Measurements of atmospheric dimethyl sulfide and carbon disulfide in the western Atlantic boundary layer

Shipboard measurements of atmospheric dimethyl sulfide were made during two transects along the east coast of the United States and at several stations in the Gulf of Maine. Limited measurements of carbon disulfide and hydrogen sulfide are also reported. The mean DMS mixing ratio was 29 pptv (=25, n=84, median 19 pptv) during the Atlantic transects, and 101 pptv (=67, n=77, median 79 pptv) in the Gulf of Maine. Distinct diurnal variations were found in the DMS data from the transects. The meteorology of the study area appears to control day-to-day differences in the magnitude of these diurnal variations, although rapid daytime oxidation is suggested in some cases. Diurnal variations were also evident in near-shore stations in the Gulf of Maine due to nocturnal boundary-layer inversion. Diurnal variation was not evident at other sites in the Gulf due to large scale changes in the atmospheric circulation pattern, which effectively masked any effects due to oxidation processes. Model simulations confirm that the DMS levels and diurnal variation found during the transects are not consistent with atmospheric oxidation processes alone. Atmospheric CS₂ and H₂S mixing ratios were less than 3 pptv during the transects, except for a single period of higher CS₂ mixing ratios (reaching 11 pptv) during advection of continental air. Calculations of the flux of oceanic sulfur to the eastern United States show that the contribution of natural sulfur to the North American sulfur budget is small compared to anthropogenic sources.     

Estimation of hydroxyl radical concentrations in the marine atmospheric boundary layer using a reactive atmospheric tracer

Hydroxyl radical (OH) concentrations in the atmospheric boundary layer over a number of remote ocean locations are calculated from the measured diurnal variation in atmospheric dimethylsulfide (DMS). By using averaged DMS data sets from extended periods, the calculation yields OH concentrations averaged over periods from several days to weeks. These average OH concentrations range from 7×10⁵ to 2.9×10⁶ molecules cm^-3, corresponding to midday maxima of 3 to 12×10⁶ molecules cm^-3. The lowest values correspond to studies with the lowest light intensity (Antarctic summer and South Atlantic winter), and the highest values to regions with probable anthropogenic influence. In addition to the long term averages, daily average OH levels can be calculated for most days in a two week period from a cruise in the tropical eastern Pacific. These calculations are in good argeement with global average OH levels derived from other tracers, and are consistent with model OH calculations when allowance is made for variation in ambient ozone levels between the studies. Estimates of gas exchange made from the diurnal variation of DMS suggest that either the gas exchange coefficient of DMS or the boundary layer mixing depth may have been overestimated in past analyses.     

Effects of mesoscale sea-surface temperature fronts on the marine atmospheric boundary layer

A numerical modelling study is presented focusing on the effects of mesoscale sea-surface temperature (SST) variability on surface fluxes and the marine atmospheric boundary-layer structure. A basic scenario is examined having two regions of SST anomaly with alternating warm/cold or cold/warm water regions. Conditions upstream from the anomaly region have SST values equal to the ambient atmosphere temperature, creating an upstream neutrally stratified boundary layer. Downstream from the anomaly region the SST is also set to the ambient atmosphere value. When the warm anomaly is upstream from the cold anomaly, the downstream boundary layer exhibits a more complex structure because of convective forcing and mixed layer deepening upstream from the cold anomaly. An internal boundary layer forms over the cold anomaly in this case, generating two distinct layers over the downstream region. When the cold anomaly is upstream from the warm anomaly, mixing over the warm anomaly quickly destroys the shallow cold layer, yielding a more uniform downstream boundary-layer vertical structure compared with the warm-to- cold case. Analysis of the momentum budget indicates that turbulent momentum flux divergence dominates the velocity field tendency, with pressure forcing accounting for only about 20% of the changes in momentum. Parameterization of surface fluxes and boundary-layer structure at these scales would be very difficult because of their dependence on subgrid-scale SST spatial order. Simulations of similar flow over smaller scale fronts (<5 km) suggest that small-scale SST variability might be parameterized in mesoscale models by relating the effective heat flux to the strength of the SST variance.     

冷空气过程对黄海东海区域海洋大气边界层结构影响的个例分析

利用一次冷空气过程的14组GPS探空数据,采用位温梯度法确定了冷空气过境前后大气边界层高度,并分析了冷空气过程对大气边界层结构的影响。结果表明：冷锋过境加大了海洋大气边界层的静力不稳定度,使边界层内对流活动增强,且锋面过后距离锋面越近的区域边界层的静力不稳定度越大;冷锋过境使边界层的平均高度升高,边界层顶处逆温梯度增大。结合ERA-Interim再分析资料,分析认为大气边界层高度与静力稳定度（海气温差）存在显著的正相关关系（相关系数达0.73）,海气温差越大,大气边界层高度越高。     

Double-layer structure in the boundary layer over the baltic sea

Double-layered structures found over the Baltic Sea are investigated using radiosoundings and lidar measurements. Situations with double-layer structures are also simulated with the regional model REMO in a realistic manner. The double layer consists of two adjacent well-mixed layers, with a sharp inversion in between.Results from radiosoundings show that the double-layer structure over the Baltic Sea mainly occurs during the autumn with thermally unstable stratification near the surface. The structure is present in about 50 % of the radiosoundings performed during autumn. The presence of the double-layer structure cannot be related to any specific wind direction, wind speed or sea surface temperature.The lidar measurements give a more continuous picture of the time evolution of the double-layer structure, and show that the top of the lower layer is not a rigid lid for vertical transport. Two possible explanations of the double-layer structure are given, (i) the structure is caused by `advection' of land boundary-layer air over the convective marine boundary layer or, (ii) by development of Sc clouds in weak frontal zones connected to low pressure systems. Also the forming of Cu clouds is found to be important for the development of a double-layer structure.     

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.     

Characteristics of intermittent turbulence in the upper stable boundary layer over Greenland

The experiment IGLOS (Investigation of the Greenland Boundary Layer Over Summit) was conducted in June and July 2002 in the central plateau of the Greenland inland ice. The German research aircraft Polar2, equipped with the turbulence measurement system Meteopod, was used to investigate turbulence and radiation flux profiles near research station “Summit Camp”. Aircraft measurements are combined with measurements of radiation fluxes and turbulent quantities made from a 50 m tower at Summit Camp operated by Eidgenössische Technische Hochschule (ETH) Zürich. During all six flight missions, well-developed stable boundary layers were found. Even in high-wind conditions, the surface inversion thickness did not exceed roughly 100 m. The turbulent height of the stable boundary layer (SBL) was found to be much smaller than the surface inversion thickness. Above the surface layer, significant turbulent fluxes occurred only intermittently in intervals on the order of a few kilometres. Turbulent event fraction in the upper SBL shows the same dependence on gradient Richardson number as reported for near-surface measurements. Clear-air longwave radiation divergence was always found to contribute significantly to the SBL heat budget. In low-wind cases, radiative cooling even turned out to be dominant.     

解耦的海洋性层积云顶边界层湍流与云微物理特征

基于POST观测计划中获得的海洋性层积云顶边界层内高频气象资料和云微物理资料,在选取解耦个例基础上研究解耦边界层湍流和云微物理特征及成因。结果表明,过渡层的大气静力稳定度较强,抑制向上浮力做功,使得湍流动能迅速消耗殆尽,实现边界层解耦。湍流动能最大值出现在云内,主要与云顶降温、大云滴下落沉降拖曳带来的下沉气流增强及云底之上附近凝结增长潜热释放产生向上浮力作用有关。近地面层的浮力项和切变项对湍流动能都起到增强作用,并以切变项的贡献更为显著,云内的湍流动能是以浮力项贡献为主。过渡层附近存在向下的热通量,抑制了热量向上输送和向上浮力项的增强,促进解耦发生。云内存在向上感热通量,其最大值及其出现高度主要与云顶冷却和云中下部的凝结潜热加热有关。云顶之上湿层促进了潜热通量的向下输送,增强了云内水汽含量,为解耦边界层云的发展起到正反馈作用。云顶浮力倒转引起的云中湍流混合呈现非均匀性,并进一步导致绝热或超绝热液滴出现,促进凝结和碰并增长的增强,同时云顶之上湿层进一步对云中的微物理增长起到了重要的推动作用。云底因夹卷混合表现为均匀混合特征。     

WRF模式对青藏高原那曲地区大气边界层模拟适用性研究

采用WRF（Weather Research and Forecasting）模式4种边界层参数化方案对青藏高原那曲地区边界层特征进行了数值模拟,并利用"第三次青藏高原大气科学试验"在青藏高原那曲地区5个站点的观测资料对模拟结果进行验证,分析不同参数化方案在那曲地区的适用性。研究表明,YSU、MYJ、ACM2和BouLac方案对2 m气温和地表温度的模拟偏低。BouLac方案模拟的地表温度偏差较小。通过对能量平衡各分量的对比分析发现,温度模拟偏低可能是向下长波辐射模拟偏低以及感热通量和潜热通量交换过强导致的。对于边界层风、位温和相对湿度垂直结构的模拟,局地方案的模拟效果均优于非局地方案。BouLac方案对那曲地区近地层温度、边界层内位温和相对湿度的垂直分布模拟效果较好。      

On the turbulence structure in the stable boundary layer over the Greenland ice sheet

Turbulence measurements from a 30 m tower in the stably stratifiedboundary layer over the Greenland ice sheet are analyzed. The observationsinclude profile and eddy-correlation measurements at various levels. Atfirst, the analysis of the turbulence data from the lowest level (2 m aboveground) shows that the linear form of the non-dimensional wind profile(m) is in good agreement with the observations for z/L <0.4, whereL represents the Obukhov length. A linear regression yieldsm=1+5.8z/L. The non-dimensional temperature profile (h) at the2m level shows no tendency to increase with increasing stability. The datafrom the upper levels of the tower are analyzed in terms of both localscaling and surface-layer scaling. The m and the h values show atendency to level off at large stability (z/>0.4) where represents the local Obukhov length. Hence, the linear form of the functions is no longer appropriate under such conditions. The bestcorrespondence to the data can be achieved when using the expression ofBeljaars and Holtslag for m and h. The vertical profiles of theturbulent fluxes, the wind velocity variances and temperature variance arealso determined. The momentum flux profile and the profiles of wind speedvariances are in general agreement with other observations if a welldeveloped low-level wind maximum occurs, and the height of this maximum isused as a height scale.     

Detection of iodine monoxide radicals in the marine boundary layer using laser induced fluorescence spectroscopy

A Laser Induced Fluorescence (LIF) instrument has been developed to detect iodine monoxide (IO) radicals in the atmosphere. An all solid-state Nd:YAG pumped Ti:Sapphire laser operating at approximately 445 nm was used to excite the (2,0) band of the IO A²Π_3/2 ← X²Π_3/2 electronic transition, with off-resonance fluorescence in the (2,5) band detected at 521 nm. The sensitivity of the instrument was determined by calibration. IO (between 10 and 150 pptV) was generated following the 184.9 nm photolysis of N₂O/CF₃I/N₂ mixtures with O₃ actinometry used to determine the photolysis flux. The detection limit was determined to be 0.3 pptV for a 300 s integration period, with an uncertainty of 23% (1σ). The instrument was deployed in August/September 2006 during the RHaMBLe (Reactive Halogens in the Marine Boundary Layer) campaign in Roscoff, France. Located on a small jetty, a few metres from the water’s edge at high tide, the instrument measured significant levels of IO on 11 days, with a maximum of 27.6 ± 3.2 pptV observed on one day (averaged over 10 s) representing the highest IO mixing ratio recorded in the marine boundary layer to date. IO displayed a clear diurnal profile with a maximum at low tide during the daytime. These results represent the first point measurements of IO in the atmosphere by LIF.     

Measurement of IO radical concentrations in the marine boundary layer using a cavity ring-down spectrometer

An open-path cavity ring-down spectroscopy (CRDS) instrument for measurement of atmospheric iodine monoxide (IO) radicals has been tested in the laboratory and subsequently deployed in Roscoff on the north-west coast of France as part of the Reactive Halogens in the Marine Boundary Layer (RHaMBLe) project in September 2006. In situ measurements are reported of local IO mixing ratios in the marine boundary layer. To obtain these mixing ratios, accurate absorption cross sections of IO are required at the selected wavelengths used for spectroscopic measurements. Absorption cross sections at the bandheads of the IO A²Π_3/2–X²Π_3/2 (3,0) and (2,0) vibronic bands were thus verified by a combination of spectral simulation methods, inter-comparison of prior determinations of cross-sections at high and low spectral resolution, and by measurement of rates of loss of IO by its self-reaction. The performance of the open-path CRDS instrument was tested by measuring concentrations of NO₂ in ambient air, both within and outside the laboratory, with results that were in excellent agreement with a previously validated continuous wave CRDS apparatus for NO₂ detection. During the RHaMBLe campaign, the open-path CRDS instrument was located within a few metres of the shoreline and operated at wavelengths close to 435 nm to detect the absorption of light by trace levels of IO. The IO mixing ratios were obtained on two days, peaked close to low tide, and were approximately 5–10 times higher than values calculated from column densities previously reported by long-path, differential optical absorption spectroscopy (DOAS) in coastal regions. The typical detection limit of the instrument was estimated to be 10 pptv of IO, with some fluctuation around this value depending on the conditions of wind and atmospheric aerosol particles, and the total accumulation time was 30 s for each data point. The observations of relatively high concentration of IO, compared to the values previously reported by DOAS, are consistent with the concurrent observations using a LIF (Laser induced Fluorescence) instrument (Whalley et al. in press). The first such measurements of localized IO by CRDS and LIF should contribute to an improved understanding of the chemistry of halogen compounds and the formation of iodine oxide aerosol particles in the marine boundary layer.     

利用COSMIC掩星资料研究青藏高原地区大气边界层高度

以往关于青藏高原边界层的研究都是基于个别站点的常规观测,对青藏高原边界层的整体性认识受限。GPS掩星资料具有测量精度高和垂直分辨率高的特性,其廓线中含有大量有价值的边界层信息。利用2007—2013年COSMIC掩星资料,通过计算大气折射率最小梯度来确定边界层高度,并用无线电探空资料对结果进行了检验。在此基础上,对青藏高原地区边界层高度的特征及其形成机制展开了研究,比较了COSMIC掩星确定的边界层高度和ERA-Int的差别,讨论了最小梯度法用于边界层研究的不确定性。结果表明:青藏高原上COSMIC掩星和无线电探空数据检测的边界层高度相关系数为0.786,平均值偏差为0.049 km,均方根误差为0.363 km,COSMIC掩星数据检测的边界层高度和无线电探空的结果非常接近。青藏高原上边界层高度呈现西高东低的分布特征,高原中西部边界层高度主要为1.8—2.3 km,而高原东部边界层为1.4—1.8 km,最大值在高原西南部。青藏高原地区边界层有明显的季节差异,冬季高原上大部分地区边界层高度超过2.0 km;春季大部分地区高度降低,但在受印度季风影响的高原南部有明显的抬升,最大值可超过3.0 km;夏季高原上边界层高度开始升高,大部分地区超过1.8 km;秋季又开始回落。青藏高原以北塔克拉玛干沙漠和高原以南印度季风活动区是两个高值区,北部的沙漠地区边界层高度在夏季最高,南部印度季风活动区在季风爆发前（4月）达到全年最大值。青藏高原中西部地区有水平风辐合以及广泛的上升运动,为边界层的发展提供了动力条件,而东部的下沉运动对边界层的发展有抑制作用。青藏高原边界层各个季节的空间分布与地表感热通量分布一致。COSMIC掩星资料确定的边界层高度和ERA-Int相比,空间分布基本一致但ERA-Int边界层高度明显偏低。当有系统性强逆温存在的时候,或者云中液态水或冰水含量较大时,用最小梯度法检测的边界层高度不确定性增加。      

Hydroperoxides in the marine troposphere over the Atlantic Ocean

Hydrogen peroxide (H₂O₂) and organic hydroperoxides (ROOH) were measured on board of theRV Polarstern during its cruise across the Atlantic Ocean from 20 October to 12 November 1990 (54° N to 51° S latitude) by the enzyme fluorometric method. The H₂O₂ mixing ratio varied from below the detection limit of about 0.12 ppbv up to 3.89 ppbv, showing a latitudinal dependence with generally higher values around the equator and decreasing values poleward. The shape of the latitudinal H₂O₂ distribution agrees well with an analytical steady state expression for H₂O₂ using the measured H₂O and O₃ distribution and a wind dependent H₂O₂ deposition rate. The ROOH mixing ratio varied from below the detection limit of about 0.08 ppbv up to 1.25 ppbv with qualitatively the same latitudinal dependence as H₂O₂. The observed ratio ROOH/(ROOH + H₂O₂) varied between 0.17 and 0.98 showing higher values at the lowest H₂O₂ mixing ratios at high latitudes. The measured H₂O₂ mixing ratio shows a significant diurnal variation with a maximum around 14:00 local time, explicable by a superposition of the photochemical H₂O₂ production with a constant H₂O₂ deposition rate. Four independent estimations of the average effective H₂O₂ deposition rate inferred from the H₂O₂ decrease in the night, from the midday H₂O₂ production deficit (as derived from comparison with a photochemical model and from the daily ozone loss), and from the offset in the latitudinal H₂O₂ distribution, were consistent. An episode of maximum H₂O₂ concentration suggests the possibility of its formation in clouds.     

The role of surface heterogeneity in modelling the stable boundary layer

High-resolution numerical simulations are performed for three nights for each of two areas of the United Kingdom. Area-averaging techniques are then used to calculate effective stability functions for turbulence parametrizations in models using typical mesoscale and global spatial resolutions. Comparisons are made with parametrizations commonly used in numerical weather prediction models. The present results do not suggest that significant enhancement of the stability functions above 50 m is justified. Closer to the surface, more significant enhancement is observed in some regions. It is shown that the amount of enhancement is related to the variability of the orography.     

A numerical study of daily transitions in the convective boundary layer

We examine daily (morning–afternoon) transitions in the atmospheric boundary layer based on large-eddy simulations. Under consideration are the effects of the stratification at the top of the mixed layer and of the wind shear. The results describe the transitory behaviour of temperature and wind velocity, their second moments, the boundary-layer height Z _m (defined by the maximum of the potential temperature gradient) and its standard deviation σ _m , the mixed-layer height z _i (defined by the minimum of the potential temperature flux), entrainment velocity W _e, and the entrainment flux H _i . The entrainment flux and the entrainment velocity are found to lag slightly in time with respect to the surface temperature flux. The simulations imply that the atmospheric values of velocity variances, measured at various instants during the daytime, and normalized in terms of the actual convective scale w_*, are not expected to collapse to a single curve, but to produce a significant scatter of observational points. The measured values of the temperature variance, normalized in terms of the actual convective scale Θ_*, are expected to form a single curve in the mixed layer, and to exhibit a considerable scatter in the interfacial layer.     

Continental impact on marine boundary layer coarse particles over the Atlantic Ocean between Europe and Antarctica

Aerosol samples were collected in the Atlantic marine boundary layer between the English Channel and Antarctica during November–December 1999. The composition of coarse (aerodynamic diameter 1–3 μm) individual aerosol particles was studied using the SEM/EDX method. The major particle types observed were fresh sea salt, sea-salt particles reacted partly or totally with sulphuric acid or nitric acid, Mg-sulphate, Ca-sulphate, mixed aluminosilicates and sea salt, aluminosilicates, Ca-rich particles and Fe-rich particles. The relative fractions of sea-salt particles with moderate or strong Cl depletion were high near the coasts of Europe (65–74%) and Northern Africa (44–87%), low far from the coast of Western Africa (10–20%) and very low in remote sea areas between Africa and Antarctica (1%). The Cl depletion was strongest when air masses arrived from the direction of anthropogenic pollution sources. The fractions of Mg-sulphate particles were high (18–25%) in 2 samples near Europe. The Mg-sulphate particles were probably formed as a result of fractional recrystallization of sea-salt particles in which Cl was substituted by sulphate. It remained unclear whether these particles were formed in the atmosphere or during and after sampling. The relative fractions of particles from continental sources were quite low (10–15%) near Europe, very high (25–78%) near the coast of Northwestern Africa and very low in the remote sea areas (0–2%). Most of the continental particles were aluminosilicates and some of them were internally mixed with sea salt. Near the coast of Northwestern Africa, the main source of aluminosilicates was Saharan dust, and near the Gulf of Guinea, emissions from biomass burning were also mixed with aluminosilicates and sea salt.     

Nocturnal stable boundary layer height model and its application

A model of the evolution of the nocturnal stable boundary layer height, based on the heat conservation equation for a turbulent flow, is presented. This model is valid for nights with weak winds and little cloudiness in rural areas. The model includes an expression of vertical profile of potential temperature within the boundary layer, which is obtained using micrometeorological information from Prairie Grass, Wangara and O'Neill Projects. The expression turned out to be a second-grade polynomial of the dimensionless height of the nocturnal stable boundary layer. The resulting model is a function of the Monin–Obukhov length, the surface potential temperature of air and the roughness length. This model was satisfactorily compared with micrometeorological data. It was applied at three stations of Argentina, using surface hourly meteorological information. From the results that were obtained, the monthly average values of the stable boundary layer thickness were analysed. The maximum monthly average values occur during the cold season and the minimum ones take place during the hot season. It was observed that the monthly average thickness increases with latitude.     

利用探空资料估算青藏高原及下游地区大气边界层高度

利用三个时段的探空加密试验资料,分别采用气块法和Richardson数法来估算青藏高原及下游地区的对流边界层和稳定边界层的高度特征。结果显示:(1)高原中部对流边界层结构的出现概率高于高原东侧及下游地区,而四川盆地稳定边界层结构的出现概率远高于高原和长江中游。(2)高原中部和东侧的对流边界层高度春季高而夏季低,其中高原中部的对流边界层高度高于高原东侧;四川盆地和长江中游的对流边界层高度冬季低、夏季高,而高原东侧的对流边界层高度的变化趋势则相反;四川盆地的对流边界层高度低于长江中游。(3)高原的稳定边界层高度春季高而夏季低;冬季四川盆地的稳定边界层高度高于高原东侧和长江中游,而夏季长江中游的稳定边界层高度高于高原东侧和四川盆地,冬夏差异导致的稳定边界层高度的变化幅度在长江中游最明显,四川盆地次之,而高原东侧最小。(4)高原东侧及下游地区的平均边界层高度的日变化具有相似的演变特征,平均边界层高度在白天高夜间低,而高原中部的平均边界层高度在日出左右较低,之后随时间逐渐增高,并在晚上达到最大值;高原的平均边界层高度的日变化振幅大于下游地区,且其日变化振幅随站点海拔高度的增加而增大。     

Turbulent mixing of reactive gases in the convective boundary layer

We study the interactions of chemistry and turbulent mixing of tracersin the convective boundary layer with a second-order closure model,including higher order chemistry terms. In order to limit the number of predictive equations we prescribe the profiles for ¯w¯, ¯w¯ ¯ and the lengthscale l. However, for model validation we treat temperature and humidity asinert tracers, and compare the results with profiles observed during theAir Mass Transformation Experiment, and with similarity expressions for thesurface layer. We find good agreement of the mean profiles, but the (co-)variances are slightly underpredicted. Furthermore, the model usesdiagnostic equations expressing third moments of concentration in terms ofsecond moments and their vertical derivatives. They are compared withlarge-eddy model results, showing good agreement and, therefore, thesimplifications are justified. The model is applied to the transport of two gases subject to one bimolecular reaction. The importance of concentration correlations on themean transformation rate is studied. For two gases diffusing in oppositedirections we find for moderate and fast chemistry a 50% and90% decreased transformation rate due to the negatively correlatedconcentrations. These values are similar to large-eddy results of Schumannand Sykes et al. For two bottom-up tracers we find that the covariance ofboth reactive species is either positive or negative, increasing or reducingthe effective transformation rate depending on the Damköhler number (the ratio of the turbulent and the chemistry timescale). A significantdirect influence of chemistry on the flux divergence is found in bothcases. According to the model the effective transport to mid-levels of theboundary layer is increased when two reactive tracers diffuse in oppositedirections, and decreased in the case of two bottom-up tracers.