Estimates of the Chemical Budget for Ozone at Waliguan Observatory

Waliguan Observatory (WO) is an in-land Global Atmosphere Watch (GAW) baseline station on the Tibetan plateau. In addition to the routine GAW measurement program at WO, measurements of trace gases, especially ozone precursors, were made for some periods from 1994 to 1996. The ozone chemical budget at WO was estimated using a box model constrained by these measured trace gas concentrations and meteorological variables. Air masses at WO are usually affected by the boundary layer (BL) in the daytime associated with an upslope flow, while it is affected by the free troposphere (FT) at night associated with a downslope flow. An anti-relationship between ozone and water vapor concentrations at WO is found by investigating the average diurnal cycle pattern of ozone and water vapor under clear sky conditions. This relationship implies that air masses at WO have both the FT and BL characteristics. Model simulations were carried out for clear sky conditions in January and July of 1996, respectively. The chemical characteristics of mixed air masses (MC) and of free tropospheric air masses (FT) at WO were investigated. The effects of the variation in NO_x and water vapor concentrations on the chemical budget of ozone at WO were evaluated for the considered periods of time. It was shown that ozone was net produced in January and net destroyed in July for both FT and MC conditions at WO. The estimated net ozone production rate at WO was –0.1 to 0.4 ppbv day^–1 in FT air of January, 0.0 to 1.0 ppbv day^–1 in MC air of January, –4.9 to –0.2 ppbv day^–1 in FT air of July, and –5.1 to 2.1 ppbv day^–1 in MC air of July.     

On the Spatial Distribution and Seasonal Variation of Lower-Troposphere Ozone over Europe

Surface ozone data from 25 Europeanlow-altitude sites and mountain sites located between79°N and 28°N were studied. The analysiscovered the time period March 1989–February 1993.Average summer and winter O₃ concentrations inthe boundary layer over the continent gave rise togradients that were strongest in the north-west tosouth-east direction and west-east direction, respectively. WintertimeO₃ ranged from 19 to 27 ppbover the continent, compared to about 32 ppb at thewestern border, while for summer the continentalO₃ values ranged between 39 and 56 ppb and theoceanic mixing ratios were around 37 ppb. In the lowerfree troposphere average wintertime O₃ mixingratios were around 38 ppb, with only an 8 ppbdifference between 28°N and 79°N. For summerthe average O₃ levels decreased from about 55 ppbover Central Europe to 32 ppb at 79°N. Inaddition, O₃ and O_x(= O₃ + NO₂)in polluted and clean air were compared. Theamplitudes of the seasonal ozone variations increasedin the north-west to south-east direction, while thetime of the annual maximum was shifted from spring (atthe northerly sites) to late summer (at sites inAustria and Hungary), which reflected the contributionof photochemical ozone production in the lower partsof the troposphere.     

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.     

Cloud condensation nuclei measurements at Mace Head, Ireland, over the period 1994–2002

Analyses of cloud condensation nuclei (CCN) number concentrations (cm^− 3) measured at the Mace Head Atmospheric Research Station, near Carna, County Galway, Ireland, using a DH Associates Model M1 static thermal diffusion cloud chamber over the period from March 1994 to September 2002 are presented in this work. Air masses are defined as being ‘marine’ if they originate from a wind direction of 180–300° and ‘continental’ air masses are defined as originating from a wind direction of 45–135°. Air masses without such filtering were classified as ‘undefined’ air masses. Air masses were found to be dominated by marine sector air, re-affirming Mace Head as a baseline atmospheric research station. CCN levels for specific air masses at Mace Head were found to be comparable with earlier studies both at Mace Head and elsewhere. Monthly averaged clean marine (wind direction of 180–300° and black carbon absorption coefficient < 1.425 Mm^− 1) CCN and marine CCN varied between 15–247 cm^− 3 and 54–670 cm^− 3, respectively. As expected, significant increases in number concentration were found in continentally sourced CCN over that of marine CCN and were found to follow a log-normal distribution significantly tighter than that of clean marine air masses. No significant trend was found for CCN over the 9-year period. While polluted continental air masses showed a slight increase in CCN concentrations over the winter months, most likely due to increased fuel usage and a lower mixed boundary layer, the dominance of marine sector air arriving at Mace Head, which generally consists of background CCN concentrations, reduced seasonal differences for polluted air. Marine air showed a distinct seasonal pattern, with elevated values occurring over the spring and summer seasons. This is thought to be due to enhanced biogenic aerosol production as a result of phytoplankton bloom activity in the North Atlantic.     

华南地区春季平流层入侵对对流层低层臭氧影响的模拟研究

利用中尺度大气化学模式WRF/Chem对2013年3月6日华南地区一次平流层入侵事件及其对对流层低层臭氧的影响进行模拟研究。通过加入UBC（Upper Boundary Condition）上边界处理方案,弥补WRF/Chem模式未考虑平流层臭氧化学反应的不足。结合臭氧探空廓线资料、地面O₃、CO、NO_x、相对湿度、温度和风速等观测资料以及再分析资料对模拟结果进行定量评估,结果表明模式能较为真实地模拟本次平流层入侵过程。模拟分析进一步揭示:（1）副热带高空急流是本次平流层入侵的主要原因。当华南地区处在副热带急流入口区左侧下沉区域时,平流层入侵将富含臭氧的干燥空气输送到对流层中低层。（2）本次平流层入侵对对流层低层臭氧收支有重要影响,导致香港地区近地层臭氧体积混合比浓度明显上升,如塔门站夜间臭氧浓度升高21.3 ppb（1 ppb=1×10-9）。地面气象场和化学物种的分析进一步确认了平流层入侵的贡献。（3）采用动力学对流层顶高度时零维箱式模型和Wei公式计算得到的平流层入侵通量相当,分别为-1.42×10-3 kg m-2 s-1和-1.59×10-3 kg m-2 s-1,这一结果与前人研究相吻合,且与采用热力学对流层顶高度计算所得到的结果具有可比性。     

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.     

Surface ozone over the Atlantic ocean

During the past decade (1977–1986) ozone has been measured on board various research vessels between 83° N and 76° S. The latitudinal distribution shows nearly twice as high an ozone concentration in the Northern Hemisphere (30 ppb) as compared to the Southern Hemisphere (15 ppb). North of 70° N the concentration rapidly drops to low values (17 ppb). In the ITCZ we frequently observed a pronounced secondary maximum which indicates that tropospheric-stratospheric exchange may be induced by deep convection. During other occasions, ozone concentrations below 10 ppb were observed in the ITCZ. Secondary maxima are also found in each hemisphere, indicating the latitudes in which stratospheric-tropospheric exchange occurs. One such pronounced maximum is found between 60 and 65° N. A comparable counterpart does not, however, occur in the respective southern latitudes.The observations, although incomplete, allow for conclusions with respect to seasonal cycles, at least in some of the latitudes or for some part of the year. In the tropics differences between continental and maritime yearly cycles are found. In mid to high northern latitudes, a sharp maximum is found which extends to the summer months over continental European sites. In the respective southern latitudes, such a spring maximum is only indicated.Comparison with occasionally concurrent Kr 85 measurements by other groups allows for the conclusion that in some cases photochemically produced ozone in polluted air masses is seen over the Atlantic ocean.     

A Seasonal Comparison of the Ozone Photochemistry in Clean and Polluted Air Masses at Mace Head, Ireland

Measurements of the sum of peroxy radicals [HO₂ + RO₂],NO_x (NO + NO₂) and NO_y (the sum of oxidisednitrogen species) made at Mace Head, on the Atlantic coast of Ireland in summer 1996 and spring 1997 are presented. Together with a suite of ancillary measurements, including the photolysis frequencies of O₃ O(¹D)(j(O¹D)) and NO₂ (j(NO₂)), the measured peroxy radicals are used to calculate meandailyozone tendency (defined as the difference of the in-situphotochemical ozone production and loss rates); these values are compared with values derived from the photochemical stationary state (PSS) expression. Although the correlation between the two sets of values is good, the PSS values are found to be significantly larger than those derived from the peroxy radical measurements, on average, in line with previous published work. Possible sources of error in these calculations are discussed in detail. The data are further divided up into five wind sectors, according to the instantaneous wind direction measured at the research station. Calculation of mean ozone tendencies by wind sector shows that ozone productivity was higher during spring (April–May) 1997 than during summer (July–August) 1996across all airmasses, suggesting that tropospheric photochemistry plays an important role in the widely-reported spring ozone maximum in the Northern Hemisphere. Ozone tendencies were close to zero for the relatively unpolluted south-west, west and north-west wind sectors in the summer campaign, whereas ozone productivity was greatest in the polluted south-east sector for both campaigns. Daytime weighted average ozone tendencies were +(0.3± 0.1) ppbv h^–1 for summer 1996 and +(1.0± 0.5) ppbvh^–1 for spring 1997. These figures reflect the higher mixing ratios of ozone precursors in spring overall, as well as the higher proportion of polluted air masses from the south-east arriving at the site during the spring campaign. The ozone compensation point, where photochemical ozone destruction and production processes are in balance, is calculated to be ca. 14 pptv NO for both campaigns.     

Oxidation of sulfur dioxide by ozone in highly dispersed water droplets

The oxidation of S(IV) by ozone in aqueous solution has been studied in highly dispersed droplets in a laboratory could chamber at 20°C. Urban air was used. Ozone concentrations ranged from 15 to 120 ppb, orders of magnitude lower than most previous works. Rate constants were derived from differential product determination, rather than from rates of decay of bulk reactants. Comparison of the results obtained here with those from previous work indicates that the reaction rate is indeed first order with respect to ozone. The second-order reaction rate constants found in this work lay between, or close to, other recent results, and could be represented by:k=1.23×10⁴ [H⁺]^–0.51 M^–1 s^–1.     

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.     

Carboxylic Acids: Seasonal Variation and Relation to Chemical and Meteorological Parameters

Formic and acetic acid measured as daily averages in 1993–1994show equal and highly correlated concentrations up to 3 ppb in the summer(May–August). In the winter (October–March) the formicacid/acetic acid ratio was 0.6 and the formic acid concentrations wereusually below 1 ppb. In winter the carboxylic acids correlate withO_x, NO_y, SO₂ and particulatesulphur. The main sources are suggested to be ozonolysis of anthropogenicalkenes and reactions between peroxyacetyl radicals and RO₂radicals. In spring–summer the carboxylic acids correlate withO₃, O_x, HNO₃, PAN,NO_y, SO₂, particulate sulphur and temperature.In addition to the sources of the winter a contribution from ozonolysis ofbiogenic alkenes is likely. Quite similar formic acid/acetic acid ratios forall wind directions suggest that the source(s) are atmospheric oxidationprocesses distributed over large areas. The highest concentrations occurringfor winds from east to south and the correlation with e.g., particulatesulphur indicate chemical production in polluted air masses during longrange transport.     

Long-term measurements of surface ozone in the German Democratic Republic

The ozone concentration near the earth's surface has been measured at some stations in the GDR for more than 30 yr using the wet chemical method. Even at rural stations the ozone data show a significant linear increase by about 1–3% yr^–1. The ozone increase being stronger in summer than in winter is assumed to be due to photochemical ozone production from increasing anthropogenic emissions of trace gases that are transported over long distances. A weaker ozone increase by only about 0.2% per year was observed in the free troposphere (5.5 km) from balloon-soundings at Lindenberg within the period 1975–1984. If the ozone trends continue, the ozone concentration near the surface and its seasonal amplitude will have doubled around the turn of the century as compared to the mid-fifties.     

Determination of the Tropospheric Hydroxyl Radical by Liquidphase Scrubbing and HPLC: Preliminary Results

A preliminary study was carried out toexamine the feasibility of measuring tropospherichydroxyl radicals (OH) by liquidphase scrubbing andhigh performance liquid chromatography (HPLC). Thepotential advantages of this approach are itssimplicity, portability, and low expense. Thesampling system employs glass bubblers to trapatmospheric OH into a buffered solution of salicylicacid (o-hydroxybenzoic acid, OHBA). Rapidreaction of OH with OHBA produces a stable fluorescentproduct, 2,5-dihydroxybenzoic acid (2,5-DHBA), whichis determined by reverse-phase HPLC and fluorescencedetection. Our preliminary field results indicatethat this method is most suitable for OH measurementsin clean tropospheric air, where interferences fromother atmospheric species appear to be negligible orminor relative to polluted air. In clean air, thesampling period is about 45–90 minutes, which yieldsa detection limit of approximately 3–6 ×10⁵ radicalscm^-3. During an OHintercomparison experiment at the Caribou samplingsite in Colorado, our liquidphase scrubber method wascompared with the ion-assisted mass spectrometry (MS)method. Our results were within the same range asthose of the ion-assisted MS method (1–5 ×10⁶ radicals cm^-3) within our precision atthat time (about ±30–50%). Preliminary testsin Pullman, WA indicated that the method might alsofunction in moderately polluted air by acidifying thescrubbing solution or by adding a scavenger tosuppress interferences. In Pullman, mid-day OHconcentrations were usually in the range of 2–20 ×10⁶ radicals cm^-3. Nighttime OHconcentrations were always low, either at or slightlyabove the detection limit.     

Fluxes of gases and particles above a deciduous forest in wintertime

Eddy-correlation measurements of the vertical fluxes of ozone, carbon dioxide, fine particles with diameter near 0.1 m, and particulate sulfur, as well as of momentum, heat and water vapor, have been taken above a tall leafless deciduous forest in wintertime. During the experimental period of one week, ozone deposition velocities varied from about 0.1 cm s^–1 at night to more than 0.4 cm s^-1 during the daytime, with the largest variations associated primarily with changes in solar irradiation. Most of the ozone removal took place in the upper canopy. Carbon dioxide fluxes were directed upward due to respiration and exhibited a strong dependence on air temperature and solar heating. The fluxes were approximately zero at air temperatures less than 5 °C and approached 0.8 mg m^–2 s^–1 when temperatures exceeded 15 °C during the daytime. Fine-particle deposition rates were large at times, with deposition velocities near 0.8 cm s^–1 when turbulence levels were high, but fluxes directed upward were found above the canopy when the surface beneath was covered with snow. Diffusional processes seemed to dominate fine-particle transfer across quasilaminar layers and subsequent deposition to the upper canopy. Deposition velocities for particulate sulfur were highly variable and averaged to a value small in magnitude as compared to similar measurements taken previously over a pine forest in summer.     

Tropospheric Ozone at High Latitudes in Clean and Polluted Air Masses, a Climatological Study

Several years of continuous measurements of surfaceozone at Norwegian monitoring sites are studied in aclimatological way. The monitoring sites are at rurallocations extending from 58°N, a few hundredkilometers from the European continent and into theArctic at 79°N. The ozone observations are sorted intoclasses of integrated NO_x emissions along 96 h backtrajectories. The average seasonal cycles of ozone areestimated for each class separately. The differencesindicate the change from the background air due toanthropogenic emissions. The average seasonal cycle ofozone in the cleanest air masses showed a maximum inspring and a minimum during summer and autumn at allsites, but the spring maximum was more pronounced atthe southernmost locations. Polluted air masses showedan ozone deficit during winter and a surplus duringsummer. The deviation from the background was clearlylinked to the integrated NO_x emission along thetrajectories. In summer the calculations indicate thatthe number of ozone molecules formed per NO_x moleculedrops with increasing emissions. The average seasonalcycle of ozone at Birkenes for different transportsectors indicate that the most pronounced ozoneformation takes place in air masses from E-Europe/Russia.     

Impact of Air Transport on Seasonal Variations and Trends of Surface Ozone at Kislovodsk High Mountain Station

The impact of air transport on the surface ozone variations is analyzed at Kislovodsk High Mountain Station for the period 1989–1996 on the basis of 2D back trajectories. It was shown that the contribution of photochemical and dynamical processes is different for the different seasons. In summer months the surface ozone concentration is governed by photochemical ozone production in semi polluted air from the regions of Northern Caspian, Southern Ural and Volga region. Time of the seasonal ozone maximum appearance is defined by joint influence of the processes of photochemical production and destruction in the eastern sectors and advection from Ukraine and Central Europe. The value of the seasonal minimum is determined by the processes of ozone destruction in the air coming from northeastern direction in the stable frontal zone. Distribution of sectors of the air transport changes from year to year and it can partly explain strong negative trend of the surface ozone concentration at the site.     

Chemistry of Halogen Oxides in the Troposphere: Comparison of Model Calculations with Recent Field Data

Reactive halogen species (RHS = X, XO, HOX, OXO; X = Cl, Br, I) are known to have an important influence on the chemistry in the polar boundary layer (BL), where they are responsible for ozone depletion events in spring. Recent field campaigns at Mace Head, Ireland, and the Dead Sea, Israel, identified for the first time iodine oxide (IO) at mixing ratios of up to 6.6 ppt and 90 ppt bromine oxide (BrO), respectively, by DOAS also at lower latitudes. These results intensified the discussion about the role of the RHS in the mid-latitude BL.Photochemical box model calculations show that the observed IO mixing ratios can destroy ~0.45 ppb ozone per hour. This is comparable to the rates of the known O₃-loss processes in the boundary layer. The model studies also reveal that IO, at these levels, has a strong influence on the BL photochemistry, increasing the OH/HO₂- and the NO₂/NO - ratios. In combination these changes lead to a reduction of the photochemical ozone formation, which - in addition - reduces ozone mixing ratios by up to 0.15 ppb/h.The studies for the Dead Sea case give no information on the heterogeneous process responsible for the bromine release, but they show that a total of 2 – 4 ppb of total bromine have to be released to explain the observed complete depletion of 60 ppb ozone in 2 – 3 hours.     

Connection between ozone concentration and atmosphere circulation at peak Moussala

Connection between ozone concentration and atmosphere circulation is investigated based on measurements at BEO station, peak Moussala (2,925 m a.s.l.), for the period 09 August 2006 to 29 January 2008. Ozone concentration data are collected with UV-analyzer “Environnement O₃ 42” and meteo data with weather station “Vaisala”. There are measurements of ⁷Be. Data from NOAA HYSPLIT model for particle trajectories are also used. Eight wind directions and three ranges of wind velocities are employed in the analysis. A comparison of ozone concentrations in upward and downward air transport according to HYSPLIT model is made. The number of cases with ozone concentration above 63 ppb has been counted. Mann–Whitney nonparametric test is employed as a basic statistical method. Correlation between atmosphere pressure and tropospheric ozone content is made. The same is done for ⁷Be and ozone. The main conclusion is that there is not any local or regional pollution effect detectable at peak Moussala, but most of the ozone measured is due to emissions of hydrocarbons and NO _x over a larger region. There could be some regional sources of ozone building substances in southwest direction from peak Moussala. Air transported from the north quarter has higher ozone concentrations compared to the south quarter. In vertical direction, upward transport of air masses shows higher values of ozone concentration. Higher wind velocity is associated with low ozone concentrations at peak Moussala. The annual course of ozone concentration has summer maximum and winter minimum. There is right connection between air pressure and ozone concentration. The same is valid for the correlation between ⁷Be and ozone. Diurnal ozone course shows daytime maximum in winter and nighttime maximum in summer.     

Influence of Transport over a Mountain Ridge on the Chemical Composition of Marine Aerosols during the ACE-2 Hillcloud Experiment

Aerosol chemical composition and trace gas measurements were made at twolocations on the northeastern peninsula of Tenerife during the ACE-2HILLCLOUD experiment, between 28 June and 23 July 1997. Measurementswere made of coarse (#gt;2.5 m aerodynamic diameter) and fine (#lt; 2.5m) aerosol Cl^–, NO₃ ^–,SO₄ ^2–, non-sea saltSO₄ ^2– (NSSS),CH₃SO₃ ^– (MSA) andNH₄ ⁺, and gas phase dimethylsulphide (DMS), HCl,HNO₃, SO₂, CH₃COOH, HCOOH andNH₃. Size distributions were measured using a cascadeimpactor. Results show that in marine air masses NSSS and MSA wereformed via DMS oxidation, with additional NSSS present in air massescontaining a continental component. Using a Eulerian box model approachfor aerosols transported between upwind and downwind sites, a mean NSSSproduction rate of 4.36 × 10^–4 gm^–3 s^–1 was calculated for daytimeclear sky periods (highest insolation), with values for cloudy periodsduring daytime and nighttime of 3.55 × 10^–4 and2.40 × 10^–4 g m^–3s^–1, respectively. The corresponding rates for MSA were6.23 × 10^–6, 8.49 × 10^–6and 6.95 × 10^–6 g m^–3s^–1, respectively. Molar concentration ratios forMSA/NSSS were 8.7% (1.8–18.2%) and 1.9%(1.3–3.5%) in clean and polluted air masses, respectively.Reactions occurring within clouds appeared to have a greater influenceon rates of MSA production, than of NSSS, while conversely daytime gasphase reactions were more important for NSSS. For MSA, nighttimein-cloud oxidation rates exceeded rates of daytime gas phase productionvia OH oxidation of DMS. NSSS, MSA and ammonium had trimodal sizedistributions, with modes at 0.3, 4.0 and >10.0 m (NSSS andNH₄ ⁺), and 0.3, 1.5 and 4.0 m (MSA). Nosignificant production of other aerosol species was observed, with theexception of ammonium, which was formed at variable rates dependent onneutralisation of the aerosol with ammonia released from spatiallynon-uniform surface sources. Seasalt components were mainly present incoarse particles, although sub-micrometre chloride was also measured.Losses by deposition exceeded calculated expectations for all species,and were highest for the seasalt fraction and nitrate.     

Comparison of Measured Ozone Production Efficiencies in the Marine Boundary Layer at Two European Coastal Sites under Different Pollution Regimes

Ozone production efficiencies (E_N), which can be defined as the netnumber of ozone molecules produced per molecule of NO_xoxidised, have been calculated from measurements taken during three intensive field campaigns (one in the spring, EASE 96, and two in the summer, EASE 97 and TIGER 95), at two European coastal sites (Mace Head, Ireland (EASE) and Weybourne, Norfolk (TIGER)) impacted by polluted air masses originating from both the U.K. and continental Europe, as well as relatively clean oceanic air masses from the Arctic and Atlantic. From a detailed wind sector analysis of the EASE 96 and 97 data it is clear that two general types of pollution regime were encountered at Mace Head. The calculated ozone production efficiency in clean oceanic air masses was approximately 65, which contrasted to more polluted air, from the U.K. and the continental European plume, where the efficiency decreased to between 4 and 6. The latter values of E_Nagree well with literature measurements conducted downwind of various urban centres in the U.S. and Europe, which are summarised in a wide-ranging review table. The E_N value calculated for clean oceanic air is effectivelyan upper limit, owing to the relatively rapid deposition of HNO₃ tothe ocean. Consideration of the variation of E_N with NO_x forthe three campaigns suggests that ozone production efficiency is relatively insensitive to both geographical location and season. The measuredE_N values are also compared with values derived from steady-state expressions. An observed anti-correlation between E_N and measured ozone tendencyis briefly discussed.