OH Burden Research Articles

Influences of hydroxyl radicals (OH) on top-down estimates of the global and regional methane budgets

Abstract. The hydroxyl radical (OH), which is the dominant sink of methane (CH4), plays a key role in closing the global methane budget. Current top-down estimates of the global and regional CH4 budget using 3D models usually apply prescribed OH fields and attribute model–observation mismatches almost exclusively to CH4 emissions, leaving the uncertainties due to prescribed OH fields less quantified. Here, using a variational Bayesian inversion framework and the 3D chemical transport model LMDz, combined with 10 different OH fields derived from chemistry–climate models (Chemistry–Climate Model Initiative, or CCMI, experiment), we evaluate the influence of OH burden, spatial distribution, and temporal variations on the global and regional CH4 budget. The global tropospheric mean CH4-reaction-weighted [OH] ([OH]GM-CH4) ranges 10.3–16.3×105 molec cm−3 across 10 OH fields during the early 2000s, resulting in inversion-based global CH4 emissions between 518 and 757 Tg yr−1. The uncertainties in CH4 inversions induced by the different OH fields are similar to the CH4 emission range estimated by previous bottom-up syntheses and larger than the range reported by the top-down studies. The uncertainties in emissions induced by OH are largest over South America, corresponding to large inter-model differences of [OH] in this region. From the early to the late 2000s, the optimized CH4 emissions increased by 22±6 Tg yr−1 (17–30 Tg yr−1), of which ∼25 % (on average) offsets the 0.7 % (on average) increase in OH burden. If the CCMI models represent the OH trend properly over the 2000s, our results show that a higher increasing trend of CH4 emissions is needed to match the CH4 observations compared to the CH4 emission trend derived using constant OH. This study strengthens the importance of reaching a better representation of OH burden and of OH spatial and temporal distributions to reduce the uncertainties in the global and regional CH4 budgets.

Inter-model comparison of global hydroxyl radical (OH) distributions and their impact on atmospheric methane over the 2000–2016 period

Abstract. The modeling study presented here aims to estimate how uncertainties in global hydroxyl radical (OH) distributions, variability, and trends may contribute to resolving discrepancies between simulated and observed methane (CH4) changes since 2000. A multi-model ensemble of 14 OH fields was analyzed and aggregated into 64 scenarios to force the offline atmospheric chemistry transport model LMDz (Laboratoire de Meteorologie Dynamique) with a standard CH4 emission scenario over the period 2000–2016. The multi-model simulated global volume-weighted tropospheric mean OH concentration ([OH]) averaged over 2000–2010 ranges between 8.7×105 and 12.8×105 molec cm−3. The inter-model differences in tropospheric OH burden and vertical distributions are mainly determined by the differences in the nitrogen oxide (NO) distributions, while the spatial discrepancies between OH fields are mostly due to differences in natural emissions and volatile organic compound (VOC) chemistry. From 2000 to 2010, most simulated OH fields show an increase of 0.1–0.3×105 molec cm−3 in the tropospheric mean [OH], with year-to-year variations much smaller than during the historical period 1960–2000. Once ingested into the LMDz model, these OH changes translated into a 5 to 15 ppbv reduction in the CH4 mixing ratio in 2010, which represents 7 %–20 % of the model-simulated CH4 increase due to surface emissions. Between 2010 and 2016, the ensemble of simulations showed that OH changes could lead to a CH4 mixing ratio uncertainty of >±30 ppbv. Over the full 2000–2016 time period, using a common state-of-the-art but nonoptimized emission scenario, the impact of [OH] changes tested here can explain up to 54 % of the gap between model simulations and observations. This result emphasizes the importance of better representing OH abundance and variations in CH4 forward simulations and emission optimizations performed by atmospheric inversions.

Description and evaluation of tropospheric chemistry and aerosols in the Community Earth System Model (CESM1.2)

Abstract. The Community Atmosphere Model (CAM), version 5, is now coupled to extensive tropospheric and stratospheric chemistry, called CAM5-chem, and is available in addition to CAM4-chem in the Community Earth System Model (CESM) version 1.2. The main focus of this paper is to compare the performance of configurations with internally derived "free running" (FR) meteorology and "specified dynamics" (SD) against observations from surface, aircraft, and satellite, as well as understand the origin of the identified differences. We focus on the representation of aerosols and chemistry. All model configurations reproduce tropospheric ozone for most regions based on in situ and satellite observations. However, shortcomings exist in the representation of ozone precursors and aerosols. Tropospheric ozone in all model configurations agrees for the most part with ozonesondes and satellite observations in the tropics and the Northern Hemisphere within the variability of the observations. Southern hemispheric tropospheric ozone is consistently underestimated by up to 25%. Differences in convection and stratosphere to troposphere exchange processes are mostly responsible for differences in ozone in the different model configurations. Carbon monoxide (CO) and other volatile organic compounds are largely underestimated in Northern Hemisphere mid-latitudes based on satellite and aircraft observations. Nitrogen oxides (NOx) are biased low in the free tropical troposphere, whereas peroxyacetyl nitrate (PAN) is overestimated in particular in high northern latitudes. The present-day methane lifetime estimates are compared among the different model configurations. These range between 7.8 years in the SD configuration of CAM5-chem and 8.8 years in the FR configuration of CAM4-chem and are therefore underestimated compared to observational estimations. We find that differences in tropospheric aerosol surface area between CAM4 and CAM5 play an important role in controlling the burden of the tropical tropospheric hydroxyl radical (OH), which causes differences in tropical methane lifetime of about half a year between CAM4-chem and CAM5-chem. In addition, different distributions of NOx from lightning explain about half of the difference between SD and FR model versions in both CAM4-chem and CAM5-chem. Remaining differences in the tropical OH burden are due to enhanced tropical ozone burden in SD configurations compared to the FR versions, which are not only caused by differences in chemical production or loss but also by transport and mixing. For future studies, we recommend the use of CAM5-chem configurations, due to improved aerosol description and inclusion of aerosol–cloud interactions. However, smaller tropospheric surface area density in the current version of CAM5-chem compared to CAM4-chem results in larger oxidizing capacity in the troposphere and therefore a shorter methane lifetime.

Effects of Climate-induced Changes in Isoprene Emissions after the eruption of Mount Pinatubo

The eruption of Mt. Pinatubo in June 1991 was the largest in the twentieth century. One of its effects was to produce cooler and drier conditions in the years following the eruption. We present the results of an integrated model study of the effect of these climatic changes on the emissions of isoprene from the biosphere. Our emissions model simulations showed that global isoprene emissions were reduced by 9% from 1990 to 1992. When incorporated into our model of global atmospheric chemistry this reduction of isoprene emissions led to an increase in the tropospheric OH burden of 2%. This caused an increase in the removal of methane via oxidation by OH of up to 5 Tg per year. This could have contributed to the observed changes in methane growth rate at this time.

Tropospheric ozone evolution between 1890 and 1990

In this study, we have performed a set of simulations to detail the evolution of tropospheric ozone from 1890 to 1990. The simulations are compared with available measurements for present‐day conditions and earlier. Using our best estimates of ozone precursors emissions (based on the work by van Aardenne et al. (2001)), we have found a tropospheric ozone burden increase of 71 Tg between 1890 and 1990, an increase of ∼30%. When no anthropogenic emissions in 1890 are considered, this burden increase reaches 88 Tg. The ozone lifetime is shown to have decreased by ∼30%, especially after 1930. It is also shown that the net chemical production in the lower troposphere exceeded that in the free troposphere for the first time in the 1950–1970 period. In addition, the ozone production in this study increased rapidly between 1890 and 1930 and from 1970 to 1990. However, the ozone production efficiency in the troposphere is shown to have decreased during the 20th century, making the troposphere less NOx limited. Finally, a decrease in the OH burden is estimated to be on the order of 8%, matched by a similar increase in the CO lifetime.

Intracellular myoglobin loading worsens H2O2-induced, but not hypoxia/reoxygenation-induced, in vitro proximal tubular injury.

Intracellular iron reportedly mediates many forms of tissue injury, including ischemic and myohemoglobinuric acute renal failure. This action may be explained by the ability of iron to catalyze the formation of the highly toxic hydroxyl radical (.OH) from H2O2 via the Fenton/Haber-Weiss reactions. To assess whether renal tubular myoglobin/iron loading, induced by a physiological mechanism (endocytosis), alters its susceptibility to O2 deprivation/reoxygenation- and H2O2-mediated injury, rats were infused with myoglobin or its vehicle (5% dextrose, control rats), and after 2 hours, proximal tubular segments (PTSs) were isolated for study. This infusion caused substantial myoglobin endocytic uptake (approximately 25 micrograms/mg PTS protein), and it doubled PTS catalytic iron content (assessed by bleomycin assay). Nevertheless, PTS viability (percent lactate dehydrogenase release) was minimally affected (4% to 6% increase), and an increased .OH burden (assessed by the salicylate trap method) did not appear to result. Deferoxamine addition, reported to protect against in vivo acute renal failure, paradoxically increased .OH levels (approximately 25%) in myoglobin-loaded, but not control, PTSs. Conversely, dimethylthiourea (an .OH scavenger) depressed .OH (by approximately 80%) in all PTSs. Myoglobin/iron loading modestly increased PTS vulnerability to exogenous H2O2 addition (P < .001). However, tubular susceptibility to hypoxia (15 and 30 minutes)/reoxygenation injury was not affected. .OH levels appeared to fall in response to both forms of injury, suggesting decreased .OH production and/or .OH scavenging. To assess whether myoglobin decreases .OH levels in the presence of Fenton reactants, myoglobin and six other test proteins were incubated with Fe2+/H2O2. Myoglobin decreased .OH levels by approximately 70%, a significantly greater decrement than was observed with the other proteins tested.(ABSTRACT TRUNCATED AT 250 WORDS)

Reaction of N2O5 on tropospheric aerosols: Impact on the global distributions of NOx, O3, and OH

Using a three‐dimensional global model of the troposphere, we show that the heterogeneous reactions of NO3 and N2O5 on aerosol particles have a substantial influence on the concentrations of NOx, O3, and OH. Due to these reactions, the modeled yearly average global NOx burden decreases by 50% (80% in winter and 20% in summer). The heterogeneous removal of NOx in the northern hemisphere (NH) is dominated by reactions on aerosols; in the tropics and southern hemisphere (SH), with substantial smaller aerosol concentrations, liquid water clouds can provide an additional sink for N2O5 and NO3. During spring in the NH subtropics and at mid‐latitudes, O3‐concentrations are lowered by 25%. In winter and spring in the subtropics of the NH calculated OH concentrations decreased by up to 30%. Global tropospheric average O3 and OH burden (the latter weighted with the amount of methane reacting with OH) can drop by about 9% each. By including reactions on aerosols, we are better able to simulate observed nitrate wet deposition patterns in North America and Europe. O3 concentrations in springtime smog situations are shown to be affected by heterogeneous reactions, indicating the great importance of chemical interactions resulting from NOx and SO2 emissions. However, a preliminary analysis shows that under present conditions a change in aerosol concentrations due to limited SO2 emission control strategies (e.g., reductions by a factor of 2 in industrial areas) will have only a relatively minor influence on O3 concentrations. Much larger reductions in SO2 emissions may cause larger increases in surface O3 concentrations, up to a maximum of 15%, if they are not accompanied by a reduction in NOx or hydrocarbon emission.