Dudley E Shallcross

Gas-phase ultraviolet absorption cross-sections and atmospheric lifetimes of several C2C5 alkyl nitrates

Abstract Gas-phase ultraviolet absorption cross-sections of ethyl nitrate, 1-propyl nitrate, 2-propyl nitrate, 2-methyl, 1-propyl nitrate, 1-butyl nitrate and 1-pentyl nitrate have been measured over the wavelength range 220–340 nm using a dual-beam, diode array spectrometer. Each alkyl nitrate spectrum appears to be the sum of at least two Gaussian-shaped absorptions with an intense π−π ∗ band extending from 190–240 nm having a shoulder between 250–340 nm due to a π−π ∗ system. The absorption cross-sections recorded for ethyl nitrate, 1-propyl nitrate, 2-propyl nitrate and 1-butyl nitrate are within 10% of previous data: those of 2-methyl, 1-propyl nitrate and 1-pentyl nitrate have been measured for the first time. For ethyl nitrate, absorption cross-sections between 280–340 nm in the tail of the near-ultraviolet band declined with decreasing temperature from 298-233 K. At two-dimensional numerical model of tropospheric chemistry was used to calculate atmospheric lifetimes with respect to photodissociation and OH radical reaction that are markedly dependent on season, latitude and altitude. Relatively long, surface level atmospheric lifetimes of several days to weeks confirm that the C2ue5f8C5 alkyl nitrates may act as temporary reservoirs for NOx and suggest that they may also constitute a significant fraction of total reactive odd-nitrogen, NOy, particularly during winter at northern hemisphere high latitudes.

Modelling NOx from lightning and its impact on global chemical fields

Abstract We have used a three-dimensional off-line chemical transport model (CTM) to assess the impact of lightning emissions in the free troposphere both on NO x itself and on other chemical species such as O 3 and OH. We have investigated these effects using two lightning emission scenarios. In the first, lightning emissions are coupled in space and time to the convective cloud top height calculated every 6xa0h by the CTMs moist convection scheme. In the second, lightning emissions are calculated as a constant, monthly mean field. The models performance against observed profiles of NO x and O 3 in the Atlantic and Pacific ocean improves significantly when lightning emissions are included. With the inclusion of these emissions, the CTM produces a significant increase in the NO x concentrations in the upper troposphere, where the NO x lifetime is long, and a smaller increase in the lower free troposphere, where the surface NO x sources dominate. These changes cause a significant increase in the O 3 production in the upper troposphere and hence higher calculated O 3 there. The model indicates that lightning emissions cause local increases of over 50 parts per 10 12 by volume (pptv) in NO x , 200xa0pptv in HNO 3 and 20 parts per 10 9 by volume (ppbv) (>40%) in O 3 . In addition, a smaller increase of O 3 in the lower troposphere occurs due to an increase in the downward transport of O 3 . The O 3 change is accompanied by an increase in OH which is more pronounced in the upper troposphere with a corresponding reduction in CO. The method of emission employed in the model does not appear to have a significant effect globally. In the upper troposphere (above about 300xa0hPa) NO x concentrations are generally lower with monthly mean emissions, because of the de-coupling of emissions from the models convection scheme, which vents NO x aloft more efficiently in the coupled scheme. Below the local convective outflow altitude, NO x concentrations are larger when using the monthly mean emissions than when coupled to the convection scheme, because the more dilute emissions, and nighttime emissions, lead to a slower NO x destruction rate. Only minor changes are predicted in the monthly average fields of O 3 if we emit lightning as a monthly constant field. However, the method of emission becomes important when we make a direct comparison of model results with time varying data. These differences should be taken into account when a direct comparison of O 3 with measurements collected at particular times and locations is attempted.

The vertical distribution of NO3 in the atmospheric boundary layer

Abstract Recent urban measurements suggest that NO 3 concentrations vary significantly with altitude in the lowest few hundred metres of the atmosphere. Calculations using a one-dimensional boundary layer model show that NO 3 concentrations are small near the ground and increase with altitude to a maximum near the top of the nocturnal boundary layer (NBL). These results show that the NBL is not well mixed, and that where there are surface sources and sinks two-box models of the NBL are inadequate, and surface measurements are not representative and may lead to an underestimate of the oxidising capacity of the atmosphere.

A Lagrangian study of the three-dimensional transport of boundary-layer tracers in an idealised baroclinic-wave life-cycle

The role of baroclinic-wave driven chemical transport is examined using the framework of a Lagrangiantrajectory model.The Lagrangian motion of transported trace gases are closely monitored through labelledboundary-layer tracers binned accordingto their latitudinal locations.From a set of 14-day Lagrangian paths, the mechanistically liftedsubtropical boundary-layer tracers track along tilted poleward paths, whilethe subsiding high-latitude tracers track along tilted equator-ward paths.The most significantmovements of tracers occur between days 6 and8. The vertical and latitudinal displacements during this time interval are3 km and 15° latitude. During a baroclinic-wave life-cycle,boundary-layertracers can either ascend vertically from 1 km to 7 km or descend to thesurface, while they arelatitudinally transported from 37° to 73° and from 45° to near 15° during the poleward and equator-ward motions,respectively.Vertical mixing of tracers occurs vigorously at mid-latitudes, where more than50%, by day 7, and a maximum of 70%, between days 9 and 10, of the boundary layer tracers have beentransported intothe free troposphere during the baroclinic-wave life-cycle.A clear 3D picture emerges from a Lagrangian analysis.Each time the tracer travels equator-ward, it descends, whilewhen it travels poleward, it ascends.Almost all of the low latitude tracers show tilted upward and poleward paths,while high latitude tracersshows downward tilted and equator-ward path.The maximum vertical displacement between poleward andequator-ward tracers are shown in mid-latitudes.Two types of the tilted upward and poleward paths are generally seen in thelatitude-height projections:anti-clockwise and clockwise paths.Both types of path transport tracers upward, however, the anti-clockwise pathsdeliver tracersequator-ward, while the clockwise paths deliver tracers poleward.Hence, whenlow latitude warm air arrives at mid-latitude, it can pick up enhanced tracerconcentrationand carry them on either poleward or equator-ward.

Formulation and Evaluation of IMS, an Interactive Three-Dimensional Tropospheric Chemical Transport Model 2. Model Chemistry and Comparison of Modelled CH4, CO, and O3 with Surface Measurements

In part two of this series of papers on the IMS model, we present the chemistry reaction mechanism usedand compare modelled CH4, CO, and O3 witha dataset of annual surface measurements. The modelled monthly and 24-hour mean tropospheric OH concentrationsrange between 5–22 × 105 moleculescm−3, indicating an annualaveraged OH concentration of about 10 × 105 moleculescm−3. This valueis close to the estimated 9.7 ± 0.6 × 105 moleculescm−3 calculated fromthe reaction of CH3CCl3 with OH radicals.Comparison with CH4 generally shows good agreementbetween model and measurements, except for the site at Barrow where modelledwetland emission in the summer could be a factor 3 too high.For CO, the pronounced seasonality shown in the measurements is generally reproduced by the model; however, the modelled concentrations are lower thanthe measurements. This discrepancy may due to lower the CO emission,especially from biomass burning,used in the model compared with other studies.For O3, good agreement between the model and measurements is seenat locations which are away from industrial regions. The maximum discrepancies between modelled results and measurementsat tropical and remote marine sites is about 5–10 ppbv,while the discrepancies canexceed 30 ppbv in the industrial regions.Comparisons in rural areas at European and American continental sites arehighly influenced by the local photochemicalproduction, which is difficult to model with a coarse global CTM.The very large variations of O3 at these locations vary from about15–25 ppbv in Januaryto 55–65 ppbv in July–August. The observed annual O3amplitude isabout 40 ppbv compared with about 20 ppbv in the model. An overall comparison of modelled O3 with measurements shows thatthe O3seasonal surface cycle is generally governed bythe relative importance of two key mechanisms that drivea springtime ozone maximum and asummertime ozone maximum.

A model study of the potential role of the reaction BrO + OH in the production of stratospheric HBr

We have used a constrained one-dimensional photochemical model to investigate the effect of a potential minor channel of the fast reaction between BrO + OH to produce HBr. There is no direct evidence for this reaction but the analogous yield of HCl from ClO + OH is thought to be about 5%. With only a 1–2% yield of HBr the modelled HBr mixing ratio between 20–30 km increases from around 0.5 parts per 1012 by volume (pptv) to 1–2 pptv. This brings the model into agreement with recent balloon-borne observations of stratospheric HBr. Should BrO + OH produce HBr with around 1–2% yield then this reaction will dominate HBr production between 20–35 km. As the main loss of HBr is reaction with OH this will lead to steady state HBr:BrO partitioning which is independent of other species, and temperature.

Formulation and evaluation of IMS, an interactive three-dimensional tropospheric chemical transport Model 1. Model emission schemes and transport processes

In part one of this series of papers on a new integrated modelling system(IMS),the interactive three-dimensional chemical transport model (CTM),we presenta detailed description of the interactive emission scheme for biogenic speciesand outlinethe datasets used for anthropogenic species. In addition, we describe thetransportscheme employed in this model.The biogenic emission schemes incorporate the high-resolution Olson World Ecosystem data (Olson, 1992),the satellite-sensedterrestrial vegetation data from AVHRR (A Very High Resolution Radiometer) (Brown et al., 1985),and the CZCS (Coastal Zone Color Scanner) data (Erickson and Eaton, 1993).These datasetsprovide seasonal variations in surface biogenic emissions.The emission schemesare tested against other estimates (e.g., GEIA) and equilibrium emissions. A comparison of terrestrial biogenic fluxes,both the spatial and temporal (seasonal) variation of modelled surfacenet primary production, is consistent with the geographicalvariations of the global vegetation index (GVI) distribution derived fromAVHRR.The annual net primary production is 76000 Tg C yr−1, whichcompares wellwith the 40500–78000 Tg C yr−1 estimated by Melilloet al. (1993).This indicates that the model works well in capturing spatial andseasonal variations in the terrestrial vegetation. The modelled surface vegetation fluxes are verified against data from Guenther et al. (1995). While thecomparison shows agenerally good agreement in terms of the temporal and spatial distributionsof isoprene (530 Tg yr−1), large discrepancies are seen overthetropical locations which often exhibit strong seasonality in rainfalland very small variation in temperature. These differences indicatethata large difference in the estimation between an empirical relation and an LSMcalculation occursif an area in which seasonal distribution of rainfall is the main factor whichdeterminesthe type of vegetation. In this paper, we assess(results are discussed in following papers)the role of changing surface biogenic distributions insurface-to-atmosphere biogenic fluxes (both ocean-to-atmosphere and land-to-atmosphere).

A study of the self reaction of CH2ClO2 and CHCl2O2 radicals at 298 K

A low-pressure discharge-flow system equipped with laser-induced fluorescence (LIF) detection of NO2 and resonance-fluorescence detection of OH has been employed to study the self reactions CH2ClO2 + CH2ClO2 products (1) and CHCl2O2 + CHCl2O2 products (2), at T = 298 K and P = 1–3 Torr. Possible secondary reactions involving alkoxy radicals are identified. We report the phenomenological rate constants (kobs) n n nk1obs = (4.1 ± 0.2) × 10−12 cm3 molecule−1 s−1k2obs = (8.6 ± 0.2) × 10−12 cm3 molecule−1 s−1 n n n nand the rate constants derived from modelling the decay profiles for both peroxy radical systems, which takes into account the proposed secondary chemistry involving alkoxy radicals n n nk1 = (3.3 ± 0.7) × 10−12 cm3 molecule−1 s−1k2 = (7.0 ± 1.8) × 10−12 cm3 molecule−1 s−1 n n n nA possible mechanism for these self reactions is proposed and QRRK calculations are performed for reactions (1), (2) and the self-reaction of CH3O2, CH3O2 + CH3O2 products (3). These calculations, although only semiquantitative, go some way to explaining why both k1 and k2 are a factor of ten larger than k3 and why, as suggested by the products of reaction (1) and (2), it seems that the favored reaction pathway is different from that followed by reaction (3). The atmospheric fate of the chlorinated peroxy species, and hence the impact of their precursors (CH3Cl and CH2Cl2), in the troposphere are briefly discussed. HC(O)Cl is identified as a potentially important reservoir species produced from the photooxidation of these precursors.

Formulation and evaluation of IMS, an interactive three-dimensional tropospheric chemical transport model 3. Comparison of modelled C2-C5 hydrocarbons with surface measurements

In part 3 of this series of papers on a new 3-D global troposphericchemical transport model, using an Integrated Modelling System (IMS), anevaluation of the model performance in simulating global distributions andseasonal variations for volatile organic compounds (VOCs) in the atmosphere,is presented. Comparisons of model OH concentrations with previous modelstudies show consistent modelled OH levels from the subtropics tomidlatitudes, while more discrepancies occur over the tropical lowlatitudes, with IMS predicting the highest levels of OH. The close agreementbetween modelled OH concentrations over midlatitudes, where high surfaceNOxand VOC concentrations are also found, is indicative of the strongphotochemical coupling between NOx, VOCs and O3 overthese latitudes. IMSOH concentrations in the Northern Hemisphere (NH) midlatitudes during summerare generally lower than available measurements, implying that models ingeneral are underestimating OH levels at this location and time of year.Substantial differences between model OH concentrations over low latitudesclearly highlight areas of uncertainty between models. IMS OH concentrationsare the highest in general of the models compared, one possible reason isthat biogenic emissions of species such as isoprene and monoterpenes arehighest in IMS, leading to higher O3 levels and hence higher OH.Generally, the IMS VOC concentrations show a similar seasonality to themeasurements at most locations. In general though, IMS tends to underestimatethe NH wintertime VOC maximum and overestimate the NH summertime VOCminimum. Such an overestimate in summer could be due to IMSunderestimating OH levels, or an overestimation of VOC emissions or possiblya problem with model transport, all of these possibilities are explored.Except for n-pentane, the model underprediction of a VOC maximum during theNH winter month strongly suggests a missing emission mechanism in the modelor an underestimate of an existing one. It is very likely that there is alack of time varying emission sources in the model to account for theseasonal change in emission behaviour such as increasing energy usage (e.g.,electricity and gas), road transportation, engine performance, and otheranthropogenic factors which show strong seasonal characteristics. Theanomalous overprediction of wintertime n-pentane compared with its closesummertime prediction with the measurements suggest that emissions in thiscase may be too high.

Modelling the impacts of aircraft traffic on the chemical composition of the upper troposphere

Abstract A three-dimensional chemistry-transport model has been used to assess the impacts of aircraft traffic in the upper troposphere. Aircraft engines emit NOx, which has the potential to perturb the chemical composition at the flight altitude paths, i.e. at a 10–12 km height for subsonic flights. The model used includes a comprehensive chemistry scheme, so perturbations to other species apart from NOx could also be examined. More specifically, the model showed that the monthly mean increase for NOx due to aircraft is around 60 pptv (parts per trillion volume (30 per cent increase) and for HNO3 it is 100 pptv (30 per cent increase). Consequently, O3 is enhanced by 2500 pptv (5 per cent increase) due to aircraft traffic. To assess the regional and temporal variations in the perturbations, a time series analysis above a central European grid cell located at 47 °N 18 °E has also been performed. The analysis has indicated that the local perturbations can be much larger than the monthly mean values and can reach 200 pptv for NOx, 150 pptv for HNO3 and 5000 pptv for O3.