Marine Boundary-Layer Variability Over The Indian Ocean During Indoex (1998)

The variability in boundary-layerstructure over the Indian Ocean during a north-eastmonsoon and the factors influencing it areinvestigated. This study was made possible as acomponent of the Indian Ocean Experiment (INDOEX),conducted from February 19 to March 30, 1998. The dataused are, surface-layer mean and turbulencemeasurements of temperature, humidity and wind, andvertical soundings of temperature and humidity.Significant spatio-temporal variability was observedin the boundary-layer structure throughout the cruise.The ITCZ was characterized as the region withstrongest winds and maximum surface turbulent fluxesof momentum and heat. One of the important findingsfrom this study was a strong influence of continentalair masses on the boundary-layer structure in theNorthern Hemisphere, even at a distance of 600 km offthe Indian coast. This was generally evident in theform of an elevated plume of dry continental airbetween altitudes of 1500 m and 2700 m. Advection ofcontinental aerosols in this layer presents potentialfor significant entrainment into shallow clouds inthis region, which eventually feed deeper clouds atthe ITCZ. This finding provides an explanation foranomalous higher aerosol concentrations found duringprevious studies. The structure of the marineboundary layer was influenced by various factors suchas proximity to land, an anomalous warm pool in theocean and the ITCZ. In the southern hemisphere, theboundary-layer height was primarily governed bysurface-layer sensible heat flux and was found to behighest in the vicinity of the ITCZ. North of theequator it was strongly influenced by land-air-seainteractions. In addition to this synoptic modulation,there was also a significant diurnal variability inthe boundary-layer height.     

On the Role of Lightning NOx in the Formation of Tropospheric Ozone Plumes: A Global Model Perspective

A series of ozone transects measured each year from 1987 to 1990 over thewestern Pacific and eastern Indian oceans between mid-November andmid-Decembershows a prominent ozone maximum reaching 50–80 ppbv between 5 and 10 kmin the 20° S–40° S latitude band. This maximum contrasts with ozonemixing ratios lower than20 ppbv measured at the same altitudes in equatorial regions. Analyses witha globalchemical transport model suggest that these elevated ozone values are part ofa large-scale tropospheric ozone plume extending from Africa to the western Pacific acrosstheIndian ocean. These plumes occur several months after the peak in biomassburninginfluence and during a period of high lightning activity in the SouthernHemispheretropical belt. The composition and geographical extent of these plumes aresimilar to theozone layers previously encountered during the biomass burning season in thisregion.Our model results suggest that production of nitrogen oxides from lightningstrokes sustains the NO_x (= NO+NO₂) levels and the ozonephotochemical productionrequired in the upper troposphere to form these persistent elevated ozonelayers emanating from biomass burning regions.     

Studying the City Plume of Berlin on 20 July 1998 with Three Different Modelling Approaches

In this paper we try to identify and describe the specifics of the Berlin city plume characterised by a zone of enhanced photochemical activity downwind of the urban area, where the major emissions of ozone precursors (NO_x and VOC) take place. Two Eulerian CTM systems (EURAD and REM3/CALGRID) and one Euler–Lagrangian model (LaMM5) are applied to thearea of Berlin/Brandenburg to investigate the processes involved in the evolution of the Berlin plume inherent to the models. The study focuses on 20 July 1998,the first special observation period (SOP) during BERLIOZ. The examination includes (1) the role of turbulence and transport and the role of mass flux to the surface during the evolution of the plume, (2) the import situation of the ground based measurement sites, (3)the terms of the ozone budget equation and their contribution to precursor distribution and ozone formation, (4) the substructures of the plume defined by its chemical regimes. Main results show that the complex meteorological conditions during this SOP with considerable turbulent transports demand an Euler–Lagrangian approach to determine the source receptor relationshipsfor the ground based measurement sites. These relations reveal that only a minor part (5%) of the air approaching Pabstthum and the more downwind (north-western) stations origins from Berlin and that the mutual transport between all sites was limited. The latter result afflicts the joint interpretation of data from different stations because the presumption to measure the same air mass is violated in many cases for this SOP. The CTM results reveal that the local net production of ozone within the plume was controlled by transport ad chemical processes both owning the same importance but tending to counteract each other. At this SOP far distance sources of ozone precursors originating from the Bohemian basin did not interfere the Berlin city plume although they generally have the potential to do so.     

Ozone and PAN Formation Inside and Outside of the Berlin Plume – Process Analysis and Numerical Process Simulation

During the BERLIOZ field phase on 20 July 1998 a 40 km wide ozone-plume 30 to 70 km north of Berlin in the lee of the city was detected. The ozone mixing ratio inside the plume was app. 15 ppb higher than outside, mainly caused by high ozone precursor emissions in Berlin, resulting in a net chemical ozone production of 6.5 ppb h^–1, which overcompensates ozone advection of –3.6 ppb h^–1 andturbulent diffusion of –1.1 ppb h^–1. That means, although moreozone leaves the control volume far in the lee of Berlin than enters it at the leeside cityborder and although turbulent diffusion causes a loss of ozone in the leeside control volume the chemical production inside the volume leads to a net ozone increase. Using a semi-Lagrangian mass budget method to estimate the net ozone production, 5.0 ppb h^–1 are calculated for theplume. This means a fraction of about 20% of ozone in the plume is producedby local emissions, therefore called `home made' by the Berlin emissions. For the same area KAMM/DRAIS simulations using an observation based initialisation, results in a net production rate between 4.0 and 6.5 ppbh^–1, while the threefold nested EURAD model gives 6.0 ppbh^–1. The process analysis indicates in many cases goodagreement (10% or better) between measurements and simulations not only in the ozone concentrations but also with respect to the physical and chemical processes governing the total change. Remaining differences are caused by different resolution in time and space of the models and measurements as well as by errors in the emission calculation.The upwind-downwind differences in PAN concentrations are partly similar to those of ozone, because in the BERLIOZ case they are governed mainly by photochemical production. While in the stable boundary layer at night and windward of Berlin 0.1 to 0.3 ppb are detected, in the centre of the plume at noon concentrations between 0.75 ppb and 1.0 ppb are measured. The O₃/PAN ratio is about 80 to 120 and thus due to the relatively lowPAN concentrations significantly higher than found in previous studies. The low PAN formation on 20 July, was mainly restricted by the moderate nonmethane hydrocarbon levels, whereas high PAN concentrations of 3.0 ppb on 21 July, are caused by local production in the boundary layer and by large scale advection aloft.     

Vertical ozone fluxes and related deposition parameters over agricultural and forested landscapes

The spatial variability and temporal behavior of the vertical flux of ozone have been investigated from turbulence measurements collected on aircraft flight legs in the daytime period during two consecutive summer experimental field programs. The data were obtained during horizontal flight legs conducted over agricultural crops and forested land in three different regions of the eastern United States.Results from individual experimental cases and statistics derived from all cases in each region are presented. Ozone flux generally exhibited a significant height dependency. The strongest negative (downward) fluxes in the lowest-level flight legs were primarily attributed to the uptake of ozone by the surface and vegetative cover. Fluxes were near-zero in the middle of the convective boundary layer (CBL) in the afternoon period. As ozone flux was proportional to concentration, slightly stronger fluxes were found in low-level urban plume segments where ozone concentrations were 10–20 ppb higher than in the surrounding area. The derived deposition velocity showed no such bias as a function of position across the urban plume. Ozone flux differences were not apparent between the more heavily forested sections and the primarily agricultural cropland areas in these regions. During the afternoon period when no clear temporal trend was evident, means from values obtained below 0.15Z _i (Z _i being the CBL height) were -0.421 and -0.431 ppb m^-2 s^-1 for ozone flux and 0.81 and 0.82 cm s^-1 for the derived mean deposition velocity in the southeastern Pennsylvania and central Ohio areas, respectively. These experimental results for ozone provide support to a dry deposition parameterization module which computes grid-area averaged deposition velocities for use in regional-scale models.On assignment from the National Oceanic and Atmospheric Administration, U.S. Department of Commerce.     

土壤热异常影响地表能量平衡的个例分析和数值模拟

The statistical relationship between soil thermal anomaly and short-term climate change is presented based on a typical case study. Furthermore, possible physical mechanisms behind the relationship are revealed through using an off-line land surface model with a reasonable soil thermal forcing at the bottom of the soil layer.In the first experiment, the given heat flux is 5 W m-2 at the bottom of the soil layer (in depth of 6.3 m)for 3 months, while only a positive ground temperature anomaly of 0.06℃ can be found compared to the control run. The anomaly, however, could reach 0.65℃ if the soil thermal conductivity was one order of magnitude larger. It could be even as large as 0.81℃ assuming the heat flux at bottom is 10 W m-2. Meanwhile, an increase of about 10 W m-2 was detected both for heat flux in soil and sensible heat on land surface, which is not neglectable to the short-term climate change. The results show that considerable response in land surface energy budget could be expected when the soil thermal forcing reaches a certain spatial-tem poral scale. Therefore, land surface models should not ignore the upward heat flux from the bottom of the soil layer. Moreover, integration for a longer period of time and coupled land-atmosphere model are also necessary for the better understanding of this issue.     

Studies of Oxidant Production at the Weybourne Atmospheric Observatory in Summer and Winter Conditions

Detailed studies have been made of the behaviour of gases and radicals involved in the production of oxidants at the Weybourne Atmospheric Observatory in both summertime and wintertime conditions. In June 1995 the range of meteorological conditions experienced varied such that ozone destruction was observed in clean northerly air flows reaching Weybourne down the North Sea from the Arctic, and ozone production was observed in varying degrees in air with different loadings of nitrogen oxides and other precursors. The transition point for ozone destruction to ozone production occurred at a nitric oxide concentration of the order of 50 pptv. Plumes of polluted air from various urban areas in the U.K. were experienced in the June campaign at Weybourne. Quantitative studies of ozone production in a plume from the Birmingham conurbation on 18 June 1995 showed that the measurement of ozone production agreed well with calculated production rates from the product of the nitric oxide and peroxy radical concentrations (r2=0.9). In wintertime conditions (October–November 1994) evidence was also found for oxidant production, defined as the sum of O3+NO2. At this time of year the peroxy radical concentrations (RO2) were much lower than observed in the summertime and the nitric oxide (NO) was much higher. There was still sufficient RO2 during the day, however, for a slow accumulation of oxidant. Confirmatory evidence for this comes from the diurnal co-variance of (O3+NO2) with PAN, an excellent tracer of tropospheric photochemistry. The same type of covariance occurs in summer between PAN and ozone. The results obtained in these series of measurements are pertinent to understanding the measures necessary to control production of regional photochemical air pollution, and to the production of ozone throughout the northern hemisphere in winter.     

人为扰动对陆面水分能量的影响——以沩水河流域为例

以沩水河流域为例,基于陆面模式CLM4.5,建立了综合考虑作物种植、地下水开采及灌溉等人类活动的流域陆面水文模型。利用所发展模型,针对1981~2012年,取500 m空间分辨率,探讨人为扰动对陆面过程的影响。研究表明:1）地下水侧向流使得中下游地区地下水位有所提高,平原地下水埋深分布在4 m左右,山区埋深可达到几十米;模拟的叶面积指数较静态MODIS叶面积指数偏大1左右,由此使得种植区月蒸腾量提高约10 mm,土壤蒸发和地表产流有所减少;在灌溉作用下,作物叶面积指数略增长,蒸散发稍有提高,而在假设水稻采用漫灌的情况下,水库灌溉补偿了作物生长产生的水消耗,提高了该区域土壤湿度,增加潜热通量;研究区地下水开采存在但其水文效应并不显著。2）土地覆盖变化自1990年有较大变动,1990~2000年以林地为主,2000年后以耕地为主,其中,1990~2000年土地覆盖类型变化不明显,2001~2012年耕地面积呈先减少再增加又减少的趋势,林地面积则先增加再减少又增加,耕地与林地在2012年所占比例基本持平;同一土地类型内,植被类型变化较为明显,导致陆面水文模拟结果差异较大。     

Intraseasonal Oscillation of Tropospheric Ozone over the Indian Summer Monsoon Region

Boreal summer intraseasonal oscillation(BSISO) of lower tropospheric ozone is observed in the Indian summer monsoon(ISM) region on the basis of ERA-Interim reanalysis data and ozonesonde data from the World Ozone and Ultraviolet Radiation Data Centre. The 30–60-day intraseasonal variation of lower-tropospheric ozone shows a northwest–southeast pattern with northeastward propagation in the ISM region. The most significant ozone variations are observed in the Maritime Continent and western North Pacific. In the tropics, ozone anomalies extend from the surface to 300 hPa; however, in extratropical areas, it is mainly observed under 500 hPa. Precipitation caused by BSISO plays a dominant role in modulating the BSISO of lower-tropospheric ozone in the tropics, causing negative/positive ozone anomalies in phases 1–3/5–6. As the BSISO propagates northeastward to the western North Pacific, horizontal transport becomes relatively more important, increasing/reducing tropospheric ozone via anticyclonic/cyclonic anomalies over the western North Pacific in phases 3–4/7–8.As two extreme conditions of the ISM, most of its active/break events occur in BSISO phases 4–7/1–8 when suppressed/enhanced convection appears over the equatorial eastern Indian Ocean and enhanced/suppressed convection appears over India, the Bay of Bengal, and the South China Sea. As a result, the BSISO of tropospheric ozone shows significant positive/negative anomalies over the Maritime Continent, as well as negative/positive anomalies over India, the Bay of Bengal,and the South China Sea in active/break spells of the ISM. This BSISO of tropospheric ozone is more remarkable in break spells than in active spells of the ISM, due to the stronger amplitude of BSISO in the former.     

Ozone concentration characteristics at a high-elevation forest site

Summary Atmospheric ozone concentrations have been monitored at a subalpine forest ecosystem site, 3180m above mean sea level (msl), and at a 2680m msl forest-steppe ecotone site 15km to the southeast. Ozone concentrations were monitored at three heights above the ground on a 30m tower at the higher elevation site, and on a 10m tower in a large meadow downwind of this site.The research reported here concentrates on the data for the first six months of 1990–1994. The diel amplitude of ozone concentrations is very small in winter, increasing as the seasons advance to June. Following snowmelt in late May or June, the nighttime minima decrease under the forest canopy. The highest monthly mean ozone concentrations occur in April or May, depending on the year. Episodal high concentrations were recorded during spring months when upper level atmospheric low pressure troughs and cut-off low centers occurred. These contributions peak in May, and add about 10% to normal May background ozone concentrations, varying widely from year to year. Spectral representations of ozone concentrations, illustrate the suppression of the diel peak during spring months, when stratospheric-tropospheric intrusion patterns show no significant diel cycle.With 9 Figures     

Direct measurements of nitrogen oxides and ozone fluxes over grassland

Using the eddy correlation method, fluxes of nitric oxide, nitrogen dioxide, ozone, water, and sensible heat were measured at a site 20 km north of Denver, Colorado over mature crested wheat grass, 0.75 m high in late June and early July. During this period the weather was fair with no synoptic disturbances. In the early morning a well-mixed diluted urban pollution plume traversed the site, by late morning aged pollution had mixed downward into the local boundary layer, and by afternoon the air came from a relatively unpolluted area of the high plains. The mean trace gas concentrations reflect this repeated pattern of local air flow. The fluxes of the trace gases were influenced both by the variation of the means and by other factors including temperature and biological activity. Ozone fluxes were found to be always negative and proportional to the mean, with an average deposition velocity for this case of about 0.006 m s^-1. For the oxides of nitrogen this simple treatment was not appropriate. Both deposition and emission were observed, generally deposition predominated in the morning and emission in the afternoon with observed variations in the fluxes of NO_x=NO+NO₂ from –0.3 to +0.2 ppbv m s^-1.The National Center for Atmospheric Research is sponsored by the National Science Foundation     

On the Importance of Reliable Background Concentrations of Ozone for Regional Scale Photochemical Modelling

Ozone throughout the troposphere is subject of significant temporal and spatial variability due to photochemical production in the planetary boundary layer and free troposphere, stratospheric intrusions, convective events and long range transport. However, high resolving observations of ozone in the troposphere are generally rare today. That is of special disadvantage for limited area models, which represent mathematically a differential equation system with an initial and boundary problem. As ozone concentrations usually increase from the earth surface to the stratosphere, a proper choice of the background ozone concentrations is necessary to reproduce or even predict the amount and distribution of ozone in a specific region of interest.In this paper the impact of background concentrations of ozone on regional scale model results is analysed during a summer smog episode over Europe. For this purpose ozone is artificially partitioned into individual categories. For each category, transport and chemical transformation is calculated separately. Initial and boundary concentrations of ozone dominate total ozone concentrations increasingly with height. But also in the planetary boundary layer they contribute with more than 30% to thetotal ozone changes and are therefore far from being negligible. Moderately modified assumptions of background ozone concentrations reveal an uncertainty of near surface ozone concentrations of 5–15%depending on the weather situation.     

Chemical effect of carbonaceous aerosols on the diurnal cycle of MBL ozone at a tropical site: measurements and simulations

During late austral summer and winter 1998, black carbon (BC) aerosols were monitored with an Aethalometer at 2 sites of La Réunion Island (Indian Ocean): Saint‐Denis, the main city and Sainte‐Rose, a quite uninhabited region situated at the east coast. BC concentration data at Saint‐Denis show a marked diurnal cycle, which may be primarily attributed to traffic. The background data found at night‐time display average BC concentrations, ranging from about 80 to 250 ng/m³ whereas during the day, BC concentrations increase by a factor of at least 4. In comparison, BC concentrations vary in the range of 10 to 60 ng/m³ at Sainte‐Rose. Ozone concentration was also measured at Saint‐Denis using a Dasibi photometer and found to be at significant levels (means: 16.5–23 ppbv in April and 28.5–34 ppbv in September). A noticeable increase of ozone concentrations during the day points out the build‐up of pollutants enhancing photochemical transformations. However, during traffic pollution peaks, ozone concentration displays systematic depletion. The comparison of ozone and BC measurements at both seasons points to some possible effects of heterogeneous interaction of ozone and its precursors with BC particles. These interactions were also simulated with a 0D time‐dependent chemistry model using conditions of a polluted site. The measured ozone concentration characteristics (mean concentration and range of variation) are well simulated in the presence of BC. Our model results show that at La Réunion Island adsorption of ozone and its precursors onto BC aerosol particles could be one of the important steps determining ozone concentration characteristics, especially in absence of photochemistry during night‐time.     

Field observations of stratified atmospheric flow above an obstacle

In a study using the plume from the Four Corners power plant, near Farmington, N.M., lee waves were observed during times when the plume flowed across the Hogback. Wavelengths were typically about 1.2 km; wave amplitudes were more variable, ranging from 20 to 100 m. The observed amplitudes imply an obstacle that is broader and shallower than is actually the case. This is in agreement with laboratory studies that show the existence of regions of complex flow both upstream and downstream from an obstacle, which have the effect of broadening the region over which laminar flow occurs. Visual observation, measurement of the plume cross-sectional area both upstream and downstream from the Hogback, and measurement of plume aerosol concentrations show that turbulent and eddy flow over and downwind from the Hogback increase the rate of mixing of the plume with the surrounding atmosphere. This in turn increases the rate at which plume components come into contact with the ground.     

EARLINET observations of the 14–22-May long-range dust transport event during SAMUM 2006: validation of results from dust transport modelling

We observed a long-range transport event of mineral dust from North Africa to South Europe during the Saharan Mineral Dust Experiment (SAMUM) 2006. Geometrical and optical properties of that dust plume were determined with Sun photometer of the Aerosol Robotic Network (AERONET) and Raman lidar near the North African source region, and with Sun photometers of AERONET and lidars of the European Aerosol Research Lidar Network (EARLINET) in the far field in Europe. Extinction-to-backscatter ratios of the dust plume over Morocco and Southern Europe do not differ. Ångström exponents increase with distance from Morocco. We simulated the transport, and geometrical and optical properties of the dust plume with a dust transport model. The model results and the experimental data show similar times regarding the appearance of the dust plume over each EARLINET site. Dust optical depth from the model agrees in most cases to particle optical depth measured with the Sun photometers. The vertical distribution of the mineral dust could be satisfactorily reproduced, if we use as benchmark the extinction profiles measured with lidar. In some cases we find differences. We assume that insufficient vertical resolution of the dust plume in the model calculations is one reason for these deviations.     

On the dynamics of photochemical smog over the Swiss Middleland — Results of the first POLLUMET field experiment

Summary The Swiss Middleland is a 300×50 km large plain embedded between the Jura, whose highest elevations are between 1000 and 2000m MSL, and the Alps, whose highest peaks are about 400m MSL. Because this plain is the main residence area of Switzerland with a great variety of emission sources, it is also a location with high photosmog concentrations during the summer months.Within the framework of the Swiss POLLUMET (Air Pollution and Meteorology) programme, an initial summer smog field experiment was carried out during July 1990 with the participation of different research groups from Switzerland and Germany. The measurements showed that the ozone concentrations within the atmospheric boundary layer were remarkably variable. The highest concentrations in the upper mixed layer varied between 100 and 130 ppb. The background ozone concentration in the upper atmospheric boundary layer increased from day to day. However, a clear indication of long-range transport could not be found. Remarkable local and regional concentration differences are not only based on the complex structure of the large emission sources (highways, urban plumes). They are also the result of the interaction of convectively driven motion systems like slope and valley winds and mountain-plain circulation.With 14 Figures     

Human appropriation of land for food: The role of diet

Human appropriation of land for food production has fundamentally altered the Earth system, with impacts on water, soil, air quality, and the climate system. Changes in population, dietary preferences, technology and crop productivity have all played important roles in shaping today’s land use. In this paper, we explore how past and present developments in diets impact on global agricultural land use. We introduce an index for the Human Appropriation of Land for Food (HALF), and use it to isolate the effects of diets on agricultural land areas, including the potential consequences of shifts in consumer food preferences. We find that if the global population adopted consumption patterns equivalent to particular current national per capita rates, agricultural land use area requirements could vary over a 14-fold range. Within these variations, the types of food commodities consumed are more important than the quantity of per-capita consumption in determining the agricultural land requirement, largely due to the impact of animal products and in particular ruminant species. Exploration of the average diets in the USA and India (which lie towards but not at global consumption extremes) provides a framework for understanding land use impacts arising from different food consumption habits. Hypothetically, if the world were to adopt the average Indian diet, 55% less agricultural land would be needed to satisfy demand, while global consumption of the average USA diet would necessitate 178% more land. Waste and over-eating are also shown to be important. The area associated with food waste, including over-consumption, given global adoption of the consumption patterns of the average person in the USA, was found to be twice that required for all food production given an average Indian per capita consumption. Therefore, measures to influence future diets and reduce food waste could substantially contribute towards global food security, as well as providing climate change mitigation options.     

Modelling of the long-range transport of peroxyacetylnitrate to Scandinavia

Model calculations and field measurements have shown that when air masses accumulate emissions of hydrocarbons and nitrogen oxides from sources in continental Europe and then move towards Scandinavia without any synoptic scale break-up of the atmospheric boundary layer (e.g. frontal passages), elevated PAN concentrations in southern Norway or Sweden in the range 1–5 ppb may be caused by long-range transport. The model calculations showed that over sea, the persistence of PAN was comparable to that of ozone in an ageing air mass when the temperatures were fairly low (5–10°C). At higher temperatures the thermal decomposition of PAN made the compound less persistent than ozone. Over land, the situation may be different since the ground removal is typically three times more efficient for ozone than for PAN.According to the model, the concentration of PAN did not change very much when an ageing air mass was exposed to moderate emissions of hydrocarbons, nitrogen oxides, or both. The concentration of PAN decreased less than the concentration of ozone when an ageing air mass was exposed to high emissions of nitrogen oxides.