On springtime high ozone events in the lower troposphere from Southeast Asian biomass burning

Ozone peaks with mixing ratios as high as 138 ppbv were observed in the lower troposphere (2.5–4.5 km) over Hong Kong in spring. Simultaneously observed high humidity suggests that this enhanced ozone was not the result of transport from the upper troposphere. Back trajectory analysis suggests that these enhancements resulted from lateral transport. Air masses arriving at the altitude of the ozone peaks appear to have passed over continental Southeast Asia where the bulk of biomass burning occurs at this time of the year (February–April). We hypothesize that biomass burning in this region provided the necessary precursors for the observed ozone enhancement. As far as we know this is the first observation of highly enhanced ozone layers associated with biomass burning in continental Southeast Asia.     

Aqueous-phase reactive uptake of dicarbonyls as a source of organic aerosol over eastern North America

We use a global 3-D atmospheric chemistry model (GEOS-Chem) to simulate surface and aircraft measurements of organic carbon (OC) aerosol over eastern North America during summer 2004 (ICARTT aircraft campaign), with the goal of evaluating the potential importance of a new secondary organic aerosol (SOA) formation pathway via irreversible uptake of dicarbonyl gases (glyoxal and methylglyoxal) by aqueous particles. Both dicarbonyls are predominantly produced in the atmosphere by isoprene, with minor contributions from other biogenic and anthropogenic precursors. Dicarbonyl SOA formation is represented by a reactive uptake coefficient γ = 2.9 × 10^?3 and takes place mainly in clouds. Surface measurements of OC aerosol at the IMPROVE network in the eastern U.S. average 2.2 ± 0.7 μg C m^?3 for July–August 2004 with little regional structure. The corresponding model concentration is 2.8 ± 0.8 μg C m^?3, also with little regional structure due to compensating spatial patterns of biogenic, anthropogenic, and fire contributions. Aircraft measurements of water-soluble organic carbon (WSOC) aerosol average 2.2 ± 1.2 μg C m^?3 in the boundary layer (<2 km) and 0.9 ± 0.8 μg C m^?3 in the free troposphere (2–6 km), consistent with the model (2.0 ± 1.2 μg C m^?3 in the boundary layer and 1.1 ± 1.0 μg C m^?3 in the free troposphere). Source attribution for the WSOC aerosol in the model boundary layer is 27% anthropogenic, 18% fire, 28% semi-volatile SOA, and 27% dicarbonyl SOA. In the free troposphere it is 13% anthropogenic, 37% fire, 23% semi-volatile SOA, and 27% dicarbonyl SOA. Inclusion of dicarbonyl SOA doubles the SOA contribution to WSOC aerosol at all altitudes. Observed and simulated correlations of WSOC aerosol with other chemical variables measured aboard the aircraft suggest a major SOA source in the free troposphere compatible with the dicarbonyl mechanism.     

Seasonal variability of radon-derived fetch regions for Sado Island,Japan, based on 3 years of observations: 2002–2004

Three years of hourly atmospheric radon measurements at Sado Island (Japan) are discussed and compared with corresponding measurements at Gosan (South Korea), and Hok Tsui (China). In conjunction with back trajectory analysis, Sado radon data are used to characterise the seasonal variability in fetch regions of air masses subject to extremes of terrestrial influence. In winter, fetch regions of air masses that have experienced the greatest terrestrial influence covered southern Siberia; in summer, the terrestrial fetch was dominated by Japan; throughout the remaining months the terrestrial fetch encompassed the Korean Peninsula and far eastern China. Summer radon data are then used to estimate the radon flux from central Honshu (23.5 mBq m^?2 s^?1), which varied regionally between 10.6 and 47.9 mBq m^?2 s^?1. The Sado radon record reported here completes a 4-site, multi-year dataset of hourly radon concentrations across East Asia and the central Pacific (spanning 16° of latitude), which constitutes a unique evaluation tool for transport and mixing schemes of atmospheric and chemical transport models.     

Atmospheric mercury species in the European Arctic: Measurements and modelling

Concentrations of different species of mercury in arctic air and precipitation have been measured at Ny-Ålesund (Svalbard) and Pallas (Finland) during 1996–1997. Typical concentrations for vapour phase mercury measured at the two stations were in the range of 0.7–2 ng m⁻³ whereas particulate mercury concentrations were below 5 pg m⁻³. Total mercury in precipitation was in the range 3–30 ng l⁻¹. In order to evaluate the transport and deposition of mercury to the arctic from European anthropogenic sources, the Eulerian transport model HMET has been modified and extended to also include mercury species. A scheme for chemical conversion of elemental mercury to other species of mercury and deposition characteristics of different mercury species have been included in the model. European emission inventories for three different forms of Hg (Hg⁰, HgCl₂ and Hg_p) have been implemented in the numerical grid system for the HMET model.     

Dry deposition of sulphur over eastern South Africa

Dry deposition of sulphur is estimated in three climatic regions of Mpumalanga, South Africa, using the inferential method. Data from June 1996 to May 1997 are used at Elandsfontein and Palmer on the industrialised highveld, as well as data from two-week monitoring campaigns in the late-winter and in summer at Blyde on the eastern escarpment and at Skukuza in the lowveld. Total dry deposition rates for sulphur range across the Mpumalanga highveld from 13.1 kg ha^-1 a^-1 at Elandsfontein to 3.1 kg ha^-1 a^-1 at Palmer, are associated with the strong SO₂ gradient between the two stations and are attributed mostly to dry deposition of sulphur from SO₂. The deposition flux varies less from Palmer eastward over the escarpment and the lowveld and ranges from 3.9 kg S ha^-1 a^-1 at Blyde to 3.3 kg S ha^-1 a^-1 at Skukuza. A weak seasonal variation in sulphur dry deposition flux occurs on the central highveld with the maximum in summer and the minimum in winter. Conversely, the maximum sulphur dry deposition on the periphery of the highveld, the escarpment and in the lowveld occurs in winter with the minimum in summer. More than 80% of the dry deposition of sulphur in Mpumalanga occurs during daytime in all seasons.     

Historical deposition of mercury and selected trace elements to high-elevation National Parks in the Western U.S. inferred from lake-sediment cores

Atmospheric deposition of Hg and selected trace elements was reconstructed over the past 150 years using sediment cores collected from nine remote, high-elevation lakes in Rocky Mountain National Park in Colorado and Glacier National Park in Montana. Cores were age dated by ²¹⁰Pb, and sedimentation rates were determined using the constant rate of supply model. Hg concentrations in most of the cores began to increase around 1900, reaching a peak sometime after 1980. Other trace elements, particularly Pb and Cd, showed similar post-industrial increases in lake sediments, confirming that anthropogenic contaminants are reaching remote areas of the Rocky Mountains via atmospheric transport and deposition. Preindustrial (pre-1875) Hg fluxes in the sediment ranged from 5.7 to 42 μg m^?2 yr^?1 and modern (post-1985) fluxes ranged from 17.7 to 141 μg m^?2 yr^?1. The average ratio of modern to preindustrial fluxes was 3.2, which is similar to remote lakes elsewhere in North America. Estimates of net atmospheric deposition based on the cores were 3.1 μg m^?2 yr^?1 for preindustrial and 11.7 μg m^?2 yr^?1 for modern times. Current-day measurements of wet deposition range from 5.0 to 8.6 μg m^?2 yr^?1, which are lower than the modern sediment-based estimate of 11.7 μg m^?2 yr^?1, perhaps owing to inputs of dry-deposited Hg to the lakes.     

An analysis of seasonal surface dust aerosol concentrations in the western US (2001–2004): Observations and model predictions

Long-term surface observations indicate that soil dust represents over 30% of the annual fine (particle diameter less than 2.5 μm) particulate mass in many areas of the western US; in spring and summer, it represents an even larger fraction. There are numerous dust-producing playas in the western US, but surface dust aerosol concentrations in this region are also influenced by dust of Asian origin. This study examines the seasonality of surface soil dust concentrations at 15 western US sites using observations from the Interagency Monitoring of PROtected Visual Environments (IMPROVE) network from 2001 to 2004. Average soil concentrations in particulate matter less than 10 μm in diameter (PM₁₀) were lowest in winter and peaked during the summer months at these sites; however, episodic higher-concentration events (>10 μg m⁻³) occurred in the spring, the time of maximum Asian dust transport to the western US. Simulated surface dust concentrations from the Navy Aerosol Analysis and Prediction System (NAAPS) suggested that long-range transport from Asia dominates surface dust concentrations in the western US in the spring, and that, although some long-range transport does occur throughout the year (1–2 μg m⁻³), locally generated dust plays a larger role in the region in summer and fall. However, NAAPS simulated some anomalously high concentrations (>50 μg m⁻³) of local dust in the fall and winter months over portions of the western US. Differences between modeled and observed dust concentrations were attributed to overestimation of total observed soil dust concentrations by the assumptions used to convert IMPROVE measurements into PM₁₀ soil concentrations, lack of inhibition of model dust production in snow-covered regions, and lack of seasonal agricultural sources in the model.     

Vertical profiles of ultrafine to supermicron particles measured by aircraft over Osaka metropolitan area in Japan

Intensive aircraft- and ground-based measurements of ultrafine to supermicron particles in the Osaka metropolitan area, Japan, were carried out on 17–19 March 2003, in order to investigate vertical profiles of size-resolved particles in the urban atmosphere. Differently sized particles were observed at different altitudes on 19 March. Relatively higher concentrations of ultrafine particles (31 nm) and submicron particles (0.3–0.5 μm) were measured (100–200 cm⁻³) at altitudes of 300 and 600 m, whereas supermicron particles (2–5 μm) were present (300–600 cm⁻³) at higher altitudes (1300 m in the morning and 2200 m in the afternoon). The chemical composition analysis showed that supermicron particles evidently comprised mainly soil particles mixed internally with anthropogenic species such as carbonaceous components and sulfate. Numerical simulation using the Chemical weather FORecasting System (CFORS) suggested the long-range transport of soil dust and black carbon from the Asian continent. Total number concentrations of particles sized 10–875 nm ranged from 4.8×10³ to 3.0×10⁴ cm⁻³ at an altitude of 300 m and from 7.3×10² to 4.8×10³ cm⁻³ at an altitude of 1300 m. Total number concentrations of particles sized 10–875 nm correlated very well with NO_X concentrations, and, therefore, ultrafine and submicron particles were likely emitted from urban activities such as car traffic and vertically transported. Number size distributions at lower altitudes obtained by aircraft measurements were similar to those obtained by ground measurements, with modal diameters of 20–30 nm on 18 March and about 50 nm on 19 March.     

Estimation of wind blown dust emissions in Europe and its vicinity

The assessment of the wind blown dust emission for Europe and selected regions of North Africa and Southwest Asia was carried out using a mesoscale model. The mesoscale model was parameterized based on the current literature review. The model provides data on PM₁₀ emission from several dust reservoirs (anthropogenic, agriculture, semi- and natural) with spatial resolution of 10 × 10 km and temporal resolution of 1 h. The spatial variability of PM₁₀ emission depends on soil texture, land cover/land use as well as meteorological conditions. Lands covered with water or permanently wet were excluded from the model. The land covered with vegetation is treated as dust reservoir whose dust emission capacity depends on the type of vegetation and cover. The dust reservoirs are divided into reservoirs with stable and unstable surface. The changes of emission in time depend on meteorological parameters.The wind blown dust emission should be treated as a non-continuous spatio-temporal process. The emissions are estimated with high uncertainty. The estimated PM₁₀ yearly total load emitted by wind from the European territory is highly differentiated in space and time and is equal to 0.74 Tg. The total load of PM₁₀ emitted by wind from North African and Southwest Asian land surface located in the vicinity of European boundaries is assessed as nearly 50% (0.43 Tg) of the total load estimated for the whole Europe.The average yearly PM₁₀ emission factor for Europe was estimated at 0.139 Mg km^?2.The PM₁₀ emission from agricultural areas is estimated at 52% of the total wind blown emission from the domain of the European Union project “Improving and applying methods for the calculation of natural and biogenic emissions and assessment of impacts to the air quality” - NatAir.PM₁₀ emission factor for natural areas of Europe is estimated at 0.021 Mg km^?2. Appropriate factors for agricultural areas and anthropogenic areas are 0.157 Mg km^?2 and 0.118 Mg km^?2, respectively. The latter two factors are probably underestimated due to omitting in the model of other dust emission mechanisms than aeolian erosion.     

Long range transport of fine particle windblown soils and coal fired power station emissions into Hanoi between 2001 to 2008

Fine particulate matter (PM_2.5), source fingerprints and their contributions have been measured and reported previously at Hanoi, Vietnam, from 25 April 2001 to 31 December 2008. In this study back trajectories are used to identify long range transport into Hanoi for two of these sources, namely, windblown dust (Soil) from 12 major deserts in China and emissions from 33 coal fired power plants (Coal) in Vietnam and China. There were 28 days of extreme Soil events with concentrations greater than 6 μg m^?3 and 25 days of extreme Coal with concentrations greater than 30 μg m^?3 from a total of 748 sampling days during the study period. Through the use of back trajectories it was found that long range transport of soil from the Taklamakan and Gobi desert regions (more than 3000 km to the north west) accounted for 76% of the extreme events for Soil. The three local Vietnamese power stations contributed to 15% of the extreme Coal events, while four Chinese power stations between 300 km and 1700 km to the north-east of Hanoi contributed 50% of the total extreme Coal events measured at the Hanoi sampling site.     

Impact of anthropogenic heat on urban climate in Tokyo

This study quantifies the contribution through energy consumption, to the heat island phenomena and discussed how reductions in energy consumption could mitigate impacts on the urban thermal environment. Very detailed maps of anthropogenic heat in Tokyo were drawn with data from energy statistics and a very detailed digital geographic land use data set including the number of stories of building at each grid point. Animated computer graphics of the annual and diurnal variability in Tokyo's anthropogenic heat were also prepared with the same data sources. These outputs characterize scenarios of anthropogenic heat emission and can be applied to a numerical simulation model of the local climate. The anthropogenic heat flux in central Tokyo exceeded 400 W m⁻² in daytime, and the maximum value was 1590 W m⁻² in winter. The hot water supply in offices and hotels contributed 51% of this 1590 W m⁻². The anthropogenic heat flux from the household sector in the suburbs reached about 30 W m⁻² at night. Numerical simulations of urban climate in Tokyo were performed by referring to these maps. A heat island appeared evident in winter because of weakness of the sea breeze from Tokyo Bay. At 8 p.m., several peaks of high-temperature appeared, around Otemachi, Shinjuku and Ikebukuro; the areas with the largest anthropogenic heat fluxes. In summer the shortwave radiation was strong and the influence of anthropogenic heat was relatively small. In winter, on the other hand, the shortwave radiation was weak and the influence of anthropogenic heat was relatively large. The effects of reducing energy consumption, by 50% for hot water supply and 100% for space cooling, on near surface air temperature would be at most −0.5°C.     

Simultaneous use of relaxed eddy accumulation and gradient flux techniques for the measurement of sea-to-air exchange of dimethyl sulphide

The sea-to-air flux of the biogenic volatile sulphur compound dimethyl sulphide was assessed with the relaxed eddy accumulation (REA) and the gradient flux (GF) techniques from a stationary platform in the coastal Atlantic Ocean. Fluxes varied between 2 and 16 μmol m⁻² d⁻¹. Fluxes derived from REA were on average 7.1±5.03 μmol m⁻² d⁻¹, not significantly different from the average flux of 5.3±2.3 μmol m⁻² d⁻¹ derived from GF measurements. Gas transfer velocities were calculated from the fluxes and seawater DMS concentrations. They were within the range of gas transfer rates derived from the commonly used parameterizations that relate gas transfer to wind speed.     

Inventory of aerosol and sulphur dioxide emissions from India. Part II—biomass combustion

A spatially resolved biomass burning data set, and related emissions of sulphur dioxide and aerosol chemical constituents was constructed for India, for 1996–1997 and extrapolated to the INDOEX period (1998–1999). Sources include biofuels (wood, crop waste and dung-cake) and forest fires (accidental, shifting cultivation and controlled burning). Particulate matter (PM) emission factors were compiled from studies of Indian cooking stoves and from literature for open burning. Black carbon (BC) and organic matter (OM) emissions were estimated from these, accounting for combustion temperatures in cooking stoves. Sulphur dioxide emission factors were based on fuel sulphur content and reported literature measurements. Biofuels accounted 93% of total biomass consumption (577 MT yr⁻¹), with forest fires contributing only 7%. The national average biofuel mix was 56 : 21 : 23% of fuelwood, crop waste and dung-cake, respectively. Compared to fossil fuels, biomass combustion was a minor source of SO₂ (7% of total), with higher emissions from dung-cake because of its higher sulphur content. PM_2.5 emissions of 2.04 Tg yr⁻¹ with an “inorganic fraction” of 0.86 Tg yr⁻¹ were estimated. Biomass combustion was the major source of carbonaceous aerosols, accounting 0.25 Tg yr⁻¹ of BC (72% of total) and 0.94 Tg yr⁻¹ of OM (76% of total). Among biomass, fuelwood and crop waste were primary contributors to BC emissions, while dung-cake and forest fires were primary contributors to OM emissions. Northern and the east-coast India had high densities of biomass consumption and related emissions. Measurements of emission factors of SO₂, size resolved aerosols and their chemical constituents for Indian cooking stoves are needed to refine the present estimates.     

Source apportionment of secondary organic aerosol during a severe photochemical smog episode

The UCD/CIT air quality model was modified to predict source contributions to secondary organic aerosol (SOA) by expanding the Caltech Atmospheric Chemistry Mechanism to separately track source apportionment information through the chemical reaction system as precursor species react to form condensable products. The model was used to predict source contributions to SOA in Los Angeles from catalyst-equipped gasoline vehicles, non-catalyst equipped gasoline vehicles, diesel vehicles, combustion of high sulfur fuel, other anthropogenic sources, biogenic sources, and initial/boundary conditions during the severe photochemical smog episode that occurred on 9 September 1993. Gasoline engines (catalyst+non-catalyst equipped) were found to be the single-largest anthropogenic source of SOA averaged over the entire model domain. The region-wide 24-h average concentration of SOA produced by gasoline engines was predicted to be 0.34 μg m⁻³ with a maximum 24-h average concentration of 1.81 μg m⁻³ downwind of central Los Angeles. The region-wide 24-h average concentration of SOA produced by diesel engines was predicted to be 0.02 μg m⁻³, with a maximum 24-h average concentration of 0.12 μg m⁻³ downwind of central Los Angeles. Biogenic sources are predicted to produce a region-wide 24-h average SOA value of 0.16 μg m⁻³, with a maximum 24-h average concentration of 1.37 μg m⁻³ in the less-heavily populated regions at the northern and southern edges of the air basin (close to the biogenic emissions sources). SOA concentrations associated with anthropogenic sources were weakly diurnal, with slightly lower concentrations during the day as mixing depth increased. SOA concentrations associated with biogenic sources were strongly diurnal, with higher concentrations of aqueous biogenic SOA at night when relative humidity (RH) peaked and little biogenic SOA formation during the day when RH decreased.     

Identification and assessment of ship emissions and their effects in the harbour of Göteborg,Sweden

The effect of ship emissions in the urban environment of Göteborg has been studied by multivariate analysis. The simultaneous measurements of relevant gases and sub-micron particles make identification of ship plumes possible. Increased concentrations of these species due to ship emissions are quantified for ships entering the inner part of the harbour. Annual depositions of SO₂ and NO₂ are estimated to be 220 and 115 kg km⁻² yr⁻¹, respectively. Exposure of transient particles (less than 0.1 μm in diameter) to this part of the harbour increased by a factor of 3 in number concentration when a ship plume was recorded. Ni, Pb, V and Zn are shown to have positive correlation with NO emissions from ships.     

The impact of transpacific transport of mineral dust in the United States

We use a global chemical transport model (GEOS-Chem) to estimate the impact of transpacific transport of mineral dust on aerosol concentrations in North America during 2001. We have implemented two dust mobilization schemes in the model (GOCART and DEAD) and find that the best simulation of North American surface observations with GEOS-Chem is achieved by combining the topographic source used in GOCART with the entrainment scheme used in DEAD. This combination restricts dust emissions to year-round arid areas but includes a significant wind threshold for dust mobilization. The model captures the magnitude and seasonal cycle of observed surface dust concentrations over the northern Pacific. It simulates the free tropospheric outflow of dust from Asia observed in the TRACE-P and ACE-Asia aircraft campaigns of spring 2001. It reproduces the timing and distribution of Asian dust outbreaks in North America during April–May. Beyond these outbreaks we find persistent Asian fine dust (averaging 1.2 μg m⁻³) in surface air over the western United States in spring, with much weaker influence (0.25 μg m⁻³) in summer and fall. Asian influence over the eastern United States is 30–50% lower. We find that transpacific sources accounted for 41% of the worst dust days in the western United States in 2001.     

Secondary organic aerosol importance in the future atmosphere

In order to investigate the secondary organic aerosol (SOA) response to changes in biogenic volatile organic compounds (VOC) emissions in the future atmosphere and how important will SOA be relative to the major anthropogenic aerosol component (sulfate), the global three-dimensional chemistry/transport model TM3 has been used. Emission estimates of biogenic VOC (BVOC) and anthropogenic gases and particles from the literature for the year 2100 have been adopted.According to our present-day model simulations, isoprene oxidation produces 4.6 Tg SOA yr⁻¹, that is less than half of the 12.2 Tg SOA yr⁻¹ formed by the oxidation of other BVOC. In the future, nitrate radicals and ozone become more important than nowadays, but remain minor oxidants for both isoprene and aromatics. SOA produced by isoprene is estimated to almost triple, whereas the production from other BVOC more than triples. The calculated future SOA burden change, from 0.8 Tg at present to 2.0 Tg in the future, is driven by changes in emissions, oxidant levels and pre-existing particles. The non-linearity in SOA formation and the involved chemical and physical feedbacks prohibit the quantitative attribution of the computed changes to the above-mentioned individual factors. In 2100, SOA burden is calculated to exceed that of sulfate, indicating that SOA might become more important than nowadays. These results critically depend on the biogenic emissions and thus are subject to the high uncertainty associated with these emissions estimated due to the insufficient knowledge on plant response to carbon dioxide changes. Nevertheless, they clearly indicate that the change in oxidants and primary aerosol caused by human activities can contribute as much as the change in BVOC emissions to the increase of the biogenic SOA production in the future atmosphere.     

A scheme for black carbon and sulphate aerosols tested in a hemispheric scale,Eulerian dispersion model

Production, transport and deposition of sulphate and black carbon (BC) are simulated separating aerosol modes by production mechanisms, thus facilitating calculation of aerosol physical properties. The scheme uses explicit sulphur chemistry with prescribed oxidants and aerosol transformation based on Brownian coagulation rates, and is implemented in a 3-D, hemispheric-scale transport model with off-line meteorology from ECMWF. The components are DMS, SO₂, sulphate and BC. Simulations are made for the year 1988. Predicted results for sulphate agree well with the measurements, except for considerable underestimations in the Arctic. In Europe SO₂ is slightly overestimated and sulphate is slightly underestimated in cold seasons, whilst trends are less clear in North America. Compared to many other models, we estimate a smaller effective oxidation rate for SO₂ due to reduced rates in cold clouds; a shorter turnover time for sulphate (3.7 d) due to a probably too large below-cloud scavenging ratio; and slightly smaller sulphate column burdens. Our BC results are similar to Liousse et al.’s (1996, J. Geophys. Res. 101, 19.411–19.432) except in USA, whilst the concentrations in remote areas and the turnover time (3.7 d) are considerably smaller than Cooke and Wilson’s (1996, J. Geophys. Res. 101, 190.395–19.409). Agreements with available measurements are quite close to large anthropogenic emissions (including USA), but they are considerably underestimated in Arctic winter. Transition from hydrophobic to hydrophilic BC due to coagulation is swift (6% h^-1). Sensitivity tests emphasize that sub-cloud scavenging coefficients rely on careful assumptions about size distributions, and that more research is needed on sulphate production in ice-clouds. Emphasis should also be put on production of sulphate and hydrophilic BC boundary-layer clouds; inclusion of hydrophobic accumulation mode BC and sub-grid transition to hydrophilic BC in emitting grid squares.     

Organic trace gas composition of the marine boundary layer over the northwest Indian Ocean in April 2000

In April 2000 atmospheric trace gas measurements were performed on the western Indian Ocean on a cruise of the Dutch research vessel Pelagia from the Seychelles (5°S, 55°E) to Djibouti (12°N, 43°E). The measurements included analysis of dimethyl sulfide (DMS), acetone and acetonitrile every 40 s using PTR-MS (proton-transfer-reaction mass spectrometry) and gas chromatographic analyses of C₂–C₇ hydrocarbons in air samples taken during the cruise. The measurements took place at the end of the winter monsoon season and the sampled air masses came predominantly from the Southern Hemisphere, resulting in low concentrations of some long-lived hydrocarbons, halocarbons, acetone (350 pptv) and acetonitrile (120 pptv). On three consecutive days a diurnal cycle in DMS concentration was observed, which was used to estimate the emission of DMS (1.5±0.7×10¹³ molecules m⁻² s⁻¹) and the 24 h averaged concentration of hydroxyl (OH) radicals (1.4±0.7×10⁶ molecules cm⁻³). A strongly increased DMS concentration was found at a location where upwelling of deeper ocean waters took place, coinciding with a marked decrease in acetone and acetonitrile. In the northwestern Indian Ocean a slight increase of some trace gases was noticed showing a small influence of pollution from Asia and from northeast Africa as indicated with back trajectory calculations. The air masses from Asia had elevated acetonitrile concentrations showing some influence of biomass burning as was also found during the 1999 Indian Ocean Experiment, whereas the air masses from northeast Africa seemed to have other sources of pollution.     

Estimation of direct radiative forcing due to non-methane hydrocarbons

The direct radiative forcing due to non-methane hydrocarbons (NMHCs) has not previously been quantified. We use new measurements of infrared absorption cross-sections and a narrow band radiative transfer model to estimate a forcing. An upper limit to the global mean anthropogenic forcing is likely to be in the region of 0.015 W m^-2, less than 1% of the forcing due to other greenhouse gases. However, taking account of the natural NMHC loading and the vertical profile of these gases the actual radiative forcing is likely to be somewhat less than this.