Secondary organic aerosols are believed to make an important contribution to the atmospheric radiation budget, though the exact mechanisms responsible for their formation are still uncertain. We compute a remarkably large contribution of hydroperoxides from the oxidation of biogenic monoterpenes. Although currently neglected in aerosol models, the hydroperoxides lead to 63% of our calculated global secondary organic aerosol formation, whereas carboxylic acids, usually considered as the dominant secondary organic aerosol precursors, were responsible for only 26%. Detailed studies are needed to determine the implications of this aerosol formation pathway for the total atmospheric aerosol burden and its impacts on climate.
In this study we identify recurring plumes of tropospheric NO2 originating from Africa and Indonesia during the monsoon transition periods over the central Indian Ocean (CIO, 5°N 30°S, 55°E 95°E), based on GOME satellite observations and global model (MATCH-MPIC) simulations. Despite the relatively short lifetime of NOx, these strong plumes can develop due to the pronounced anti-cyclonic circulation over the CIO, and the weak maritime convection, which limits vertical mixing. Model results indicate that the plumes are mainly transported in the middle troposphere (MT). Thus, the CIO in the Southern Hemisphere (SH) is not always as pristine as found in INDOEX during the winter monsoon.
A sensitivity study of the treatment of isoprene and related parameters in 3D atmospheric models was conducted using the global model of tropospheric chemistry MATCH-MPIC. A total of twelve sensitivity scenarios which can be grouped into four thematic categories were performed. These four categories consist of simulations with different chemical mechanisms, different assumptions concerning the deposition characteristics of intermediate products, assumptions concerning the nitrates from the oxidation of isoprene and variations of the source strengths. The largest differences in ozone compared to the reference simulation occured when a different isoprene oxidation scheme was used (up to 30-60% or about 10 nmol/mol). The largest differences in the abundance of peroxyacetylnitrate (PAN) were found when the isoprene emission strength was reduced by 50% and in tests with increased or decreased efficiency of the deposition of intermediates. The deposition assumptions were also found to have a significant effect on the upper tropospheric HOx production. Different implicit assumptions about the loss of intermediate products were identified as a major reason for the deviations among the tested isoprene oxidation schemes. The total tropospheric burden of O3 calculated in the sensitivity runs is increased compared to the background methane chemistry by 26±9 Tg(O3) from 273 to an average from the sensitivity runs of 299 Tg(O3). Thus, there is a spread of ±35% of the overall effect of isoprene in the model among the tested scenarios. This range of uncertainty and the much larger local deviations found in the test runs suggest that the treatment of isoprene in global models can only be seen as a first order estimate at present, and points towards specific processes in need of focused future work.
Production of nitrogen oxides (NOx = NO + NO2) by lightning (LtNOx) is the most uncertain among the global NOx sources, with recent estimates ranging from about 1-20 Tg(N)/yr. Previous studies of LtNOx have focused mainly on its role in the tropospheric NOy (reactive nitrogen) and O3 budgets. We show that the global mean OH concentration is also very sensitive to LtNOx. Furthermore, despite the fact that the largest changes in NOx due to lightning are in the upper troposphere, where reactions with OH are generally slower, we find that the sensitivity of the mean tropospheric lifetime of methane (CH4) and methylchloroform (CH3CCl3) to assumptions about LtNOx are as large as the sensitivity of the tropospheric O3 burden. Thus, an improved understanding of LtNOx will be important for our ability to accurately simulate the tropospheric oxidizing efficiency and its changes over time.
We have developed a global three-dimensional chemistry-meteorology model for studies of ozone and hydrocarbons in the troposphere, called Model of Atmospheric Transport and Chemistry-Max-Planck-Institute for Chemistry Version (MATCH-MPIC). The model currently calculates the distributions of 54 species and 141 reactions using a new flexible chemical integration method in connection with a fast general Rosenbrock solver. The reactions can be easily expanded for future studies with the model. The model includes updated emission inventories, an explicit dry deposition scheme, online photolysis rates, extensive budgeting capabilities and a correction for the so-called “mass-wind inconsistency” problem. One-year simulations at two different horizontal resolutions, approximately 1.9° × 1.9° (T63) and 5.6° × 5.6° (T21), both with 28 vertical levels, are extensively evaluated with available observations from surface stations, ozonesondes, and field campaigns. The model is generally able to reproduce the observations of ozone to within 10 nmol/mol, but it overestimates upper tropospheric ozone at northern high-latitude stations in winter and spring and tends to underestimate the summer maximum in the free troposphere. In the tropics the chemical tropopause is sometimes too low. Generally, the low-resolution run yields only slightly worse agreement with observations compared to the higher-resolution run, thus making it suitable for further sensitivity studies. In a simulation using different meteorological data, O3 agrees much better with observations in the upper troposphere, possibly because of the higher resolution near the tropopause. The net ozone production integrated over all tropospheric regions with a net production and loss separately reveals that the calculated chemically induced redistribution of ozone in the troposphere is 2–3 times larger than the net stratospheric influx. Even in the upper troposphere, photochemical production is of similar magnitude to the stratospheric influence. [The full set of evaluation plots can be found at http://www.mpch-mainz.mpg.de/~kuhlmann/matcheval]
A global three-dimensional model for studies of the tropospheric chemistry of HOx, ozone, and their precursors is thorougly evaluated with available observations of 15 species from surface stations and/or aircraft campaigns. The effect of grid-resolution was studied by comparing the measurements to model runs at a high (1.9° × 1.9°) and a reduced resolution. This study is a follow-up to a previous paper where the model was described and results for ozone were discussed. A statistical analysis of the complete comparison of each species is also presented in order to serve as a quantitative baseline for future evaluations. The seasonality and magnitude of the CO abundance is well simulated except for an overestimate of up to 20 nmol/mol in southern low latitudes. The tropospheric mean methane lifetime is calculated to be 8.7 and 9.1 years for the high and low resolution run, respectively, in agreement with recent estimates. Industrial emissions of alkanes from North America appear to be too low in the current data set. Indications for a biogenic source of propane were found by comparing with measurements over the Amazon rain forest. Alkene emissions from oceans are underestimated in the model indicating that they are higher than previous studies suggested. Nitrogen species are mostly reproduced within a factor of 2 or better, but a general bias to underestimate HNO3 and to overpredict PAN, especially in remote regions could also be found. NO was well simulated in the lower and mid troposphere, but significantly understimated in the upper troposphere. A sensitivity run shows that studies are needed to better constrain the rate constants for the thermal equilibrium of PAN. Acetone is often underestimated indicating that additional wide-spread sources, likely oceanic, are present. The comparison with other oxygenated species (acetaldehyde, methanol, formic and acetic acid) indicates severe gaps in our understanding of the present budgets of these species. In particular the underestimate of acetaldehyde implies large missing sources (order 100 Tg/yr) of this compound. [The full set of evaluation plots can be found at http://www.mpch-mainz.mpg.de/~kuhlmann/matcheval]
The first global tropospheric forecasts of O3 and its precursors have been used in the daily flight planning of field measurement campaigns. The 3-D chemistry-transport model MATCH-MPIC is driven by meteorological data from a weather center (NCEP) to produce daily 3-day forecasts of the global distributions Of O3 and related gases, as well as regional CO tracers. This paper describes the forecast system and its use in three field campaigns, MINOS, CONTRACE and INDOEX. An overview is given of the forecasts by MATCH-MPIC and by three other chemical weather forecast models (EURAD, ECHAM, and FLEXPART), focusing on O3 and CO. Total CO and regional CO tracers were found to be the most valuable gases for flight planning, due to their relatively well-defined anthropogenic source regions and lifetimes of one to a few months. CO was in good agreement with the observations on nearly all the flights (generally r > 0.7, and the relative RMS differences for the deviations from the means was less than 20%). In every case in which the chemical weather forecasts were primarily responsible for the flight plans, the targeted features were observed. Three forecasted phenomena are discussed in detail: outflow from Asia observed in the Mediterranean upper troposphere during MINOS, outflow from North America observed in the middle troposphere over northern Europe during CONTRACE, and the location of the chemical ITCZ over the Indian Ocean during INDOEX. In particular it is shown that although intercontinental pollution plumes such as those observed during MINOS and CONTRACE occur repeatedly during the months around the campaigns, their frequency is sufficiently low (similar to 10-30% of the time) that global chemical weather forecasts are important for enabling them to be observed during limited-duration field campaigns. The MATCH-MPIC chemical weather forecasts, including an interface for making customized figures from the output, are available for community use via http://www.mpch-mainz.mpg.de/~lawrence/forecasts.html.
Ozone is an air quality problem today for much of the world's population. Regions can exceed the ozone air quality standards (AQS) through a combination of local emissions, meteorology favoring pollution episodes, and the clean-air baseline levels of ozone upon which pollution builds. The IPCC 2001 assessment studied a range of global emission scenarios and found that all but one projects increases in global tropospheric ozone during the 21st century. By 2030, near-surface increases over much of the northern hemisphere are estimated to be about 5 ppb (+2 to +7 ppb over the range of scenarios). By 2100 the two more extreme scenarios project baseline ozone increases of >20 ppb, while the other four scenarios give changes of -4 to +10 ppb. Even modest increases in the background abundance of tropospheric ozone might defeat current AQS strategies. The larger increases, however, would gravely threaten both urban and rural air quality over most of the northern hemisphere.
The global sulphur cycle has been simulated using a general circulation model with a focus on the source and oxidation of atmospheric dimethylsulphide (DMS). The sensitivity of atmospheric DMS to the oceanic DMS climatology, the parameterisation of the sea-air transfer and to the oxidant fields have been studied. The importance of additional oxidation pathways (by O3 in the gas- and aqueous-phases and by BrO in the gas phase) not incorporated in global models has also been evaluated. While three different climatologies of the oceanic DMS concentration produce rather similar global DMS fluxes to the atmosphere at 24-27 Tg-S/yr, there are large differences in the spatial and seasonal distribution. The relative contributions of OH and NO3 radicals to DMS oxidation depends critically on which oxidant fields are prescribed in the model. Oxidation by O3 appears to be significant at high latitudes in both hemispheres. Oxidation by BrO could be significant even for BrO concentrations at sub-pptv levels in the marine boundary layer. The impact of such refinements on the DMS chemistry onto the indirect radiative forcing by anthropogenic sulphate aerosols is also discussed.
Formaldehyde (HCHO) is an important intermediate product in the photochemical degradation of methane and non-methane volatile organic compounds. In August 2001, airborne formaldehyde measurements based on the Hantzsch reaction technique were performed during the Mediterranean INtensive Oxidant Study, MINOS. The detection limit of the instrument was 42 pptv (1 sigma) at a time resolution of 180 s (10-90was 30eastern Mediterranean Sea average HCHO concentrations were of the order of 1500 pptv, in reasonable agreement with results from a three-dimensional global chemical transport model of the lower atmosphere including non-methane volatile organic compound (NMVOC) chemistry. Above the boundary layer HCHO mixing ratios decreased with increasing altitude to a minimum level of 250 pptv at about 7 km. At higher altitudes (above 7 km) HCHO levels showed a strong dependency on the airmass origin. In airmasses from the North Atlantic/North American area HCHO levels were of the order of 300 pptv, a factor of 6 higher than values predicted by the model. Even higher HCHO levels, increasing to values of the order of 600 pptv at 11 km altitude, were observed in easterlies transporting air affected by the Indian monsoon outflow towards the Mediterranean basin. Only a small part (similar to30 pptv) of the large discrepancy between the model results and the measurements of HCHO in the free troposphere could be explained by a strong underestimation of the upper tropospheric acetone concentration by up to a factor of ten by the 3D-model. Therefore, the measurement-model difference in the upper troposphere remains unresolved, while the observed dependency of HCHO on airmass origin might indicate that unknown, relatively long-lived NMVOCs - or their reaction intermediates - associated with biomass burning are at least partially responsible for the observed discrepancies.
This study presents measurements of acetonitrile, benzene, toluene, methanol and acetone made using the proton-transfer-reaction mass spectrometry (PTR-MS) technique at the Finokalia ground station in Crete during the Mediterranean INtensive Oxidant Study (MINOS) in July August 2001. Three periods during the campaign with broadly consistent back trajectories are examined in detail. In the first, air was advected from Eastern Europe without significant biomass burning influence ( mean acetonitrile mixing ratio 154 pmol/mol). In the second period, the sampled air masses originated in Western Europe, and were advected approximately east-south-east, before turning southwest over the Black Sea and north-western Turkey. The third well-defined period included air masses advected from Eastern Europe passing east and south of/over the Sea of Azov, and showed significant influence by biomass burning ( mean acetonitrile mixing ratio 436 pmol/mol), confirmed by satellite pictures. The mean toluene: benzene ratios observed in the three campaign periods described were 0.35, 0.37 and 0.22, respectively; the use of this quantity to determine air mass age is discussed. Methanol and acetone were generally well-correlated both with each other and with carbon monoxide throughout the campaign. Comparison of the acetone and methanol measurements with the MATCH-MPIC model showed that the model underestimated both species by a factor of 4, on average. The correlations between acetone, methanol and CO implied that the relatively high levels of methanol observed during MINOS were largely due to direct biogenic emissions, and also that biogenic sources of acetone were highly significant during MINOS ( similar to 35both species may be due, at least in part, to missing biogenic emissions.
The MINOS (Mediterranean INtensive Oxidant Study) campaign was an international, multi-platform field campaign to measure long-range transport of air-pollution and aerosols from South East Asia and Europe towards the Mediterranean basin during August 2001. High pollution events were observed during this campaign. For the Mediterranean region enhanced tropospheric nitrogen dioxide (NO2) and formaldehyde (HCHO), which are precursors of tropospheric ozone (O3), were detected by the satellite based GOME (Global Ozone Monitoring Experiment) instrument and compared with airborne in situ measurements as well as with the output from the global 3D photochemistry-transport model MATCH-MPIC (Model of Atmospheric Transport and CHemistry - Max Planck Institute for Chemistry). The increase of pollution in that region leads to severe air quality degradation with regional and global implications.
The present generation of tropospheric chemistry models applies horizontal and vertical model resolutions that are sufficiently fine to represent synoptic-scale processes. In this study we compare simulations of a tropopause folding event on 20-21 June 2001 from six tropospheric ozone models with tropospheric ozone profiles observed at Garmisch-Partenkirchen (Germany). The event involves air masses of stratospheric origin and of North Atlantic and North American tropospheric origin. Two coupled chemistry-climate models, three chemistry-transport models, and one chemistry-trajectory model participate in the intercomparison. The models do not explicitly include stratospheric chemistry, and stratospheric ozone is parameterized instead. The horizontal resolution of the Eulerian models, T42 (2.8° × 2.8°) or finer, appears adequate to represent two prominent features, namely, the stratospheric intrusion descending from the upper troposphere to about 4 km altitude on the first day and an ozone-poor air mass of marine origin in the lower troposphere on the second day. The ozone distribution from the Lagrangian model is less representative because of an insufficient air parcel density. Major discrepancies between model results and observations are the underestimation of ozone levels in the intrusion, too strong downward transport of ozone between the lower stratosphere and the upper troposphere on the first day, and too fast and deep descent of the intrusion. Accurate representation of ozone levels in the intrusion depends directly on the accuracy of the simulated ozone in the lower stratosphere. Additionally, for Eulerian models a relatively coarse vertical resolution in the tropopause region may add to inaccuracies in the simulated ozone distributions.
During the Mediterranean Intensive Oxidant Study MINOS in August 2001, 87 air samples were collected at the ground-based station Finokalia (35°19'N, 25°40'E) on the north coast of Crete and subsequently analysed by GC-MS. The analysis includes various hydrocarbons, organo-halogens, HCFCs and CFCs. These compounds have a wide variety of sources and sinks and a large range of atmospheric lifetimes. We evaluated the characteristics of the sampling site in terms of proximity to individual sources by plotting the measured variability of these species against lifetime. The resulting linear relationship suggests that the sampling site is representative of intermediate conditions between a remote site and one that is in the vicinity of a wide variety of sources. Our analysis of air mass origin and chemical ratios also shows that several distinct anthropogenic sources influenced the atmospheric composition over Crete. Propane observations are compared to a global model to assess the fossil fuel related emission inventory. Although the model reproduces the general pattern of the propane variations, the model mixing ratios are systematically too low by a factor of 1.5 to 3, probably due to an underestimation of the propane emissions from east European countries in the underlying global database EDGAR. Another important finding was that methyl chloroform, a compound banned under the Montreal protocol, showed significant enhancements from background, which were well correlated with CFC-113. This suggests continued use and emission of methyl chloroform by one or more European countries. We also discuss the observed variations of methyl bromide and suggest that the significant peak observed on 12 August 2001 reflects heavy agricultural use as a soil fumigant in Italy.
The distribution and budget of tropospheric NOx over Asia, especially India, are examined using the global 3D chemistry meteorology model MATCH-MPIC and GOME NO2 columns. Enhanced abundances of NO2 over China and northeast India are reproduced by the model, as are the pronounced maxima during biomass burning periods, though somewhat underestimated. The mean NO2 column over India is also reproduced, though the model has trouble with the seasonal cycle for unknown reasons. Model sensitivity tests for the Indian region indicate that the scaled sensitivity to changes in the local NOx source is 60-70% for lower tropospheric NOx and is only 10-25% for tropospheric O3, indicating that moderate reductions or increases in current NOx emissions are not expected to lead to large changes in regional O3 levels. In the upper troposphere, during winter nearly all of the NOx comes from remote sources, while in summer deep convection causes the upper troposphere to become sensitive to local surface emissions (~40-50% scaled sensitivity) and lightning NOx production (~10-20%). The regional lifetime of NOx estimated for India, based on MATCH output is about 15-23 h, comparable to the lifetime of NOx over China (14-21 h), while over Indonesia (23-43 h) and North Asia (21-47 h), it is longer and highly seasonal. Implications of these results are discussed.
The balance of effects that vertical transport associated with deep cumulus convection has on tropospheric O3 is discussed. We first show theoretically that convective mixing of O3 can substantially reduce its column mean lifetime over clean regions, while a much smaller increase is generally expected over polluted regions. The global chemistry-transport model MATCH-MPIC confirms this, computing a 67however, that the net effect of convective transport of all trace gases (O3 and precursors together) is a 12contrast to previous literature, our results indicate that the enhanced O3 production due to precursor transport from polluted regions significantly outweighs the reduction in O3 lifetime due to mixing over clean regions.
Using the proton transfer reaction mass spectrometry (PTR-MS) technique, isoprene and its oxidation products were measured in a productive woodland savanna (Calabozo site, during the wet and dry seasons) and a less productive grassland savanna (La Gran Sabana, Parupa site). The measured protonated masses in the PTR-MS, postulated compounds, and daytime average volume mixing ratios at the Calabozo site during the wet season are: 69 isoprene (1.62 nmol/mol), 71 methyl vinyl ketone + methacrolein (0.98 nmol/mol), 83 3-methyl furan + unsaturated C-5-hydroxycarbonyls (0.12 nmol/mol), and 101 isoprene hydroperoxides (0.16 nmol/mol). Significant diurnal cycles of the hydrocarbon concentrations were observed, with distinct characteristics between sites and seasons. Two times lower levels of isoprene were observed during the dry season. At the Parupa site measured concentrations of all masses were about three times lower than at the Calabozo site during the wet season, and significant transport of isoprene from upwind forests was observed. Comparison with a photochemical box model revealed that surface deposition is likely a significant sink for isoprene and its oxidation products. An isoprene source of 2.1-3.2 x 10(6) molec/cm(3)/s and an HO concentration of 4.1-6.0 x 10(5) molec/cm(3) averaged over 24 hours were needed to match the observed mixing ratios. Assuming a mixed boundary layer of 1500 m and an isoprene source half the strength during the 5 months dry season, a global emission of isoprene to the atmosphere from tropical savannas between 53 and 79 Tg C/yr can be calculated from our results if the Calabozo site is representative.
A fundamental difficulty in 3-D models is addressed which can arise due to inconsistencies between advection schemes and winds. It is shown that a model will fail to meet certain basic criteria, desired of 3-D transport models, when the density field computed by the advection scheme from the winds differs from the implied density field based on the surface pressure and the sigma (or hybrid) coordinates. To allow a rigorous mathematical formulation, the focus is on the example of a mass flux advection scheme in a model where the winds and surface pressure are derived from different advection schemes (e.g. a spectral scheme in a climate model or a westher centre model); however, in principle the discussion applies to nearly any situation in which the pressure levels change in a model. To illustrate the potential severity of such problems, a mass conserving grid- to-grid transformation scheme is constructed which only uses the current tracer mass mixing-ratio distribution It is shown that only one solution exists that is comprehensively valid for any arbititary tracer distribution. and that this type of correction introduces an additional undesired artificial vertical diffusion component into the model transport that increases with increasing tracer mass mixing-ratio gradients and may exceed the physical vertical transport itself. It is demonstrated that the results of any supplementary fix, either grid-to grid transformation, are generally unacceptable for global modelling applications. From this, it is concluded that the only alternative which can produce reliable results for any arbitrary tracer is to maintain a consistent grid throughout the entire model time step, where all changes in pressure levels due to modelled advection exactly match the changes implied by the surface pressure at the next time step. Although this is already done in some models, this would require significant changes in the structure of the advection scheme or its input wind fields in several other contemporary general circulation and chemistry transport models.
We examine the impact of heterogeneous chemistry involving liquid aerosol and ice particles on net ozone (O3) production rates under conditions representative of the midlatitude upper troposphere (UT) and lowermost stratosphere (LS). We demonstrate that heterogeneous effects are controlled by nitrogen oxides (NOx) and by the location of the air masses relative to the tropopause (TP). The net effect of heterogeneous chemistry is to decrease net O3 production below the TP (via heterogeneous HO2 loss) and to cause O3 destruction above the TP (via heterogeneous chlorine (Cl) activation). In the UT, gas phase chemistry due to non-methane hydrocarbons (NMHCs) can become as important for O3 chemistry as heterogeneous reactions, and removal of HO2 by particles can become more important than changes of hydrogen oxides (HOx) through heterogeneous bromine (Br) chemistry. In the humid LS, Cl activation can become sufficiently large, so that O-3 depletion occurs at all conceivable values of NOx. Such cold and humid conditions occur frequently enough to reduce the average ozone production rates in the midlatitude LS by more than 10%.
The global mean OH concentration ([ OH](GM)) has been used as an indicator of the atmospheric oxidizing efficiency and its changes over time. It is also used for evaluating the performance of atmospheric chemistry models by comparing with other models or with observationally-based reference [OH](GM) levels. We contend that the treatment of this quantity in the recent literature renders it problematic for either of these purposes. Several different methods have historically been used to compute [OH](GM) : weighting by atmospheric mass or volume, or by the reaction with CH4 or CH3CCl3. In addition, these have been applied over different domains to represent the troposphere. While it is clear that this can lead to inconsistent [OH](GM) values, to date there has been no careful assessment of the differences expected when [OH](GM) is computed using various weightings and domains. Here these differences are considered using four different 3D OH distributions, along with the weightings mentioned above applied over various atmospheric domains. We find that the [OH](GM) values computed based on a given distribution but using different domains for the troposphere can result in differences of 10weightings can lead to differences of up to 30which is commonly stated for [OH](GM) or its trend. Thus, at present comparing [OH](GM) values from different studies does not provide clearly interpretable information about whether the OH amounts are actually similar or not, except in the few cases where the same weighting and domain have been used in both studies. We define the atmospheric oxidizing efficiency of OH with respect to a given gas as the inverse of the lifetime of that gas, and show that this is directly proportional to the [OH](GM) value weighted by the reaction with that gas, where the proportionality constant depends on the temperature distribution and the domain. We find that the airmass-weighted and volume-weighted [OH](GM) values, in contrast, are generally poor indicators of the global atmospheric oxidizing efficiency with respect to gases such as CH4 and CH3CCl3 with a strong temperature dependence in their reaction with OH. We recommend that future studies provide both the airmass-weighted and the CH4-reaction-weighted [OH](GM) values, over the domain from the surface to a climatological tropopause. The combination of these values helps to reduce the chance of coincidental agreement between very different OH distributions. Serious evaluations of modeled OH concentrations would best be done with airmass-weighted [OH](GM) broken down into atmospheric sub-compartments, especially focusing on the tropics, where the atmospheric oxidizing efficiency is the greatest for most gases.
A new condensed isoprene oxidation mechanism for global atmospheric modeling (MIM) was derived from a highly detailed master chemical mechanism (MCM). In a box model intercomparison covering a wide range of boundary layer conditions the MIM was compared with the MCM and with five other condensed mechanisms, some of which have already been used in global modeling studies of nonmethane hydrocarbon chemistry. The results of MCM and MIM were generally in good agreement, but the other tested mechanisms exhibited substantial differences relative to the MCM as well as relative to each other. Different formation yields, reactivities and degradation pathways of organic nitrates formed in the course of isoprene oxidation were identified as a major reason for the deviations. The relevance of the box model results for chemistry transport models is discussed, and the need for a validated reference mechanism and for an improved representation of isoprene chemistry in global models is pointed out.
Excerpts from the abstract of the original paper commented on:It is our opinion that over the past 20 years, some atmospheric scientists have used a scientifically incorrect method for finding the contribution of methane to changes in ozone. [...] An approach was introduced around 1980 that has been widely used since then, with the valid assumption that in the lower troposphere the methane smog reactions are the only source of gross ozone production, P(methane). [...] The object of this paper is to solve for how methane changes ozone in the troposphere and stratosphere. Crutzen [1973] gave the first treatment of ozone formation in the global troposphere via methane photooxidation. He considered radical attack on methane to produce CH, followed by three reaction sequences that go from CH, to CO,, referred to here as the sequence (SEQ) method. The present study substantially extends Crutzen's sequence method. [...] A detailed derivation is given here, branching ratios are evaluated, and the results are presented as two complicated, closed, algebraic equations, which are valid from the surface up to the middle stratosphere. The SEQ method contains only ozone changes caused by the presence of methane. [...]