Global OH Concentration Research Articles

Monitoring global tropospheric OH concentrations using satellite observations of atmospheric methane

Abstract. The hydroxyl radical (OH) is the main tropospheric oxidant and the main sink for atmospheric methane. The global abundance of OH has been monitored for the past decades using atmospheric methyl chloroform (CH3CCl3) as a proxy. This method is becoming ineffective as atmospheric CH3CCl3 concentrations decline. Here we propose that satellite observations of atmospheric methane in the short-wave infrared (SWIR) and thermal infrared (TIR) can provide an alternative method for monitoring global OH concentrations. The premise is that the atmospheric signature of the methane sink from oxidation by OH is distinct from that of methane emissions. We evaluate this method in an observing system simulation experiment (OSSE) framework using synthetic SWIR and TIR satellite observations representative of the TROPOMI and CrIS instruments, respectively. The synthetic observations are interpreted with a Bayesian inverse analysis, optimizing both gridded methane emissions and global OH concentrations. The optimization is done analytically to provide complete error accounting, including error correlations between posterior emissions and OH concentrations. The potential bias caused by prior errors in the 3-D seasonal OH distribution is examined using OH fields from 12 different models in the ACCMIP archive. We find that the satellite observations of methane have the potential to constrain the global tropospheric OH concentration with a precision better than 1 % and an accuracy of about 3 % for SWIR and 7 % for TIR. The inversion can successfully separate the effects of perturbations to methane emissions and to OH concentrations. Interhemispheric differences in OH concentrations can also be successfully retrieved. Error estimates may be overoptimistic because we assume in this OSSE that errors are strictly random and have no systematic component. The availability of TROPOMI and CrIS data will soon provide an opportunity to test the method with actual observations.

Impacts of bromine and iodine chemistry on tropospheric OH and HO<sub>2</sub>: comparing observations with box and global model perspectives

Abstract. The chemistry of the halogen species bromine and iodine has a range of impacts on tropospheric composition, and can affect oxidising capacity in a number of ways. However, recent studies disagree on the overall sign of the impacts of halogens on the oxidising capacity of the troposphere. We present simulations of OH and HO2 radicals for comparison with observations made in the remote tropical ocean boundary layer during the Seasonal Oxidant Study at the Cape Verde Atmospheric Observatory in 2009. We use both a constrained box model, using detailed chemistry derived from the Master Chemical Mechanism (v3.2), and the three-dimensional global chemistry transport model GEOS-Chem. Both model approaches reproduce the diurnal trends in OH and HO2. Absolute observed concentrations are well reproduced by the box model but are overpredicted by the global model, potentially owing to incomplete consideration of oceanic sourced radical sinks. The two models, however, differ in the impacts of halogen chemistry. In the box model, halogen chemistry acts to increase OH concentrations (by 9.8 % at midday at the Cape Verde Atmospheric Observatory), while the global model exhibits a small increase in OH at the Cape Verde Atmospheric Observatory (by 0.6 % at midday) but overall shows a decrease in the global annual mass-weighted mean OH of 4.5 %. These differences reflect the variety of timescales through which the halogens impact the chemical system. On short timescales, photolysis of HOBr and HOI, produced by reactions of HO2 with BrO and IO, respectively, increases the OH concentration. On longer timescales, halogen-catalysed ozone destruction cycles lead to lower primary production of OH radicals through ozone photolysis, and thus to lower OH concentrations. The global model includes more of the longer timescale responses than the constrained box model, and overall the global impact of the longer timescale response (reduced primary production due to lower O3 concentrations) overwhelms the shorter timescale response (enhanced cycling from HO2 to OH), and thus the global OH concentration decreases. The Earth system contains many such responses on a large range of timescales. This work highlights the care that needs to be taken to understand the full impact of any one process on the system as a whole.

The effect of stratospheric sulfur from Mount Pinatubo on tropospheric oxidizing capacity and methane

AbstractThe eruption of Mount Pinatubo in 1991 injected a large amount of SO2 into the stratosphere, which formed sulfate aerosols. Increased scattering and absorption of UV radiation by the enhanced stratospheric SO2 and aerosols decreased the amount of UV radiation reaching the troposphere, causing changes in tropospheric photochemistry. These changes affected the oxidizing capacity of the atmosphere and the removal rate of CH4 in the years following the eruption. We use the three‐dimensional chemistry transport model TM5 coupled to the aerosol microphysics module M7 to simulate the evolution of SO2 and sulfate aerosols from the Pinatubo eruption. Their effect on tropospheric photolysis frequencies and concentrations of OH and CH4 is quantified for the first time. We find that UV attenuation by stratospheric sulfur decreased the photolysis frequencies of both ozone and NO2 by about 2% globally, decreasing global OH concentrations by a similar amount in the first 2 years after the eruption. SO2 absorption mainly affects OH primary production by ozone photolysis, while aerosol scattering also alters OH recycling. The effect of stratospheric sulfur on global OH and CH4 is dominated by the effect of aerosol extinction, while SO2 absorption contributes by 12.5% to the overall effect in the first year after the eruption. The reduction in OH concentrations causes an increase in the CH4 growth rate of 4 and 2 ppb/yr in the first and second years after the eruption, respectively, contributing 11 Tg to the 27 Tg observed CH4 burden change in late 1991 and early 1992.

Interpreting methane variations in the past two decades using measurements of CH<sub>4</sub> mixing ratio and isotopic composition

Abstract. The availability δ13C-CH4 measurements from atmospheric samples has significantly improved in recent years, which allows the construction of time series spanning up to about 2 decades. We have used these measurements to investigate the cause of the methane growth rate decline since 1980, with a special focus on the period 1998–2006 when the methane growth came to a halt. The constraints provided by the CH4 and δ13C-CH4 measurements are used to construct hypothetical source and sink scenarios, which are translated into corresponding atmospheric concentrations using the atmospheric transport model TM3 for evaluation against the measurements. The base scenario, composed of anthropogenic emissions according to EDGAR 4.0, constant emissions from natural sources, and a constant atmospheric lifetime, overestimates the observed global growth rates of CH4 and δ13C-CH4 by, respectively, 10 ppb yr−1 and 0.02‰ yr−1 after the year 2000. It proves difficult to repair this inconsistency by modifying trends in emissions only, notably because a temporary reduction of isotopically light sources, such as natural wetlands, leads to a further increase of δ13C-CH4. Furthermore, our results are difficult to reconcile with the estimated increase of 5 Tg CH4 yr−1 in emissions from fossil fuel use in the period 2000–2005. On the other hand, we find that a moderate (less than 5% per decade) increase in the global OH concentration can bring the model in agreement with the measurements for plausible emission scenarios. This study demonstrates the value of global monitoring of methane isotopes, and calls for further investigation into the role OH and anthropogenic emissions to further improve our understanding of methane variations in recent years.

Removal of methyl chloroform in a coastal salt marsh of eastern China

The atmospheric burden of methyl chloroform (CH 3CCl 3) is still considerable due to its long atmospheric lifetime, although CH 3CCl 3 emissions have declined considerably since it was included into the Montreal Protocol. Moreover, CH 3CCl 3 emissions are used to estimate hydroxyl radical (OH) levels, trends, and hemispheric distributions, and thus the mass balance of the trace gas in the atmosphere is critical for characterizing OH concentrations. Salt marshes may be a potential sink for CH 3CCl 3 due to its anoxic environment and abundant organic matter in sediments. In this study, seasonal dynamics of CH 3CCl 3 fluxes were measured using static flux chambers from April 2004 to January 2005, along an elevational gradient of a coastal salt marsh in eastern China. To estimate the contribution of higher plants to the gas flux, plant aboveground biomass was experimentally harvested and the flux difference between the treatment and the intact was examined. In addition, the flux was analyzed in relation to soil and weather conditions. Along the elevational gradient, the salt marsh generally acted as a net sink of CH 3CCl 3 in the growing season (from April to October). The flux of CH 3CCl 3 ranged between −3.38 and −32.03 nmol m −2 d −1 (positive for emission and negative for consumption), and the maximum negative rate occurred at the cordgrass marsh. However, the measurements made during inundation indicated that the mudflat was a net source of CH 3CCl 3. In the non-growing season (from November to March), the vegetated marsh was a minor source of CH 3CCl 3 when soil was frozen, the emission rate ranging from 3.43 to 7.77 nmol m −2 d −1. However, the mudflat was a minor sink of CH 3CCl 3 whether it was frozen or not in the non-growing season. Overall, the coastal salt marsh in eastern China was a large sink for the gas, because the magnitude of consumption rate was lager than that of emission, and because the duration of the growing season was longer than that of the non-growing season. Plant aboveground biomass had a great effect on the flux. Comparative analysis showed that the direction and magnitude of the effect of higher plants on the flux of CH 3CCl 3 depended on timing of sampling vegetation type. In the growing season the plant biomass decreased the gas flux and acted as a large sink of the gas, whereas it presented as a minor source in the non-growing season. However, the mechanism underlying plant uptake process is not clear. The CH 3CCl 3 flux was positively related to the dissolved salt concentration and organic matter content in soil, as well as light intensity, but it was negatively related to soil temperature, sulfate concentrations, and initial ambient atmospheric concentrations of CH 3CCl 3. Our observations have important implications for estimation of the tropospheric lifetime of CH 3CCl 3 and global OH concentration from the global budget concentration of CH 3CCl 3.

Laboratory study on heterogeneous decomposition of methyl chloroform on various standard aluminosilica clay minerals as a potential tropospheric sink

Abstract. Methyl chloroform (1,1,1-trichloroethane, CH3CCl3) was found to decompose heterogeneously on seven types of standard clay minerals (23 materials) in dry air at 313 K in the laboratory. All reactions proceeded through the elimination of HCl; CH3CCl3 was converted quantitatively to CH2=CCl2. The activities of the clay minerals were compared via their pseudo-first-order reaction rate constants (k1). A positive correlation was observed between the k1 value and the specific surface area (S) of clay minerals, where the S value was determined by means of the general Brunauer-Emmett-Teller (BET) equation. The k1 value was anti-correlated with the value of n, which was a parameter of the general BET equation and related to the average pore size of the clay minerals, and correlated with the water content that can be removed easily from the clay minerals. The reaction required no special pretreatment of clay minerals, such as heating at high temperatures; hence, the reaction can be expected to occur in the environment. Photoillumination by wavelengths present in the troposphere did not accelerate the decomposition of CH3CCl3, but it induced heterogeneous photodecomposition of CH2=CCl2. The temperature dependence of k1, the adsorption equilibrium coefficient of CH3CCl3 and CH2=CCl2, and the surface reaction rate constant of CH3CCl3 were determined for an illite sample. The k1 value increased with increasing temperature. The amount of CH3CCl3 adsorbed on the illite during the reaction was proportional to the partial pressure of CH3CCl3. The reaction was sensitive to relative humidity and the k1 value decreased with increasing relative humidity. However, the reaction was found to proceed at a relative humidity of 22% at 313 K, although the k1 value was about one-twentieth of the value in non-humidified air. The conditions required for the reaction may be present in major desert regions of the world. A simple estimation indicates that the possible heterogeneous decomposition of CH3CCl3 on the ground surface in arid regions is worth taking into consideration when inferring the tropospheric lifetime of CH3CCl3 and global OH concentration from the global budget concentration of CH3CCl3.

CHASER: A global chemical model of the troposphere 2. Model results and evaluation

We present results from a global three‐dimensional chemical model for the troposphere, named Chemical AGCM for Study of Atmospheric Environment and Radiative Forcing (CHASER). This model has been developed to simulate the tropospheric photochemistry involving the O3‐HOx‐NOx‐CH4‐CO photochemical system and oxidation of nonmethane hydrocarbons (NMHCs), based on an atmospheric general circulation model (AGCM). In this paper, we present results from a climatological run of the model, and evaluate them with observational data. The simulation was conducted at the model horizontal resolution of T21 (5.6° × 5.6°) with 32 vertical layers from the surface to about 40 km altitude. The model reproduces well the observed vertical profiles of CO and NMHCs as ethane, propane, and acetone in almost all cases. The simulated seasonal cycle of surface CO in the biomass‐burning region (South America) shows a good agreement with observation. In the case of nitric oxide (NO), the model generally reproduces the observed vertical profiles well, though it appears to somewhat underestimate NO in the upper troposphere in the low latitudes. Although the model overestimates nitric acid (HNO3) in some cases like other model studies, the calculated HNO3 is generally within the range of observations. Peroxyacetyl nitrate (PAN) appears to be generally simulated well, but overestimated by the model in some remote regions. In the simulation of O3, the vertical profiles and seasonal variations observed in polluted and remote sites are well reproduced. The ozone distributions calculated in the biomass‐burning‐related regions as South America, Africa, and the South Atlantic show good agreements with observations. However, in the midlatitudes, the model tends to overestimate O3 in the upper troposphere in winter‐springtime, maybe indicating the need of improving the model's horizontal resolution. The model calculates a net chemical ozone production of 397 TgO3/yr in the global troposphere (4895 TgO3/yr production and 4498 TgO3/yr destruction). The estimated net O3 flux from the stratosphere is 593 TgO3/yr, well within the range suggested by recent studies. The calculated global OH concentration leads to a global mean CH4 lifetime of 7.9 years in this simulation.

Reply [to “Comment on “Global OH trend inferred from methylchloroform measurements” by Maarten Krol et al.”

Journal of Geophysical Research: AtmospheresVolume 106, Issue D19 p. 23159-23164 CommentariesFree Access Reply [to “Comment on “Global OH trend inferred from methylchloroform measurements” by Maarten Krol et al.”] Maarten Krol, Maarten KrolSearch for more papers by this authorPeter Jan van Leeuwen, Peter Jan van LeeuwenSearch for more papers by this authorJos Lelieveld, Jos LelieveldSearch for more papers by this author Maarten Krol, Maarten KrolSearch for more papers by this authorPeter Jan van Leeuwen, Peter Jan van LeeuwenSearch for more papers by this authorJos Lelieveld, Jos LelieveldSearch for more papers by this author First published: 01 October 2001 https://doi.org/10.1029/2001JD900039Citations: 8AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL References Cunnold, D. M., R. G. Prinn, Comment on “Tropospheric OH in a three-dimensional chemical tracer model: An assessment based on observation of CH3CCl3” by C. M. Spivakovsky et al, J. Geophys. Res., 96, 17,391– 17,393, 1991. Karlsdottir, S., I. S. A. Isaksen, Changing methane lifetime: Possible cause for reduced growth, Geophys. Res. Lett., 27, 93– 97, 2000. Krol, M. C., P. J. Van Leeuwen, J. Lelieveld, Global OH trend inferred from methylchloroform measurements, J. Geophys. Res., 103, 10,697– 10,711, 1998. Prinn, R. G., andJ. Huang, Comment on “Global OH trend inferred from methylchloroform measurements” by Maarten Krol et al.,J. Geophys. Res., 106(D19), 2001. Prinn, R. G., et al., Global average concentration and trend for hydroxyl radicals deduced from ALE/GAGE trichloroethane (methyl chloroform) data for 1978–1990, J. Geophys. Res., 97, 2445– 2461, 1992. Prinn, R. G., R. F. Weiss, B. R. Miller, J. Huang, F. N. Alya, D. M. Cunnold, P. J. Fraser, D. E. Hartley, P. G. Simmonds, Atmospheric trends and lifetime of CH3CCl3 and global OH concentrations, Science, 269, 187– 190, 1995. Spivakovsky, C. M., Reply to comment by D. M. Cunnold and R. G. Prinn on “Tropospheric OH in a three-dimensional chemical tracer model: An assessment based on observations of CH3CCl3”, J. Geophys. Res., 96, 17395– 17398, 1991. Citing Literature Volume106, IssueD1916 October 2001Pages 23159-23164 ReferencesRelatedInformation

Changing trends in tropospheric methane and carbon monoxide: A sensitivity analysis of the OH-radical

A sensitivity analysis is performed in order to study recently observed changes in atmospheric methane and carbon monoxide trends. For the analysis we have adapted a one-dimensional transport/chemistry model in order to comply with changes in vertical transport, stratosphere-troposphere flux of ozone, the water vapour cycle and the short-wave radiative transfer. In addition we have formulated an improved relationship which expresses the steady state OH concentration in terms of longer lived compounds which has a fair agreement with the one-dimensional model results. An analysis of the observed changes and trends in methane and carbon monoxide shows that both emissions and changes in global OH concentrations can be main causes for the observed changes. Average methane emissions have slowed down, particularly in the NH, in the last five years, though perhaps not very significantly. Carbon monoxide emissions are decreasing faster in the last couple of years than in the period 1983–1990. The study suggests that climate fluctuations (tropospheric water vapour, temperature and convective activity) and the stratospheric ozone depletion (tropospheric UV radiation) have a significant influence on tropospheric composition and thus on trends in methane and carbon monoxide concentrations.

Kinetics of the reactions of OH with methyl chloroform and methane: Implications for global tropospheric OH and the methane budget

The hydroxyl radical plays a central role in the oxidation of methane, larger organics including methyl chloroform, and CO in the troposphere. Hence a knowledge of the kinetics of such reactions is essential in estimating atmospheric lifetimes of these organics as well as global OH concentrations. We report here fast flow discharge studies of the absolute rate constants for both the OH‐CH3CCl3 and OH‐CH4 reactions from 278–378 K in He at 1.05±0.05 Torr pressure. For CH3CCl3, . The rate constants at 298 K and 277 K, 1.0 and 0.73×10−14 cm3 molecule−1 s−1 respectively, are approximately 15% and 5% below the values now used to estimate global OH concentrations. The use of these lower rate constants in computations of global OH based on CH3CCl3 emissions and ambient measurements will increase the estimated OH concentrations, particularly at higher temperatures found in warmer equatorial regions. For CH4, . At 298 K and 277 K respectively, this gives values which are ∼23% and 28% below the values which have generally been used in tropospheric models. Our results therefore support the recent suggestion that the methane flux may have been overestimated in the past.

The two‐dimensional diagnostic model for tropospheric OH: An uncertainty analysis

Tropospheric OH is believed to play a major role in atmospheric photochemistry, and, via its chemical interactions, OH ultimately may affect the climate, stratospheric ozone levels, and the pH of rain. A valuable tool for predicting the global OH abundance as a function of latitude and altitude is the two‐dimensional diagnostic model for tropospheric OH. This model can also be applied to budget studies of species which interact with OH, such as CH4, CO, and O3. In addition to laboratory‐obtained kinetic data, important variables, including the rate constants for several reactions, the rate of heterogeneous removal of soluble species, and the global abundance and spatial variability in H2O, O3, CO, and NOx. The uncertainties in these parameters imply over a factor of 3 variability in the calculated global OH concentration as well as significant uncertainties in the latitudinal and vertical OH distribution.