Aerosol physical,chemical and optical properties during the Rocky Mountain Airborne Nitrogen and Sulfur study

During the Rocky Mountain Airborne Nitrogen and Sulfur (RoMANS) study, conducted during the spring and summer of 2006, a suite of instruments located near the eastern boundary of Rocky Mountain National Park (RMNP) measured aerosol physical, chemical and optical properties. Three instruments, a differential mobility particle sizer (DMPS), an optical particle counter (OPC), and an aerodynamic particle sizer (APS), measured aerosol size distributions. Aerosols were sampled by an Interagency Monitoring of Protected Visual Environments (IMPROVE) sampler and a URG denuder/filter-pack system for compositional analysis. An Optec integrating nephelometer measured aerosol light scattering. The spring time period had lower aerosol concentrations, with an average volume concentration of 2.2 ± 2.6 μm³ cm^?3 compared to 6.5 ± 3.9 μm³ cm^?3 in the summer. During the spring, soil was the single largest constituent of PM_2.5 mass, accounting for 32%. During the summer, organic carbon accounted for 60% of the PM_2.5 mass. Sulfates and nitrates had higher fractional contributions in the spring than the summer. Variability in aerosol number and volume concentrations and in composition was greater in the spring than in the summer, reflecting differing meteorological conditions. Aerosol scattering coefficients (b_sp) measured by the nephelometer compared well with those calculated from Mie theory using size distributions, composition data and modeled RH dependent water contents.     

Secondary organic aerosol formation from the oxidation of aromatic hydrocarbons in the presence of dry submicron ammonium sulfate aerosol

A laboratory study was conducted to examine formation of secondary organic aerosols. A smog chamber system was developed for studying gas–aerosol interactions in a dynamic flow reactor. These experiments were conducted to investigate the fate of gas and aerosol phase compounds generated from hydrocarbon–nitrogen oxide (HC/NO_x) mixtures irradiated in the presence of fine (<2.5 μm) particulate matter. The goal was to determine to what extent photochemical oxidation products of aromatic hydrocarbons contribute to secondary organic aerosol formation through uptake on pre-existing inorganic aerosols in the absence of liquid water films. Irradiations were conducted with toluene, p-xylene, and 1,3,5-trimethylbenzene in the presence of NO_x and ammonium sulfate aerosol, with propylene added to enhance the production of radicals in the system. The secondary organic aerosol yields were determined by dividing the mass concentration of organic fraction of the aerosol collected on quartz filters by the mass concentration of the aromatic hydrocarbon removed by reaction. The mass concentration of the organic fraction was obtained by multiplying the measured organic carbon concentration by 2.0, a correction factor that takes into account the presence of hydrogen, nitrogen, and oxygen atoms in the organic species. The mass concentrations of ammonium, nitrate, and sulfate concentrations as well as the total mass of the aerosols were measured. A reasonable mass balance was found for each of the aerosols. The largest secondary organic aerosol yield of 1.59±0.40% was found for toluene at an organic aerosol concentration of 8.2 μm⁻³, followed by 1.09±0.27% for p-xylene at 6.4 μg m⁻³, and 0.41±0.10% for 1,3,5-trimethylbenzene at 2.0 μg m⁻³. In general, these results agree with those reported by Odum et al. and appear to be consistent with the gas–aerosol partitioning theory developed by Pankow. The presence of organic in the aerosol did not affect significantly the hygroscopic properties of the aerosol.     

Particulate contributions to light extinction and local forcing at a rural Illinois site

The light extinction and direct forcing properties of the atmospheric aerosol were investigated for a midwestern rural site (Bondville, IL) using field measurements, a semi-empirical light extinction model, and a radiative transfer code. Model inputs were based on the site measurements of the physical and chemical characteristics of atmospheric aerosol during the spring, summer, fall and winter of 1994. The light scattering and extinction coefficients were calculated and apportioned using the elastic light scattering interactive efficiency (ELSIE) model (Sloane and Wolff, 1985, Atmospheric Environment 19(4), 669–680). The average efficiencies calculated for organic carbon (OC, carbon measured as organic multiplied by 1.2) ranged from 3.81 m²/g OC at lower relative humidities (<63%) to 6.90 m²/g OC at higher relative humidities (>75%) while sulfate (assumed as ammonium sulfate) efficiencies ranged from 1.23 m²/g (NH₄)₂SO₄ to 5.78 m²/g (NH₄)₂SO₄ for the same range of relative humidities. Radiative transfer calculations showed that the rural aerosol at Bondville is most likely to have an overall negative (cooling) forcing effect on climate. Elemental carbon (EC), however, acts to counter sulfate forcing to a degree that has a significant seasonal variation, primarily due to the seasonal variation in the sulfate concentrations. Taking the loading to be the mean summer EC+ammonium sulfate loading and assuming [EC]/[(NH₄)₂SO₄] to be zero in one case (i.e. no soot present) and 0.025 (summer mean at Bondville) in another leads to a 37% difference in calculated forcing.     

Carbonaceous and inorganic composition in long-range transported aerosols over northern Japan: Implication for aging of water-soluble organic fraction

To better understand the influence of sources and atmospheric processing on aerosol chemical composition, we collected atmospheric particles in Sapporo, northern Japan during spring and early summer 2005 under the air mass transport conditions from Siberia, China and surrounding seas. The aerosols were analyzed for inorganic ions, organic carbon (OC), elemental carbon (EC), water-soluble organic carbon (WSOC), and the major water-soluble organic compound classes (i.e., dicarboxylic acids and sugars). SO₄^2? is the most abundant inorganic constituent (average 44% of the identified inorganic ion mass) followed by NH₄⁺ (21%) and NO₃^? (13%). Concentrations of OC, EC, and WSOC ranged from 2.0–16, 0.24–2.9, and 0.80–7.9 μg m^?3 with a mean of 7.4, 1.0, and 3.1 μg m^?3, respectively. High OC/EC ratios (range: 3.6–19, mean: 8.7) were obtained, however WSOC/OC ratios (0.23–0.69, 0.44) do not show any significant diurnal changes. These results suggest that the Sapporo aerosols were already aged, but were not seriously affected by local photochemical processes. Identified water-soluble organic compounds (diacids + sugars) account for <10% of WSOC. Based on some marker species and air mass back trajectory analyses, and using stable carbon isotopic compositions of shorter-chain diacids (i.e., C₂–C₄) as photochemical aging factor of organic aerosols, the present study suggests that a fraction of WSOC in OC is most likely influenced by aerosol aging, although the OC loading in aerosols may be more influenced by their sources and source regions.     

Size-resolved sulfate and ammonium measurements in marine boundary layer over the North and South Pacific

Marine background levels of non-sea-salt- (nss-) SO₄²⁻ (5.0–9.7 neq m⁻³), NH₄⁺ (2.1–4.4 neq m⁻³) and elemental carbon (EC) (40–80 ngC m⁻³) in aerosol samples were measured over the equatorial and South Pacific during a cruise by the R/V Hakuho-maru from November 2001 to March 2002. High concentrations of nss-SO₄²⁻ (47–94 neq m⁻³), NH₄⁺ (35–94 neq m⁻³) and EC (130–460 ngC m⁻³) were found in the western North Pacific near the coast of the Asian continent under the influence of the Asian winter monsoon. Particle size distributions of ionic components showed that the equivalent concentrations of nss-SO₄²⁻ were balanced with those of NH₄⁺ in the size range of 0.06<D<0.22 μm, whereas the concentration ratios of NH₄⁺ to nss-SO₄²⁻ in the size range of D>0.22 μm were decreased with increase in particle size. We estimated the source contributions of those aerosol components in the marine background air over the equatorial and South Pacific. Biomass burning accounted for the large fraction (80–98% in weight) of EC and the minor fraction (2–4% in weight) of nss-SO₄²⁻. Marine biogenic source accounted for several tens percents of NH₄⁺ and nss-SO₄²⁻. In the accumulation mode, 70% of particle number existed in the size range of 0.1<D<0.2 μm. In the size rage of 0.06<D<0.22 μm, the dominant aerosol component of (NH₄)₂SO₄ would be mainly derived from the marine biogenic sources.     

Heterogeneous conversion of nitric acid to nitrous acid on the surface of primary organic aerosol in an urban atmosphere

Nitrous acid (HONO), nitric acid (HNO₃), and organic aerosol were measured simultaneously atop an 18-story tower in Houston, TX during August and September of 2006. HONO and HNO₃ were measured using a mist chamber/ion chromatographic technique, and aerosol size and chemical composition were determined using an Aerodyne quadrupole aerosol mass spectrometer. Observations indicate the potential for a new HONO formation pathway: heterogeneous conversion of HNO₃ on the surface of primary organic aerosol (POA). Significant HONO production was observed, with an average of 0.97 ppbv event^?1 and a maximum increase of 2.2 ppb in 4 h. Nine identified events showed clear HNO₃ depletion and well-correlated increases in both HONO concentration and POA-dominated aerosol surface area (SA). Linear regression analysis results in correlation coefficients (r²) of 0.82 for HONO/SA and 0.92 for HONO/HNO₃. After correction for established HONO formation pathways, molar increases in excess HONO (HONO_excess) and decreases in HNO₃ were nearly balanced, with an average HONO_excess/HNO₃ value of 0.97. Deviations from this mole balance indicate that the residual HNO₃ formed aerosol-phase nitrate. Aerosol mass spectral analysis suggests that the composition of POA could influence HONO production. Several previously identified aerosol-phase PAH compounds were enriched during events, suggesting their potential importance for heterogeneous HONO formation.     

Water-soluble organic nitrogen (WSON) in size-segregated atmospheric particles over the Eastern Mediterranean

Atmospheric water-soluble organic nitrogen (WSON) was determined on size-segregated aerosol particles collected during a two years period (2005–2006) in a remote marine location in the Eastern Mediterranean (Finokalia, Crete island). Average concentration of WSON was 5.5 ± 3.9 nmol m^?3 and 11.6 ± 14.0 nmol m^?3 for coarse (PM_1.3-10) and fine (PM_1.3) mode respectively, corresponding to 13% of Total Dissolved Nitrogen (TDN) in both modes. Air masses origin and correlation with tracers of natural and anthropogenic sources indicate that combustion process (biomass burning and fossil fuel) and African dust play an important role in regulating levels of WSON in both coarse and fine aerosol fractions. Chemical speciation of organic nitrogen pool was attempted by analyzing 47 fine aerosol samples (PM₁) for 17 free amino acids (N-FAA), dimethylamine (DMA) and trimethylamine (TMA). The average concentration of N-FAA was 0.5 ± 0.5 nmol m^?3, while the average concentration of DMA was 0.2 ± 0.8 nmol m^?3, TMA was below detection limit. The percentage contribution of N-FAA and DMA to WSON was 2.1 ± 2.3% and 0.9 ± 3.4%, respectively.     

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.     

Drop size-dependent chemical composition of clouds and fogs. Part II: Relevance to interpreting the aerosol/trace gas/fog system

Size-resolved fog drop chemical composition measurements were obtained during a radiation fog campaign near Davis, California in December 1998/January 1999 (reported in Reilly et al., Atmos. Environ. 35(33) (2001) 5717; Moore et al., Atmos. Environ. this issue). Here we explore how knowledge of this size-dependent drop composition—particularly from the newly developed Colorado State University 5-Stage cloud water collector—helps to explain additional observations in the fog environment. Size-resolved aerosol measurements before and after fog events indicate relative depletion of large (>2 μm in diameter) particles during fog accompanied by a relative increase in smaller aerosol particle concentrations. Fog equivalent air concentrations suggest that entrainment of additional particles and in-fog sedimentation contributed to observed changes in the aerosol size distribution. Calculated deposition velocities indicate that sedimentation was an important atmospheric removal mechanism for some species. For example, nitrite typically has a larger net deposition velocity than water and its mass is found preferentially in the largest drops most likely to sediment rapidly. Gas–liquid equilibria in fog for NO₃⁻/HNO₃, NH₄⁺/NH₃, and NO₂⁻/HONO were examined. While these systems appear to be close to equilibrium or relative equilibrium during many time periods, divergences are observed, particularly for low liquid water content (<0.1 g m⁻³) fogs and in different drop sizes. Knowledge of the drop size-dependent composition provided additional data useful to the interpretation of these deviations. The results suggest that data from multi-stage cloud water collectors are useful to understanding fog processes as many depend upon drop size.     

Gaseous emission during the composting of pig feces from Chinese Ganqinfen system

The Ganqinfen system – a process of manually cleaning animal feces by means of a shovel – is a widely used manure separating method in Chinese pig farms. Ganqinfen pig feces and chopped corn stalks were mixed at the ratio of 7:1, and composted in 1.5 m³ rotting boxes for 70 d. Evolution of CH₄, N₂O and NH₃ during composting, and the effects of turning and covering, were studied in this research. Results showed that 20–39% and 0.5–4% of total nitrogen were lost in the form of NH₃ and N₂O respectively, and 0.1–0.9% of initial organic carbon was emitted as CH₄. Turning enhanced air exchange in the piles, thus decreasing CH₄ emission by 83–93% and shortening the maturing period. When trials were finished, all non-turned piles were separated to three layers by moisture content. This structure caused the N₂O losses of non-turning treatments to be 6–12.7 times higher than that of turning treatments. Covering materials reduced air exchange at the surface of the pile, thus decreasing the O₂ supply and consequently increasing CH₄ production by 33–45%. Covering also reduced NH₃ emission by 4–34%. For the composting of Ganqinfen pig feces, we suggest that a program of turning twice weekly without covering will result in compost that is sufficiently matured after 6 wk with the lowest resultant greenhouse gas emission.     

HNO3 losses within the cyclone inlet of a diffusion-denuder system under simulated marine environments

We evaluated the loss of HNO₃ within a Teflon-coated aluminum cyclone of an annular diffusion denuder atmospheric sampling system (ADS) under simulated marine conditions. To simulate marine environment, the cyclones were pre-coated with NaCl aerosol droplets. Loss of vapor-phase HNO₃ within the NaCl-coated cyclone was generally greater than 30% at relative humidities (RH) of 60 and 80% and as large as 67% when the cumulative HNO₃ dosages were lower than 3 μg. In contrast, there was little loss of HNO₃ (<8%) in cyclones with no NaCl coating at RHs ranging from 0 to 80%, at HNO₃ air concentrations of 4.3±1.6 μg m⁻³, and at cumulative HNO₃ dosages of greater than 5 μg. However, at lower HNO₃ cumulative dosages (<3 μg), losses in the non-coated cyclones were strongly influenced by RH, ranging from 9% in dry air to 58% at 80% RH. The enhanced loss of HNO₃ in the NaCl-coated cyclone was most likely caused by the reaction between HNO₃ and NaCl on the cyclone wall.     

Aerosol scattering properties in northern China

The aerosol scattering properties were investigated at two continental sites in northern China in 2004. Aerosol light scattering coefficient (σ_sp) at 525 nm, PM₁₀, and aerosol mass scattering efficiencies (α) at Dunhuang had a mean value of 165.1±148.8 M m⁻¹, 157.6±270.0 μg m⁻³, and 2.30±3.41 m² g⁻¹, respectively, while these values at Dongsheng were, respectively, 180.2±151.9 M m⁻¹, 119.0±112.9 μg m⁻³, and 1.87±1.41 m² g⁻¹. There existed a seasonal variability of aerosol scattering properties. In spring, at Dunhuang PM₁₀, σ_sp, and α were 184.1±211.548 μg m⁻³, 126.3±89.6 M m⁻¹, and 1.05±0.97 m² g⁻¹, respectively, and these values at Dongsheng were 146.4±142.1 μg m⁻³, 183.4±81.7 M m⁻¹, and 1.98±1.52 m² g⁻¹, respectively. However, in winter at Dunhuang PM₁₀, σ_sp, and α were 158.1±261.4 μg m⁻³, 303.3±165.2 M m⁻¹, and 3.17±1.93 m² g⁻¹, respectively, and these values at Dongsheng were 155.7±170.1 μg m⁻³, 304.4±158.1 M m⁻¹, and 2.90±1.72 m² g⁻¹, respectively. σ_sp and α in winter were higher than that in spring at both the sites, which coincides with the characteristics of dust aerosol and pollution aerosol. Overall, the dominant aerosol types in spring and winter at both sites in northern China are dust aerosol and pollution aerosol, respectively.     

Atmospheric nitrogen fluxes at the Belgian coast: 2004–2006

Daily and seasonal variations in dry and wet atmospheric nitrogen fluxes have been studied during four campaigns between 2004 and 2006 at a coastal site of the Southern North Sea at De Haan (Belgium) located at coordinates of 51.1723° N and 3.0369° E. Concentrations of inorganic N-compounds were determined in the gaseous phase, size-segregated aerosol (coarse, medium, and fine), and rainwater samples. Dissolved organic nitrogen (DON) was quantified in rainwater. The daily variations in N-fluxes of compounds were evaluated with air-mass backward trajectories, classified into the main air-masses arriving at the sampling site (i.e., continental, North Sea, and Atlantic/UK/Channel).The three, non-episodic campaigns showed broadly consistent fluxes, but during the late summer campaign exceptionally high episodic N-deposition was observed. The average dry and wet fluxes for non-episodic campaigns amounted to 2.6 and 4.0 mg N m^?2 d^?1, respectively, whereas during the episodic late summer period these fluxes were as high as 5.2 and 6.2 mg N m^?2 d^?1, respectively.Non-episodic seasons/campaigns experienced average aerosol fluxes of 0.9–1.4 mg N m^?2 d^?1. Generally, the contribution of aerosol NH₄⁺ was more significant in the medium and fine particulate fractions than that of aerosol NO₃^?, whereas the latter contributed more in the coarse fraction, especially in continental air-masses. During the dry mid-summer campaign, the DON contributed considerably (～15%) to the total N-budget.Exceptionally high episodic aerosol-N inputs have been observed for the late summer campaign, with especially high deposition rates of 3.6 and 2.9 mg N m^?2 d^?1 for Atlantic/UK/Channel and North Sea-continental (mixed) air-masses, respectively. During this pollution episode, the flux of NH₄⁺ was dominating in each aerosol fraction/air-mass, except for coarse continental aerosols. High deposition of gaseous-N was also observed in this campaign with an average total N-flux of 2–2.5-times higher than in other campaigns.     

Size-resolved,real-time measurement of water-insoluble aerosols in metropolitan Atlanta during the summer of 2004

During the month of August 2004, the size-resolved number concentration of water-insoluble aerosols (WIA) from 0.25 to 2.0 μm was measured in real-time in the urban center of Atlanta, GA. Simultaneous measurements were performed for the total aerosol size distribution from 0.1 to 2.0 μm, the elemental and organic carbon mass concentration, the aerosol absorption coefficient, and the aerosol scattering coefficient at a dry (RH=30%) humidity. The mean aerosol number concentration in the size range 0.1–2.0 μm was found to be 360±175 cm⁻³, but this quantity fluctuated significantly on time scales of less than one hour and ranged from 25 to 1400 cm⁻³ during the sample period. The mean WIA concentration (0.25–2.0 μm) was 13±7 cm⁻³ and ranged from 1 to 60 cm⁻³. The average insoluble fraction in the size range 0.25–2.0 μm was found to be 4±2.5% with a range of 0.3–38%. The WIA population was found to follow a consistent diurnal pattern throughout the month with concentration maxima concurring with peaks in vehicular traffic flow. WIA concentration also responded to changes in meteorological conditions such as boundary layer depth and precipitation events. The temporal variability of the absorption coefficient followed an identical pattern to that of WIA and ranged from below the detection limit to 55 Mm⁻¹ with a mean of 8±6 Mm⁻¹. The WIA concentration was highly correlated with both the absorption coefficient and the elemental carbon mass concentration, suggesting that WIA measurements are dominated by fresh emissions of elemental carbon. For both the total aerosol and the WIA size distributions, the maximum number concentration was observed at the smallest sizes; however the WIA size distribution also exhibited a peak at 0.45 μm which was not observed in the total population. Over 60% of the particles greater than 1.0 μm were observed to be insoluble in the water sampling stream used by this instrumentation. Due to the refractive properties of black carbon, it is highly unlikely that these particles could be composed of elemental carbon, suggesting a crustal source for super-micron WIA.     

Ultraviolet radiation spatio-temporal characteristics derived from the ground-based measurements taken in China

Ultraviolet (UV) radiation and broadband solar radiation (R_s) measured from January 2005 to June 2006 at 31 stations in Chinese Ecosystem Research Network (CERN) were used to investigate the spatio-temporal characteristics of UV radiation and UV fraction (the ratio of UV radiation to R_s) in China. Results indicated that the seasonal variations of UV radiation and R_s were consistent with the solar activities, which reached their lower values during winter period, and increased throughout the spring, peaking in June or July, in most sites. The Meiyu weather system and Southwest Monsoon produced different variation characteristics of UV radiation and R_s in subtropical and tropical regions. The UV fraction values showed a similar seasonal trend as that of UV radiation, which was mainly determined by the seasonal change of the aerosol optical depth (AOD) and water vapor content in atmosphere. The seasonal variations of UV fraction were much smoother in southern China due to high water vapor content over the whole year.The UV radiation showed an increasing trend from east to west in China. In the western area, a simple increasing trend was observed from north to south, with the largest annual mean daily UV value 0.91 MJ m⁻² appearing in the Qinghai-Tibet Plateau area. In east China there was a low center that appeared in the subtropical region due to high aerosol burden, with the lowest value 0.41 MJ m⁻² observed in Yanting site. Two high centers were located in the tropics with higher solar altitudes and the north desert region with low atmospheric attenuation. The largest values of UV fraction appeared in the tropical and subtropical regions due to higher relative humidity (RH) in these areas. However, the smallest value did not appear in the north desert region where the RH was the lowest, they were found at the Luancheng site which featured relatively low humidity and abundant fine aerosols. The variations in the UV fraction were not such distinctive as those of the UV radiation in China due to the more complex influences of aerosol and water vapor.     

Characterisation of fresh particulate vehicular exhausts near a Paris high flow road

Simultaneous continuous measurements of PM_2.5, PM₁₀, black carbon mass (BCae), Black smoke (BS) and particle number density (N) were conducted in the close vicinity of a high traffic road around Paris during a three-month period beginning in August 1997. In parallel some aerosol collection was performed on filters in order to assess the black carbon (BC), organic carbon (OC) and water soluble organic fractions (WSOC) of the freshly emitted traffic aerosols. The high hourly concentrations of PM_2.5 (39±20 μg m⁻³), BCae (14±7 μg m⁻³), and N (220,000±115,000 cm⁻³), were found to be well correlated with each other. On average PM_2.5 represented 66±13% of PM₁₀ and appears to be composed primarily of BC (43±20%). On the contrary no correlation was found between PM_2.5 and the coarse (PM₁₀–PM_2.5) mass fractions which was attributed to resuspension processes by vehicles. Black carbon mass concentrations obtained from both filter analyses (BC) and Aethalometre data (BCae) show a good agreement suggesting that the Aethalometre calibration based on a black carbon specific attenuation coefficient (σ) of 19 m² g⁻¹ is well adapted to nearby roadside measurements. Daily BC (used as a surrogate for fine particles) concentrations and wind speed were found to be anti-correlated. Average daily variations of BC could be related to traffic intensity and regime as well as to the boundary layer height. As expected for freshly emitted traffic aerosols, filter analyses indicated a high BC/TC ratio (29±5%) and a low mean WSOC/OC ratio (12.5±5%) for the bulk aerosol. For these two ratios no day/night differences were observed, the sampling station being probably too close to traffic to evidence photochemical modification of the aerosol phase. Finally, a linear relationship was found between BC and BS hourly concentrations (BC=0.10×BS+1.18; r²=0.93) which offers interesting perspectives to retrieve BC concentrations from existing BS archives.     

Atmospheric corrosion effects of HNO3—Influence of temperature and relative humidity on laboratory-exposed copper

The effect of HNO₃ on the atmospheric corrosion of copper has been investigated at varied temperature (15–35 °C) and relative humidity (0–85% RH). Fourier transform infrared (FT-IR) spectroscopy and X-ray diffraction (XRD) confirmed the existence of cuprite and gerhardtite as the two main corrosion products on the exposed copper surface. For determination of the corrosion rate and for estimation of the deposition velocity (V_d) of HNO₃ on copper, gravimetry and ion chromatography has been employed. Temperature had a low effect on the corrosion of copper. A minor decrease in the mass gain was observed as the temperature was increased to 35 °C, possibly as an effect of lower amount of cuprite due to a thinner adlayer on the metal surface at 35 °C. The V_d of HNO₃ on copper, however, was unaffected by temperature. The corrosion rate and V_d of HNO₃ on copper was the lowest at 0% RH, i. e. dry condition, and increased considerably when changing to 40% RH. A maximum was reached at 65% RH and the mass gain remained constant when the RH was increased to 85% RH. The V_d of HNO₃ on copper at ⩾65% RH, 25 °C and 0.03 cm s⁻¹ air velocity was as high as 0.15±0.03 cm s⁻¹ to be compared with the value obtained for an ideal absorbent, 0.19±0.02 cm s⁻¹. At sub-ppm levels of HNO₃, the corrosion rate of copper decreased after 14 d and the growth of the oxide levelled off after 7 d of exposure.     

Extent of H-atom abstraction from OH + p-cymene and upper limits to the formation of cresols from OH + m-xylene and OH + p-cymene

Aromatic hydrocarbons are important constituents of vehicle exhaust and of non-methane volatile organic compounds in ambient air in urban areas. It has recently been proposed that dealkylation is a significant pathway for the OH radical-initiated reactions, leading to the formation of phenolic compounds and/or oxepins (Noda, J., Volkamer, R., Molina, M.J., 2009. Dealkylation of alkylbenzenes: a significant pathway in the toluene, o-, m-, and p-xylene + OH reaction. Journal of Physical Chemistry A 113, 9658–9666.). We have investigated the formation of cresols from the reactions of OH radicals with m-xylene and p-cymene, and obtain upper limits of <1% for formation of each cresol isomer from OH + m-xylene and <2% for formation of each cresol isomer from OH + p-cymene. In addition, we have measured the formation yield of 4-methylacetophenone (the major product formed subsequent to H-atom abstraction from the CH(CH₃)₂ group) in the OH + p-cymene reaction to be 14.8 ± 3.2%, and estimate that H-atom abstraction from the CH₃ and CH(CH₃)₂ groups in p-cymene accounts for 20 ± 4% of the overall OH radical reaction. We also used a relative rate technique to measure the rate constant for the reaction of OH radicals with 4-methylacetophenone to be (4.50 ± 0.43) × 10^?12 cm³ molecule^?1 s^?1 at 297 ± 2 K.     

Biogenic VOC emissions from fresh leaf mulch and wood chips of Grevillea robusta (Australian Silky Oak)

The emissions of VOC from freshly cut and shredded Grevillea robusta (Australian Silky Oak) leaves and wood have been measured. The VOC emissions from fresh leaf mulch and wood chips lasted typically for 30 and 20 h respectively, and consisted primarily of ethanol, (E)-2-hexenal, (Z)-3-hexen-1-ol and acetaldehyde. The integrated emissions of the VOCs were 0.38±0.04 g kg⁻¹ from leaf mulch, and 0.022±0.003 g kg⁻¹ from wood chips. These emissions represent a source of VOCs in urban and rural air that has previously been unquantified and is currently unaccounted for. These VOCs from leaf mulch and wood chips will contribute to both urban photochemistry and secondary organic aerosol formation. Any CH₄ emissions from leaf mulch and wood chips were <1×10⁻¹¹ g g dry mass⁻¹ s⁻¹.     

Characterisation of PM10 atmospheric aerosols for the wet season 2005 at two sites in East Africa

Ambient daily PM₁₀ aerosol samples were collected at two sites in Tanzania in May and June 2005 (during the wet season), and their chemical characteristics were studied. The sites were a rural site in Morogoro and an urban kerbside site in Dar es Salaam. A Gent PM₁₀ stacked filter unit sampler with sequential Nuclepore polycarbonate filters, providing fine and coarse size fractions, and a PM₁₀ sampler with quartz fibre filters were deployed. Parallel collections of 24 h were made with the two samplers and the number of these collections was 13 in Morogoro and 16 in Dar es Salaam. The average mass concentration of PM₁₀ was 27 ± 11 μg/m³ in Morogoro and 51 ± 21 μg/m³ in Dar es Salaam. In Morogoro, the mean concentrations of organic carbon (OC), elemental carbon (EC), and water-soluble organic carbon (WSOC) were 6.8, 0.51, and 2.8 μg/m³, respectively. In contrast, higher mean concentrations (11.9, 4.6, and 3.3 μg/m³, respectively) were obtained for Dar es Salaam. At both sites, species and elements, such as black carbon, NH₄⁺, non-sea-salt SO₄^2?, K, and Ni (and at Dar es Salaam also V, As, Br, and Pb) were mainly present in the fine size fraction. The common crustal and sea-salt elements, including Na, Mg, Al, Si, Cl, Ca, Ti, Mn, Fe, and Sr, and also NO₃^? and P (and to a lesser extent Cu and Zn) were concentrated in the coarse particles. Aerosol chemical mass closure indicated that the PM₁₀ mass in Morogoro consisted, on average, of 48% organic matter (OM), 44% crustal matter, 4% sea salt, and 2% EC, while in Dar es Salaam OM, crustal matter, sea salt, and EC represented 37%, 32%, 9%, and 9% of the PM₁₀ mass. The contributions of the secondary inorganic aerosol (non-sea-salt sulphate, nitrate, and ammonium) were small, i.e., only 5% in total at each site. Carbonaceous materials and crustal matter were thus the most important components of the PM₁₀ mass. It is suggested that biomass burning is a major contributor to the OM; at Dar es Salaam there is also a very substantial contribution from traffic. A source apportionment calculation indicated that 68% of the OC at this site originated from traffic exhaust versus 32% from charcoal burning. The crustal matter at Morogoro is likely mainly attributable to soil dust resuspension, whereas in Dar es Salaam it is likely mostly resuspended road dust.