Ten years of atmospheric methane observations at a high elevation site in Western China

In this paper, the continuous (1994–2001) and discrete air sample (1991–2001) measurements of atmospheric CH₄ from the Waliguan Baseline Observatory located in western China (36°17′N, 100°54′E, 3816 m asl) are presented and characterized. The CH₄ time series show large episodic events on the order of 100 ppb throughout the year. During spring, a diurnal cycle with average amplitude of 7 ppb and a morning maximum and late afternoon minimum is observed. In winter, a diurnal cycle with average amplitude of 14 ppb is observed with an afternoon maximum and morning minimum. Unlike most terrestrial observational sites, no obvious diurnal patterns are present during the summer or autumn. A background data selection procedure was developed based on local horizontal and vertical winds. A selected hourly data set representative of “baseline” conditions was derived with approximately 50% of the valid hourly data. The range of CH₄ mixing ratios, annual means, annual increases and mean annual cycle at Waliguan during the 1992–2001 were derived from discrete and continuous data representative of “baseline” conditions and compared to air samples collected at other Northern Hemisphere sites. The range of CH₄ monthly means of 1746–1822 ppb, average annual means of 1786.7±10.8 ppb and mean annual increase of 4.5±4.2 ppb yr⁻¹ at Waliguan were inline with measurements from sites located between 30° and 60°N. There were variations observed in the CH₄ annual increase patterns at Waliguan that were slightly different from the global pattern. The mean CH₄ annual cycle at Waliguan shows an unusual pattern of two gentle peaks in summer and February along with two small valleys in early winter and spring and a mean peak-to-peak amplitude of 11 ppb, much smaller than amplitudes observed at most other mid- and high-northern latitude sites. The Waliguan CH₄ data are strongly influenced by continental Asian CH₄ emissions and provide key information for global atmospheric CH₄ models.     

Spatial gradients and source apportionment of volatile organic compounds near roadways

Concentrations of 55 volatile organic compounds (VOCs) (C₂–C₁₂) are reported near a highway in Raleigh, NC. Thirty-minute samples were collected at eight locations, ranging from approximately 10–100 m perpendicular from the roadway. The highest concentrations of VOCs were generally measured closest to the roadway, and concentrations decreased exponentially with increasing distance from the roadway. The highest mean concentration for individual VOCs were for ethylene (3.10 ppbv) (mean concentration at x = 13 m), propane (2.27 ppbv), ethane (1.91 ppbv), isopentane (1.54 ppbv), toluene (0.95 ppbv), and n-butane (0.89 ppbv). Concentrations at the nearest roadway location (x = 13 m) were generally between 2.0 and 1.5 times those from the farthest roadway location (x = 92 m). The data were apportioned into four source categories using the EPA Chemical Mass Balance Model (CMB8.2): motor vehicle exhaust, compressed natural gas, propane gas, and evaporative gasoline. The majority of the VOCs resulted from motor vehicle exhaust (67 ± 12%) (% of total VOC at x = 13 m ± S.D.). Compressed natural gas, propane gas, and evaporative gasoline accounted for approximately 15%, 7% and 1% of the total VOC emissions, respectively, at x = 13 m.     

The sensitivity of modeled ozone to the temporal distribution of point, area, and mobile source emissions in the eastern United States

Ozone remains one of the most recalcitrant air pollution problems in the US. Hourly emissions fields used in air quality models (AQMs) generally show less temporal variability than corresponding measurements from continuous emissions monitors (CEM) and field campaigns would imply. If emissions control scenarios to reduce emissions at peak ozone forming hours are to be assessed with AQMs, the effect of emissions' daily variability on modeled ozone must be understood. We analyzed the effects of altering all anthropogenic emissions' temporal distributions by source group on 2002 summer-long simulations of ozone using the Community Multiscale Air Quality Model (CMAQ) v4.5 and the Carbon Bond IV (CBIV) chemical mechanism with 12 km resolution. We find that when mobile source emissions were made constant over the course of a day, 8-h maximum ozone predictions changed by ±7 parts per billion by volume (ppbv) in many urban areas on days when ozone concentrations greater than 80 ppbv were simulated in the base case. Increasing the temporal variation of point sources resulted in ozone changes of +6 and −6 ppbv, but only for small areas near sources. Changing the daily cycle of mobile source emissions produces substantial changes in simulated ozone, especially in urban areas at night; results suggest that shifting the emissions of NO_x from day to night, for example in electric powered vehicles recharged at night, could have beneficial impacts on air quality.     

Long-term variation of PM2.5 levels and composition at rural, urban, and roadside sites in Hong Kong: Increasing impact of regional air pollution

Long-term study of air pollution plays a decisive role in formulating and refining pollution control strategies. In this study, two 12-month measurements of PM_2.5 mass and speciation were conducted in 00/01 and 04/05 to determine long-term trend and spatial variations of PM_2.5 mass and chemical composition in Hong Kong. This study covered three sites with different land-use characteristics, namely roadside, urban, and rural environments. The highest annual average PM_2.5 concentration was observed at the roadside site (58.0±2.0 μg m⁻³ (average±2σ) in 00/01 and 53.0±2.7 μg m⁻³ in 04/05), followed by the urban site (33.9±2.5 μg m⁻³ in 00/01 and 39.0±2.0 μg m⁻³ in 04/05), and the rural site (23.7±1.9 μg m⁻³ in 00/01 and 28.4±2.4 μg m⁻³ in 04/05). The lowest PM_2.5 level measured at the rural site was still higher than the United States’ annual average National Ambient Air Quality Standard of 15 μg m⁻³. As expected, seasonal variations of PM_2.5 mass concentration at the three sites were similar: higher in autumn/winter and lower in summer. Comparing PM_2.5 data in 04/05 with those collected in 00/01, a reduction in PM_2.5 mass concentration at the roadside (8.7%) but an increase at the urban (15%) and rural (20%) sites were observed. The reduction of PM_2.5 at the roadside was attributed to the decrease of carbonaceous aerosols (organic carbon and elemental carbon) (>30%), indicating the effective control of motor vehicle emissions over the period. On the other hand, the sulfate concentration at the three sites was consistent regardless of different land-use characteristics in both studies. The lack of spatial variation of sulfate concentrations in PM_2.5 implied its origin of regional contribution. Moreover, over 36% growth in sulfate concentration was found from 00/01 to 04/05, suggesting a significant increase in regional sulfate pollution over the years. More quantitative techniques such as receptor models and chemical transport models are required to assess the temporal variations of source contributions to ambient PM_2.5 mass and chemical speciation in Hong Kong.     

A study of the atmospheric reaction of CH3S with O3 as a function of temperature

The atmospheric reaction of the methylthiyl radical (CH₃S) with O₃ was investigated as a function of temperature (259–381 K), in the pressure range of 25–300 Torr, using the technique of laser photolysis/laser-induced fluorescence. The resulting Arrhenius expression, with an uncertainty of ±2σ, was k₁(T=259–381 K)=(1.02±0.30)×10⁻¹² exp[(432±77) K/T] cm³ molecule⁻¹ s⁻¹. The obtained rate constant k₁ was independent of pressure over the limited range employed. Our results are compared with the previous studies carried out, at single temperature and as a function of temperature, by different techniques.     

A review of surface ozone in the polar regions

Surface ozone records from ten polar research stations were investigated for the dependencies of ozone on radiative processes, snow-photochemisty, and synoptic and stratospheric transport. A total of 146 annual data records for the Arctic sites Barrow, Alaska; Summit, Greenland; Alert, Canada; Zeppelinfjellet, Norway; and the Antarctic stations Halley, McMurdo, Neumayer, Sanae, Syowa, and South Pole were analyzed. Mean ozone at the Northern Hemisphere (NH) stations (excluding Summit) is ∼5 ppbv higher than in Antarctica. Statistical analysis yielded best estimates for the projected year 2005 median annual ozone mixing ratios, which for the Arctic stations were 33.5 ppbv at Alert, 28.6 ppbv at Barrow, 46.3 ppbv ppb at Summit and 33.7 ppbv at Zeppelinfjellet. For the Antarctic stations the corresponding ozone mixing ratios were 21.6 ppbv at Halley, 27.0 ppbv at McMurdo, 24.9 ppbv at Neumayer, 27.2 ppbv at Sanae, 29.4 ppbv at South Pole, and 25.8 ppbv at Syowa. At both Summit (3212 m asl) and South Pole (2830 m asl), annual mean ozone is higher than at the lower elevation and coastal stations. A trend analysis revealed that all sites in recent years have experienced low to moderate increases in surface ozone ranging from 0.02 to 0.26 ppbv yr⁻¹, albeit none of these changes were found to be statistically significant trends. A seasonal trend analysis showed above-average increases in ozone during the spring and early summer periods for both Arctic (Alert, Zeppelinfjellet) and Antarctic (McMurdo, Neumayer, South Pole) sites. In contrast, at Barrow, springtime ozone has been declining. All coastal stations experience springtime episodes with rapid depletion of ozone in the boundary layer, attributable to photochemically catalyzed ozone depletion from halogen chemistry. This effect is most obvious at Barrow, followed by Alert. Springtime depletion episodes are less pronounced at Antarctic stations. At South Pole, during the Antarctic spring and summer, photochemical ozone production yields frequent episodes with enhanced surface ozone. Other Antarctic stations show similar, though less frequent spring and summertime periods with enhanced ozone. The Antarctic data provide evidence that austral spring and summertime ozone production in Antarctica is widespread, respectively, affects all stations at least through transport events. This ozone production contributes to a several ppbv enhancement in the annual mean ozone over the Antarctic plateau; however, it is not the determining process in the Antarctic seasonal ozone cycle. Although Summit and South Pole have many similarities in their environmental conditions, this ozone production does not appear to be of equal importance at Summit. Amplitudes of diurnal, summertime ozone cycles at these polar sites are weaker than at lower latitude locations. Amplitudes of seasonal ozone changes are larger in the Southern Hemisphere (by ∼5 ppbv), most likely due to less summertime photochemical ozone loss and more transport of ozone-rich air to the Arctic during the NH spring and summer months.     

Simulation of aerosol radiative properties with the ORISAM-RAD model during a pollution event (ESCOMPTE 2001)

The aim of this study is to present the organic and inorganic spectral aerosol module-radiative (ORISAM-RAD) module, allowing the 3D distribution of aerosol radiative properties (aerosol optical depth, single scattering albedo and asymmetry parameter) from the ORISAM module. In this work, we test ORISAM-RAD for one selected day (24th June) during the ESCOMPTE (expérience sur site pour contraindre les modèles de pollution atmosphérique et de transport d’emissions) experiment for an urban/industrial aerosol type. The particle radiative properties obtained from in situ and AERONET observations are used to validate our simulations. In a first time, simulations obtained from ORISAM-RAD indicate high aerosol optical depth (AOD)0.50–0.70±0.02 (at 440 nm) in the aerosol pollution plume, slightly lower (10–20%) than AERONET retrievals. In a second time, simulations of the single scattering albedo (ω_o) have been found to well reproduce the high spatial heterogeneities observed over this domain. Concerning the asymmetry parameter (g), ORISAM-RAD simulations reveal quite uniform values over the whole ESCOMPTE domain, comprised between 0.61±0.01 and 0.65±0.01 (at 440 nm), in excellent agreement with ground based in situ measurements and AERONET retrievals. Finally, the outputs of ORISAM-RAD have been used in a radiative transfer model in order to simulate the diurnal direct radiative forcing at different locations (urban, industrial and rural). We show that anthropogenic aerosols strongly decrease surface solar radiation, with diurnal mean surface forcings comprised between −29.0±2.9 and −38.6±3.9 W m⁻², depending on the sites. This decrease is due to the reflection of solar radiations back to space (−7.3±0.8<ΔF_TOA<−12.3±1.2 W m⁻²) and to its absorption into the aerosol layer (21.1±2.1<ΔF_ATM<26.3±2.6 W m⁻²). These values are found to be consistent with those measured at local scale.     

Processes affecting the movement of organochlorine pesticides (OCPs) between soil and air in an industrial site in Turkey

Soil and atmospheric concentrations, dry deposition and soil-air gas exchange of organochlorine pesticides (OCPs) were investigated at an industrial site in Aliaga, Izmir, Turkey. Current-use pesticides, endosulfan and chlorpyrifos, had the highest atmospheric levels in summer and winter. Summertime total (gas + particle) OCP concentrations in air were higher, probably due to increased volatilization at higher temperatures and seasonal local/regional applications of current-use pesticides. Particle deposition fluxes were generally higher in summer than in winter. Overall average dry particle deposition velocity for all the OCPs was 4.9 ± 4.1 cm s⁻¹ (average ± SD). ΣDDXs (sum of p,p′-DDT, p,p′-DDD, and p,p′-DDE) were the most abundant OCPs in Aliaga soils (n = 48), probably due to their heavy historical use and persistence. Calculated fugacity ratios and average net gas fluxes across the soil-air interface indicated volatilization for α-CHL, γ-CHL, heptachlorepoxide, cis-nonachlor, trans-nonachlor, and p,p′-DDT in summer, and for α-CHL, γ-CHL, trans-nonachlor, endosulfan sulfate, and p,p′-DDT in winter. For the remaining OCPs, soil acted as a sink during both seasons. Comparison of the determined fluxes showed that dry particle, gas-phase, and wet deposition are significant OCP input mechanisms to the soil in the study area.     

Atmospheric transport pathways from the Bilibino nuclear power plant to Alaska

The Bilibino nuclear power plant (68°03′N, 166°20′E, 340 m asl) in northeastern Siberia is the closest Russian nuclear power plant to the USA. We used an isentropic trajectory model to estimate the probability that air in the Bilibino region would be transported to Alaska following a hypothetical accident. This estimate is based on the meteorological data from 1991 to 1995. Our calculations indicate that the probability that air in the Bilibino region will be transported to Alaska is approximately 6–16%, averaged over the entire year. This probability doubles in the summer and early fall with a maximum in August of 12–33%. For the entire year the mean, median, and minimum transport times from the plant to Alaska are 4, 3.5 and 1 d, respectively. Since rapid transport (1–2 d) could bring air parcels containing short-lived radionuclides, these events potentially represent the greatest risk to inhabitants of Alaska.     

Global baseline pollution studies XI: Congener specific determination of polychlorinated biphenyls (PCB) and occurrence of alpha- and gamma-hexachlorocyclohexane (HCH), 4,4′-DDE and 4,4′-DDT in continental air

The hexachlorocyclohexane-isomers (HCH), hexachlorobenzene (HCB), the polychlorinated biphenyls (PCB), 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (4,4′-DDT) and 1,1-dichloro-2,2-bis(p-chlorophenyl)ethene (4,4′-DDE) have been measured in urban and rural air around the city of Ulm (F. R. G., 48.4° N, 10.0° E). The sampling stations are typical for continental air in the westerlies of the northern hemisphere including local and regional influences. The analytical method consists of adsorptive sampling of large volumes (1000 m³) of air on silica gel, solvent desorption with CH₂Cl₂, preseparation of the collected chlorinated C₆/C₁₄ hydrocarbons by liquid adsorption chromatography on silica gel, and high resolution capillary gas chromatography with electron capture-(HRGC/ECD) or mass-selective detection (HRGC/MSD). The concentrations found in the lower troposphere under different meteorological conditions reflect regional input and long range transport. The levels found range from 1 pg/m³ for 4,4′-DDT to 10 ng/m³ for gamma HCH.     

Neptunium(V) adsorption to calcite

The migration behavior of the actinyl ions U(VI)O₂²⁺, Np(V)O₂⁺ and Pu(V,VI)O₂^(+,2+) in the geosphere is to a large extend controlled by sorption reactions (inner- or outer-sphere adsorption, ion-exchange, coprecipitation/structural incorporation) with minerals. Here NpO₂⁺ adsorption onto calcite is studied in batch type experiments over a wide range of pH (6.0–9.4) and concentration (0.4 μM–40 μM) conditions. pH is adjusted by variation of CO₂ partial pressure. Adsorption is found to be pH dependent with maximal adsorption at pH 8.3 decreasing with increasing and decreasing pH. pH dependence of adsorption decreases with increasing Np(V) concentration. EXAFS data of neptunyl adsorbed to calcite and neptunyl in the supernatant shows differences in the Np(V)-O-yl distance, 1.85 ± 0.01 Å for the adsorbed and 1.82 ± 0.01 Å for the solution species. The equatorial environment of the neptunyl in solution shows about 5 oxygen neighbours at 2.45 ± 0.02 Å. For adsorbed neptunyl there are also about 5 oxygen neighbours at 2.46 ± 0.01 Å. An additional feature in the adsorbed species' R-space spectrum can be related to carbonate neighbours, 3 to 6 carbon backscatterers (C-eq) at 3.05 ± 0.03 Å and 3 to 6 oxygen backscatterers (O-eq2) at 3.31 ± 0.02 Å. The differences in the Np(V)-O-yl distance and the C-eq and O-eq2 backscatterers which are only present for the adsorbed species indicate inner-sphere bonding of the adsorbed neptunyl species to the calcite surface. Experiments on adsorption kinetics indicate that after a fast surface adsorption process a continuous slow uptake occurs which may be explained by incorporation via surface dissolution and reprecipitation processes. This is also indicated by the part irreversibility of the adsorption as shown by increased K_D values after desorption compared to adsorption.     

WRF-Chem simulation of East Asian air quality: Sensitivity to temporal and vertical emissions distributions

This study develops fine temporal (seasonal, day-of-week, diurnal) and vertical allocations of anthropogenic emissions for the TRACE-P inventory and evaluates their impacts on the East Asian air quality prediction using WRF-Chem simulations in July 2001 at 30-km grid spacing against available surface measurements from EANET and NEMCC. For NO₂ and SO₂, the diurnal and vertical redistributions of emissions play essential roles, while the day-of-week variation is less important. When all incorporated, WRF-Chem best simulates observations of surface NO₂ and SO₂ concentrations, while using the default emissions produces the worst result. The sensitivity is especially large over major cities and industrial areas, where surface NO₂ and SO₂ concentrations are reduced by respectively 3–7 and 6–12 ppbv when using the scaled emissions. The incorporation of all the three redistributions of emissions simulates surface O₃ concentrations higher by 4–8 ppbv at night and 2–4 ppbv in daytime over broad areas of northern, eastern and central China. To this sensitivity, the diurnal redistribution contributes more than the other two.     

C2–C8 NMHCs over the Eastern Mediterranean: Seasonal variation and impact on regional oxidation chemistry

More than 2500 measurements of C₂–C₈ non-methane hydrocarbons (NMHCs) have been conducted at Finokalia sampling station on the island of Crete over a thirty-month period (September 2003–February 2006), to investigate the factors controlling NMHC levels and estimate their role in the oxidizing capacity of the Eastern Mediterranean atmosphere. Atmospheric concentrations of NMHCs range from below the detection limit (5 pptv) to a few ppbv and present a hydroxyl radical (OH) driven seasonal pattern with lower values during summer. The diel variability was also influenced by the reaction of the NMHC with the OH radical, exhibiting a nighttime maximum and a midday or early afternoon minimum. Long-lived compounds demonstrate higher concentrations under the influence of the northern sector (European continent), indicating that besides chemistry, transport significantly contributes to NMHCs levels in the area. Based on the observed NMHCs diurnal cycles, mean OH radical levels of 3.5 × 10⁶ molecules cm⁻³ have been derived for May–October period.     

Do aerosols act as catalysts in the OH radical initiated atmospheric oxidation of volatile organic compounds?

Smog chamber/FTIR techniques were used to study the relative reactivity of OH radicals with methanol, ethanol, phenol, C₂H₄, C₂H₂, and p-xylene in 750 Torr of air diluent at 296±2 K. Experiments were performed with, and without, 500–8000 μg m⁻³ (4000–50 000 μm² cm⁻³ surface area per volume) of NaCl, (NH₄)₂SO₄ or NH₄NO₃ aerosol. In contrast to the recent findings of Oh and Andino (Atmospheric Environment 34 (2000) 2901, 36 (2002) 149; International Journal of Chemical Kinetics 33 (2001) 422) there was no discernable effect of aerosol on the rate of loss of the organic compounds via reaction with OH radicals. Gas kinetic theory arguments cast doubt upon the findings of Oh and Andino. The available data suggest that the answer to the title question is “No”. As part of this work the rate constants for reactions of OH radicals with methanol, ethanol, and phenol in 750 Torr of air at 296 K were determined to be: k_OH+CH3OH=(8.12±0.54)×10⁻¹³, k_OH+C2H5OH=(3.47±0.32)×10⁻¹² and k_OH+phenol=(3.27±0.31)×10⁻¹¹ cm³ molecule⁻¹ s⁻¹.     

Estimation and validation of PM2.5/PM10 exhaust and non-exhaust emission factors for practical street pollution modelling

In order to carry out efficient traffic and air quality management, validated models and PM emission estimates are needed. This paper compares current available emission factor estimates for PM₁₀ and PM_2.5 from emission databases and different emission models, and validates these against eight high quality street pollution measurements in Denmark, Sweden, Germany, Finland and Austria.The data sets show large variation of the PM concentration and emission factors with season and with location. Consistently at all roads the PM₁₀ and PM_2.5 emission factors are lower in the summer month than the rest of the year. For example, PM₁₀ emission factors are in average 5–45% lower during the month 6–10 compared to the annual average.The range of observed total emission factors (including non-exhaust emissions) for the different sites during summer conditions are 80–130 mg km⁻¹ for PM₁₀, 30–60 mg km⁻¹ for PM_2.5 and 20–50 mg km⁻¹ for the exhaust emissions.We present two different strategies regarding modelling of PM emissions: (1) For Nordic conditions with strong seasonal variations due to studded tyres and the use of sand/salt as anti-skid treatment a time varying emission model is needed. An empirical model accounting for these Nordic conditions was previously developed in Sweden. (2) For other roads with a less pronounced seasonal variation (e.g. in Denmark, Germany, Austria) methods using a constant emission factor maybe appropriate. Two models are presented here.Further, we apply the different emission models to data sets outside the original countries. For example, we apply the “Swedish” model for two streets without studded tyre usage and the “German” model for Nordic data sets. The “Swedish” empirical model performs best for streets with studded tyre use, but was not able to improve the correlation versus measurements in comparison to using constant emission factors for the Danish side. The “German” method performed well for the streets without clear seasonal variation and reproduces the summer conditions for streets with pronounced seasonal variation. However, the seasonal variation of PM emission factors can be important even for countries not using studded tyres, e.g. in areas with cold weather and snow events using sand and de-icing materials. Here a constant emission factor probably will under-estimate the 90-percentiles and therefore a time varying emission model need to be used or developed for such areas.All emission factor models consistently indicate that a large part (about 50–85% depending on the location) of the total PM₁₀ emissions originates from non-exhaust emissions. This implies that reduction measures for the exhaust part of the vehicle emissions will only have a limited effect on ambient PM₁₀ levels.     

Monoterpene emissions and carbonyl compound air concentrations during the blooming period of rape (Brassica napus)

An increasing percentage of agricultural land in Germany is used for oil seed plants. Hence, rape has become an important agricultural plant (in Saxony 1998: 12% of the farmland) in the recent years. During flowering of rape along with intensive radiation and high temperatures, a higher production and emission of biogenic VOC was observed. The emissions of terpenes were determined and more importantly, high concentrations of organic carbonyl compounds were observed during this field experiment. All measurements of interest have been carried out during two selected days with optimal weather conditions. It is found that the origin or the mechanism of formation of different group of compounds had strong influence on the day to day variation of their concentrations. The emission flux of terpenes from flowering rape plants was determined to be 16–32 μg h⁻¹ m⁻² (30–60 ng h⁻¹ per g dry plant––540–1080 ng h⁻¹ per plant), in total. Limonene, α-thujene and sabinene were the most important compounds (about 60% of total terpenes). For limonene and sabinene reference emission rates (M_S) and temperature coefficients were determined: β_limonene=0.108 K⁻¹ and M_S=14.57 μg h⁻¹ m⁻²; β_sabinene=0.095 K⁻¹ and M_S=5.39 μg h⁻¹ m⁻².The detected carbonyl compound concentrations were unexpectedly high (maximum formaldehyde concentration was 18.1 ppbv and 3.4 ppbv for butyraldehyde) for an open field. Possible reasons for these concentrations are the combination of primary emission from the plants induced by high temperature and high ozone stress, the secondary formation from biogenically and advected anthropogenically emitted VOC at high radiation intensities and furthered by the low wind speeds at this time.     

Impact of the Saharan dust outbreaks on the ambient levels of total suspended particles (TSP) in the marine boundary layer (MBL) of the Subtropical Eastern North Atlantic Ocean

Six years (1998–2003) of measurements of ambient air concentrations of total suspended particulate (TSP) measured at a rural background monitoring station in Tenerife (Canary Islands), the El Río station (ER, 28°08′35″N, 16°39′20″W, 500 m a.s.l.) were studied. African dust outbreaks were objectively identified using a new quantitative tool, called the African Index. This index indicates the percentage of time that an air mass remained over an African region at one of three possible height intervals of the lower troposphere. After identifying these episodes, a study of the background TSP levels at the ER station and of direct and indirect (those which cause vertical deposition of dust) African air mass intrusion impacts was performed. Taking into account both direct and indirect episodes, a total of 322 days of African dust intrusion were objectively identified (a mean of 54 episodes per year) in the period 1998–2003, some of them caused by “transition episodes” or “return African air masses”. A subjective method confirmed that 256 of these days were caused by direct impacts of African dust on the ER station. A mean TSP value of 21.6 μg m⁻³ was found at the station during this period. All the episodes occurred when the TSP concentration was >28.5 μg m⁻³. The TSP background (14 μg m⁻³) can be assumed to be representative of the MBL of the Eastern North Atlantic subtropical region. The highest number of dust gravitational settlement (or indirect) episodes occurs in summer, but the highest contribution of these episodes to the TSP levels is in March with a monthly mean TSP contribution of up to 30.5 μg m⁻³.     

Flow patterns influencing the seasonal behavior of surface ozone and carbon monoxide at a coastal site near Hong Kong

Surface O₃ and CO were measured at Cape D’Aguilar, Hong Kong during the period of January 1994 to December1996 in order to understand the temporal variations of surface O₃ and CO in East Asia–West Pacific region. The isentropic backward trajectories were used to isolate different air masses reaching the site and to analyze the long-range transport and photochemical buildup of O₃ on a regional scale. The results show that the diurnal variation of surface O₃ was significant in all seasons with daily O₃ production being about 20 ppbv in fall and 10 ppbv in winter, indicating more active photochemical processes in the subtropical region. The distinct seasonal cycles of O₃ and CO were found with a summer minimum (16 ppbv)–fall maximum (41 ppbv) for O₃ and a summer minimum (116 ppbv)–winter maximum (489 ppbv) for CO. The isentropic backward trajectory cluster analyses suggest that the air masses (associated with regional characteristics) to the site can be categorized into five groups, which are governed by the movement of synoptic weather systems under the influence of the Asian monsoon. For marine-originated air masses (M-SW, M-SE and M-E, standing for marine-southwest, marine-southeast and marine-east, respectively) which always appear in summer and spring, the surface O₃ and CO have relatively lower mixing ratios (18, 16 and 30 ppbv for O₃, 127, 134 and 213 ppbv for CO), while the continental air masses (C-E and C-N, standing for continent-east and continent-north, respectively) usually arrive at the site in winter and fall seasons with higher O₃ (43 and 48 ppbv) and CO (286 and 329 ppbv). The 43 ppbv O₃ and 286 ppbv CO are representative of the regionally polluted continental outflow air mass due to the anthropogenic activity in East Asia, while 17 ppbv O₃ and 131 ppbv CO can be considered as the signature of the approximately clean marine background of South China Sea. The very high CO values (461–508 ppbv) during winter indicate that the long-range transport of air pollutants from China continent is important at the monitoring site. The fall maximum (35–46 ppbv) of surface O₃ was believed to be caused by the effects of the weak slowly moving high-pressure systems which underlie favorable photochemical production conditions and the long-range transport of aged air masses with higher O₃ and its precursors.     

Ambient formaldehyde and its contributing factor to ozone and OH radical in a rural area

Formaldehyde (HCHO), as well as correlative pollutants was measured from 1 to 31 July in 2007 at Mazhuang, a rural site located in the east of China. Gaseous HCHO was scrubbed from the air with an acidic 2,4-dinitrophenylhydrazine (DNPH) solution, which leaded to the reaction of HCHO with DNPH and produced a stable product, 2,4-dinitrophenylhydrazone, followed by online analysis by high-performance liquid chromatography (HPLC) coupled with Ultraviolet detector. During the observation period, mixing ratios of HCHO ranged from 0.2 ppbv to 6.2 ppbv, with an average of 1.5 ± 0.67 ppbv. HCHO shows an evident diurnal variation, the maximum appeared during 12:00–14:00. The average concentration diurnal variations of measured HCHO, ozone (O₃), Methylhydroperoxides (MHP, CH₃OOH), hydrogen peroxide (H₂O₂), nitrogen oxides (NO_x) and meteorological parameters were compared. The similar variations of HCHO, O₃ and radiation imply that photo-oxidation of hydrocarbons might be the major source for HCHO. Based on the maximum incremental reactivity (MIR) coefficient of HCHO, the calculation shows that HCHO contributes about 20% to total observed O₃ during the study period. In order to compare the contributions of O₃, HCHO and HONO to OH radical, photolysis rate parameters (J-values) of the three compounds were calculated by the Tropospheric Ultraviolet and Visible (TUV) Radiation Model (4.4 version). Based on the comparison, this study reaches the conclusion that O₃ is the dominant source of OH radical at Mazhuang. This study also uses P(HCHO)/P(O₃) which represents the ratio of contrbutions of HCHO and O₃ to OH radical, to discuss the action of HCHO in OH radical soucers. The result shows that P(HCHO)/P(O₃) is 12.5% on average, with the maximum of 21.0% at 13:00_P.M. and minimum of 7.5% before 9:00_A.M. and after 17:00_P.M..Therefore HCHO is also an important source of OH radical and cannot be ignored.     

Production of boundary layer ozone from tropical American Savannah biomass burning emissions

Boundary layer ozone and carbon monoxide were measured at a savannah site in the Orinoco river basin, during the dry and wet seasons. CO and O₃ concentrations recorded around noontime show a good linear correlation, suggesting that the higher ozone levels observed during the dry season are photochemically produced during the oxidation of reactive hydrocarbons in the presence of NO_x both emitted by biomass burning. The rate of photochemical ozone production in the boundary layer ozone by biomass burning calculated from the production ratio ΔO₃/ΔCO (0.17±0.01 v : v) and the amount of CO produced by fires (0.26–1.3 mole m⁻² dry season⁻¹), ranges from 0.6 to 2.6 ppbv h⁻¹ for 8 h of daylight. This O₃ production rate is in fairly good agreement with the value derived from RO₂ radical measurements made in the Venezuelan savannah during the dry season. The net boundary layer production of O₃ from all tropical America savannah fires is estimated to range between 0.28 and 0.36 Tmol O₃ per year, which is about 3 times higher than the O₃ produced from pollution sources in the eastern United States during the summer. An extrapolation to all of the world's savannah would indicate a net boundary layer ozone production of about 1.2 Tmol yr⁻¹. This is discussed in the context of the overall global budget of tropospheric ozone.