Oceanic Emissions Research Articles

Methyl iodide: Atmospheric budget and use as a tracer of marine convection in global models

We simulate the oceanic and atmospheric distribution of methyl iodide (CH3I) with a global 3‐D model driven by assimilated meteorological observations from the Goddard Earth Observing System of the NASA Data Assimilation Office and coupled to an oceanic mixed layer model. A global compilation of atmospheric and oceanic observations is used to constrain and evaluate the simulation. Seawater CH3I(aq) in the model is produced photochemically from dissolved organic carbon, and is removed by reaction with Cl− and emission to the atmosphere. The net oceanic emission to the atmosphere is 214 Gg yr−1. Small terrestrial emissions from rice paddies, wetlands, and biomass burning are also included in the model. The model captures 40% of the variance in the observed seawater CH3I(aq) concentrations. Simulated concentrations at midlatitudes in summer are too high, perhaps because of a missing biological sink of CH3I(aq). We define a marine convection index (MCI) as the ratio of upper tropospheric (8–12 km) to lower tropospheric (0–2.5 km) CH3I concentrations averaged over coherent oceanic regions. The MCI in the observations ranges from 0.11 over strongly subsiding regions (southeastern subtropical Pacific) to 0.40 over strongly upwelling regions (western equatorial Pacific). The model reproduces the observed MCI with no significant global bias (offset of only +11%) but accounts for only 15% of its spatial and seasonal variance. The MCI can be used to test marine convection in global models, complementing the use of radon‐222 as a test of continental convection.

The distribution of the chlorinated solvents dichloromethane, perchloroethylene, and trichloroethylene in the global atmosphere.

Dichloromethane, perchloroethylene, and trichloroethylene are commercially important chlorinated solvents whose health and environmental impacts are under scrutiny in the industrial world. Their distributions in the global atmosphere have been computed based on data from the Reactive Chlorine Emissions Inventory (RCEI) project using the Global Balance Environment (GLOBE) model, a 3-D radiative-dynamical-chemical model. Their atmospheric lifetimes, scaled to an observed methyl chloroform lifetime of 4.8 years, are 158 days, 105 days, and 4.3 days, respectively. They have strong interhemispheric gradients, with maximum zonal mean surface concentrations in the winter mid-latitude northern hemisphere of approximately 40 ppt, 9 ppt, and 2.5 ppt, respectively. Their spatial distributions show significant seasonal variability, and are sensitive to vertical mixing by cumulus convection and horizontal mixing by synoptic-scale turbulence. While the model interhemispheric exchange time (1.0 years) and computed atmospheric lifetimes are very sensitive to sub-grid scale diffusion, interhemispheric gradients of the chlorinated solvents are not. The simulated results suggest a greater importance for oceanic emissions of perchloroethylene and trichloroethylene than has previously been assumed.

People

Volatile organic compounds (VOCs) that are emitted from biomass burning, vehicle exhaust, and industrial emissions play a vital role in the formation of ozone (O3) and secondary organic aerosols (SOA). Since VOCs are the precursors of O3 and aerosol pollution which have become the world's most emergent environmental problems, a field measurement study focused on VOCs was carried out from 27 August to 10 October 2018 in a rural coastal site in Hong Kong. During the campaign, 13 VOC species were detected continuously with proton-transfer-reaction quadrupole mass spectrometry, and their effects on photochemical air pollution were studied. Methanol was the most abundant species among the measured VOCs (average concentration, 3.73 ± 3.26 ppb), and higher concentrations of oxygenated VOCs were found than reported in previous studies of atmospheric chemistry in rural areas. Diurnal variations were observed in the concentrations of various VOC species, indicating that the VOC concentrations were influenced by photochemical reactions. The amount of O3 formation was estimated based on the maximum incremental reactivity scale of the VOCs. The top five contributors to O3 formation in Hong Kong (in order) were isoprene (13.46 μg/m3), methyl ethyl ketone (12.74 μg/m3), xylene (8.52 μg/m3), acetaldehyde (8.22 μg/m3), and acrolein (4.32 μg/m3). Receptor model positive matrix factorization (PMF) was used to identify the dominant emission sources and evaluate their corresponding contributions to VOCs. Five major VOC sources were identified with the PMF method, including (1) industry and vehicle-related sources (8.1%), (2) biogenic emissions (5.5%), (3) biomass burning (63.7%), (4) secondary formation (9.2%), and (5) ship-related emissions (13.5%). The source apportionment results from PMF analysis show that the sampling site at the southeastern tip of Hong Kong was strongly influenced by urban plumes from the Guangdong–Hong Kong–Macao Greater Bay Area/Pearl River Delta region and by oceanic emissions.

Atmospheric methyl iodide: High correlation with surface seawater temperature and its implications on the sea‐to‐air flux

Intensive measurements of atmospheric methyl iodide taken at high, middle, and low latitudes over a period of 3 years have provided evidence for its photochemical production in seawater and given new information that sea‐to‐air transport of CH3I is mainly controlled by surface seawater temperature (SST). These findings suggest a highly localized production and distribution of CH3I in the surface microlayer. As a result, the oceanic emission of CH3I is likely to be larger than previous estimates based on the classical two‐layer model. Owing to the SST dependence of atmospheric CH3I concentration, its impact on tropospheric or stratospheric ozone depletion would be increased by El Niño or future global warming.

Chemical characterization of submicron organic aerosols in the tropical trade winds of the caribbean using gas chromatography/mass spectrometry

Chemical characterization of submicron ( D p<0.6 μm) organic aerosols in the tropical trade winds in the Caribbean was performed. Samples were collected with a Lundgren cascade impactor at different locations in and around the island of Puerto Rico. The majority of the samples were collected at the Lighthouse at Fajardo. The GC/MS results were compiled into a database. The compounds identified were consistently observed in most of the samples and seem to have a natural origin. The families of compounds most consistently detected were esters (fatty acid esters and phosphates), fatty acids, and normal as well as branched hydrocarbons. The modality of extracted ion from full-scan MS data did not show the presence of PAHs in any of the samples analyzed from the coastal and ocean sites. Results tend to indicate that a substantial fraction of the submicron organic aerosol found in the tropical trade winds of the Caribbean may be associated with natural oceanic emissions; therefore, any treatment of the total aerosol should consider the contribution of this natural background aerosol.

Tropospheric concentrations of the chlorinated solvents, tetrachloroethene and trichloroethene, measured in the remote northern hemisphere

A fully automated twin ECD gas chromatograph system with sample enriching adsorption–desorption primary stage was deployed on two field campaigns – Ny-Ålesund, Svalbard, Arctic Norway (July–September 1997), and the RRS Discovery CHAOS cruise of the northeast Atlantic (April–May 1998). Concentrations of an extensive set of halocarbons were detected at hourly intervals at pptv levels. We present here the results obtained for the chlorinated solvents, tetrachloroethene (PCE) and trichloroethene (TCE). Average baseline PCE and TCE concentrations of 1.77 and 0.12 pptv, respectively, were recorded in Ny-Ålesund. During pollution incidences, concentrations rose to 5.61 (PCE) and 3.18 pptv (TCE). The cruise data showed average concentrations ranging from 4.26 (PCE) and 1.66 pptv (TCE) for air masses originating over the North Atlantic and Arctic open oceans, to maxima of 15.59 (PCE) and 17.51 pptv (TCE) for polluted air masses from Northern Europe. The data sets emphasise the difficulties in defining remote sites for background tropospheric halocarbon measurements, as Ny-Ålesund research station proved to be a source of tetrachloroethene. The data also suggest possible oceanic emissions of trichloroethene in the sub-tropical ocean.

Global distribution of N2O emissions from aquatic systems: natural emissions and anthropogenic effects

Context Abstract: Atmospheric concentrations of nitrous oxide, a greenhouse gas, are increasing due to human activities. Our analysis suggests that a third of global anthropogenic N2O emission is from aquatic sources (rivers, estuaries, continental shelves) and the terrestrial sources comprise the remainder. Over 80% of aquatic anthropogenic N2O emissions are from the Northern Hemisphere mid-latitudes consistent with the geographic distribution of N fertilizer use, human population and atmospheric N deposition. These N inputs to land have increased aquatic as well as terrestrial anthropogenic N2O emissions because a substantial portion enters aquatic systems and results in increased N2O production. Thus, wise management of N in the terrestrial environment could help reduce/control both aquatic and terrestrial N2O emissions.Main Abstract: The global distribution of N2O emissions from rivers, estuaries, continental shelves, and oceans are compared to each other, and to terrestrial emissions, using existing gridded inventories. Rivers, estuaries and continental shelves (1.9 Tg N y−1) account for about 35% of total aquatic N2O emissions; oceanic emissions comprise the remainder. Oceanic N2O emissions are approximately equally distributed between the Northern and Southern Hemispheres; however, over 90% of emissions from estuaries and rivers are in the Northern Hemisphere. N2O emissions from rivers, estuaries, and continental shelves combined equal oceanic emissions in both the 20°–45°N and 45°–66°N latitudinal zones. Over 90% of river and estuary emissions are considered anthropogenic (1.2 Tg N y−1); only 25% of continental shelf emissions are considered anthropogenic (0.1 Tg N y−1); oceanic emissions are considered natural. Overall, approximately one third of both aquatic and of terrestrial emissions are anthropogenic.Natural terrestrial emissions are highest in tropical latitudes while natural aquatic emissions are relatively evenly distributed among latitudinal zones. Over half of both the anthropogenic terrestrial and aquatic emissions occur between 20° and 66°N. Anthropogenic N inputs to the terrestrial environment drive anthropogenic N2O emissions from both land and aquatic ecosystems, because a substantial portion of the anthropogenic N applied to watersheds enters rivers, estuaries and continental shelves.

A strong source of methyl chloride to the atmosphere from tropical coastal land.

Methyl chloride (CH3Cl), the most abundant halocarbon in the atmosphere, has received much attention as a natural source of chlorine atoms in the stratosphere. The annual global flux of CH3Cl has been estimated to be around 3.5 Tg on the grounds that this must balance the loss through reaction with OH radicals (which gives a lifetime for atmospheric CH3Cl of 1.5 yr). The most likely main source of methyl chloride has been thought to be oceanic emission, with biomass burning the second largest source. But recent seawater measurements indicate that oceanic fluxes cannot account for more than 12% of the estimated global flux of CH3Cl, raising the question of where the remainder comes from. Here we report evidence of significant CH3Cl emission from warm coastal land, particularly from tropical islands. This conclusion is based on a global monitoring study and spot measurements, which show enhancement of atmospheric CH3Cl in the tropics, a close correlation between CH3Cl concentrations and those of biogenic compounds emitted by terrestrial plants, and OH-linked seasonality of CH3Cl concentrations in middle and high latitudes. A strong, equatorially located source of this nature would explain why the distribution of CH3Cl is uniform between the Northern and Southern hemispheres, despite their differences in ocean and land area.

Sulphur in the mix

The precursor of the gas dimethylsulphide (DMS) is produced by phytoplankton. Oceanic emissions of this compound put sulphur into the atmosphere, where it is thought to exert a cooling effect on climate. A new study shows that the depth of surface-layer ocean mixing, which itself depends on climatic factors such as wind and temperature, seems to be a determinant of the level of DMS production.

Natural emissions of chlorine‐containing gases: Reactive Chlorine Emissions Inventory

Although there are many chlorine‐containing trace gases in the atmosphere, only those with atmospheric lifetimes of 2 years or fewer appear to have significant natural sources. The most abundant of these gases are methyl chloride, chloroform, dichloromethane, perchloroethylene, and trichloroethylene. Methyl chloride represents about 540 parts per trillion by volume (pptv) Cl, while the others together amount to about 120 pptv Cl. For methyl chloride and chloroform, both oceanic and land‐based natural emissions have been identified. For the other gases, there is evidence of oceanic emissions, but the roles of the soils and land are not known and have not been studied. The global annual emission rates from the oceans are estimated to be 460 Gg Cl/yr for CH3Cl, 320 Gg Cl/yr for CHCl3, 160 Gg Cl/yr for CH2Cl2, and about 20 Gg Cl/yr for each of C2HCl3, and C2Cl4. Land‐based emissions are estimated to be 100 Gg Cl/yr for CH3Cl and 200 Gg Cl/yr for CHCl3. These results suggest that the oceans account for about 12% of the global annual emissions of methyl chloride, although until now oceans were thought to be the major source. For chloroform, natural emissions from the oceans and lands appear to be the major sources. For further research, the complete database compiled for this work is available from the archive, which includes a monthly emissions inventory on a 1° × 1° latitude‐longitude grid for oceanic emissions of methyl chloride.

Composite global emissions of reactive chlorine from anthropogenic and natural sources: Reactive Chlorine Emissions Inventory

Emission inventories for major reactive tropospheric Cl species (particulate Cl, HCl, ClNO2, CH3Cl, CHCl3, CH3CCl3, C2Cl4, C2HCl3, CH2Cl2, and CHClF2) were integrated across source types (terrestrial biogenic and oceanic emissions, sea‐salt production and dechlorination, biomass burning, industrial emissions, fossil‐fuel combustion, and incineration). Composite emissions were compared with known sinks to assess budget closure; relative contributions of natural and anthropogenic sources were differentiated. Model calculations suggest that conventional acid‐displacement reactions involving S(IV) + O3, (IV) + O3 H2O2, and H2SO4 and HNO3 scavenging account for minor fractions of sea‐salt dechlorination globally. Other important chemical pathways involving sea‐salt aerosol apparently produce most volatile chlorine in the troposphere. The combined emissions of CH3Cl from known sources account for about half of the modeled sink, suggesting fluxes from known sources were underestimated, the OH sink was overestimated, or significant unidentified sources exist. Anthropogenic activities (primarily biomass burning) contribute about half the net CH3Cl emitted from known sources. Anthropogenic emissions account for only about 10% of the modeled CHCl3 sink. Although poorly constrained, significant fractions of tropospheric CH2Cl2 (25%), C2HCl3 (10%), and C2Cl4 (5%) are emitted from the surface ocean; the combined contributions of C2Cl4 and C2HCl3 from all natural sources may be substantially higher than the estimated oceanic flux.

Processes controlling the distribution of aerosol particles in the lower marine boundary layer during the First Aerosol Characterization Experiment (ACE 1)

The goals of the International Global Atmospheric Chemistry (IGAC) Program's First Aerosol Characterization Experiment (ACE 1) are to determine and understand the properties and controlling factors of the aerosol in the remote marine atmosphere that are relevant to radiative forcing and climate. A key question in terms of this goal and the overall biogeochemical sulfur cycle is what factors control the formation, growth, and evolution of particles in the marine boundary layer (MBL). To address this question, simultaneous measurements of dimethylsulfide (DMS), sulfur dioxide (SO2), the aerosol chemical mass size distribution, and the aerosol number size distribution from 5 to 10,000 nm diameter were made on the National Oceanic and Atmospheric Administration (NOAA) ship Discoverer. From these data we conclude that the background MBL aerosol during ACE 1 often was composed of four distinct modes: an ultrafine (UF) mode (Dp = 5–20 nm), an Aitken mode (Dp = 20–80 nm), an accumulation mode (Dp = 80–300 nm), and a coarse mode (Dp &gt; 300 nm). The presence of UF mode particles in the MBL could be explained by convective mixing between the free troposphere (FT) and the MBL associated with cloud pumping and subsidence following cold frontal passages. There was no evidence of major new particle production in the MBL. Oceanic emissions of DMS appeared to contribute to the growth of Aitken and accumulation mode particles. Coarse mode particles were comprised primarily of sea salt. Although these particles result from turbulence at the air‐sea interface, the instantaneous wind speed accounted for only one third of the variance in the coarse mode number concentration in this region.

Photochemical box modeling of long‐range transport from North America to Tenerife during the North Atlantic Regional Experiment (NARE) 1993

A photochemical trajectory model was used to reproduce the concentrations of several trace gases measured at Izaña/Tenerife during a selected episode of August 1993 with long‐range transport from the North American continent across the Atlantic Ocean. The set of initial conditions necessary to match the observations is analyzed and compared with available data in the literature. Initial and final concentrations of several hydrocarbons are in reasonable agreement with other measurements made at similar latitudes, and the CO/NOy and NMHC/NOy ratios are within the range of those observed from airborne measurements over the eastern United States and Canada. Important exceptions are the light olefines ethene and propene which may originate from oceanic emissions. The simulated concentration of ozone at Izaña is very similar to the initial concentration used, and there is no indication for photochemical net ozone production during long‐range transport. The abundance of NOx at the end of the simulation is largely due to decomposition of peroxyacetyl nitrate during the descent of the air before reaching Izaña. The results differ significantly from those of a steady state calculation which was made for comparison, and the height evolution of the trajectory was found to be a critical parameter. A parameter variation study revealed that significant net ozone production only occurs for very low initial ozone concentrations and/or NOy levels that are well above those observed during the period at Izaña at night. This study also emphasizes the important role of water vapor for the tropospheric ozone budget and the role ozone itself plays in its photochemical balance.

Ammonium in coastal Antarctic aerosol and snow: Role of polar ocean and penguin emissions

Year‐round aerosol samples collected in the boundary layer at coastal Antarctic sites (Dumont D'Urville, Neumayer, and Halley) indicate a seasonal cycle of ammonium concentrations with a minimum in winter (April–September). A large intersite difference appears in the summer (November–February) maxima values, from ∼12.5 ng m−3 at Neumayer to 140–230 ng m−3 at Dumont D'Urville. At Dumont D'Urville, ammonium concentrations are the largest ever reported from Antarctic sites, and the large summer maxima are associated with large enrichments with respect to sea salt for potassium and calcium. In addition, seasonal ammonium variations at Dumont D'Urville are in phase with a well‐marked seasonal cycle of oxalate concentrations which exhibit maxima of 5–10 ng m−3 in spring and summer and minima of less than 0.5 ng m−3 in winter. Such a composition of aerosols present in the boundary layer at Dumont D'Urville in summer is linked to the presence of a large Adélie penguin population from the end of October to March at the site. Ornithogenic soils (defined as guano‐enriched soils), together with the bacterial decomposition of uric acid, are a source of ammonium, oxalate, and cation (such as potassium and calcium) aerosol, in addition to a subsequent large ammonia loss from ornithogenic soils to the atmosphere. The total breeding population of 5 million Adélie penguins widely distributed around the Antarctic continent may emit, at most, some 2.5 × 10−4 Mt of NH3‐N during the summer months. In contrast, Halley and Neumayer Stations are far less exposed to penguin colony emissions. At Neumayer, ammonium concentrations peak from January to March and are in phase with the increase of biogenic sulfur species. Here the NH4+/(MSA + nss SO4−) molar ratio is close to 13% in summer aerosol and to 40% in winter aerosol. Using this summer ratio, which may be related to ammonia and sulfur oceanic emissions occurring south of 50°S in summer and estimated DMS emissions in these regions at this time, we derive an upper limit of 0.064 Mt NH3‐N emitted per year by the high‐latitude Southern Ocean in summer. This study indicates a very limited ammonia neutralization of acidic sulfate aerosols at high southern latitudes, except in the vicinity of ornithogenic soils occupied by large penguin colonies.

Organic aerosols in the Caribbean trade winds: A natural source?

Mass concentrations of carbonaceous and inorganic components of submicron (aerodynamic diameters &lt; 0.6 μm) aerosol particles were measured at Cape San Juan, a coastal site located on the extreme northeastern tip of Puerto Rico. Additionally, carbonaceous aerosols and condensation nuclei (CN) were measured offshore during a limited shipboard sampling experiment at an Atlantic Ocean site away from the coast. Both sites were exposed to trade winds during the sampling periods. Under these conditions no obvious upwind anthropogenic sources should impact the sites. From the analyses of collected filter samples, mass concentrations of major inorganic anions and filterable particulate organic aerosol material (OCp) were determined. The derived OCp concentrations may represent lower limits because of the uncertainty in the collection of semivolatile organic material lost from the particles during sampling. Analyses of Cape San Juan samples show that (1) mass concentrations of submicron OCp (average about 390 ng m−3) exceed sulfate concentrations (average about 270 ng m−3); (2) this organic aerosol material is water soluble (and the particles act as effective cloud condensation nuclei); and (3) primary combustion aerosol does not appear to be a major contributor to OCp. The fact that OCp concentrations measured at the coastal site are similar to OCp concentrations (330–400 ng m−3) measured at an Atlantic Ocean site removed from the coast suggests that a substantial fraction of the OCp in the Caribbean trade winds may be associated with natural oceanic emissions. Results imply that background organic marine aerosol should be included in estimating the indirect forcing of climate by anthropogenic sulfate aerosol.

Atmospheric sulphur and cloud condensation nuclei in marine air in the Southern Hemisphere

Measurements of atmospheric sulphur species made in Southern Ocean air, at the Cape Grim Baseline Air Pollution Station, are reviewed in an attempt to discern the role played by oceanic emissions of dimethyl sulphide (DMS) as a source of cloud condensation nuclei (CCN). Consistent with conclusions reached by others, our data indicate that the connection between DMS concentration and CCN concentration is neither simple nor direct, being mediated through a range of chemical pathways and intermediate species that are subject to considerable variability over timescales ranging from minutes to months. Physical and meteorological processes are no less important than chemical processes as sources of complexity in the DMS to CCN transformation process. Moreover, the considerable uncertainty that currently exists about both the number of chemical pathways involved in DMS oxidation, and the kinetic coefficients associated with the proposed pathways, make quantitative modelling studies problematic. Nevertheless, synthesis of a large body of data available from Cape Grim and other Southern Ocean sites does permit some refinement of our understanding of the DMS–CCN connection. Here, these data are employed to illustrate the current state of knowledge about the connections between DMS, CCN and cloud properties at Cape Grim, and to highlight the many complexities that underlie these connections.

Carbonyl sulfide (COS) in the surface ocean and the atmospheric COS budget

Carbonyl sulfide (COS) mixing ratioswere measured in the marine atmosphere and in airequilibrated with surface sea water during severalcruises in the North Sea and western NorthAtlantic. In April 1994, North Sea waters weresupersaturated with respect to the atmosphere,resulting in oceanic emissions of COS. Saturationratios varied between the equilibrium value of one inthe central North Sea and high values of >15 in theElbe Estuary. We observed weak diel cycles of surfacewater COS during a three day drift station. During theunderway parts of the cruise, diel COS variations weremasked by the high geographical variability of COSconcentrations in the German Bight. In August 1994, weobserved a pronounced diel cycle of COS off theFlorida coast with saturation ratios varying betweenthe equilibrium value of one in the early morning andmaximum values of four to five in the afternoon. InMarch 1995, we found COS supersaturation as well asextensive undersaturation in the western NorthAtlantic between Norfolk, VA, and Bermuda. Suchundersaturation in marine surface waters results inregional and seasonal uptake of atmospheric COS. Basedon our data and those of other researchers, weestimate the global oceanic COS net emission to bebetween 1.3 and 2.5 Gmol yr-1. This estimate is significantly smaller than previous ones which had notconsidered the possibility of COS uptake by theoceans. COS hydrolysis in the ocean has a significantinfluence on the atmospheric turnover time of COS,which we estimate to be 5.7 yr. This may contribute tothe lack of an observable increase in atmospheric COSlevels despite substantial anthropogenic emissions.

Ozone, hydroperoxides, oxides of nitrogen, and hydrocarbon budgets in the marine boundary layer over the South Atlantic

The NASA GTE TRACE A mission sampled air over the South Atlantic and western Indian Oceans. Thirteen flight legs were flown within the marine boundary layer (MBL). The MBL was typically the cleanest air sampled (e.g., CH4 &lt; 1680 ppb, CO &lt; 70 ppb, C2H6 &lt; 400 ppt, C3H8 &lt; 40 ppt, NOx &lt; 15 ppt, and midday NO &lt; 5 ppt) but was overlain by polluted air. The photochemistry of the MBL was influenced by oceanic emissions, surface deposition, and entrainment of pollutants from aloft. Chemical budgets were constructed for several species in the MBL in order to investigate these effects and are presented for ethane, ethylene, propane, propylene, n‐butane, formic acid (HFo), methylhydroperoxide (CH3OOH), oxides of nitrogen (i.e., NO, NO2, PAN, HNO3), hydrogen peroxide (H2O2), and ozone (O3). A photochemical point model was used to evaluate local chemical production and loss. An entrainment model was used to assess material exchange between the lower free troposphere (FT) and the MBL and a resistance deposition model was used to evaluate material exchange across the air‐sea interface. The results suggested the ocean to be the source of measured alkenes in the MBL and to be the most likely source of the shorter‐lived alkanes: propane and n‐butane. Ethane was the only hydrocarbon for which input from aloft may have exceeded its photochemical destruction. The estimated hydrocarbon sources from the ocean were in agreement with prior analyses. Transport from the lower FT together with surface loss could not account for measured concentrations of CH2O, HFo, and HNO3. The transport of peroxyacetylnitrate (PAN) from the FT to the MBL exceeded the rate of HNO3 production and was more than sufficient to maintain observed NOx levels without having to invoke an oceanic source for NO. The flux of NOx, PAN, and HNO3 was in balance with the surface deposition flux of HNO3. However, the predicted rates of HNO3 formation from the oxidation of NO2 and HNO3 entrainment from aloft were inadequate to maintain observed levels of HNO3 unless HNO3 was partitioned between the gas phase and a more slowly depositing aerosol phase. The estimated dry deposition flux of HNO3 to the South Atlantic during TRACE A, 2–4 × 109 molecules cm−2 s−1, was about 10 times the annual average estimate for this region. The destruction of O3 within the MBL was found to be exceeded by transport into the MBL from aloft, 6 ± 2 × 1010 compared to 11 ± 10 × 1010 molecules cm−2 s−1. The principal O3 destruction process was mediated by the formation and surface deposition of H2O2 and CH3OOH, 4 ± 4 × 1010 and 1.1 ± 0.5 × 1010 molecules cm−2 s−1. The direct loss of O3 to the sea surface was estimated to be 1.7 ± 0.2 × 1010 molecules cm−2 s−1. CH3OOH was lost to the sea and transported into the FT from the MBL. Its first‐order loss rate was estimated to be 7 × 10−6 s−1 for a mean MBL height of 700 m. H2O2 and CH2O losses from the MBL were estimated at rates of 1.3 × 10−5 s−1 for both species. The inclusion of surface deposition improved the agreement between predicted and measured concentrations of HNO3, CH3OOH, H2O2, and CH2O. However, model CH2O remained significantly greater than that measured in the MBL.

Atmospheric dimethylsulfide and sulfur species in aerosol and rainwater at a coastal site in New Zealand

Aspects of the atmospheric speciation of sulfur were investigated at a coastal site on the North Island of New Zealand between December 1990 and November 1992. Gaseous dimethylsulfide (DMS) as well as methanesulfonic acid (MSA) and non‐sea‐salt sulfate (NSSS) in aerosol and rainwater samples were determined. Atmospheric DMS concentrations exhibited seasonal variations in the range 0.25 nmole m−3 to 14.9 nmole m−3, with a median of 3.75 nmole m−3. Also, DMS levels varied diurnally and with meteorological variables such as wind speed and cloud coverage. Aerosol MSA concentrations similarly ranged seasonally from 0.02 to 1.40 nmole m−3 (median 0.36 nmole m−3). Aerosol NSSS concentrations varied from −1.04 to 15.9 nmole m−3, with a median value of 7.92 nmole m−3. MSA and NSSS concentrations in “clean” rainwater samples ranged from 0.03 to 0.31 μeq L−1 (median 0.04 μeq L−1) and 0.83 to 5.44 μeq L−1 (median 1.37 μeq L−1), respectively. The MSA:NSSS molar ratios varied seasonally from 0.44 to 21.4% (median 3.28%) in aerosols of marine character and from 2.38 to 22.3% (median 6.40%) for rainwater samples. Atmospheric DMS and MSA concentrations at this coastal location in New Zealand are comparable to those observed at other remote southern hemisphere sites. In contrast, NSSS concentrations are somewhat elevated, possibly incorporating an unknown contribution from heterogeneous SO2 oxidation. With respect to DMS flux calculations an oceanic emission rate of 83 mg S m−2 yr−1 would be required to account for oxidative losses (70 mg S m−2 yr−1) and leakage out of the marine boundary layer. The sum of wet and dry deposition for MSA and NSSS amounts to 65 mg S m−2 yr−1. The small difference between DMS oxidative losses and total MSA + NSSS deposition likely stems from an underestimate of the wet deposition rate for NSSS.

Nitrous oxide in coastal waters

We determined atmospheric and dissolved nitrous oxide (N2O) in the surface waters of the central North Sea, the German Bight, and the Gironde estuary. The mean saturations were 104 ± 1% (central North Sea, September 1991), 101 ± 2% (German Bight, September 1991), 99 ± 1% (German Bight September 1992), and 132% (Gironde estuary, November 1991). To evaluate the contribution of coastal areas and estuaries to the oceanic emissions we assembled a compilation of literature data. We conclude that the mean saturations in coastal regions (with the exception of estuaries and regions with upwelling phenomena) are only slightly higher than in the open ocean. However, when estuarine and coastal upwelling regions are included, a computation of the global oceanic N2O flux indicates that a considerable portion (approximately 60%) of this flux is from coastal regions, mainly due to high emissions from estuaries. We estimate, using two different parameterizations of the air‐sea exchange process, an annual global sea‐to‐air flux of 11–17 Tg N2O. Our results suggest a serious underestimation of the flux from coastal regions in widely used previous estimates.