Measurement and analysis of atmospheric concentrations of isoprene and its reaction products in central Texas

A field experiment was conducted in August 1998 to investigate the concentrations of isoprene and isoprene reaction products in the surface and mixed layers of the atmosphere in Central Texas. Measured near ground-level concentrations of isoprene ranged from 0.3 (lower limit of detection – LLD) to 10.2 ppbv in rural regions and from 0.3 to 6.0 ppbv in the Austin urban area. Rural ambient formaldehyde levels ranged from 0.4 ppbv (LLD) to 20.0 ppbv for 160 rural samples collected, while the observed range was smaller at Austin (0.4–3.4 ppbv) for a smaller set of samples (37 urban samples collected). Methacrolein levels did not vary as widely, with rural measurements from 0.1 ppbv (LLD) to 3.7 ppbv and urban concentrations varying between 0.2 and 5.7 ppbv. Isoprene flux measurements, calculated using a simple box model and measured mixed-layer isoprene concentrations, were in reasonable agreement with emission estimates based on local ground cover data. Ozone formation attributable to biogenic hydrocarbon oxidation was also calculated. The calculations indicated that if the ozone formation occurred at low VOC/NO_x ratios, up to 20 ppbv of ozone formed could be attributable to biogenic photooxidation. In contrast, if the biogenic hydrocarbon reaction products were formed under low NO_x conditions, ozone production attributable to biogenics oxidation would be as low as 1 ppbv. This variability in ozone formation potentials implies that biogenic emissions in rural areas will not lead to peak ozone levels in the absence of transport of NO_x from urban centers or large rural NO_x sources.     

Ozone uptake by citrus trees exposed to a range of ozone concentrations

The Citrus genus includes a large number of species and varieties widely cultivated in the Central Valley of California and in many other countries having similar Mediterranean climates. In the summer, orchards in California experience high levels of tropospheric ozone, formed by reactions of volatile organic compounds (VOC) with oxides of nitrogen (NO_x). Citrus trees may improve air quality in the orchard environment by taking up ozone through stomatal and non-stomatal mechanisms, but they may ultimately be detrimental to regional air quality by emitting biogenic VOC (BVOC) that oxidize to form ozone and secondary organic aerosol downwind of the site of emission. BVOC also play a key role in removing ozone through gas-phase chemical reactions in the intercellular spaces of the leaves and in ambient air outside the plants. Ozone is known to oxidize leaf tissues after entering stomata, resulting in decreased carbon assimilation and crop yield. To characterize ozone deposition and BVOC emissions for lemon (Citrus limon), mandarin (Citrus reticulata), and orange (Citrus sinensis), we designed branch enclosures that allowed direct measurement of fluxes under different physiological conditions in a controlled greenhouse environment. Average ozone uptake was up to 11 nmol s^?1 m^?2 of leaf. At low concentrations of ozone (40 ppb), measured ozone deposition was higher than expected ozone deposition modeled on the basis of stomatal aperture and ozone concentration. Our results were in better agreement with modeled values when we included non-stomatal ozone loss by reaction with gas-phase BVOC emitted from the citrus plants. At high ozone concentrations (160 ppb), the measured ozone deposition was lower than modeled, and we speculate that this indicates ozone accumulation in the leaf mesophyll.     

Fungi and bacteria in mould-damaged and non-damaged office environments in a subarctic climate

The fungi and bacterial levels of the indoor air environments of 77 office buildings were measured in winter and a comparison was made between the buildings with microbe sources in their structures and those without such sources. Penicillium, yeasts, Cladosporium and non-sporing isolates were the commonest fungi detected in the indoor air and in settled dust, in both the mould-damaged and control buildings. Aspergillus ochraceus, Aspergillus glaucus and Stachybotrys chartarium were found only in environmental samples from the mould-damaged buildings. Some other fungi, with growth requiring of water activity, a_w, above 0.85, occurred in both the reference and mould-damaged buildings, but such fungi were commoner in the latter type of buildings. The airborne concentrations of Penicillium, Aspergillus versicolor and yeasts were the best indicators of mould damage in the buildings studied. Penicillium species and A. versicolor were also the most abundant fungi in the material samples. This study showed that the fungi concentrations were very low (2–45 cfu m⁻³ 90% of the concentrations being <15 cfu m⁻³) in the indoor air of the normal office buildings. Although the concentration range of airborne fungi was wider for the mould-damaged buildings (2–2470 cfu m⁻³), only about 20% of the samples exceeded 100 cfu m⁻³. The concentrations of airborne bacteria ranged from 12 to 540 cfu m⁻³ in the control buildings and from 14 to 1550 cfu m⁻³ in the mould-damaged buildings. A statistical analysis of the results indicated that bacteria levels are generally <600 cfu m⁻³ in office buildings in winter and fungi levels are <50 cfu m⁻³. These normal levels are applicable to subarctic climates for urban, modern office buildings when measurements are made using a six-stage impactor. These levels should not be used in evaluations of health risks, but elevated levels may indicate the presence of abnormal microbe sources in indoor air and a need for additional environmental investigations.     

Peroxyacetyl nitrate and peroxypropionyl nitrate in Porto Alegre,Brazil

For 41 days between 25 May 1996 and 27 March 1997, peroxyacetyl nitrate (PAN) and peroxypropionyl nitrate (PPN) have been measured by electron capture gas chromatography at Santa Rita near Porto Alegre, RS, Brazil, where light-duty vehicles used either ethanol or a gasoline–MTBE blend. Daily maximum concentrations ranged from 0.19 to 6.67 ppb for PAN and 0.06 to 0.72 ppb for PPN. Linear regression of maximum PPN vs. maximum PAN yielded a slope of 0.105±0.004 (R²=0.974). Diurnal variations of ambient PAN often followed those of ozone with respect to time of day but not with respect to amplitude. This was reflected in the large relative standard deviations associated with the study-averaged PAN/ozone concentration ratio, 0.037±0.105 (ppb/ppb, n=789) and the maximum PAN/maximum ozone concentration ratio, 0.028±0.015 (ppb/ppb, range 0.005–0.078, n=41). On several days PAN accounted for large fractions of the total ambient NO_x in the late morning and afternoon hours, e.g., PAN/NO_x⩽0.58 and PAN/(NO_x–NO) ⩽0.76 on 27 March 1997. The amount of PAN lost by thermal decomposition (TPAN) was comparable in magnitude to that present in ambient air. The ratios TPAN/(PAN+TPAN) were up to 0.53, 0.67 and 0.64 during the warm afternoons of 25, 26 and 27 March 1997, respectively. The highest calculated value of TPAN was 5.6 ppb on 27 March 1997. On that day the 24 h-averaged value of TPAN (1.01 ppb) was nearly the same as that of PAN (1.09 ppb). Using computer kinetic modeling (SAPRC 97 chemical mechanism) and sensitivity analysis of VOC incremental reactivity, we ranked VOC present in Porto Alegre ambient air for their importance as precursors to PAN and to PPN. Using as input data the averages of VOC concentrations measured in downtown Porto Alegre during the ca. 1 yr period March 1996–April 1997, we calculated that the most important precursors to PAN and PPN were the SAPRC 97 model species ARO2 (which includes the aromatics xylenes, trimethylbenzenes, ethyltoluenes, etc.), which accounted for ca. 17% of the total PAN and total PPN formation potentials. Overall, the results indicate a major role for aromatics and alkenes and a minor role for acetaldehyde and ethanol as precursors to peroxyacyl nitrates in the Porto Alegre urban area.     

Size and seasonal distributions of airborne bioaerosols in commuting trains

Aerobiological studies in commuting trains in northern Taiwan were carried out from August, 2007 until July, 2008. Two six-stage (>7 μm, 4.7～7 μm, 3.3～4.7 μm, 2.1～3.3 μm, 1.1～2.1 μm, 0.65～1.1 μm) cascade impactors of 400 orifices were used to collect viable bacteria and fungi, respectively. The levels of carbon monoxide (CO), carbon dioxide (CO₂), formaldehyde (HCHO), temperature, and relative humidity in the commuting trains were also recorded during the sampling period. Results show that bacterial concentrations ranged from 25 to 1530 CFU m^?3, and averaged 417 CFU m^?3. The fungal concentrations ranged from 45 to 1906 CFU m^?3, and averaged 413 CFU m^?3. Additionally, the highest fractions occurred in the fifth stage (1.1～2.1 μm) for both bacteria and fungi. The respirable fractions, R_b and R_f, for bacteria and fungi were 62.8% and 81.4%, respectively, which are higher than those in other studies. Furthermore, the bacterial concentration reached its highest level in autumn, and its lowest level in winter. However, the fungal concentration was highest in spring and lowest in winter. Though the total bacterial or fungal concentration did not exceed the recommendation standard in Taiwan, the relatively high respirable fraction in commuting trains probably implies a higher adverse health risk for sensitive commuters. This study further conducted multiple regression analysis to determine the relationship of various stage fractions of airborne bacteria and fungi with indoor air pollutants (CO and HCHO) and environmental parameters (CO₂, temperature, and relative humidity). The correlation coefficients of multiple regression analysis for total bacteria and fungi concentrations with indoor air pollutants and environmental parameters were 0.707 (p < 0.00376) and 0.612 (p < 0.00471), respectively. There are currently no formally regulated laws for indoor air quality (IAQ) in Taiwan, and this preliminary study can provide references to the Taiwan government on IAQ management.     

Ozone and increased nitrogen supply effects on the yield and nutritive quality of Trifolium subterraneum

The influence of ambient ozone (O₃) concentrations and nitrogen (N) fertilization, singly and in combination, on the growth and nutritive quality of Trifolium subterraneum was assessed. This is an important O₃-sensitive species of great pastoral value in Mediterranean areas. Plant material was enclosed in open-top chambers (OTCs). Three O₃ levels were established: Filtered air with O₃ concentrations below 15 ppb (CFA), non-filtered air with O₃ concentrations in the range of ambient levels (NFA), and non-filtered air supplemented with 40 ppb O₃ over ambient levels (NFA+). Similarly, three N levels were defined: 5, 15 and 30 kg ha⁻¹. The increase in O₃ exposure induced a reduction of the clover aerial green biomass and an increase of senescent biomass. Ozone effects were more adverse in the root system, inducing an impairment of the aerial/subterranean biomass ratio. Compared with the CFA treatment, nutritive quality of aerial biomass was 10 and 20% lower for NFA and NFA+ treatments, respectively, due to increased concentrations of acid detergent fiber, neutral detergent fiber and lignin. The latter effect appears to be related to senescence acceleration. The increment in N supplementation enhanced the increase of ADF concentrations in those plants simultaneously exposed to ambient and above-ambient O₃ concentrations, and reduced the incremental rate of foliar senescence induced by the pollutant.     

Surface ozone and NOx concentrations at a high altitude Mediterranean site,Greece

Ozone was measured in six- and NO_x in five sampling periods in 1996–97, mostly during summer, at a 1070 m altitude site in northern Peloponnese. Mean values in each sampling period ranged from 43–48 ppb exceeding the European Union 24 h plant protection standard. The background ozone concentration of 43 ppb derived from the correlation of ozone with NO^′_x also exceeded the EU plant protection standard. Ozone exhibited maxima in the afternoon and minima during the night; in certain 24–48 h periods, however, the ozone concentrations remained practically constant; in these short periods air mass back trajectories indicated air masses which originated in north Africa. NO^′_x concentrations had maximum of 24 h around noon. Their mean concentrations ranged from 0.5–0.7 ppb, smaller than respective concentrations in north-central Europe.     

Characterization of n-alkanes in PM2.5 of the Taipei aerosol

Ambient concentrations of n-alkanes with carbon number ranging from 17 to 36 were determined for PM_2.5 samples collected in Taipei city during September 1997–February 1998. The measured concentrations of particulate n-alkanes were in the range of 69–702 ng m⁻³, considerably higher than the concentration levels observed in Los Angeles and Hong Kong. The concentration distributions of n-alkanes homologues obtained in this study exhibited peaks at C₁₉, C₂₄ or C₂₅. This suggests that fossil fuel utilization, such as vehicular exhaust and lubricant residues, was an important contributor to the Taipei aerosol. Source apportionment of PM_2.5 was conducted using carbon preference index (CPI, defined as the ratio of the total concentration of particulate n-alkanes with odd carbon number to that with even carbon number) and U : R ratio (the concentration ratio of unresolved components to resolved components obtained from chromatograms). The low CPI value (0.9–1.9) and high U : R ratio (2.6–6.4) for each sample further confirmed that fossil fuel utilization was the major source of n-alkanes in ambient PM_2.5 of Taipei city. Estimates from these results showed that 69–93% of the n-alkanes in PM_2.5 of the Taipei aerosol originated from vehicular exhaust. The higher concentration level of particulate n-alkanes in the Taipei aerosol was mainly a result of vehicular emissions.     

Observational study of ozone pollution at a rural site in the Yangtze Delta of China

Ozone and related trace gases (CO, NO_x, and SO₂) were measured from June 1999 to July 2000 at a rural site in the Yangtze Delta of China, a region of intensive anthropogenic activity. Elevated ozone levels were frequently observed during the study period, with the highest frequency in late spring and early summer. Over a 1 yr period, 21 d were found to have ozone concentrations exceeding the new US 8-h 80 ppb health standard. Calculation of the “SUM06” exposure index also shows relatively high (>15 ppm h) values for each season except winter. At these levels ozone may have adverse effects on human health as well as agricultural crops. Analysis of meteorological data shows that the high ozone days were associated with large-scale stagnation, intense solar radiation, and minimum rainfall. Large-scale back trajectories indicate a slow-moving/re-circulating airmass during the episodic days. Examination of chemical data shows that the observed daytime high ozone concentrations were due to downward mixing of ozone-rich air, in situ photochemical formation, and in some cases, advection to the site of aged plumes. The very high CO levels (and high CO to NO_x ratios) were found to coincide with many of the ozone episodes, suggesting a contribution from sources of emission involving incomplete combustion. It is suggested that the burning of biomass (e.g., biofeuls and crop residues) may be an important source for the observed high CO and O₃ values.     

Assessment of metal concentrations in atmospheric particles from Burnaby Lake,British Columbia,Canada

Trace metals were assessed in atmospheric particulates at Burnaby Lake, in the greater Vancouver area of British Columbia to assess concentrations, particle size distributions and deposition rates to an urban watershed. Week-long samples were collected over a period of 18 weeks in 1995 using a 13 stage low pressure impactor (LPI). Samples were analysed using inductively coupled plasma atomic emission spectroscopy (ICP). Aluminum, boron, calcium, iron, magnesium, manganese, sodium and strontium had a similar time series pattern and particle size distribution. For these metals, maximum concentrations occurred during weeks of low precipitation and exhibited a large peak in mid June. Their particle size distribution was mostly dominated by a large peak between 1.7–18.4 μm with a secondary peak at <0.08 μm. Metal concentrations were generally one to three orders of magnitude higher than those measured in a rural location 100 km away from Burnaby Lake but similar to those measured in urban Taipei, Taiwan. Concentrations of the highly toxic metals, arsenic, cadmium and lead were within current air quality guidelines, however boron exceeded the Ontario Ministry of Environment ambient air quality standard in two of the 16 samples. Deposition velocities ranged between 0.22 and 13 cm s⁻¹ with the largest values corresponding to the coarse particle mode. Mean deposition rates ranged between 4.0 μg m⁻² d⁻¹ and 650 mg m⁻² d⁻¹. Depending on the metal, yearly loadings to the watershed ranged from 90 kg to several thousand tonnes. Calcium, aluminum, boron and magnesium had the highest metal loadings to the watershed. Manganese also had relatively high loadings, a reflection of the high traffic density in the area. The relatively high metal deposition rates indicate that metal contribution from atmospheric sources may represent a significant portion of the total metal load to the Burnaby Lake watershed.     

The continuous field measurements of soluble aerosol compositions at the Taipei Aerosol Supersite,Taiwan

The characteristics of ambient aerosols, affected by solar radiation, relative humidity, wind speed, wind direction, and gas–aerosol interaction, changed rapidly at different spatial and temporal scales. In Taipei Basin, dense traffic emissions and sufficient solar radiation for typical summer days favored the formation of secondary aerosols. In winter, the air quality in Taipei Basin was usually affected by the Asian continental outflows due to the long-range transport of pollutants carried by the winter monsoon. The conventional filter-based method needs a long time for collecting aerosols and analyzing compositions, which cannot provide high time-resolution data to investigate aerosol sources, atmospheric transformation processes, and health effects. In this work, the in situ ion chromatograph (IC) system was developed to provide 15-min time-resolution data of nine soluble inorganic species (Cl⁻, NO₂⁻, NO₃⁻, SO₄²⁻, Na⁺, NH₄⁺, K⁺, Mg²⁺ and Ca²⁺). Over 89% of all particles larger than approximately 0.056 μm were collected by the in situ IC system. The in situ IC system is estimated to have a limit of detection lower than 0.3 μg m⁻³ for the various ambient ionic components. Depending on the hourly measurements, the pollutant events with high aerosol concentrations in Taipei Basin were associated with the local traffic emission in rush hour, the accumulation of pollutants in the stagnant atmosphere, the emission of industrial pollutants from the nearby factories, the photochemical secondary aerosol formation, and the long-range transport of pollutants from Asian outflows.     

Secondary aerosol formation through photochemical reactions estimated by using air quality monitoring data in Taipei City from 1994 to 2003

Analyses of diurnal patterns of PM₁₀ in Taipei City have been performed in this study at different daily ozone maximum concentrations (O_3,max) from 1994 to 2003. In order to evaluate secondary aerosol formation at different ozone levels, CO was used as a tracer of primary aerosol, and O_3,max was used as an index of photochemical activity. Results show that when O_3,max exceeds 120 ppb, the highest photochemical formation of secondary aerosol can be found at 15:00 (local time). The produced secondary aerosol is estimated to contribute 30 μg m⁻³ (43%) of PM₁₀ concentration, and about 77% of the estimated secondary PM₁₀ is composed of PM_2.5. The estimated maximum concentration of secondary aerosol occurs 2–3 h later than the maximum ozone concentration. As revealed in an O₃ episode, PM₁₀ and PM_2.5 vary consistently with O₃ at daytime, which suggests that they are mostly secondary aerosols produced from photochemical reactions. Data collected from Taipei aerosol supersite in 2002 indicates that for all O₃ levels, summertime PM_2.5 is composed of 23%, 20%, 9%, and 7% of organic carbon, sulfate, nitrate, and elemental carbon, respectively. Aerosol number and volume size spectra are dominated by submicron particles either from pollution transport or photochemical reactions. Secondary PM₁₀ concentrations show increasing tendencies for the time between 15:00 and 19:00 from 1994–1996 to 2001–2003. This reveals that the abatement of secondary PM₁₀ becomes more important after pronounced primary PM₁₀ reduction in a metropolis.     

Characterization of PM2.5-bound polycyclic aromatic hydrocarbons in Atlanta

Twenty-four hour PM_2.5 samples from a rural site, an urban site, and a suburban site (next to a major highway) in the metropolitan Atlanta area in December 2003 and June 2004 were analyzed for 19 polycyclic aromatic hydrocarbons (PAH). Extraction of the air samples was conducted using an accelerated solvent extraction method followed by isotope dilution gas chromatography/mass spectrometry determination. Distinct seasonal variations were observed in total PAH concentration (i.e. significantly higher concentrations in December than in June). Mean concentrations for total particulate PAHs in December were 3.16, 4.13, and 3.40 ng m^?3 for the urban, suburban and rural sites, respectively, compared with 0.60, 0.74, and 0.24 ng m^?3 in June. Overall, the suburban site, which is impacted by a nearby major highway, had higher PAH concentration than did the urban site. Total PAH concentrations were found to be well correlated with PM_2.5, organic carbon (OC), and elemental carbon (EC) in both months (r² = 0.36–0.78, p < 0.05), although the slopes from the two months were different. PAHs represented on average 0.006% of total PM_2.5 mass and 0.017% of OC in June, compared with 0.033% of total PM_2.5 and 0.14% of OC in December. Total PAH concentrations were also correlated with potassium ion (r² = 0.39, p = 0.014) in December, but not in June, suggesting that in winter biomass burning can potentially be an important source for particulate PAH. Retene was found at a higher median air concentration at the rural site than at the urban and suburban sites—unlike the rest of the PAHs, which were found at lower levels at the rural site. Retene also had a larger seasonal difference and had the weakest correlation with the rest of the PAHs measured, suggesting that retene, in particular, might be associated with biomass burning.     

Ozone response to precursor controls: comparison of data analysis methods with the predictions of photochemical air quality simulation models

Comparisons were made between the predictions of six photochemical air quality simulation models (PAQSMs) and three indicators of ozone response to emission reductions: the ratios of O₃/NO_z and O₃/NO_y and the extent of reaction. The values of the two indicator ratios and the extent of reaction were computed from the model-predicted mixing ratios of ozone and oxidized nitrogen species and were compared to the changes in peak 1 and 8 h ozone mixing ratios predicted by the PAQSMs. The ozone changes were determined from the ozone levels predicted for base-case emission levels and for reduced emissions of volatile organic compounds (VOCs) and oxides of nitrogen (NO_x). For all simulations, the model-predicted responses of peak 1 and 8 h ozone mixing ratios to VOC or NO_x emission reductions were correlated with the base-case extent of reaction and ratios of O₃/NO_z and O₃/NO_y. Peak ozone values increased following NO_x control in 95% (median over all simulations) of the high-ozone (>80 ppbv hourly mixing ratio in the base-case) grid cells having mean afternoon O₃/NO_z ratios less than 5 : 1, O₃/NO_y less than 4 : 1, or extent less than 0.6. Peak ozone levels decreased in response to NO_x reductions in 95% (median over all simulations) of the grid cells having peak hourly ozone mixing ratios greater than 80 ppbv and where mean afternoon O₃/NO_z exceeded 10 : 1, O₃/NO_y was greater than 8 : 1, or extent exceeded 0.8. Ozone responses varied in grid cells where O₃/NO_z was between 5 : 1 and 10 : 1, O₃/NO_y was between 4 : 1 and 8 : 1, or extent was between 0.6 and 0.8. The responses in such grid cells were affected by ozone responses in upwind grid cells and by the changes in ozone levels along the upwind boundaries of the modeling domains.     

C1–C8 volatile organic compounds in the atmosphere of Hong Kong: Overview of atmospheric processing and source apportionment

We present measurements of C₁–C₈ volatile organic compounds (VOCs) at four sites ranging from urban to rural areas in Hong Kong from September 2002 to August 2003. A total of 248 ambient VOC samples were collected. As expected, the urban and sub-urban sites generally gave relatively high VOC levels. In contrast, the average VOC levels were the lowest in the rural area. In general, higher mixing ratios were observed during winter/spring and lower levels during summer/fall because of seasonal variations of meteorological conditions. A variation of the air mass composition from urban to rural sites was observed. High ratios of ethyne/CO (5.6 pptv/ppbv) and propane/ethane (0.50 pptv/pptv) at the rural site suggested that the air masses over the territory were relatively fresh as compared to other remote regions. The principal component analysis (PCA) with absolute principal component scores (APCS) technique was applied to the VOC data in order to identify and quantify pollution sources at different sites. These results indicated that vehicular emissions made a significant contribution to ambient non-methane VOCs (NMVOCs) levels in urban areas (65±36%) and in sub-urban areas (50±28% and 53±41%). Other sources such as petrol evaporation, industrial emissions and solvent usage also played important roles in the VOC emissions. At the rural site, almost half of the measured total NMVOCs were due to combustion sources (vehicular and/or biomass/biofuel burning). Petrol evaporation, solvent usage, industrial and biogenic emissions also contributed to the atmospheric NMVOCs. The source apportionment results revealed a strong impact of anthropogenic VOCs to the atmosphere of Hong Kong in both urban/sub-urban and rural areas.     

The impact of an 8 h ozone air quality standard on ROG and NOx controls in Southern California

The new National Ambient Air Quality Standard for ozone in the US uses 8 h averaging for the concentration. Based on the 1993 ambient data for Southern California, 8 h averaging has a moderate tendency to move the location of the peak ozone concentration east of the location of the peak 1 h ozone concentration. Reducing the area-wide peak 8 h ozone concentration to 80 ppb would require an effective reduction of the area-wide peak 1 h ozone concentration to around 90 ppb. The Urban Airshed Model with improved numerical solvers, meteorological input based on a mesoscale model and an adjusted emissions inventory was used to study the effect of reactive organic gases (ROG) and NO_x controls on daily-maximum and peak 8 h ozone concentrations under the 26–28 August 1987 ozone episodic conditions in Southern California. The NO_x disbenefit remains prominent for the case of 8 h ozone concentration but is somewhat less prominent, especially when areal ozone exposure is considered, than the case for 1 h ozone concentration. The role of two indicators – O₃/NO_y and H₂O₂/HNO₃ – for NO_x- and ROG-sensitivity for 1 and 8 h ozone concentrations were also studied. In general, the indicator trends are consistent with model predictions, but the discriminating power of the indicators is rather limited.     

Comparison of measured and modeled surface ozone concentrations at two different sites in Europe during the solar eclipse on August 11, 1999

The effects of the solar eclipse on 11 August 1999 on surface ozone at two sites, Thessaloniki, Greece (urban site) and Hohenpeissenberg, Germany (elevated rural site) are investigated in this study and compared with model results. The eclipse offered a unique opportunity to test our understanding of tropospheric ozone chemistry and to investigate with a simple photochemical box model the response of surface ozone to changes of solar radiation during a photolytical perturbation such as the solar eclipse. The surface ozone measurements following the eclipse display a decrease of around 10–15 ppbv at the urban station of Eptapyrgio at Thessaloniki while at Hohenpeissenberg, the actual ozone data do not show any clear effect of eclipse on surface ozone. For Thessaloniki, the model results suggest that solely photochemistry can account for a significant amount of the observed surface ozone decrease during the eclipse but transport effects mask part of the photochemical effect of eclipse on surface ozone. For Hohenpeissenberg, the box model predicted an ozone decrease, due to the eclipse, of about 2 ppbv in relative agreement with the magnitude of the observed ozone decrease from the 2 h moving average while at the same time it inhibits the foreseen diurnal ozone increase. However, this modeled ozone decrease during the eclipse is small compared to the diurnal ozone variability due to transport effects, and hence, transport really masks such relative small changes. The different magnitude of the surface ozone decrease between the two sites indicates mainly the role of the NO_x levels. Measured and modeled NO and NO₂ concentrations at Hohenpeissenberg during the eclipse are also compared and indicate that the partitioning of NO and NO₂ in NO_x is influenced clearly from the eclipse. This is not observed at Thessaloniki due to local NO_x sources.     

Polycyclic aromatic hydrocarbons in air on small spatial and temporal scales – I. Levels and variabilities

Polycyclic aromatic hydrocarbons (PAHs) were measured together with inorganic air pollutants at two urban sites and one rural background site in the Banja Luka area, Bosnia and Hercegovina, during 72 h in July 2008 using a high time resolution (5 samples per day) with the aim to study the spatial and temporal variabilities and to explore the significance of averaging effects inherent to 24 h-sampling. Measurement uncertainty was quantified on basis of three independent side-by-side samplers, deployed at one of the sites.PAH abundances in the urban and rural environments differed largely. Levels at the urban sites exceeded the levels at the rural site by >100%. The discrepancy was largely dominated by emission of 3–4 ring PAHs in the city, while 5–6 ring PAHs were more evenly distributed between city sites and the hill site. During the night a higher fraction of the semivolatile PAHs might have been stored in the soil or sorbed to surfaces. PAH patterns were undistinguishable across the three sites. However, concentrations of more particle-associated substances differed significantly between the urban sites than between one of the urban sites and the rural site (3σ uncertainty). Time-averaging (on a 24 h-basis) would have masked the significant inter-site differences of half of the substances which were found at different levels (on a 4 h-basis).     

Trends over a 20-year period from 1987 to 2007 in surface ozone at the atmospheric research station,Mace Head,Ireland

Hourly measurements of baseline ozone at the Mace Head Atmospheric Research Station on the Atlantic Ocean coast of Ireland are observed when unpolluted air masses are advected to the station from across the North Atlantic Ocean. Monthly mean ozone mixing ratios in baseline air masses have risen steadily during the 1980s and 1990s reaching unprecedented levels during the early months of 1999. During the 2000s, baseline ozone mixing ratios have shown evidence of decline and stabilisation. Over the entire 20-year 1987–2007 period, the trend in annual baseline ozone has been +0.31±0.12(2−σ) ppb year⁻¹ and is highly statistically significant. Trends have been highest in the spring months and lowest in the summer months, producing a significant increase in the amplitude of the seasonal cycle. Over the shorter 1995–2007 period, we demonstrate how the growth to peak in 1999 and the subsequent decline have been driven by boreal biomass burning events during 1998/1999 and 2002/2003. The 2000s have been characterised by relatively constant baseline ozone and CH₄ levels and these may be a reasonable guide to future prospects, at least in the short term.     

Temporal variations of atmospheric carbonyls in urban ambient air and street canyons of a Mountainous city in Southwest China

Carbonyl compounds in urban ambient air and street canyons were measured from December 2008 to August 2009 in a mountainous city in southwest China (Guiyang). The formaldehyde yield from the photo-oxidation of isoprene emitted by vegetation was estimated to be in the range of 0.63–3.62 μg m^?3 from May to August, which accounted for 28.8–33.4% of ambient formaldehyde. Based on the calculation of photolysis rates and rates of reaction with the OH radical, it was found that photolysis was the predominant sink for formaldehyde and acetone in both summer and winter. For acetaldehyde, photo-oxidation by OH radicals and photolysis were the major sinks in summer while photo-oxidation by OH radicals was the dominant sink in winter. Wet precipitation was found to be an important removal process for the atmospheric carbonyls. In the urban ambient air, the average concentrations of formaldehyde, acetaldehyde, acetone and all carbonyls were 4.8 ± 2.1, 5.7 ± 3.3, 5.1 ± 2.5, and 25.1 ± 9.2 μg m^?3 (n = 139), respectively. The average concentrations of these species in street canyons were 18.8 ± 6.5, 9.4 ± 3.2, 10.9 ± 2.1, and 64.1 ± 16.3 μg m^?3 (n = 62), respectively. The significantly higher carbonyl levels on weekdays (compared to weekends) highlight the contribution of vehicle emissions to carbonyls in the street canyons.