K-Y Wang

Modelling terrestrial biogenic isoprene fluxes and their potential impact on global chemical species using a coupled LSM–CTM model

Abstract In this paper we investigate the important role of the biogenic species isoprene on tropospheric chemistry using a land surface model (LSM) and a three-dimensional (3-D) tropospheric chemistry transport model (CTM). An efficient and conservative coupling scheme is used to couple the LSM to the 3-D CTM. Annual integrations of the coupled model have been performed and the results compared with other estimates. The comparison shows that the annual global isoprene flux from terrestrial vegetation is 530 Tg C yr −1 , which is in good agreement with 503 Tg C yr −1 estimated by a high-resolution (0.5°×0.5°) vegetation model of Guenther et al. (1995, Journal of Geophysical Research 100 (D5), 8873–8892). Comparison of the seasonal variations of the surface emission distribution between the coupled model and Guenther et al. (1995) also shows close agreement. The potential impact of isoprene on the levels of tropospheric species is studied by running the same coupled model for the period of June–December but without biogenic isoprene emissions included, and the results are compared with the run which includes biogenic isoprene emissions. Our comparison indicates a significant difference in O 3 and PAN for both hemispheres. The discrepancy between the run with and without isoprene is predominantly governed by the spatial and temporal variations of terrestrial vegetation. The largest difference is seen in the summertime northern hemisphere at locations with extensive terrestrial vegetation (e.g. North America, Europe, east and southeast Asia, South America and equatorial central Africa). For O 3 , there is about a 4 ppbv increase over the oceanic areas and about an 8–12 ppbv increase over the mid-latitude land areas. For PAN, a maximum of about one order of magnitude in difference, which increases from 0.01 ppbv (without isoprene emissions) to 0.1–0.3 ppbv (with isoprene emissions), is seen in areas of extensive terrestrial vegetation.

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.

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).

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.

Improvement of a 3D CTM and a 4D variational data assimilation on a vector machine CRAY J90 through a multitasking strategy

In this paper we report on the improvement of a 3D CTM and a 4D variational data assimilation (4DDA) on a vector machine CRAY J90. Significant speedup has been achieved by applying a general multitasking strategy to both a 3D CTM and a 4DDA on the shared-memory platform of a CRAY J90. The 3D CTM has been multitasked to study many complex processes involved in the troposphere. For example, annual simulation to study the interaction between the atmosphere, biosphere (e.g., terrestrial vegetation), and oceans; while the 4DDA has been multitasked to assimilate observational data from satellites (e.g., UARS and ATMOS) and other measurements (e.g., ozonesondes and aircrafts). Evaluation of the multitasked models (both 3D CTM and 4DDA) are carried out by comparing (1) required job elapsed time, and (2) spatial and temporal distribution of long-lived and short-lived chemical species, physical fields, and photolysis rates between the single-threaded and the multitasked simulations. The agreement from the later comparisons indicate a correct multitasking strategy, while the first comparison shows a significantly reduced elapsed time. This validates the need of a multitasking strategy in complex global biogeochemical modeling and 4D chemical data assimilation.