Coastal Marine Boundary Layer Research Articles

Novel iodine chemistry in the marine boundary layer

The atmospheric chemistry of iodine is important for several reasons, including the influence of iodine oxides on the oxidising capacity of the troposphere, the formation of new particles, and the enrichment of iodine in marine aerosols and the transport of this essential dietary element to the continents. It is shown here that a substantial iodine source is I2, most likely emitted from macro‐algae at low tide. This source accounts for the daytime production of new particles in the coastal marine boundary layer, and also explains the discovery of significant night‐time levels of iodine oxides.

Laboratory studies of the homogeneous nucleation of iodine oxides

Abstract. Laboratory experimental results of iodine oxide nucleation are presented. Nucleation was induced following UV photolysis of CF3I or CH2I2 in the presence of excess ozone. Measurements were performed in a 70 L Teflon reactor with new particles detected using an Ultrafine Condensation Particle Counter, UCPC. The experimental results are interpreted using a coupled chemical - aerosol model to derive model parameters assuming single component homogeneous nucleation of OIO. The aerosol model results have been applied in an atmospheric box-model to interpret the possible implications of iodine oxide nucleation in the marine boundary layer. The model calculations demonstrate that IO and OIO concentrations reported in recent field measurements using long path absorption (Allan et al., 2000, 2001) are not sufficient to account for significant aerosol production either in the coastal or open ocean marine boundary layer using the mechanism presented. We demonstrate that inhomogeneous sources of iodine oxides, i.e. "hot" spots with elevated iodine species emissions, could account for the aerosol production bursts observed in the coastal region near Mace Head, Ireland.

Vertical and Horizontal Aerosol Scattering Fields over Bellows Beach, Oahu, during the SEAS Experiment*

Scanning lidar measurements were carried out during the Shoreline Environment Aerosol Study (SEAS) experiment (19‐30 April 2000) to characterize the aerosol scattering fields in the coastal marine boundary layer at Bellows Beach on the southeast side of Oahu, Hawaii. The sea salt was found to be well mixed throughout the mixed layer, although the depth of the trade wind mixed layer was found to vary significantly over short timescales. As expected, the frequency distribution of aerosol scatter had a lognormal distribution, with the exception of regions downwind of breaking waves, where the frequency distribution was bimodal. A spatial statistical study revealed that the island-blocking effects cause low-level clouds to develop as they approach the island, with enhanced drizzle near the coastline reaching all the way to the surface. The spray from waves breaking on an outer reef was found to be intermittent and contained to heights of 20 m (on average) for the average wind speed of 7 m s21. Sea-salt concentrations and fluxes from the breaking waves were estimated from the lidar measurements and found to be within the range of values reported by other investigators.

Atmospheric concentrations of the Cl atom, CIO radical, and HO radical in the coastal marine boundary layer

Atmospheric concentrations of chlorine atom (Cl ), chlorine monoxide radical (ClO ), and hydroxyl radical (HO ) in the coastal marine boundary layer are estimated in this study. A steady-state approach to their concentrations in equilibrium with other atmospheric chemical species is used. Measurements of atmospheric trace species, HCl, Cl 2, HCHO, H 2O 2, CH 3OOH, CH 4, CO, SO 2, NO, NO 2, and O 3, were performed at four sites in Taiwan during the spring of 1999. The results indicate that the concentrations of the Cl atom and the ClO and HO radicals decrease significantly with cloud cover. The calculated average daytime concentrations of Cl , ClO , and HO are 3×10 5, 1×10 7, and 6×10 5 molecules/cm 3, respectively. Due to the high reactivity of Cl with hydrocarbons and its concentration level competitive to that of HO , Cl should be a significant sink for hydrocarbons in these cases.

Fluxes of ammonia in the coastal marine boundary layer

Concentrations of ammonia in air and ammonium in surface water were measured from a platform in the Southern North Sea close to the Dutch coast. Fluxes were derived from the measurements applying Monin–Obukhov similarity theory and exchange velocities calculated. The fluxes and air concentrations of ammonia were compared to results obtained from the Lagrangian transport-chemistry model ACDEP with and without a parameterisation of outgoing fluxes of ammonia from the sea. The results indicate that the flux may in fact be upward during periods with low atmospheric ammonia concentrations and that the calculated overall ammonia dry deposition may be overestimated by a factor two or more in the coastal region. A more detailed study is needed in order to quantify how this may influence overall deposition to given marine waters. In some cases the deposition may solely be redistributed whereas the total deposition is only marginally influenced.

Gas‐aerosol relationships of H2SO4, MSA, and OH: Observations in the coastal marine boundary layer at Mace Head, Ireland

Atmospheric concentrations of gaseous sulfuric acid (H2SO4), methane sulfonic acid (MSA), and hydroxyl radicals (OH) were measured by chemical ionization mass spectrometry (CIMS) during the second New Particle Formation and Fate in the Coastal Environment (PARFORCE) campaign in June 1999 at Mace Head, Ireland. Overall median concentrations in marine background air were 1.5, 1.2, and 0.12 × 106 cm−3, respectively. H2SO4 was also present at night indicating significant contributions from nonphotochemical sources. A strong correlation was found between daytime OH and H2SO4 levels in clean marine air suggesting a fast local production of H2SO4 from sulfur precursor gases. Steady state balance calculations of ambient H2SO4 levels agreed with measured concentrations if either very low H2SO4 sticking coefficients (0.02–0.03) or sources in addition to the SO2 + OH reaction were assumed. Overall, variations in ambient H2SO4 levels showed no correlation with either the tidal cycle or ultrafine particle (UFP) concentrations. However, on particular days an anticorrelation between H2SO4 and UFP levels was occasionally observed providing evidence for the contribution of H2SO4 to new particle formation and/or particle growth. Gaseous MSA concentrations were inversely correlated with dew point temperature reflecting a highly sensitive gas‐particle partitioning equilibrium of this compound. The present observations seriously question the general use of MSA as a conservative tracer to infer the relative production yield of H2SO4 from dimethylsulfide (DMS) oxidation. MSA/H2SO4 concentration ratios typically ranged between 0.06 and 1.0 in marine air at ground level. Measured diel OH profiles showed a significant deviation from concurrent variations of the ozone photolysis frequency. They also showed up to 1 order of magnitude lower values compared to OH concentrations calculated with a simple photochemical box model. These differences were most pronounced during particle nucleation events occurring on sunny days around noon and at low tide. The present results suggest that both the oxidation capacity and the particle formation potential in the coastal boundary layer were significantly affected by reactions of unknown compounds prevailing in this type of environment.

Long-term climate patterns in Alaskan surface temperature and precipitation and their biological consequences

Mean monthly climate maps of Alaskan surface temperature and precipitation produced by the parameter-elevation regression on independent slopes model (PRISM) were analyzed. Alaska is divided into interior and coastal zones with consistent but different climatic variability separated by a transition region; it has maximum interannual variability but low long-term mean variability. Pacific decadal oscillation (PDO)- and El Nino Southern Oscillation (ENSO)-type events influence Alaska surface temperatures weakly (1-2/spl deg/C) statewide. PDO has a stronger influence than ENSO on precipitation but its influence is largely localized to coastal central Alaska. The strongest influence of Arctic oscillation (AO) occurs in northern and interior Alaskan precipitation. Four major ecosystems are defined. A major eco-transition zone occurs between the interior boreal forest and the coastal rainforest. Variability in insolation, surface temperature, precipitation, continentality, and seasonal changes in storm track direction explain the mapped ecosystems. Lack of westward expansion of the interior boreal forest into the western shrub tundra is influenced by the coastal marine boundary layer (enhanced cloud cover, reduced insolation, cooler surface and soil temperatures).

Simultaneous observations of nitrate and peroxy radicals in the marine boundary layer

This paper describes the most extensive set of simultaneous measurements of the concentrations of nitrate (NO3) and peroxy (sum of HO2+RO2, R=alkyl and acyl) radicals to date. The measurements were made in the coastal marine boundary layer over the North Sea, at the Weybourne Atmospheric Observatory on the North Norfolk coast during the spring and autumn of 1994. In spring the average nighttime concentration of NO3 measured by differential optical absorption spectroscopy, was about 10 parts per trillion (ppt) (maximum 25 ppt). The corresponding peroxy radical concentration, measured by the chemical amplifier technique, averaged about 2 ppt (maximum 6 ppt), although this is likely to be an underestimate of the total radical concentration. There is a significant positive correlation between the two sets of radicals, which has not been reported previously. A box model of the marine boundary layer is used to show that this correlation arises from the processing of reactive organic species by NO3. During spring the relatively long lifetime of NO3 (up to 18 min) at night is controlled by reaction with dimethyl sulflde (DMS), and the model predicts significant production of HNO3, methyl tiomethylen (CH3SCH2O2) and other peroxy radicals, HCHO, and eventually sulfate. A nighttime production rate for the hydroxyl (OH) of about 2×104 molecules cm−3 s−1 is estimated. During one night in autumn the NO3 lifetime of about 3 min is too short to be explained by reaction with unsaturated hydrocarbons, but is satisfactorily accounted for by the heterogeneous loss of N2O5 on deliquesced aerosols in relatively polluted conditions.

Airborne observations of thermal mesoscale circulations in the coastal marine boundary layer

Detailed observations obtained from airborne meteorological measurements in an area along the Baltic coast in the southeast of Sweden are presented. The measurements were taken during late spring/early summer when the water in the Baltic is generally cool compared to typical daytime land surface temperatures which generated significant surface temperature contrasts between land and water. The measurements show the coastal marine boundary layer to be very far from the homogeneity often assumed in numerical models. The cases presented focus mainly on mesoscale circulations driven or modified by this temperature contrast and related turbulence structure such as thermal jets and sea breeze circulations and show a high degree of temporal as well as small‐scale spatial variability. Even though the synoptic scale wind direction in this area is highly variable, due to traveling mid‐latitude cyclones, the surface temperature patterns show great consistency from day to day, with the coldest water always being along the coast with a pronounced minimum localized at the southeastern part of the coastline. It is not clear to what extent the sea surface temperature inhomogeneities are responsible for some of the observed small‐scale variability, nor is it clear if they are results of ocean processes alone or are interacting in any significant way with the marine boundary layer close to the coast.

A study of the sulfur cycle in the Antarctic marine boundary layer

Atmospheric sampling was conducted aboard R/V Polar Duke from March 21 to April 27, 1986, in the southern Drake Passage and in the coastal waters west of the Antarctic Peninsula. Average atmospheric concentrations (n = 6) of sulfur dioxide (SO2) vapor, aerosol methanesulfonate (CH3SO3−), and aerosol non‐sea‐salt sulfate (nss−SO4=) were 0.36, 0.22, and 0.34 nmol m−3, respectively. Cascade impactor data indicated that more than 85% of the masses of both CH3SO3− and nss−SO4= were on particles of <0.25‐μm mean aerodynamic radius. Average concentrations (n = 8) of CH3SO3− and nss−SO4= in precipitation were 0.09 and 0.57 μM, respectively. Box model calculations based on this limited data suggest a total sulfur removal rate from the local marine boundary layer of 0.3–7.2 μmol m−2 day−1. This removal approximately balances the oceanic source of dimethylsulfide (DMS) sulfur of 1.2–4.4 μmol m−2 day−1 (±100%) estimated by Berresheim [1987] from data obtained during the same cruise. The box model calculations also suggest that heterogeneous SO2 oxidation may be the major pathway for DMS to sulfate conversion (∼75%) and/or that other sources of sulfate may not be negligible. Observed mineral aerosol and 222Rn concentrations were low but slightly elevated above “pure” remote marine background levels, suggesting a weak but discernible continental character in sampled air. Uncertainties are large owing to the limited sampling time, study area, and numbers of samples obtained, but overall, the sulfur cycle in the coastal Antarctic marine boundary layer appears not to differ remarkably from that in other remote marine regions.