caused by industrial activities at mid‐latitudes in the northern hemisphere and by biomass burning in the tropics since the middle of the 19th century have produced an increase in the abundance of tropospheric ozone along with a reduction in the oxidizing capacity of the atmosphere (globally averaged OH concentration reduced by 17% and methane lifetime enhanced by 1.5 years). These perturbations in tropospheric ozone result in a change in annually averaged radiative forcing of 0.37 W m−2 (0.62 W m−2 in the northern hemisphere during the summer months). Future changes (1990–2050) in tropospheric ozone associated with population increase and economic development (primarily in developing countries) are expected to be largest in the tropics, specifically in South and Southeast Asia. Further changes in the oxidizing capacity of the atmosphere could be small if the abundance of tropospheric water vapor increases as a result of anticipated climate change.

Measurements of dimethyl sulfide, dimethyl sulfoxide, dimethyl sulfone, and aerosol ions at Palmer Station, Antarctica

In January and February 1994, measurements of dimethylsulfide (DMS) in air and seawater were conducted at Palmer Station, Antarctica, during project SCATE (Sulfur Chemistry in the Antarctic Troposphere Experiment). Corresponding values ranged between 6 and 595 pptv (median: 94 pptv) and 0.7 and 3.7 nM (median: 2 nM), respectively. Atmospheric DMS levels were significantly enhanced during a storm episode in connection with large‐scale low‐pressure systems passing through the study area. DMS sea‐to‐air fluxes ranged between 0.03 and 19 (median: 1.1) μmol m−2 d−1 based on seawater DMS and wind speed measurements. The atmospheric DMS lifetime is estimated to be 9 days based on a 24‐hour averaged OH concentration of 1.1 × 105 cm−3. Atmospheric dimethylsulfoxide (DMSO) and dimethylsulfone (DMSO2) mixing ratios varied mostly between <0.2 and 15 pptv. These variations consisted of many short‐term spikes which may be explained by downward mixing of these compounds from the free troposphere [Davis et al., this issue]. The DMSO photochemical lifetime with respect to the 24‐hour average OH concentration is approximately 25 hours in the coastal Antarctic troposphere in summer. However, present calculations show that the dominant sink of both DMSO and DMSO2 could be surface loss, yielding atmospheric lifetimes of a few hours for both compounds [see also Davis et al., this issue]. A good correlation was found between DMSO and DMSO2. Aerosol MSA and excess sulfate concentrations also showed a good correlation, with a slope of 0.73. However, DMS was not correlated with any of its measured oxidation products, probably because of its long atmospheric residence time over Antarctica.

Measurements of the Hydroxyl Radical in a Manipulated but Realistic Indoor Environment

Using an indirect technique to obtain a time-integrated signal, we have detected and quantified the hydroxyl radical (OH) in a commercial building. For the purpose of the measurements, the conditions in the office setting were manipulated, but they were representative of conditions that occur naturally. During monitoring periods when the concentrations of both ozone (O3) and d-limonene were elevated and the air exchange rate moderate, we found the average indoor OH concentration to be approximately 7 × 105 molecule/cm3. This value is lower than typical outdoor midday values (∼5 × 106 molecule/cm3) but larger than outdoor nighttime values. The results confirm that reactions among O3 and olefins can generate meaningful quantities of OH under conditions that commonly occur indoors. In turn, OH initiates a complex series of reactions that generate still other radicals and ultimately produce species that can adversely affect human health and artifacts in indoor environments.

Is the amplitude of the methane seasonal cycle changing?

Twelve years of atmospheric CH 4 data from a globally distributed set of sampling sites ranging in latitude from 82°N to 90°S are analyzed to determine if a statistically significant trend in the peak-to-peak seasonal cycle amplitude exists. Two analysis methods are used. Agreement between amplitude trends determined by the two methods is reasonable. Trends in amplitude determined from sampling sites of similar latitude are not consistent, except in the high southern latitudes, so we focus on the latitude zone 30–90°S, where CH 4 destruction is the primary factor determining the seasonal cycle amplitude. In this high southern hemisphere latitude zone, the trend in seasonal cycle amplitude is not statistically different from zero. Statistical uncertainties in the peak-to-peak seasonal cycle amplitude suggest that the observations are consistent with the change in the amplitude expected due to the increasing burden of atmospheric CH 4, but not with an increase in the globally averaged OH concentration.

Rate constants for the gas-phase reaction of the hydroxyl radical with a series of dimethylbenzaldehydes and trimethylphenols at atmospheric pressure

Rate constants for three dimethylbenzaldehydes and two trimethylphenols have been determined for the OH reactions at 298±2 K and atmospheric pressure using a relative rate method. The OH reaction rate constants were placed on an absolute basis using the literature rate constant for 1,2,4-trimethylbenzene of (3.25±0.5)×10−11 cm3 molecule−1s−1). The measured rate constants were (in units of cm3 molecule−1 s−1) 2,4-dimethyl-benzaldehyde, (4.32±0.67)×10−11; 2,5-dimethylbenzaldehyde, (4.37±0.68)×10−11; 3,4-dimethylbenzaldehyde, (2.14±0.34)×10−11; and 2,3,5- trimethylphenol, (12.5±1.9)×10−11, 2,3,6-trimethylphenol, (11.8±1.8)×10−11. Using an average OH concentration of 8.7×105 molecule cm−3, the estimated atmospheric lifetimes are ca. 7.5 h for 2,4- and 2,5-dimethylbenzaldehydes, ca. 15 h for 3,4-dimethylbenzaldehyde, ca. 2.5 h for 2,3,5- and 2,3,6-trimethylphenols. The reactivities of the trimethylphenols exceed those of the dimethyl-benzaldehydes by more than a factor of 3. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 523–525, 1997.

Estimation of hydroxyl radical concentrations in the marine atmospheric boundary layer using a reactive atmospheric tracer

Hydroxyl radical (OH) concentrations in the atmospheric boundary layer over a number of remote ocean locations are calculated from the measured diurnal variation in atmospheric dimethylsulfide (DMS). By using averaged DMS data sets from extended periods, the calculation yields OH concentrations averaged over periods from several days to weeks. These average OH concentrations range from 7×105 to 2.9×106 molecules cm-3, corresponding to midday maxima of 3 to 12×106 molecules cm-3. The lowest values correspond to studies with the lowest light intensity (Antarctic summer and South Atlantic winter), and the highest values to regions with probable anthropogenic influence. In addition to the long term averages, daily average OH levels can be calculated for most days in a two week period from a cruise in the tropical eastern Pacific. These calculations are in good argeement with global average OH levels derived from other tracers, and are consistent with model OH calculations when allowance is made for variation in ambient ozone levels between the studies. Estimates of gas exchange made from the diurnal variation of DMS suggest that either the gas exchange coefficient of DMS or the boundary layer mixing depth may have been overestimated in past analyses.

Hydrocarbon ratios during PEM‐WEST A: A model perspective

A useful application of the hydrocarbon measurements collected during the Pacific Exploratory Mission (PEM‐West A) is as markers or indices of atmospheric processing. Traditionally, ratios of particular hydrocarbons have been interpreted as photochemical indices, since much of the effect due to atmospheric transport is assumed to cancel by using ratios. However, an ever increasing body of observatonial and theoretical evidence suggests that turbulent mixing associated with atmospheric transport influences certain hydrocarbon ratios significantly. In this study a three‐dimensional mesoscale photochemical model is used to study the interaction of photochemistry and atmospheric mixing on select hydrocarbons. In terms of correlations and functional relationships between various alkanes, the model results and PEM‐West A hydrocarbon observations share many similar characteristics as well as explainable differences. When the three‐dimensional model is applied to inert tracers, hydrocarbon ratios andother relationships exactly follow those expected by simple dilution with model‐imposed “background air,” and the three‐dimensional results for reactive hydrocarbons are quite consistent with a combined influence of photochemistry and simple dilution. Analogous to these model results, relationships between various hydrocarbons collected during the PEM‐West A experiment appear to be consistent with this simplified picture of photochemistry and dilution affecting individual air masses. When hydrocarbons are chosen that have negligible contributions to clean background air, unambiguous determinations of the relative contributions to photochemistry and dilution can be estimated from the hydrocarbon samples. Both the three‐dimensional model results and the observations imply an average characteristic lifetime for dilution with background air roughly equivalent to the photochemical lifetime of butane for the western Pacific lower troposphere. Moreover, the dominance of OH as the primary photochemical oxidant downwind of anthropogenic source regions can be inferred from correlations between the highly reactive alkane ratios. By incorporating back‐trajectory information within the three‐dimensional model analysis, a correspondence between time and a particular hydrocarbon or hydrocarbon ratio can be determined, and the influence of atmospheric mixing or photochemistry can be quantified. Results of the three‐dimensional model study are compared and applied to the PEM‐West A hydrocarbon dataset, yielding a practical methodology for determining average OH concentrations and atmospheric mixing rates from the hydrocarbon measurements. Aircraft data taken below 2 km during wall flights east of Japan imply a diurnal average OH concentration of ∼3 × 106 cm−3. The characteristic time for dilution with background air is estimated to be ∼2.5 days for the two study areas examined in this work.

Global average concentration and trend for hydroxyl radicals deduced from ALE/GAGE trichloroethane (methyl chloroform) data for 1978–1990

Atmospheric measurements at several surface stations made between 1978 and 1990 of the anthropogenic chemical compound 1,1,1‐trichloroethane (methyl chloroform, CH3CCl3) show it increasing at a global average rate of 4.4 ± 0.2% per year (1σ) over this time period. The measured trends combined with industrial emission estimates are used in an optimal estimation inversion scheme to deduce a globally averaged CH3CCl3 tropospheric (and total atmospheric) lifetime of 5.7 (+0.7, −0.6) years (1σ) and a weighted global average tropospheric hydroxyl radical (OH) concentration of (8.7 ± 1.0) × 105 radical cm−3 (1σ). Inclusion of a small loss rate to the ocean for CH3CCl3 of 1/85 year−1 does not affect the stated lifetime but lowers the stated OH concentration to (8.1 ± 0.9) × 105 radical cm−3 (1σ). The rate of change of the weighted global average OH concentration over this time period is determined to be 1.0 ± 0.8% per year (1σ) which has major implications for the oxidation capacity of the atmosphere and more specifically for methane (CH4), which like CH3CCl3 is destroyed primarily by OH radicals. Because the weighting strongly favors the tropical lower troposphere, this deduced positive OH trend is qualitatively consistent with hypothesized changes in tropical tropospheric OH and ozone concentrations driven by tropical urbanization, biomass burning, land use changes, and long‐term warming. We caution, however, that our deduced rate of change in OH assumes that current industry estimates of anthropogenic emissions and our absolute calibration of CH3CCl3 are accurate. The CH3CCl3 measurements at our tropical South Pacific station (Samoa) show remarkable sensitivity to the El Nino‐Southern Oscillation (ENSO), which we attribute to modulation of cross‐equatorial transport during the northern hemisphere winter by the interannually varying upper tropospheric zonal winds in the equatorial Pacific. Thus measurements of this chemical compound have led to the discovery of a previously unappreciated aspect of tropical atmospheric tracer transport.

Investigation on the effect of electric fields on soot formation and flame structure of diffusion flames

The influences of DC and AC electric fields, at frequencies up to 1.48 MHz and maximum strength of about 6 kV/cm, on soot formation and flame structure were investigated by using a counterflow type acetylene diffusion flame. The distributions of flame luminosity, soot volume fraction, its number density, its average size, flame temperature and OH concentration in the flame were measured by non-invasive detection methods.

Nonmethane hydrocarbon chemistry in the remote marine boundary layer

We present a photochemical model of the remote marine boundary layer, focusing on the role of reactive nonmethane hydrocarbons (NMHCs). Current observations constrain total NMHC atmospheric concentrations in the remote marine boundary layer only to within a factor of 25, with the major uncertainty lying in the heavier alkenes. Using our model, we deduce air‐sea fluxes of NMHCs from these observations by demanding a balance between oceanic evasion fluxes and atmospheric loss rates for these species. The total NMHC air‐sea flux so deduced lies between 1×1010 and 2×1011 molec cm−2 s−1, with the range representing either true variability or uncertainty. This is inconsistent with the total NMHC air‐sea flux ((1‐6) × 109 molec cm−2 s−1) derived using a diffusive microlayer model together with the few existing oceanic NMHC observations. Diurnally averaged OH concentrations in our model range between 1.2×106 and 4.0×105 molec cm−3; our preferred model run, with a total NMHC flux of 5.4×1010 molec cm−2 s−1, produces a diurnally averaged OH concentration of 8.4×105 molec cm−3. This agrees well with OH concentrations derived from inversions of methyl chloroform data. Our model results imply a possibly significant role for NMHCs in determining OH concentrations in the remote marine boundary layer. Definitive conclusions about the role of NMHCs are prevented, however, both by inconclusive data on the NMHCs themselves and by large chemical uncertainties, particularly that in the O(1D) quantum yield.

On the indirect determination of atmospheric OH radical concentrations from reactive hydrocarbon measurements

Measurements of nonmethane hydrocarbons (NMHCs) from urban plumes have previously been used to deduce average OH concentrations by monitoring the decay of the NMHCs from initial values and comparing their time rates of decay to the decay rate of a relatively inert tracer emitted from the same source. Under the assumption of no interaction between photochemistry and transport, these observationally derived indirect OH concentrations are lower for NMHCs with successively higher reactivities toward OH. In this study, analytical solutions to simple flow situations and a three‐dimensional mesoscale model are used to test this assumption. When turbulent transport is parameterized in terms of diffusion coefficients, the models yield results that are consistent with the observations and suggest that the assumption is not generally valid. Daytime vertical mixing within the planetary boundary layer (PBL) and horizontal diffusion are two transport processes that cannot be assumed to be separable from species reactivity. The net effect is that under most daytime conditions the OH concentrations can be underpredicted by more than a factor of 2 when highly reactive NMHCs are used to derive OH concentrations in the usual manner. However, these results only apply when the species used to derive OH concentrations are emitted from a continuous source. The assumption of separability between photochemistry and transport is valid when only instantaneous or puff emissions are considered. An explanation of these effects is presented within the context of the analytical results to some simple flow systems. Results from the three‐dimensional model are used to illustrate the interaction of species reactivity and transport for more realistic parameterizations of atmospheric transport. The results of this study have important implications for experiments designed to indirectly determine OH concentrations and also for the treatment of turbulent transport in Lagrangian and Eulerian photochemical models.

The global cycle of carbon monoxide: Trends and mass balance

The annual global emissions of CO are estimated to be about 2,600 ± 600 Tg, of which about 60% are from human activities including combustion of fossil fuels and oxidation of hydrocarbons including methane. The remaining 40% of the emissions are from natural processes, mostly from the oxidation of hydrocarbons but also from plants and the oceans. Almost all the CO emitted into the atmosphere each year is removed by reactions with OH radicals (85%), by soils (10%), and by diffusion into the stratosphere. There is a small imbalance between annual emissions and removal, causing an increase of about 1% per year. It is very likely that the imbalance is due to increasing emissions from anthropogenic activities. The average concentration of CO is about 90 ppbv, which amounts to about 400 Tg in the atmosphere, and the average lifetime is about 2 months. This view of the global cycle of CO is consistent with the present estimates of average OH concentrations and the budgets of other trace gases including methane and methylchloroform. There are large remaining uncertainties that may in the future upset the apparently cohesive present budget of CO. If the present view of the global cycle of CO is correct, then it is likely that, in time, increasing levels of CO will contribute to widespread changes in atmospheric chemistry.