M. H. Smith

Marine aerosol, sea-salt, and the marine sulphur cycle: a short review

A short review of the marine aerosol size distribution and the contribution of sea-salt to this distribution is presented. The potential role of sea salt in the marine boundary layer sulphur cycle is highlighted.

Physicochemical properties of aerosols over the northeast Atlantic: Evidence for wind‐speed‐related submicron sea‐salt aerosol production

Physicochemical characteristics of submicron aerosol particles over the Northeast Atlantic (63[degrees]N, 8[degrees]W) during October/November 1989 have been examined using a thermal analytical technique and are classified according 10 air mass origin. Aerosol associated with anthropogenically influenced air masses contained typically 80% sulphate particles by number, the remainder being soot carbon and sea salt. For Arctic air masses the contribution of sulphate to the total aerosol was reduced to around 65%, due to low concentrations relative 10 sea salt which is dependent on wind speed. In situations with clean maritime air and high wind speeds, sulphate aerosol accounted for less than 25% of the total accumulation mode particles, the remainder consisting predominantly of sea salt. Arctic air masses and clean maritime air during periods of high winds were consistently acidic with inferred molar ratios of NH[sub 4][sup +]/SO[sub 4][sup =] near 0.2. The continental and modified maritime aerosol encountered was found to have molar ratios of about 0.8. Soot carbon was present in all air masses to a similar degree (5-13%). In clean air masses, submicron sea salt aerosol concentrations showed a strong exponential increase with wind speed (correlation coefficients cc [ge] 0.8), down to a dry particle radius of 0.05morexa0» [mu]m. Under these clean air conditions and high winds the sea salt aerosol dominated all particle sizes for r > 0.05 [mu]m and accounted for approximately 75% of the total concentration, suggesting that under these conditions, sea salt aerosol would comprise the primary source cloud condensation nuclei (CCN) in stratiform clouds. 30 refs., 8 figs., 4 tabs.«xa0less

Submicron particle, radon, and soot carbon characteristics over the northeast Atlantic

Atmospheric aerosol particles (0.05μm ≤ rdry ≤ 1.5μm), 222Rn, and soot carbon mass were measured on a cruise over the Northeast Atlantic (63°N, 8°W) during October and November 1989. An accumulation mode (AM) was present in all particle data and was characterized by a lognormal size distribution with parameters N cm−3 (total number concentration), rg μm (geometric mean radius) and σg (geometric standard deviation). For aerosol associated with the “cleanest” Northeast Atlantic maritime and Arctic air masses, the AM parameters were found to be 16 cm−3 ≤ N ≤ 48 cm−3, 1.32 ≤ σg ≤ 1.46, and 0.08 μm < rg < 0.09 μm, leading to AM masses of between 0.20 μg m−3 and 0.38 μg m−3. Clean background levels of soot carbon mass for maritime and Arctic air were estimated to be around 20 ng m−3 and were associated with particle size radii r ≤ 0.15 μm. Soot carbon mass showed an excellent correlation with AM number concentration (cc=0.93), demonstrating its usefulness as an air mass tracer and as an indicator of anthropogenic pollutant transport. By comparison, radon, which is often used for this purpose, exhibited a significantly poorer correlation (cc=0.60) for this region. Approximately 9% of the total AM mass was accounted for by soot carbon, regardless of air mass origin, suggesting that early winter marine aerosol in the remote North Atlantic is primarily of anthropogenic origin.

An analysis of rapid increases in condensation nuclei concentrations at a remote coastal site in western Ireland.

Massive “bursts” in condensation nuclei (CN) concentration were recorded at a remote site on the west Irish coast during campaigns in summer 1996 and spring/summer 1997. Number concentrations of 3–7 nm diameter CN were observed to rise daily from 102–103 up to ∼105 /cm3 for 1–3 hours. Data were collected as part of the Atmospheric Chemistry Studies in the Oceanic Environment program. In a previous paper the burst phenomenon was linked to the movement of the tide, and it was suggested that enhanced biogenic emissions occurred near low tide with concomitant rapid homogeneous gas phase CN formation. In this paper possible chemical mechanisms for the burst phenomenon are investigated. Two approaches are adopted. First, by assuming a 20:80 sulfate:water molar composition and calculating the number distribution using data from condensation particle counters, the total mass of CN formed during a burst is evaluated. This is compared with that mass of sulfate produced by OH-initiated dimethyl sulfide (DMS) oxidation. The procedure is termed “mass balance.” Second, a variety of chemical species are coplotted with tidal height. DMS oxidation is not believed to play a major role in CN formation at this site because (1) the mass balance calculations imply ambient DMS concentrations higher than those observed, and (2) gas phase HCl, HNO3, SO2, and NH3 did not exhibit any discernible correlation with tidal height. Further, none of the suite of observed nonmethane hydrocarbons or DMS showed a tidal relation. No mechanism has to date been convincingly identified for the burst phenomenon.

Observations of accumulation mode aerosol composition and soot carbon concentrations by means of a high‐temperature volatility technique

A high-temperature volatility system has been deployed for the measurement of the composition and concentration of the accumulation mode aerosol (0.05 μm < r < 1 μm) within the atmospheric boundary layer. This instrumentation comprises a volatility system based around a Particle Measuring Systems ASASP-X optical particle counter, which was operated together with an aethalometer for the direct observation of soot carbon concentrations. By cycling the heater tube through a range of temperatures from near ambient to over 1000°C, size-differentiated information upon aerosol composition may be obtained. Furthermore, by careful selection of analysis temperatures, discrimination is possible between elemental carbon and the more volatile fractions of the soot carbon aerosol. Observations made over the North Sea near the Dutch coast and in the central United Kingdom are presented for differing environmental conditions with soot carbon concentrations ranging from about 100 to over 6000 ng m -3 . For polluted conditions over the North Sea the volatility technique clearly showed the dominance of soot carbon particles over other aerosol components with a narrow carbon particle distribution of mode radius around 0.06 μm accounting for about 80% of all particles with radii below 0.1 μm. Under polluted conditions, only about 25% of the total soot carbon aerosol comprised elemental carbon (with the remainder consisting of more volatile material), whereas this proportion rose to around 50% in the lower carbon loadings found in a cleaner maritime air mass. The use of soot carbon loadings as a tracer of anthropogenic aerosol inputs to oceanic regions is explored on the basis of measurements from a NE Atlantic cruise.

Dimethyl sulfide and its oxidation products in the atmosphere of the Atlantic and Southern Oceans

Dimethyl sulfide, methane sulfonate, non-sea-salt sulfate and sulfur dioxide concentrations in air were obtained during a cruise between the U.K. and the Antarctic during the period October 1992–January 1993. In equatorial regions (30°N to 30°S) the atmospheric DMS concentration ranged from 3 to 46 ng (S)m−3 with an average of 18 ng(S)m−3. In the polar waters and regions south of the Falkland Islands concentrations from 3 to 714ng(S)m−3 were observed with a mean concentration of 73ng(S)m−3. Methane sulfonate concentrations were also enhanced in the vicinity of the Antarctic Peninsula and in the Weddell Sea. A simple model of DMS oxidation was used to estimate the ocean to atmosphere flux rate, and this was found to be within the range of previous estimates, with a mean value of 1011 ng(S) m−2 h−1.

A high temperature volatility technique for determination of atmospheric aerosol composition

Abstract Size segregated atmospheric aerosol composition, over the radius range 0.05μm ≤ r ≤ 1.5μm, is inferred using an optical particle counter equipped with a high temperature heater tube cycling through temperatures from 20°C to 870°C. The volatility system infers the presence of common atmospheric constituents such as H2SO4, (NH4)2SO4, NaCl, and soot carbon. This system has been successfully used to determine the physico-chemical characteristics of accumulation mode aerosol associated with air masses of different origin. Results from ship-borne and ground based measurements are presented. For continental and modified maritime air, (NH4)2SO4 was observed to be the dominant species, whilst H2SO4 was more abundant in Arctic aerosol. Sea-salt aerosol accounts for ≈75% of accumulation mode particle concentration for maritime aerosol in periods of high wind speeds. We also show that soot carbon can be identified in polluted air using this technique.

New particle formation in the marine environment

Publisher Summary nThe source of condensation nuclei (CN ) in the marine environment and their evolution into CCN (Cloud Condensation Nuclei) are the focus of this chapter. CN are formed through homogeneous nucleation of H2SO4, H2O, and possibly NH3 vapor. The surface area of existing aerosol in the marine boundary layer is thought to provide a sufficient condensation sink for H2SO4 vapor, and thus, inhibits the vapor pressure required for homogeneous nucleation from being reached. Some cases of new-particle-formation, however, are observed under conditions of very low existing aerosol surface area. The timescales for freshly formed CN (r<5 nm) to grow into CCN (r<50 nm) under typical marine boundary layer conditions are thought to exceed the lifetime of marine CCN. It is postulated that the free troposphere is the most likely location for CN and CCN formation, as the tropospheric environmental conditions promote both CN formation and their growth into CCN because of longer residence timescales. This chapter presents observations of CN formation and decay from Antarctica and at a coastal site on the North East Atlantic.

North atlantic aerosol remote concentrations measured at a hebridean coastal site

Abstract During a study of marine aerosol characteristics at a coastal site on South Uist in the Outer Hebrides, early in August 1986, an extended period with untypically low wind speeds (below 7 ms−1) was experienced. Over this period, lasting several days, very low particle concentrations were measured for all observed particle radii from 0.09 to 23.5 μm, and atmospheric visibilities of up to 80 km were noted. Particles larger than about 0.2 μm in radius showed a decline in concentration throughout this episode, suggesting that these data could yield information on particle loss rates, as well as providing a true very low wind speed background aerosol distribution for this region of the North Atlantic. Analysis of these data indicates that the decay of aerosol concentration, for all particle radii greater than 0.25 μm, may be approximated by an exponential function, and is consistent with a simple model of turbulent dry deposition of the aerosol to the ocean surface.

Physicochemical properties of atmospheric aerosol at South UIST

Four field campaigns over the period November 1993 to August 1994 were undertaken on the island of South Uist, off the northwest coast of Scotland as part of the BMCAPE project. Measurements were made of the concentration and chemical composition of aerosol particles, utilising a variety of instruments. Sulphur and nitrogen gas species were also measured throughout these campaigns, together with appropriate meteorological parameters. A variety of air mass types were encountered during the campaigns and the relationship between the physical and chemical aerosol properties are discussed in terms of air mass histories and season factors.