Brian Davison

Biogenic sulphur emissions and inferred non-sea-salt-sulphate cloud condensation nuclei in and around Antarctica

Accumulation mode aerosol properties and biogenic sulphur emissions over the South Atlantic and Antarctic Oceans are examined. Two contrasting air masses, polar and maritime, each possessing distinct aerosol properties, were encountered during the summer months. By examining aerosol volatile properties, polar air masses arriving from the Antarctic continent were shown to consist primarily Of H2SO4 in the accumulation mode size range, with inferred NH+ 4 to SO= 4 molar ratios close to zero. By comparison, air masses of temperate maritime origin were significantly neutralized with molar ratios of ≈1. These results suggest a deficit of ammonia in polar air masses compared with that in maritime air masses. Dimethyl sulphide (DMS) exhibited no correlation with its putative aerosol oxidation products, although spatial coherence in atmospheric concentrations of DMS, methane sulphonic acid (MSA), and non-sea-salt (nss)-sulphate mass was observed. Volatility analysis, used to infer nss-sulphate cloud condensation nuclei (nss-sCCN) active at a supersaturation of ≈0.2%, indicates that nss-sCCN mass and number concentration were best correlated with MSA mass (r≈0.63). Aerosol volatility identified the presence of MSA in submicron non-sea-salt aerosol; however, its contribution to the aerosol mass was small relative to the contribution of sulphuric acid and ammonium bisulphate/sulphate aerosol. The marine sulphur cycle appears strongly coupled to the sea-salt cycle with, typically, 80–90% of nss-sulphate thought to be internally mixed with sea-salt aerosol. During the austral Summer of 1992/1993, a period of strong biological productivity in the Weddell Sea and sub-Antarctic Ocean, particularly during ice-melt, the cruise-average DMS flux of 61 μg m−2 d−1 corresponded to a very modest average nss-sCCN concentration of 21 cm−3. Observed peak values of DMS flux and inferred nss-CCN concentrations during the cruise were 477 μg m−2 d−1 and 64 cm−3, respectively. Events of new particle formation were identified in the Weddell Sea and occurred under conditions of high DMS flux and low aerosol surface area.

Nitric acid from volcanoes

Abstract Atmospheric cycling of nitric acid and other nitrogen-bearing compounds is an important biogeochemical process, with significant implications for ecosystems and human health. Volcanoes are rarely considered as part of the global nitrogen cycle, but here we show that they release a previously unconsidered flux of HNO3 vapour to the atmosphere. We report the first measurements of nitric acid vapour in the persistent plumes from four volcanoes: Masaya (Nicaragua); Etna (Italy); and Villarrica and Lascar (Chile). Mean near-source volcanic plume concentrations of HNO3 range from 1.8 to 5.6 μmol m−3, an enrichment of one to two orders of magnitude over background (0.1–1.5 μmol m−3). Using mean molar HNO3/SO2 ratios of 0.01, 0.02, 0.05, and 0.07 for Villarrica, Masaya, Etna, and Lascar respectively, combined with SO2 flux measurements, we calculate gaseous HNO3 fluxes from each of these volcanic systems, and extend this to estimate the global flux from high-temperature, non-explosive volcanism to be ∼0.02–0.06 Tg (N) yr−1. While comparatively small on the global scale, this flux could have important implications for regional fixed N budgets. The precise mechanism for the emission of this HNO3 remains unclear but we suggest that thermal nitrogen fixation followed by rapid oxidation of the product NO is most likely. In explosive, ash-rich plumes NO may result from, or at least be supplemented by, production from volcanic lightning rather than thermal N fixation. We have calculated NO production via this route to be of the order of 0.02 Tg (N) yr−1.

Dimethyl sulfide, methane sulfonic acid and physicochemical aerosol properties in Atlantic air from the United Kingdom to Halley Bay

The concentrations of dimethyl sulfide in air were obtained during a cruise between the United Kingdom and the Antarctic in the period October 1992 to January 1993 using a method of sampling and analysis optimized to avoid interferences from oxidants. 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 714 ng (S) m−3 were observed, with a mean concentration of 73 ng (S) m−3. The concentrations of a range of DMS oxidation products were also obtained. No clear relationships between reactant and product concentrations were seen. Information on particle number concentration, Fuchs surface area and the thermal volatility characteristics of the ambient aerosol was obtained, but again no clear relationships with sulfur concentrations were observed. Accumulation mode particle concentrations averaged 25 cm−3 in the clean marine and polar air masses south of 58°S while background condensation nuclei (CN) concentrations were of the order of 400–600 cm−3. Simplistic calculations suggest that a particle source strength of about 20–60 particles cm−3 d−1 is required to sustain this background CN concentration. It is not clear whether boundary layer nucleation of new CN or entrainment from the free troposphere provided the source of CN. Periods of elevated CN concentrations (>4000 cm−3) were regularly observed in the boundary layer over the Weddell Sea and were attributed to “bursts” of new particle formation. However, shortly after these nucleation events the CN concentration rapidly decayed to the background level through coagulation losses, suggesting little impact on the background CN or cloud condensation nuclei (CCN) concentration.

Natural sulphur species from the North Atlantic and their contribution to the United Kingdom sulphur budget

Measurements of dimethyl sulphide (DMS) and its oxidation products, methane sulphonic acid (MSA), sulphate, dimethyl sulphoxide (DMSO), dimethyl sulphone (DMSO2), and sulphur dioxide (SO2) in the atmosphere have been made during all seasons of 1988/1990 at coastal sites in northwest Scotland and Eire. The annual average air concentration of DMS was estimated from results obtained on 92 days in 1989/1990 to be 58 ng (S) m−3 and that of MSA to be 6 ng (S) m−34. Backward air mass trajectories were used for each sampling period to explain the observed concentrations in terms of the origins of the sampled air. Seasonal and diurnal cycles were observed for DMS with maximum concentrations > 300 ng (S) m−3 occurring at night and during the spring in air of oceanic origin. Maximum MSA concentrations (up to 120 ng (S) m−3) were observed during the spring. Using a steady state approximation, the sea-to-air flux of DMS from the Atlantic north of 45° was estimated to be 0.93 μmol m−2 d−1 or 96 Gg (S) yr−1. Three methods were used to correct the observed air concentrations of the various sulphur species for anthropogenic emissions. The resultant flux of biogenic sulphur into the United Kingdom from the North Atlantic, net of any anthropogenic sulphur, was estimated to be 30 - 96 Gg (S) yr−1, or 2 - 5 % of the total anthropogenic emission rate of sulphur from the U.K. A simple 10-step model was used to describe the equilibria existing between the various sulphur species and CO2 in cloud water at cloudwater concentrations of 0.1, 0.5, and 2.5 g m−3 and the resultantacidity calculated. Values of pH as low as 3.4 were predicted, with MSA having a negligible effect and sulphate dominating. Biogenic sulphur species from the North Atlantic may therefore make a small but quantifiable contribution to the atmospheric sulphur budget of the U.K. and may play an important role in determining rainwater acidity in the west of the country.

Sources, size distribution, and downwind grounding of aerosols from Mount Etna

The number concentrations and size distributions of aerosol particles >0.3 mm diameter were measured at the summit of Mount Etna and up to 10 km downwind from the degassing vents during July and August 2004. Aerosol number concentrations reached in excess of 9 106 L1 at summit vents, compared to 4–8 104 L1 in background air. Number concentrations of intermediate size particles were higher in emissions from the Northeast crater compared to other summit crater vents, and chemical composition measurements showed that Northeast crater aerosols contained a higher mineral cation content compared to those from Voragine or Bocca Nuova, attributed to Strombolian or gas puffing activity within the vent. Downwind from the summit the airborne plume was located using zenith sky ultraviolet spectroscopy. Simultaneous measurements indicated a coincidence of elevated ground level aerosol concentrations with overhead SO2, demonstrating rapid downward mixing of the plume onto the lower flanks of the volcano under certain meteorological conditions. At downwind sites the ground level particle number concentrations were elevated in all size fractions, notably in the 2.0–7.5 mm size range. These findings are relevant for assessing human health hazard and suggest that aerosol size distribution measurements may aid volcanic risk management.

Observations of new particle production in the atmosphere of a moderately polluted site in eastern England.

Measurements of particle number density and Fuchs surface area, together with a range of gaseous pollutant concentrations, have been made in June 1995 at a coastal site in eastern England which receives air from a range of polluted and less polluted origins. Periods of enhanced local particle production were identified and found to be associated predominantly with relatively polluted air sectors. An examination of the factors contributing to homogeneous nucleation and hence new particle production suggests that those most important at this location are probably the production of hydroxyl radicals and the availability of ammonia. A numerical modeling study calculating characteristic timescales for new particle production and for condensation onto existing aerosol surfaces is able to predict periods of new particle production. The model suggests that oxidation of dimethyl sulphide and sulphur dioxide and homogeneous nucleation of sulphuric acid and water, probably in combination with ammonia, are the source of new particles at this site.

Sources of atmospheric methanesulphonate, non-sea-salt sulphate, nitrate and related species over the temperate South Pacific

Aerosol species and trace gases were collected during three intensive sampling periods representing winter (2 July to 9 August 1991), spring (29 September to 6 November 1991) and summer (15 January to 29 February 1992) at Baring Head on the southern end of New Zealands North Island. In remote marine air, mean winter, spring and summer aerosol phase concentrations (ng m−3) were, respectively, non-sea-salt sulphate (NSSS) = 115, 139 and 187, methanesulphonate (MSA) = 1.5, 23 and 48, nitrate (NOD = 293, 70 and 84, and ammonium (NH4+) = 44, 39 and 59. Mean gas phase concentrations were sulphur dioxide (SO2−) = 39,39 and 30, nitric acid (HNO3) = 18,30 and 27 and ammonia (NH3) = 74,42 and 31. Although natural sources for atmospheric reactive sulphur and nitrogen predominate in this region, evidence was also found for the existence of additional, non-oceanic sources of NOn3 and NSSS, including a long-range transport source of ammonium sulphate. Diurnal fluctuations were seen with lower SO2and NH3 concentrations at night, while nighttime sources of NSSS and HNO3 were indicated. Molar ratios of MSA/NSSS in remote air were 1.3, 16.3 and 25.7% for winter, spring and summer samples, respectively. These could be only partially explained by known DMS oxidation mechanisms, and a low, relatively constant background concentration of non-DMS NSSS appeared to be present at all times.

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.

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.

Effects of land use on surface–atmosphere exchanges of trace gases and energy in Borneo: comparing fluxes over oil palm plantations and a rainforest

This paper reports measurements of land–atmosphere fluxes of sensible and latent heat, momentum, CO2, volatile organic compounds (VOCs), NO, NO2, N2O and O3 over a 30 m high rainforest canopy and a 12 m high oil palm plantation in the same region of Sabah in Borneo between April and July 2008. The daytime maximum CO2 flux to the two canopies differs by approximately a factor of 2, 1200 mg C m−2 h−1 for the oil palm and 700 mg C m−2 h−1 for the rainforest, with the oil palm plantation showing a substantially greater quantum efficiency. Total VOC emissions are also larger over the oil palm than over the rainforest by a factor of 3. Emissions of isoprene from the oil palm canopy represented 80 per cent of the VOC emissions and exceeded those over the rainforest in similar light and temperature conditions by on average a factor of 5. Substantial emissions of estragole (1-allyl-4-methoxybenzene) from the oil palm plantation were detected and no trace of this VOC was detected in or above the rainforest. Deposition velocities for O3 to the rainforest were a factor of 2 larger than over oil palm. Emissions of nitrous oxide were larger from the soils of the oil palm plantation than from the soils of the rainforest by approximately 25 per cent. It is clear from the measurements that the large change in the species composition generated by replacing rainforest with oil palm leads to profound changes in the net exchange of most of the trace gases measured, and thus on the chemical composition of the boundary layer over these surfaces.