Remote Marine Sites Research Articles

Odds and ends of atmospheric mercury in Europe and over the North Atlantic Ocean: temporal trends of 25 years of measurements

Abstract. The global monitoring plan of the Minamata Convention on Mercury was established to generate long-term data necessary for evaluating the effectiveness of regulatory measures at a global scale. After 25 years of monitoring (since 1995), Mace Head is one of the atmospheric monitoring stations with the longest mercury record and has produced sufficient data for the analysis of temporal trends of total gaseous mercury (TGM) in Europe and the North Atlantic. Using concentration-weighted trajectories for atmospheric mercury measured at Mace Head as well as another five locations in Europe, Amderma, Andøya, Villum, Waldhof and Zeppelin, we identify the regional probabilistic source contribution factor and its changes for the period of 1996 to 2019. Temporal trends indicate that concentrations of mercury in the atmosphere in Europe and the North Atlantic have declined significantly over the past 25 years at a non-monotonic rate averaging 0.03 ngm-3yr-1. Concentrations of TGM at remote marine sites were shown to be affected by continental long-range transport, and evaluation of reanalysis back trajectories displays a significant decrease in TGM in continental air masses from Europe in the last 2 decades. In addition, using the relationship between mercury and other atmospheric trace gases that could serve as a source signature, we perform factorization regression analysis, based on positive rotatable factorization to solve probabilistic mass functions. We reconstructed atmospheric mercury concentration and assessed the contribution of the major natural and anthropogenic sources. The results reveal that the observed downward trend in the atmospheric mercury is mainly associated with a factor with a high load of long-lived anthropogenic species.

Classifying aerosol type using in situ surface spectral aerosol optical properties

Abstract. Knowledge of aerosol size and composition is important for determining radiative forcing effects of aerosols, identifying aerosol sources and improving aerosol satellite retrieval algorithms. The ability to extrapolate aerosol size and composition, or type, from intensive aerosol optical properties can help expand the current knowledge of spatiotemporal variability in aerosol type globally, particularly where chemical composition measurements do not exist concurrently with optical property measurements. This study uses medians of the scattering Ångström exponent (SAE), absorption Ångström exponent (AAE) and single scattering albedo (SSA) from 24 stations within the NOAA/ESRL Federated Aerosol Monitoring Network to infer aerosol type using previously published aerosol classification schemes.Three methods are implemented to obtain a best estimate of dominant aerosol type at each station using aerosol optical properties. The first method plots station medians into an AAE vs. SAE plot space, so that a unique combination of intensive properties corresponds with an aerosol type. The second typing method expands on the first by introducing a multivariate cluster analysis, which aims to group stations with similar optical characteristics and thus similar dominant aerosol type. The third and final classification method pairs 3-day backward air mass trajectories with median aerosol optical properties to explore the relationship between trajectory origin (proxy for likely aerosol type) and aerosol intensive parameters, while allowing for multiple dominant aerosol types at each station.The three aerosol classification methods have some common, and thus robust, results. In general, estimating dominant aerosol type using optical properties is best suited for site locations with a stable and homogenous aerosol population, particularly continental polluted (carbonaceous aerosol), marine polluted (carbonaceous aerosol mixed with sea salt) and continental dust/biomass sites (dust and carbonaceous aerosol); however, current classification schemes perform poorly when predicting dominant aerosol type at remote marine and Arctic sites and at stations with more complex locations and topography where variable aerosol populations are not well represented by median optical properties. Although the aerosol classification methods presented here provide new ways to reduce ambiguity in typing schemes, there is more work needed to find aerosol typing methods that are useful for a larger range of geographic locations and aerosol populations.

Nitro-polycyclic aromatic hydrocarbons – gas–particle partitioning, mass size distribution, and formation along transport in marine and continental background air

Abstract. Nitro-polycyclic aromatic hydrocarbons (NPAH) are ubiquitous in polluted air but little is known about their abundance in background air. NPAHs were studied at one marine and one continental background site, i.e. a coastal site in the southern Aegean Sea (summer 2012) and a site in the central Great Hungarian Plain (summer 2013), together with the parent compounds, PAHs. A Lagrangian particle dispersion model was used to track air mass history. Based on Lagrangian particle statistics, the urban influence on samples was quantified for the first time as a fractional dose to which the collected volume of air had been exposed. At the remote marine site, the 3–4-ring NPAH (sum of 11 targeted species) concentration was 23.7 pg m−3 while the concentration of 4-ring PAHs (6 species) was 426 pg m−3. The most abundant NPAHs were 2-nitrofluoranthene (2NFLT) and 3-nitrophenanthrene. Urban fractional doses in the range of < 0.002–5.4 % were calculated. At the continental site, the Σ11 3–4-ring NPAH and Σ6 4-ring PAH were 58 and 663 pg m−3, respectively, with 9-nitroanthracene and 2NFLT being the most concentrated amongst the targeted NPAHs. The NPAH levels observed in the marine background air are the lowest ever reported and remarkably lower, by more than 1 order of magnitude, than 1 decade before. Day–night variation of NPAHs at the continental site reflected shorter lifetime during the day, possibly because of photolysis of some NPAHs. The yields of formation of 2NFLT and 2-nitropyrene (2NPYR) in marine air seem to be close to the yields for OH-initiated photochemistry observed in laboratory experiments under high NOx conditions. Good agreement is found for the prediction of NPAH gas–particle partitioning using a multi-phase poly-parameter linear free-energy relationship. Sorption to soot is found to be less significant for gas–particle partitioning of NPAHs than for PAHs. The NPAH levels determined in the south-eastern outflow of Europe confirm intercontinental transport potential.

Study of surface ozone at Port Blair, India, a remote marine station in the Bay of Bengal

This paper presents seasonal variation of surface ozone monitored continuously at site of the meteorological observatory at Port Blair, a maritime site of the Bay of Bengal for the period of August, 2005–March, 2007. Present observation depicts the characteristics of surface ozone at the remote marine site and the long range transport of pollutants from three different sides i.e., Indian Subcontinent, South East Asia and Indian Ocean. Very high ozone mixing ratio (~70–80ppbv) is occasionally observed during March and November at this site. A campaign mode of observation of trace gases (surface ozone, CO, NOx, CO2), aerosol concentration and its size, UV radiation at Port Blair was made to understand the role of transport on pollutants during March 16–26, 2002. During this period of observation, a near zero surface ozone of different time scales (~few hours) has been observed several times during the period of midnight to early morning. Simultaneously NOx (NO+NO2) (~40ppbv) and carbon monoxide was observed very high (300–600ppbv) during this period. Source of this high pollutant are not expected at this remote marine sites although wind patterns, 7-days isentropic back Trajectory analysis and MATCH Model output suggest that polluted air mass has come from eastern side of Indian subcontinent.

Identification of Ion sources in rainwater of a coastal site impacted by the gas and oil industry in the southeast of Mexico

The purpose of this research was to study the chemical composition of rainwater to assess the impact of marine aerosol and anthropogenic emissions by using back air-mass trajectories. Fieldwork was done from July to November 2004, in a coastal site probably impacted by gas and oil industry in Campeche, Mexico: San Antonio Cardenas. Na+, Cl-, and Mg2+, were the most abundant ions, being sea-salt aerosol their unique source. A significant portion of K+ and Ca2+ originated from non-sea-salt sources; and the sulphate excess exceeded the background level reported for remote marine sites, suggesting that besides of marine aerosol, there was a significant contribution of SO42- from anthropogenic sources. NO3- concentrations in San Antonio Cardenas exceeded the background level reported for coastal sites with minimal anthropogenic influence and showed a significant correlation with SO42-, suggesting a common source. Agricultural practices are null, for this reason NH4+ levels were low. Mean pH value was 4.64, this acidity showed a good correlation with SO42- and NO3- levels, and with the local meteorology. A sour gas recompression plant located 10 km at NE from San Antonio Cardenas was identified as the main source of this strong acidity.

Diurnal and seasonal variations of iodocarbons (CH2ClI, CH2I2, CH3I, and C2H5I) in the marine atmosphere

[1] Four iodocarbons, chloroiodomethane (CH2ClI), diiodomethane (CH2I2), methyl iodide (CH3I), and ethyl iodide (C2H5I), were measured with an automated preconcentration gas chromatography–mass spectrometry system at two remote marine sites, Hateruma Island (24.05°N, 123.8°E) in the East China Sea and Cape Ochiishi (43.15°N, 145.5°E) on the eastern coast of Hokkaido, for 17 months, giving the first full-year high-frequency data sets for all these compounds. CH2ClI and CH2I2, which are highly photolyzed, showed remarkable diurnal variation in all seasons, with lower concentrations in the daytime, whereas CH3I and C2H5I showed no significant diurnal changes. At Cape Ochiishi, all of the iodocarbons showed clear seasonal variations and were highest in summer and autumn, which are characterized by algal blooms in the adjacent ocean. At Hateruma Island, which is surrounded by subtropical oligotrophic waters, the seasonal variations were not significant, but C2H5I and CH2I2 showed lower mixing ratios in summer. The nighttime mixing ratio of CH2ClI was strongly and positively correlated with wind speed throughout the observation period at Hateruma Island, suggesting the ubiquitous presence of CH2ClI sources, probably nonbiogenic, in the subtropical ocean. CH3I and C2H5I mixing ratios occasionally increased at Hateruma Island concurrently with winter and spring pollution events from the Asian continent, indicating possible continental anthropogenic sources or emissions from seawater off the continental coast. A rise in the CH3I baseline mixing ratio was also observed at Hateruma Island, when the air mass origin became more southerly, compared with the earlier, easterly source area, suggesting higher concentrations in equatorial regions.

Cloud condensation nuclei measurements in the marine boundary layer of the Eastern Mediterranean: CCN closure and droplet growth kinetics

Abstract. Measurements of cloud condensation nuclei (CCN) concentrations (cm−3) between 0.2 and 1.0% supersaturation, aerosol size distribution and chemical composition were performed at a remote marine site in the eastern Mediterranean, from September to October 2007 during the FAME07 campaign. Most of the particles activate at ~0.6% supersaturation, characteristic of the aged nature of the aerosol sampled. Application of Köhler theory, using measurements of bulk composition, size distribution, and assuming that organics are insoluble resulted in agreement between predicted and measured CCN concentrations within 7±11% for all supersaturations, with a tendency for CCN underprediction (16±6%; r2=0.88) at the lowest supersaturations (0.21%). Including the effects of the water-soluble organic fraction (which represent around 70% of the total organic content) reduces the average underprediction bias at the low supersaturations, resulting in a total closure error of 0.6±6%. Using threshold droplet growth analysis, the growth kinetics of ambient CCN is consistent with NaCl calibration experiments; hence the presence of aged organics does not suppress the rate of water uptake in this environment. The knowledge of the soluble salt fraction is sufficient for the description of the CCN activity in this area.

Evaluation of the global oceanic isoprene source and its impacts on marine organic carbon aerosol

Abstract. We have combined the first satellite maps of the global distribution of phytoplankton functional type and new measurements of phytoplankton-specific isoprene productivities, with available remote marine isoprene observations and a global model, to evaluate our understanding of the marine isoprene source and its impacts on organic aerosol abundances. Using satellite products to scale up data on phytoplankton-specific isoprene productivity to the global oceans, we infer a mean "bottom-up" oceanic isoprene emission of 0.31±0.08 (1σ) Tg/yr. By minimising the mean bias between the model and isoprene observations in the marine atmosphere remote from the continents, we produce a "top-down" oceanic isoprene source estimate of 1.9 Tg/yr. We suggest our reliance on limited atmospheric isoprene data, difficulties in simulating in-situ isoprene production rates in laboratory phytoplankton cultures, and limited knowledge of isoprene production mechanisms across the broad range of phytoplankton communities in the oceans under different environmental conditions as contributors to this difference between the two estimates. Inclusion of secondary organic aerosol (SOA) production from oceanic isoprene in the model with a 2% yield produces small contributions (0.01–1.4%) to observed organic carbon (OC) aerosol mass at three remote marine sites in the Northern and Southern Hemispheres. Based on these findings we suggest an insignificant role for isoprene in modulating remote marine aerosol abundances, giving further support to a recently postulated primary OC source in the remote marine atmosphere.

Correlation between cloud condensation nuclei concentration and aerosol optical thickness in remote and polluted regions

Abstract. A large number of published and unpublished measurements of cloud condensation nuclei (CCN) concentrations and aerosol optical thickness (AOT) measurements have been analyzed. AOT measurements were obtained mostly from the AERONET network, and selected to be collocated as closely as possible to the CCN investigations. In remote marine regions, CCN0.4 (CCN at a supersaturation of 0.4%) are around 110 cm−3 and the mean AOT500 (AOT at 500 nm) is 0.057. Over remote continental areas, CCN are almost twice as abundant, while the mean AOT500 is ca. 0.075. (Sites dominated by desert dust plumes were excluded from this analysis.) Some, or maybe even most of this difference must be because even remote continental sites are in closer proximity to pollution sources than remote marine sites. This suggests that the difference between marine and continental levels must have been smaller before the advent of anthropogenic pollution. Over polluted marine and continental regions, the CCN concentrations are about one order of magnitude higher than over their remote counterparts, while AOT is about five times higher over polluted than over clean regions. The average CCN concentrations from all studies show a remarkable correlation to the corresponding AOT values, which can be expressed as a power law. This can be very useful for the parameterization of CCN concentrations in modeling studies, as it provides an easily measured proxy for this variable, which is difficult to measure directly. It also implies that, at least at large scales, the radiative and microphysical effects of aerosols on cloud physics are correlated and not free to vary fully independently. While the observed strong empirical correlation is remarkable, it must still be noted that there is about a factor-of-four range of CCN concentrations at a given AOT, and that there remains considerable room for improvement in remote sensing techniques for CCN abundance.

An improvement on the dust emission scheme in the global aerosol-climate model ECHAM5-HAM

Abstract. Formulation of the dust emission scheme in the global aerosol-climate modeling system ECHAM5-HAM has been improved. Modifications on the surface aerodynamic roughness length, soil moisture and East-Asian soil properties are included in the parameterization, which result in a large impact on the threshold wind friction velocity for aeolian erosion and thus influence the simulated dust emission amount. The annual global mean of dust emission in the year 2000 is reduced by 76.5% and 2.2%, respectively, due to changes in the aerodynamic roughness length and the soil moisture. An inclusion of detailed East-Asian soil properties leads to an increase of 16.6% in the annual global mean of dust emission, which exhibits mainly in the arid and semi-arid areas of northern China and southern Mongolia. Measurements of the surface dust concentrations are collected in remote marine sites globally and in dust source regions of East Asia. The averaged relative differences between model results and measurements are reduced from 17% to 12% in global remote marine sites and from 69% to 30% in East Asia, by including the improvements. Comparisons between model results and available measurements verify a more realistic dust distribution with the improved emission scheme.

Spatial distribution and source identification of wet deposition at remote EANET sites in Japan

Wet deposition of major ions was discussed from the viewpoint of its potential sources for six remote EANET sites in Japan (Rishiri, Happo, Oki, Ogasawara, Yusuhara, and Hedo) having sufficiently high data completeness during 2000–2004. The annual deposition for each site ranged from 12.1 to 46.6 meq m −2 yr −1 for nss-SO 4 2−, from 5.0 to 21.9 meq m −2 yr −1 for NO 3 −. The ranges of annual deposition of the two ions for the sites were lower than those for urban and rural sites in Japanese Acid Deposition Survey by Ministry of the Environment, Japan, and higher than those for global remote marine sites. Factor analysis was performed on log-transformed daily wet deposition of major ions for each site. The obtained two factors were interpreted as (1) acid and soil source (or acid source for some sites), and (2) sea-salt source for all the sites. This indicates that wet deposition of ions over the remote areas in Japan has a similar structure in terms of types of sources. Factor scores of acid and soil source were relatively high during Kosa (Asian dust) events in spring in western Japan. Back-trajectories for high-deposition episodes of acid and soil source (or acid source) for the remote sites showed that episodic air masses frequently came from the northeastern area of Asian Continent in spring and winter, and from central China in summer and autumn. This indicates a large contribution of continental emissions to wet deposition of ions over the remote areas in Japan.

Long-range transport of ‘giant’ aeolian quartz grains: linkage with discrete sedimentary sources and implications for protective particle transfer

Several articles have documented the presence of large particles (>62.5μm) that have been transported great distances (>1000km) through the atmosphere. At present, we do not understand the mechanisms that lead to mineral grains being carried aloft far beyond the limits imposed by conventional gravitational settling models. Typically, marine geologists refer to ice rafting and/or advective transport by ocean currents as the dominant transport processes that deliver large mineral grains to deep sea sediments. Textural analyses of ‘giant’ quartz grains from six sedimentary deposition events collected from various sources, including a single site in the North Pacific Ocean and sites in the British Isles and France, strongly support the idea that in each case all of the grains were initially eroded from a discrete sediment type, and often a discrete point source, and subsequently transported via the atmosphere to our sampling sites up to 10 000km away. These findings suggest that at least at times erosional events fuelling long distance atmospheric transport are exceedingly specific and operate upon geologically distinctive assemblages; and are able to effect long distance transfer of large grains with no grain-to-grain collisions. This work also suggests that atmospheric transport should be included as a possible means of delivery for significant quantities of ‘giant’ sedimentary particles to remote marine sites such as the Lord Howe Rise.

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.

Control of solute concentrations in cloud and fog water by liquid water content

We measured the concentrations of Cl −, NO 3 −, and SO 4 2− in atmospheric cloudwater collected with a passive collector from a remote marine site and the concentrations of Cl −, NO 3 −, SO 4 2−, Na + and NH 4 + in cloudwater from a continental site. At the continental site, an active rotating arm collector was employed for cloudwater collection. At both locations, we observed an inverse relationship between the measured ion concentrations and the liquid water content of the clouds. The product of the observed concentrations and the liquid water content, the cloudwater loading, was found to be nearly constant. A study of published cloudwater data showed that at the majority of the examined locations this product varied little at any given site, whereas large differences exist between sites. The finding of the small variation in cloudwater loadings could be explained by the hypothesis that nucleation scavenging is the dominant process in the acquisition of solutes into hydro-meteors during fog and cloud formation.

Aerosol light scattering properties at Cape Grim, Tasmania, during the First Aerosol Characterization Experiment (ACE 1)

Large uncertainties remain in understanding the effects of ambient aerosols on climate. As part of the First Aerosol Characterization Experiment (ACE 1, November 15 to December 15, 1995), the total light scattering and backscattering coefficients by particles (σsp and σbsp) were measured at Cape Grim, Tasmania. Such measurements were made to characterize aerosol radiative forcing relevant to climate change at a southern hemisphere site. A controlled relative humidity (RH) nephelometry system (humidograph) was used to measure the dependence of σsp and σbsp upon controlled RH, upper particle diameter (Dp), and wavelength of light (λ). Parameters derived from the measurements necessary to incorporate aerosols into climate models include the dependence of light scattering on aerosol hygroscopic growth (ƒ(RH)) and the hemispheric backscatter fraction (b), both as functions of wavelength of light. Differences in parameter values during clean marine and non‐clean‐marine time periods were greatest for σsp and σbsp for Dp ≤ 1 μm and were least for ƒ(RH) and b. During clean marine conditions, for Dp ≤ 10 μm, and at λ=550 nm, means and standard deviations of key parameters measured included ƒ(RH equal to 82%) = 1.98±0.08 for σsp, ƒ(RH equal to 82%) = 1.48±0.09 for σbsp, b = 0.16±0.01 at RH equal to 22%, and b = 0.12±0.03 at RH equal to 82%. These measurements contribute to characterizing aerosol optical properties at a remote marine site, serve as a basis of comparison to polluted sites, and may provide input to climate models.

Sulphur and nitrogen compounds in variable marine/continental air masses at the southwest European coast

Measurements of atmospheric sulphur and nitrogen species were made on the west coast of Portugal, within the framework of BMCAPE project. Four research intensive campaigns took place between November 1993 and August 1994 and revealed pronounced seasonal cycles for DMS and MSA. Surprisingly high levels of DMS were detected during autumn (mean concentration: 776ngSm −3), possibly as a consequence of a later algae bloom or local marsh emissions. COS displayed a slight seasonal variation with mean concentrations ranging from 741 ng S m −3 in autumn up to 903 ng S m −3 in summer. On the basis of backward air mass trajectories and local meteorological data, samples were assigned into different classes of air masses. The lowest concentrations of SO 2, nssSO 4 2−, NH 3, NH 4 +, HNO 3, N0 3 − and NO 2 were found in air masses transported over the Atlantic Ocean and were about 3–10 times lower than those recorded in air masses from continental origin. Nevertheless, for many of the species, the mean concentrations in maritime air were still significantly higher than those previously recorded in remote marine sites, presumably owing to long-range transport of pollutants or contamination induced by mesoscale wind circulation.

A global three‐dimensional model of tropospheric sulfate

A three‐dimensional model is used to simulate the global tropospheric distributions of dimethylsulfide (DMS), SO2, SO42−, and methanesulfonic acid (MSA). The model uses meteorological input from a general circulation model (GCM) developed at the Goddard Institute of Space Studies (GISS) with 4° × 5° horizontal resolution, nine layers in the vertical, and a time resolution of 4 hours. Model results are compared with observations from surface sites, ships, and aircraft. The model reproduces generally to within 30% the observed SO2 and SO42− concentrations over the United States and Europe; these concentrations are highly sensitive to the supply of H2O2 as an in‐cloud SO2 oxidant. Sulfate concentrations and wet deposition fluxes observed at remote marine sites can be accounted for using a global DMS source of 22 Tg S yr−1 in the model. However, this source overestimates DMS air concentrations by a factor of 2 unless we assume the presence of another DMS oxidant besides OH and NO3. Inclusion of another DMS oxidant in our model also improves the simulation of the MSA to SO42− concentration ratio in marine air. Simulated SO42− concentrations in the northern hemispheric free troposphere are much lower than in previous global models and are more consistent with the few observations available. The difference reflects in part our accounting of efficient scavenging of SO2 and SO42− in wet convective updrafts. Global mean tropospheric lifetimes computed in our model are 1.0 days for DMS, 1.2 days for SO2, 3.9 days for SO42−, and 6.2 days for MSA. Fossil fuel combustion and industrial activities represent 68% of global non‐sea‐salt sulfur emissions. About 50% of SO2 globally is converted to SO42− aerosol (principally by in‐cloud oxidation) while the remainder is removed by deposition (30% by dry, 20% by wet). In‐cloud oxidation of SO2 represents 85% of the global SO42− source.

Measurements and analysis of reactive nitrogen species in the rural troposphere of Southeast United States: Southern oxidant study site SONIA

Ambient concentrations of reactive nitrogen compounds as well as total NOY were measured during June and early July 1992 at a rural site, site SONIA, in the central Piedmont region of North Carolina as a part of the Southern Oxidants Study. The measurements of reactive nitrogen species were made in an effort to provide a comprehensive understanding of nitrogen chemistry and to investigate the total nitrogen budget at the site. NO y , NO 2 and NO showed diurnal variations with maxima in the morning between 0600 and 0900 EST. The maximum NO y concentration reached was ∼ 14.5 ppbv, and the maximum concentrations of NO and NO 2 were 5.4 and 7.8 ppbv, respectively. The mean NO y mixing ratio was found to be 2.88 ± 1.58 ppbv ( n = 743) with an average daily maximum of 3.6 ppbv. The mean mixing ratios of NO and N0 2 were found to be 0.15 ± 0.29 ppbv ( n = 785) and 1.31 ± 0.99 ppbv ( n = 769). Average daily maxima of NO and N0 2 were 0.4 and 2.0 ppbv, respectively. HNO 3 and PAN showed diurnal variation with maxima in the afternoon and minimum in the night, and mean mixing ratios were found to be 0.67 ± 0.33 ppbv ( n = 250) and 0.40 ± 0.24 ppbv ( n = 578). The fractions of individual reactive nitrogen species to total NO y , were investigated and contrasted to the results from a remote marine site and rural continental sites. As in two other rural continental sites in the U.S., NO x was found to be the most abundant constituent (∼45%) of NOY; while HNO 3 was the most abundant compound in NO y measured at a remote marine site. The discrepancy between the NO y , partitioning at site SONIA and the marine site is attributed to the influence of local and regional anthropogenic sources of NO x and the continental origin of the majority of air masses encountered at the site. The NO x NO y ratio and NO z ( = NO y − NO x ) were used as an indicator of the chemical age of airmasses. The NO x NO y ratio showed strong positive correlations with the photochemical oxidants HNO 3 ( r = 0.76), PAN ( r = 0.68) and 0 3 ( r = 0.79) measured at the site. Positive correlations were found between surface wind direction and both the magnitude of NO y and the NO x NO y ratio. These correlations suggest that synoptic meteorological conditions and transport of NO x are important in the distribution of NO y and its relationship with photochemical oxidants at the site. The ozone production efficiency was illustrated by correlation of O 3 and NO z and compared with other published measurements made in the Southeast U.S., and published results from a 3D Eulerian model simulation.

Transport climatology of tropospheric ozone: Bermuda, 1988–1991

We determined the major transport patterns for Bermuda and quantified the degree to which they influenced variability in ozone concentrations by applying cluster analysis to isentropic trajectories from September 1988, through September 1991. Concentration distributions of ozone associated with these transport patterns were significantly different. The highest concentrations of ozone in each season were associated with transport off the North American continent; the lowest concentrations were during low‐level maritime transport around the Bermuda high. Using the vertical component of the isentropic trajectories, we also showed that the most extreme concentrations of ozone occurred with rapidly descending air from midtropospheric levels. This pattern was most pronounced in April and May when more than 50% of the O3 variability was related to transport differences. We conclude that this relatively remote marine site, which normally experienced low maritime ozone levels (∼30 parts per billion by volume (ppbv)), periodically entrained dry, ozone‐rich (∼55 ppbv) midtropospheric air in association with strong subsidence in high pressure behind spring low‐pressure systems. Although the ultimate source of these midtroposphere, midlatitude, elevated‐ozone concentrations is still being investigated, the synoptic meteorology associated with these transport patterns supports a significant contribution from the upper troposphere and lower stratosphere.

Precipitation composition and its variability in the southern Indian Ocean: Amsterdam Island, 1980–1987

Event precipitation samples have been collected on Amsterdam Island (37° 47′ S, 77° 31′ E) from May 1980 through January 1987 and analyzed for SO4=, NO3−, Cl−, Na+, Mg++, K+, Ca++, H+, HCOOt, and CH3COOt. The objective of this paper is to assess the processes which influence variability in the chemical composition of precipitation at this remote marine site. Back trajectories and gas phase 222Rn measurements were used to identify source regions and their relative contributions to precipitation composition. The technique of cluster analysis was applied to trajectory data as a method for determining to what degree different atmospheric flow patterns influence variability in the observed composition. The dominant source regions for chemical deposition were found to be Madagascar and/or S.E. Africa and the ocean region north and east of Amsterdam Island. A strong seasonal signal in the precipitation composition is illustrated. Processes which influence variability in the chemical composition include the following: seasonality in the source strength of biogenic precursors for non‐sea‐salt (nss) SO4=, NH4+ and carboxylic acids; seasonality in biomass‐burning continental sources for the concentration of NO3− and a portion of nss Cl−; and variations in source region driven by daily changes in meteorology, as well as seasonal and annual differences in transport and removal patterns. In addition, interannual differences in nss SO4= concentration appear to be related to fluctuations in large‐scale circulation patterns as defined by the Southern Oscillation.