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The isotopic composition of molecular hydrogen (H2) contains independent information for constraining the global H2 budget. To explore this, we have implemented hydrogen sources and sinks, including their stable isotopic composition and isotope fractionation constants, into the global chemistry transport model TM5. For the first time, a global model now includes a simplified but explicit isotope reaction scheme for the photochemical production of H2. We present a comparison of modelled results for the H2 mixing ratio and isotope composition with available measurements on seasonal to inter annual time scales for the years 2001–2007. The base model results agree well with observations for H2 mixing ratios. For δD[H2], modelled values are slightly lower than measurements. A detailed sensitivity study is performed to identify the most important parameters for modelling the isotopic composition of H2. The results show that on the global scale, the discrepancy between model and measurements can be closed by adjusting the default values of the isotope effects in deposition, photochemistry and the stratosphere-troposphere exchange within the known range of uncertainty. However, the available isotope data do not provide sufficient information to uniquely constrain the global isotope budget. Therefore, additional studies focussing on the isotopic composition near the tropopause and on the isotope effects in the photochemistry and deposition are recommended.

Would urban air quality and climate improve if we replaced the fossil fuels by molecular hydrogen (H2) as an energy carrier? A quantitative answer to this question requires a thorough understanding of the current role of H2 in the Earth’s atmosphere. On its own, H2 does not impact climate, as for example carbon dioxide or methane. However, increasing levels of H2 in the stratosphere can lead to increased ozone loss due to the formation of polar stratospheric clouds. Additionally, the atmospheric lifetime of methane could increase because both H2 and methane are removed by photochemical oxidation with the hydroxyl radical. Consequently, the lifetime of the strong greenhouse gas methane could be prolonged. During the last two decades, more and more experimental data have become available to put tighter constraints on the different sources and sinks that contribute to the global H2 cycle. However, the main removal process, dry deposition due to microbial/enzymatic decomposition of H2 in the soils, still has a rather large uncertainty between 40-99 Tg/yr globally. This is a highly uncertain number compared to the estimated overall amount of 136-166 Tg present in the troposphere. The photochemical removal of H2 from the atmosphere is estimated at 14-24 Tg/yr. Together with the estimates for the burden and dry deposition, this implies a tropospheric lifetime of H2 between 1.1-3.1 years. The atmospheric H2 is replenished by emissions from the Earth’s surfaces due to fossil fuel burning (5-25 Tg/yr), biomass burning (7-21 Tg/yr) and nitrogen fixation processes in the oceans (1-11 Tg/yr) and soils (0-11 Tg/yr). H2 is photochemically produced from methane (15-21 Tg/yr) and non-methane hydrocarbons (10-25 Tg/yr) in the atmosphere. These uncertainties suggest that at present, the global hydrogen cycle is poorly understood. However, this statement would do little justice to the scientific quality of most studies so far. The main purpose of the research in this thesis is to show that the global tropospheric budget of H2 can be constrained quite well with available measurements. The study starts with the derivation of a full hydrogen isotope chemistry scheme to use the measured deuterium content in atmospheric H2 as an additional constraint for the global budget. This new chemistry scheme is subsequently evaluated and the most important parameters in the photochemistry are identified. A condensed version of the new chemistry scheme is implemented in the global TM5 model. The new model results are verified using available measurements of H2 mixing ratios and isotopic compositions from two global flask sampling networks and the EuroHydros network. Finally, a new tropospheric budget is derived for H2. The tropospheric burden is estimated at 165±8 Tg and the removal of H2 by deposition and photochemical oxidation are estimated at 53±4 and 23±2 Tg/yr, respectively. The main (photochemical) source is estimated at 37±4 Tg/yr. From these numbers, a tropospheric lifetime of 2.2±0.2 yr for H2 is derived. These new ranges of uncertainty allow for a much more accurate evaluation of the impact of future increases in H2 emissions on air quality and climate.

Would urban air quality and climate improve if we replaced the fossil fuels by molecular hydrogen (H2) as an energy carrier? A quantitative answer to this question requires a thorough understanding of the current role of H2 in the Earth’s atmosphere. On its own, H2 does not impact climate, as for example carbon dioxide or methane. However, increasing levels of H2 in the stratosphere can lead to increased ozone loss due to the formation of polar stratospheric clouds. Additionally, the atmospheric lifetime of methane could increase because both H2 and methane are removed by photochemical oxidation with the hydroxyl radical. Consequently, the lifetime of the strong greenhouse gas methane could be prolonged. During the last two decades, more and more experimental data have become available to put tighter constraints on the different sources and sinks that contribute to the global H2 cycle. However, the main removal process, dry deposition due to microbial/enzymatic decomposition of H2 in the soils, still has a rather large uncertainty between 40-99 Tg/yr globally. This is a highly uncertain number compared to the estimated overall amount of 136-166 Tg present in the troposphere. The photochemical removal of H2 from the atmosphere is estimated at 14-24 Tg/yr. Together with the estimates for the burden and dry deposition, this implies a tropospheric lifetime of H2 between 1.1-3.1 years. The atmospheric H2 is replenished by emissions from the Earth’s surfaces due to fossil fuel burning (5-25 Tg/yr), biomass burning (7-21 Tg/yr) and nitrogen fixation processes in the oceans (1-11 Tg/yr) and soils (0-11 Tg/yr). H2 is photochemically produced from methane (15-21 Tg/yr) and non-methane hydrocarbons (10-25 Tg/yr) in the atmosphere. These uncertainties suggest that at present, the global hydrogen cycle is poorly understood. However, this statement would do little justice to the scientific quality of most studies so far. The main purpose of the research in this thesis is to show that the global tropospheric budget of H2 can be constrained quite well with available measurements. The study starts with the derivation of a full hydrogen isotope chemistry scheme to use the measured deuterium content in atmospheric H2 as an additional constraint for the global budget. This new chemistry scheme is subsequently evaluated and the most important parameters in the photochemistry are identified. A condensed version of the new chemistry scheme is implemented in the global TM5 model. The new model results are verified using available measurements of H2 mixing ratios and isotopic compositions from two global flask sampling networks and the EuroHydros network. Finally, a new tropospheric budget is derived for H2. The tropospheric burden is estimated at 165±8 Tg and the removal of H2 by deposition and photochemical oxidation are estimated at 53±4 and 23±2 Tg/yr, respectively. The main (photochemical) source is estimated at 37±4 Tg/yr. From these numbers, a tropospheric lifetime of 2.2±0.2 yr for H2 is derived. These new ranges of uncertainty allow for a much more accurate evaluation of the impact of future increases in H2 emissions on air quality and climate.

This work reassesses the global atmospheric budget of H₂ with the TM5 model. The recent adjustment of the calibration scale for H₂ translates into a change in the tropospheric burden. Furthermore, the ECMWF Reanalysis-Interim (ERA-Interim) data from the European Centre for Medium-Range Weather Forecasts (ECMWF) used in this study show slower vertical transport than the operational data used before. Consequently, more H2 is removed by deposition. The deposition parametrization is updated because significant deposition fluxes for snow, water, and vegetation surfaces were calculated in our previous study. Timescales of 1–2 h are asserted for the transport of H2 through the canopies of densely vegetated regions. The global scale variability of H₂ and δD[H2] is well represented by the updated model. H₂ is slightly overestimated in the Southern Hemisphere because too little H2 is removed by dry deposition to rainforests and savannahs. The variability in H2 over Europe is further investigated using a high-resolution model subdomain. It is shown that discrepancies between the model and the observations are mainly caused by the finite model resolution. The tropospheric burden is estimated at 165+- 8 Tg H2. The removal rates of H2 by deposition and photochemical oxidation are estimated at 53 +- 4 and 23 +- 2 Tg H2/yr, resulting in a tropospheric lifetime of 2.2 +- 0.2 year.

This work reassesses the global atmospheric budget of H2 with the TM5 model. The recent adjustment of the calibration scale for H2 translates into a change in the tropospheric burden. Furthermore, the ECMWF Reanalysis-Interim (ERA-Interim) data from the European Centre for Medium-Range Weather Forecasts (ECMWF) used in this study show slower vertical transport than the operational data used before. Consequently, more H2 is removed by deposition. The deposition parametrization is updated because significant deposition fluxes for snow, water, and vegetation surfaces were calculated in our previous study. Timescales of 1-2h are asserted for the transport of H2 through the canopies of densely vegetated regions. The global scale variability of H2 and [DH2] is well represented by the updated model. H2 is slightly overestimated in the Southern Hemisphere because too little H2 is removed by dry deposition to rainforests and savannahs. The variability in H2 over Europe is further investigated using a high-resolution model subdomain. It is shown that discrepancies between the model and the observations are mainly caused by the finite model resolution. The tropospheric burden is estimated at 165 +/- 8TgH2. The removal rates of H2 by deposition and photochemical oxidation are estimated at 53 +/- 4 and 23 +/- 2TgH2/yr, resulting in a tropospheric lifetime of 2.2 +/- 0.2year.

International audience; This work reassesses the global atmospheric budget of H2 with the TM5 model. The recent adjustment of the calibration scale for H2 translates into a change in the tropospheric burden. Furthermore, the ECMWF Reanalysis‐Interim (ERA‐Interim) data from the European Centre for Medium‐Range Weather Forecasts (ECMWF) used in this study show slower vertical transport than the operational data used before. Consequently, more H2 is removed by deposition. The deposition parametrization is updated because significant deposition fluxes for snow, water, and vegetation surfaces were calculated in our previous study. Timescales of 1–2 h are asserted for the transport of H2 through the canopies of densely vegetated regions. The global scale variability of H2 and δ[DH2] is well represented by the updated model. H2 is slightly overestimated in the Southern Hemisphere because too little H2 is removed by dry deposition to rainforests and savannahs. The variability in H2 over Europe is further investigated using a high‐resolution model subdomain. It is shown that discrepancies between the model and the observations are mainly caused by the finite model resolution. The tropospheric burden is estimated at 165±8 Tg H2. The removal rates of H2 by deposition and photochemical oxidation are estimated at 53±4 and 23±2 Tg H2/yr, resulting in a tropospheric lifetime of 2.2±0.2 year.

This work reassesses the global atmospheric budget of H₂ with the TM5 model. The recent adjustment of the calibration scale for H₂ translates into a change in the tropospheric burden. Furthermore, the ECMWF Reanalysis-Interim (ERA-Interim) data from the European Centre for Medium-Range Weather Forecasts (ECMWF) used in this study show slower vertical transport than the operational data used before. Consequently, more H2 is removed by deposition. The deposition parametrization is updated because significant deposition fluxes for snow, water, and vegetation surfaces were calculated in our previous study. Timescales of 1–2 h are asserted for the transport of H2 through the canopies of densely vegetated regions. The global scale variability of H₂ and δD[H2] is well represented by the updated model. H₂ is slightly overestimated in the Southern Hemisphere because too little H2 is removed by dry deposition to rainforests and savannahs. The variability in H2 over Europe is further investigated using a high-resolution model subdomain. It is shown that discrepancies between the model and the observations are mainly caused by the finite model resolution. The tropospheric burden is estimated at 165+- 8 Tg H2. The removal rates of H2 by deposition and photochemical oxidation are estimated at 53 +- 4 and 23 +- 2 Tg H2/yr, resulting in a tropospheric lifetime of 2.2 +- 0.2 year.

n.v.t.

The isotopic composition of molecular hydrogen (H2) contains independent information for constraining the global H2 budget. To explore this, we have implemented hydrogen sources and sinks, including their stable isotopic composition and isotope fractionation constants, into the global chemistry transport model TM5. For the first time, a global model now includes a simplified but explicit isotope reaction scheme for the photochemical production of H2. We present a comparison of modelled results for the H2 mixing ratio and isotope composition with available measurements on seasonal to inter annual time scales for the years 2001–2007. The base model results agree well with observations for H2 mixing ratios. For δD[H2], modelled values are slightly lower than measurements. A detailed sensitivity study is performed to identify the most important parameters for modelling the isotopic composition of H2. The results show that on the global scale, the discrepancy between model and measurements can be closed by adjusting the default values of the isotope effects in deposition, photochemistry and the stratosphere-troposphere exchange within the known range of uncertainty. However, the available isotope data do not provide sufficient information to uniquely constrain the global isotope budget. Therefore, additional studies focussing on the isotopic composition near the tropopause and on the isotope effects in the photochemistry and deposition are recommended.

Inverse modeling techniques used to quantify surface carbon fluxes commonly assume that the uncertainty of fossil fuel CO2 (FFCO2) emissions is negligible and that intra-annual variations can be neglected. To investigate these assumptions, we analyzed the differences between four fossil fuel emission inventories with spatial and temporal differences over Europe and their impact on the model simulated CO2 concentration. Large temporal flux variations characterize the hourly fields (~40 % and ~80 % for the seasonal and diurnal cycles, peak-to-peak) and annual country totals differ by 10 % on average and up to 40 % for some countries (i.e., the Netherlands). These emissions have been prescribed to seven different transport models, resulting in 28 different FFCO2 concentrations fields. The modeled FFCO2 concentration time series at surface sites using time-varying emissions show larger seasonal cycles (+2 ppm at the Hungarian tall tower (HUN)) and smaller diurnal cycles in summer (−1 ppm at HUN) than when using constant emissions. The concentration range spanned by all simulations varies between stations, and is generally larger in winter (up to ~10 ppm peak-to-peak at HUN) than in summer (~5 ppm). The contribution of transport model differences to the simulated concentration std-dev is 2–3 times larger than the contribution of emission differences only, at typical European sites used in global inversions. These contributions to the hourly (monthly) std-dev's amount to ~1.2 (0.8) ppm and ~0.4 (0.3) ppm for transport and emissions, respectively. First comparisons of the modeled concentrations with 14C-based fossil fuel CO2 observations show that the large transport differences still hamper a quantitative evaluation/validation of the emission inventories. Changes in the estimated monthly biosphere flux (Fbio) over Europe, using two inverse modeling approaches, are relatively small (less that 5 %) while changes in annual Fbio (up to ~0.15 % GtC yr−1) are only slightly smaller than the differences in annual emission totals and around 30 % of the mean European ecosystem carbon sink. These results point to an urgent need to improve not only the transport models but also the assumed spatial and temporal distribution of fossil fuel emission inventories.

Despite the potential of isotope measurements to improve our understanding of the global atmospheric molecular hydrogen (H2) cycle, few H2 isotope data have been published so far. Now, within the EUROpean network for atmospheric HYDRogen Observations and Studies project (EUROHYDROS), weekly to monthly air samples from six locations in a global sampling network have been analysed for H2 mixing ratio (m(H2)) and the stable isotopic composition of the H2 (δ(D,H2), hereafter referred to as δD). The time series thus obtained now cover one to five years for all stations. This is the largest set of ground station observations of δD so far. Annual average δD values are higher at the Southern Hemisphere (SH) than at the Northern Hemisphere (NH) stations; the maximum is observed at Neumayer (Antarctica), and the minimum at the non-arctic NH stations. The maximum seasonal differences in δD range from ≈18 ‰ at Neumayer to ≈45 ‰ at Schauinsland (Southern Germany); in general, seasonal variability is largest at the NH stations. The timing of minima and maxima differs per station as well. In Alert (Arctic Canada), the variations in δD and m(H2) can be approximated as simple harmonic functions with a ≈5-month relative phase shift. This out-of-phase seasonal behaviour of δD and m(H2) can also be detected, but delayed and with a ≈6-month relative phase shift, at Mace Head and Cape Verde. However, no seasonal δD cycle could be observed at Schauinsland, which likely reflects the larger influence of local sources and sinks at this continental station. At the two SH stations, no seasonal cycle could be detected in the δD data. If it is assumed that the sink processes are the main drivers of the observed seasonality in m(H2) and δD on the NH, the relative seasonal variations can be used to estimate the relative sink strength of the two major sinks, deposition to soils and atmospheric oxidation by the hydroxyl (OH) radical. For the NH coastal and marine stations this analysis suggests that the relative contribution of soil uptake to the total annual H2 removal increases with latitude.

Inverse modeling techniques used to quantify surface carbon fluxes commonly assume that the uncertainty of fossil fuel CO2 (FFCO2) emissions is negligible and that intra-annual variations can be neglected. To investigate these assumptions, we analyzed the differences between four fossil fuel emission inventories with spatial and temporal differences over Europe and their impact on the model simulated CO2 concentration. Large temporal flux variations characterize the hourly fields (~40 % and ~80 % for the seasonal and diurnal cycles, peak-to-peak) and annual country totals differ by 10 % on average and up to 40 % for some countries (i.e., the Netherlands). These emissions have been prescribed to seven different transport models, resulting in 28 different FFCO2 concentrations fields. The modeled FFCO2 concentration time series at surface sites using time-varying emissions show larger seasonal cycles (+2 ppm at the Hungarian tall tower (HUN)) and smaller diurnal cycles in summer (−1 ppm at HUN) than when using constant emissions. The concentration range spanned by all simulations varies between stations, and is generally larger in winter (up to ~10 ppm peak-to-peak at HUN) than in summer (~5 ppm). The contribution of transport model differences to the simulated concentration std-dev is 2–3 times larger than the contribution of emission differences only, at typical European sites used in global inversions. These contributions to the hourly (monthly) std-dev's amount to ~1.2 (0.8) ppm and ~0.4 (0.3) ppm for transport and emissions, respectively. First comparisons of the modeled concentrations with 14C-based fossil fuel CO2 observations show that the large transport differences still hamper a quantitative evaluation/validation of the emission inventories. Changes in the estimated monthly biosphere flux (Fbio) over Europe, using two inverse modeling approaches, are relatively small (less that 5 %) while changes in annual Fbio (up to ~0.15 % GtC yr−1) are only slightly smaller than the differences in annual emission totals and around 30 % of the mean European ecosystem carbon sink. These results point to an urgent need to improve not only the transport models but also the assumed spatial and temporal distribution of fossil fuel emission inventories.

Inverse modeling techniques used to quantify surface carbon fluxes commonly assume that the uncertainty of fossil fuel CO2 (FFCO2) emissions is negligible and that intra-annual variations can be neglected. To investigate these assumptions, we analyzed the differences between four fossil fuel emission inventories with spatial and temporal differences over Europe and their impact on the model simulated CO2 concentration. Large temporal flux variations characterize the hourly fields (~40 % and ~80 % for the seasonal and diurnal cycles, peak-to-peak) and annual country totals differ by 10 % on average and up to 40 % for some countries (i.e., the Netherlands). These emissions have been prescribed to seven different transport models, resulting in 28 different FFCO2 concentrations fields. The modeled FFCO2 concentration time series at surface sites using time-varying emissions show larger seasonal cycles (+2 ppm at the Hungarian tall tower (HUN)) and smaller diurnal cycles in summer (−1 ppm at HUN) than when using constant emissions. The concentration range spanned by all simulations varies between stations, and is generally larger in winter (up to ~10 ppm peak-to-peak at HUN) than in summer (~5 ppm). The contribution of transport model differences to the simulated concentration std-dev is 2–3 times larger than the contribution of emission differences only, at typical European sites used in global inversions. These contributions to the hourly (monthly) std-dev's amount to ~1.2 (0.8) ppm and ~0.4 (0.3) ppm for transport and emissions, respectively. First comparisons of the modeled concentrations with 14C-based fossil fuel CO2 observations show that the large transport differences still hamper a quantitative evaluation/validation of the emission inventories. Changes in the estimated monthly biosphere flux (Fbio) over Europe, using two inverse modeling approaches, are relatively small (less that 5 %) while changes in annual Fbio (up to ~0.15 % GtC yr−1) are only slightly smaller than the differences in annual emission totals and around 30 % of the mean European ecosystem carbon sink. These results point to an urgent need to improve not only the transport models but also the assumed spatial and temporal distribution of fossil fuel emission inventories.

The isotopic composition of molecular hydrogen (H2) produced by photochemical oxidation of methane (CH4) and Volatile Organic Compounds (VOCs) is a key quantity in the global isotope budget of (H2). The many individual reaction steps involved complicate its investigation. Here we present a simplified structure-activity approach to assign isotope effects to the individual elementary reaction steps in the oxidation sequence of CH4 and some other VOCs. The approach builds on and extends the work by Gerst and Quay (2001) and Feilberg et al. (2007b). The description is generalized and allows the application, in principle, also to other compounds. The idea is that the C-H and C-D bonds – seen as reactive sites – have similar relative reaction probabilities in isotopically substituted, but otherwise identical molecules. The limitations of this approach are discussed for the reaction CH4+Cl. The same approach is applied to VOCs, which are important precursors of H2 that need to be included into models. Unfortunately, quantitative information on VOC isotope effects and source isotope signatures is very limited and the isotope scheme at this time is limited to a strongly parameterized statistical approach, which neglects kinetic isotope effects. Using these concepts we implement a full hydrogen isotope scheme in a chemical box model and carry out a sensitivity study to identify those reaction steps and conditions that are most critical for the isotope composition of the final H2 product. The reaction scheme is directly applicable in global chemistry models, which can thus include the isotope pathway of H2 produced from CH4 and VOCs in a consistent way

International audience

The ability to reliably estimate CO2 fluxes from current in situ atmospheric CO2 measurements and future satellite CO2 measurements is dependent on transport model performance at synoptic and shorter timescales. The TransCom continuous experiment was designed to evaluate the performance of forward transport model simulations at hourly, daily, and synoptic timescales, and we focus on the latter two in this paper. Twenty-five transport models or model variants submitted hourly time series of nine predetermined tracers (seven for CO2) at 280 locations. We extracted synoptic-scale variability from daily averaged CO2 time series using a digital filter and analyzed the results by comparing them to atmospheric measurements at 35 locations. The correlations between modeled and observed synoptic CO2 variabilities were almost always largest with zero time lag and statistically significant for most models and most locations. Generally, the model results using diurnally varying land fluxes were closer to the observations compared to those obtained using monthly mean or daily average fluxes, and winter was often better simulated than summer. Model results at higher spatial resolution compared better with observations, mostly because these models were able to sample closer to the measurement site location. The amplitude and correlation of model-data variability is strongly model and season dependent. Overall similarity in modeled synoptic CO2 variability suggests that the first-order transport mechanisms are fairly well parameterized in the models, and no clear distinction was found between the meteorological analyses in capturing the synoptic-scale dynamics. [References: 47]

International audience; A forward atmospheric transport modeling experiment has been coordinated by the TransCom group to investigate synoptic and diurnal variations in CO 2. Model simulations were run for biospheric, fossil, and air-sea exchange of CO 2 and for SF 6 and radon for 2000-2003. Twenty-five models or model variants participated in the comparison. Hourly concentration time series were submitted for 280 sites along with vertical profiles, fluxes, and meteorological variables at 100 sites. The submitted results have been analyzed for diurnal variations and are compared with observed CO 2 in 2002. Mean summer diurnal cycles vary widely in amplitude across models. The choice of sampling location and model level account for part of the spread suggesting that representation errors in these types of models are potentially large. Despite the model spread, most models simulate the relative variation in diurnal amplitude between sites reasonably well. The modeled diurnal amplitude only shows a weak relationship with vertical resolution across models; differences in near-surface transport simulation appear to play a major role. Examples are also presented where there is evidence that the models show useful skill in simulating seasonal and synoptic changes in diurnal amplitude.

In this work, the performance of the Framework for Atmosphere-Canopy Exchange Modelling (FACEM: Pieterse et al., 2007) coupled to a Lagrangian atmospheric transport model is evaluated for carbon dioxide. Before incorporating FACEM into the Lagrangian COMET model (Vermeulen et al., 2006), its performance for the European domain is compared with the Simple Biosphere model (SiB3: Sellers et al., 1996). Overall, FACEM is well correlated to SiB3 (R2?0.60), but shows less variability for regions with predominantly bare soil. There is no significant overall bias between the models except for the winter conditions and in general for the Iberian peninsula. When coupled to the COMET transport model, both biosphere models yield similar correlations (R2?0.60) and bias relative to the 1-hourly concentration measurements for the year 2002, performed at three different sites in Europe; Cabauw (Netherlands), Hegyhatsal (Hungary) and Mace Head (Ireland). The overall results indicate that FACEM is comparable to SiB3 in terms of its applicability for atmospheric modelling studies.

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The reactivity of aromatic compounds is of great relevance to pure and applied chemical disciplines, yet existing methods for estimating gas-phase rate coefficients for their reactions with free radicals lack accuracy and universality. Here a novel approach is taken, whereby strong relationships between rate coefficients of aromatic hydrocarbons and a Randić-type topological index are investigated, optimized and developed into a method which requires no specialist software or computing power. Measured gas-phase rate coefficients for the reaction of aromatic hydrocarbons with OH radicals were correlated with a calculated Randić-type index, and optimized by including a term for side chain length. Although this method is exclusively for use with hydrocarbons, it is more diverse than any single existing methodology since it incorporates alkenylbenzenes into correlations, and can be extended towards other radical species such as O(3P) (and tentatively NO3, H and Cl). A comparison (with species common to both techniques) is made between the topological approach advocated here and a popular approach based on electrophilic subsituent constants, where it compares favourably. A modelling study was carried out to assess the impact of using estimated rate coefficients as opposed to measured data in an atmospheric model. The difference in model output was negligible for a range of NOx concentrations, which implies that this method has utility in complex chemical models. Strong relationships (e.g. for OH, R2=0.96) between seemingly diverse compounds including benzene, multisubstituted benzenes with saturated, unsaturated, aliphatic and cyclic substitutions and the nonbenzenoid aromatic, azulene suggests that the Randić-type index presented here represents a new and effective way of describing aromatic reactivity, based on a quantitative structure-activity relationship (QSAR).