Index Geophysics

Items

• Long-term trend and variability of atmospheric PM10 concentration in the Po Valley

  The limits to atmospheric pollutant concentration set by the European Commission provide a challenging target for the municipalities in the Po Valley, because of the characteristic climatic conditions and high population density of this region. In order to assess climatology and trends in the concentration of atmospheric particles in the Po Valley, a data set of PM₁₀ data from 41 sites across the Po Valley have been analysed, including both traffic and background sites (either urban, suburban or rural). Of these 41 sites, 18 with 10 yr or longer record have been analysed for long-term trend in deseasonalized monthly means, in annual quantiles and in monthly frequency distribution. A widespread significant decreasing trend has been observed at most sites, up to a few percent per year, by a generalized least squares and Theil–Sen method. All 41 sites have been tested for significant weekly periodicity by Kruskal–Wallis test for mean anomalies and by Wilcoxon test for weekend effect magnitude. A significant weekly periodicity has been observed for most PM₁₀ series, particularly in summer and ascribed mainly to anthropic particulate emissions. A cluster analysis has been applied in order to highlight stations sharing similar pollution conditions over the reference period. Five clusters have been found, two encompassing the metropolitan areas of Turin and Milan and their respective nearby sites and the other three clusters gathering northeast, northwest and central Po Valley sites respectively. Finally, the observed trends in atmospheric PM₁₀ have been compared to trends in provincial emissions of particulates and PM precursors, and analysed along with data on vehicular fleet age, composition and fuel sales. A significant basin-wide drop in emissions occurred for gaseous pollutants, contrarily to emissions of PM₁₀ and PM_2.5, whose drop was low and restricted to a few provinces. It is not clear whether the decrease for only gaseous emissions is sufficient to explain the observed drop in atmospheric PM₁₀, or if the low drop in particulate emissions is indeed due to the uncertainty in the emission inventory data for this species.

• Temporal and spatial characteristics of ozone depletion events from measurements in the Arctic

  Following polar sunrise in the Arctic springtime, tropospheric ozone episodically decreases rapidly to near-zero levels during ozone depletion events (ODEs). Many uncertainties remain in our understanding of ODE characteristics, including the temporal and spatial scales, as well as environmental drivers. Measurements of ozone, bromine monoxide (BrO), and meteorology were obtained during several deployments of autonomous, ice-tethered buoys (O-Buoys) from both coastal sites and over the Arctic Ocean; these data were used to characterize observed ODEs. Detected decreases in surface ozone levels during the onset of ODEs corresponded to a median estimated apparent ozone depletion timescale (based on both chemistry and the advection of O₃-depleted air) of 11 h. If assumed to be dominated by chemical mechanisms, these timescales would correspond to larger-than-observed BrO mole fractions based on known chemistry and assumed other radical levels. Using backward air mass trajectories and an assumption that transport mechanisms dominate observations, the spatial scales for ODEs (defined by time periods in which ozone levels ≤15 nmol mol⁻¹) were estimated to be 877 km (median), while areas estimated to represent major ozone depletions (<10 nmol mol⁻¹) had dimensions of 282 km (median). These observations point to a heterogeneous boundary layer with localized regions of active, ozone-destroying halogen chemistry, interspersed among larger regions of previously depleted air that retain reduced ozone levels through hindered atmospheric mixing. Based on the estimated size distribution, Monte Carlo simulations showed it was statistically possible that all ODEs observed could have originated upwind, followed by transport to the measurement site. Local wind speed averages were low during most ODEs (median of ~3.6 m s⁻¹), and there was no apparent dependence on local temperature.

• Atmospheric measurement of point source fossil CO2 emissions

  We use the Kapuni Gas Treatment Plant to examine methodologies for atmospheric monitoring of point source fossil fuel CO₂ (CO₂ff) emissions. The Kapuni plant, located in rural New Zealand, removes CO₂ from locally extracted natural gas and vents that CO₂ to the atmosphere, at a rate of ~0.1 Tg carbon per year. The plant is located in a rural dairy farming area, with no other significant CO₂ff sources nearby, but large, diurnally varying, biospheric CO₂ fluxes from the surrounding highly productive agricultural grassland. We made flask measurements of CO₂ and ¹⁴CO₂ (from which we derive the CO₂ff component) and in situ measurements of CO₂ downwind of the Kapuni plant, using a Helikite to sample transects across the emission plume from the surface up to 100 m above ground level. We also determined the surface CO₂ff content averaged over several weeks from the ¹⁴C content of grass samples collected from the surrounding area. We use the WindTrax plume dispersion model to compare the atmospheric observations with the emissions reported by the Kapuni plant, and to determine how well atmospheric measurements can constrain the emissions. The model has difficulty accurately capturing the fluctuations and short-term variability in the Helikite samples, but does quite well in representing the observed CO₂ff in 15 min averaged surface flask samples and in ~ one week integrated CO₂ff averages from grass samples. In this pilot study, we found that using grass samples, the modeled and observed CO₂ff emissions averaged over one week agreed to within 30%. The results imply that greater verification accuracy may be achieved by including more detailed meteorological observations and refining ¹⁴C sampling strategies.

• NOx cycle and the tropospheric ozone isotope anomaly: an experimental investigation

  The oxygen isotope composition of nitrogen oxides (NO_x) in the atmosphere is a useful tool for understanding the oxidation of NO_x into nitric acid / nitrate in the atmosphere. A set of experiments was conducted to examine change in isotopic composition of NO_x due to NO_x–O₂–O₃ photochemical cycling. At low NO_x / O₂ mixing ratios, NO_x became progressively and nearly equally enriched in ¹⁷O and ¹⁸O over time until it reached a steady state with Δ¹⁷O values of 39.3 ± 1.9&permil; and δ¹⁸O values of 84.2 ± 4&permil;, relative to the isotopic composition of the initial O₂ gas. As the mixing ratios were increased, the isotopic enrichments were suppressed by isotopic exchange between O atoms, O₂, and NO_x. A kinetic model was developed to simulate the observed data and it showed that the isotope effects occurring during O₃ formation play a dominant role in controlling NO_x isotopes and, in addition, secondary kinetic isotope effects or isotope exchange reactions are also important during NO_x cycling. The data and model were consistent with previous studies which showed that the NO + O₃ reactions occur mainly via the transfer of the terminal atoms of O₃. The model predicts that under tropospheric concentrations of NO_x and O₃, the timescale of NO_x–O₃ isotopic equilibrium ranges from hours (for ppbv NO_x / O₂ mixing ratios) to days (for pptv mixing ratios) and yields steady state Δ¹⁷O and δ¹⁸O values of 45&permil; and 117&permil; respectively (relative to Vienna Standard Mean Ocean Water (VSMOW)) in both cases. Under atmospheric conditions when O₃ has high concentrations, the equilibrium between NO_x and O₃ should occur rapidly (h) but this equilibrium cannot be reached during polar winters and/or nights if the NO_x conversion to HNO₃ is faster. The experimentally derived rate coefficients can be used to model the major NO_x–O₃ isotopologue reactions at various pressures and in isotope modeling of tropospheric nitrate.

• Spatial–temporal variations, sources, and transport of airborne inhalable metals (PM10) in urban and rural areas of northern China

  Atmospheric particle pollution is a serious environmental issue in China, especially the northern regions. Ambient air loadings (ng m^&minus;3), pollution sources and apportionment, and transport pathways of trace (Cd, Co, Cu, Ni, Pb, V, and Zn) and major (Al, Ca, Fe, and Mg) metals associated with inhalable particulate matters (PM₁₀ aerosols) were characterized in urban, rural village, and rural field areas of seven cities (from inland in the west to the coast in the east: Wuwei, Yinchuan, Taiyuan, Beijing, Dezhou, Yantai, and Dalian) across northern China by taking one 72 h sample each site within a month for a whole year (April 2010 to March 2011). Ambient PM₁₀ pollution in northern China is especially significant in the cold season (October–March) due to the combustion of coal for heating and dust storms in the winter and spring. Owing to variations in emission intensity and meteorological conditions, there is a trend of decrease in PM₁₀ levels in cities from west to east. Both air PM₁₀ and the associated metal loadings for urban and rural areas were comparable, showing that the current pattern of regional pollution in China differs from the decreasing urban–rural-background transect that is usual in other parts of the world. The average metal levels are Zn (276 ng m^&minus;3) ≫ Pb (93.7) ≫ Cu (54.9) ≫ Ni (9.37) > V (8.34) ≫ Cd (2.84) > Co (1.76). Judging from concentrations (mg kg^&minus;1), enrichment factors (EFs), a multivariate statistical analysis (principal component analysis, PCA), and a receptor model (absolute principal component scores-multiple linear regression analysis, APCS-MLR), the airborne trace metals (Zn, Pb, Cu, and Cd) in northern China were mainly anthropogenic, and mostly attributable to coal combustion and vehicle emissions with additional industrial sources. However, the Co was mostly of crustal origin, and the V and Ni were mainly from soil/dust in the western region and mostly from the petrochemical industry/oil combustion in the east. The accumulation of typical "urban metals" (Pb, Zn, Cd, and Cu) showed a trend of increase from west to east, indicating their higher anthropogenic contribution in eastern cities. The winter northwestern monsoon and westerly jet stream were the dominant forces in the long-range transport of airborne PM metals in northern China, with potentially global implications.

• Nighttime observation and chemistry of HOx in the Pearl River Delta and Beijing in summer 2006

  Nighttime HO_x chemistry was investigated in two ground-based field campaigns (PRIDE-PRD2006 and CAREBEIJING2006) in summer 2006 in China by comparison of measured and modeled concentration data of OH and HO₂. The measurement sites were located in a rural environment in the Pearl River Delta (PRD) under urban influence and in a suburban area close to Beijing, respectively. In both locations, significant nighttime concentrations of radicals were observed under conditions with high total OH reactivities of about 40–50 s⁻¹ in PRD and 25 s⁻¹ near Beijing. For OH, the nocturnal concentrations were within the range of (0.5–3) × 10⁶ cm⁻³, implying a significant nighttime oxidation rate of pollutants on the order of several ppb per hour. The measured nighttime concentration of HO₂ was about (0.2–5) × 10⁸ cm⁻³, containing a significant, model-estimated contribution from RO₂ as an interference. A chemical box model based on an established chemical mechanism is capable of reproducing the measured nighttime values of the measured peroxy radicals and $k_{ ext{OH}}$, but underestimates in both field campaigns the observed OH by about 1 order of magnitude. Sensitivity studies with the box model demonstrate that the OH discrepancy between measured and modeled nighttime OH can be resolved, if an additional RO_x production process (about 1 ppb h⁻¹) and additional recycling (RO₂ → HO₂ → OH) with an efficiency equivalent to 1 ppb NO is assumed. The additional recycling mechanism was also needed to reproduce the OH observations at the same locations during daytime for conditions with NO mixing ratios below 1 ppb. This could be an indication that the same missing process operates at day and night. In principle, the required primary RO_x source can be explained by ozonolysis of terpenoids, which react faster with ozone than with OH in the nighttime atmosphere. However, the amount of these highly reactive biogenic volatile organic compounds (VOCs) would require a strong local source, for which there is no direct evidence. A more likely explanation for an additional RO_x source is the vertical downward transport of radical reservoir species in the stable nocturnal boundary layer. Using a simplified one-dimensional two-box model, it can be shown that ground-based NO emissions could generate a large vertical gradient causing a downward flux of peroxy acetic nitrate (PAN) and peroxymethacryloyl nitrate (MPAN). The downward transport and the following thermal decomposition of these compounds can produce up to 0.3 ppb h⁻¹ radicals in the atmospheric layer near the ground. Although this rate is not sufficient to explain the complete OH discrepancy, it indicates the potentially important role of vertical transport in the lower nighttime atmosphere.

• Primary to secondary organic aerosol: evolution of organic emissions from mobile combustion sources

  A series of smog chamber experiments were conducted to investigate the transformation of primary organic aerosol (POA) and formation of secondary organic aerosol (SOA) during the photooxidation of dilute exhaust from a fleet of gasoline and diesel motor vehicles and two gas-turbine engines. In experiments where POA was present in the chamber at the onset of photooxidation, positive matrix factorization (PMF) was used to determine separate POA and SOA factors from aerosol mass spectrometer data. A 2-factor solution, with one POA factor and one SOA factor, was sufficient to describe the organic aerosol for gasoline vehicles, diesel vehicles, and one of the gas-turbine engines. Experiments with the second gas-turbine engine required a 3-factor PMF solution with a POA factor and two SOA factors. Results from the PMF analysis were compared to the residual method for determining SOA and POA mass concentrations. The residual method apportioned a larger fraction of the organic aerosol mass as POA because it assumes that all mass at <i>m / z</i> 57 is associated with POA. The POA mass spectrum for the gasoline and diesel vehicles exhibited high abundances of the C_<i>n</i>H_2<i>n</i>+1 series of ions (<i>m / z</i> 43, 57, etc.) and was similar to the mass spectra of the hydrocarbon-like organic aerosol factor determined from ambient data sets with one exception, a diesel vehicle equipped with a diesel oxidation catalyst. POA mass spectra for the gas-turbine engines are enriched in the C_<i>n</i>H_{2<i>n</i>&minus;1} series of ions (<i>m / z</i> 41, 55, etc.), consistent with the composition of the lubricating oil used in these engines. The SOA formed from the three sources exhibits high abundances of <i>m / z</i> 44 and 43, indicative of mild oxidation. The SOA mass spectra are consistent with less-oxidized ambient SV-OOA (semivolatile oxygenated organic aerosols) and fall within the triangular region of <i>f</i>₄₄ versus <i>f</i>₄₃ defined by ambient measurements. However there is poor absolute agreement between the experimentally derived SOA mass spectra and ambient OOA factors, though this poor agreement should be expected based on the variability of ambient OOA factors. Van Krevelen analysis of the POA and SOA factors for gasoline and diesel experiments reveal slopes of −0.50 and −0.40, respectively. This suggests that the oxidation chemistry in these experiments is a combination of carboxylic acid and alcohol/peroxide formation, consistent with ambient oxidation chemistry.

• High-resolution large-eddy simulations of stably stratified flows: application to subkilometer-scale turbulence in the upper troposphere–lower stratosphere

  Large-eddy simulations of stably stratified flows are carried out and analyzed using the mesoscale atmospheric model Méso-NH for applications to kilometer- and subkilometer-scale turbulence in the in the upper troposphere–lower stratosphere. Different levels of turbulence are generated using a large-scale stochastic forcing technique that was especially devised to treat atmospheric stratified flows. The study focuses on the analysis of turbulence statistics, including mean quantities and energy spectra, as well as on a detailed description of flow topology. The impact of resolution is also discussed by decreasing the grid spacing to 2 m and increasing the number of grid points to 8 × 10⁹. Because of atmospheric stratification, turbulence is substantially anisotropic, and large elongated structures form in the horizontal directions, in accordance with theoretical analysis and spectral, direct numerical simulations of stably stratified flows. It is also found that the inertial range of horizontal kinetic energy spectrum, generally observed at scales larger than a few kilometers, is prolonged into the subkilometric range, down to the Ozmidov scales that obey isotropic Kolmogorov turbulence. This study shows the capability of atmospheric models like Méso-NH to represent the turbulence at subkilometer scales.

• Spatial variations and development of land use regression models of levoglucosan in four European study areas

  Relatively little is known about long term effects of wood smoke on population health. A wood burning marker – levoglucosan – was measured using a highly standardized sampling and measurement method in four study areas across Europe (Oslo, the Netherlands, Munich/Augsburg, Catalonia) to assess within and between study area spatial variation. Levoglucosan was analyzed in addition to other components: PM_2.5, PM_2.5 absorbance, PM₁₀, polycyclic aromatic hydrocarbons (PAH), nitrogen oxides (NO_x), elemental and organic carbon (EC / OC), hopanes, steranes and elemental composition. Measurements were conducted at street, urban and regional background sites. Three two-week samples were taken per site and the annual average concentrations of pollutants were calculated using continuous measurements at one background site as a eference. Land use regression (LUR) models were developed to explain the spatial variation of levoglucosan using standardized procedures. <br><br> Much larger within than between study area contrast in levoglucosan concentration was found. Spatial variation patterns differed substantially from other measured pollutants including PM_2.5, NO_x and EC. Levoglucosan had the highest spatial correlation with ΣPAH (<I>r</i> = 0.65) and the lowest with traffic markers – NO_x, Σhopanes/steranes (<i>r</i> = &minus;0.22). The correlation of levoglucosan with potassium (K), which is also used as a wood burning marker, was moderate to low (median <i>r</i> = 0.33). Levoglucosan concentrations in the cold (heating) period were between 3 and 20 times higher compared to the warm period. The contribution of wood-smoke calculated based on levoglucosan measurements and previous European emission data to OC and PM_2.5 mass were 13 to 28% and 3 to 9% respectively in the full year. Larger contributions were calculated for the cold period. <br><br> The median model <i>R</i>² of the LUR models was 60%. In Catalonia the model <i>R</i>² was the highest (71%). The LUR models included population and natural land related variables but no traffic associated variables. <br><br> In conclusion, substantial spatial variability was found in levoglucosan concentrations particularly within study areas. Wood smoke contributed substantially to especially wintertime PM_2.5 OC and mass. The low to moderate correlation with PM_2.5 mass and traffic markers offers the potential to assess health effects of wood smoke separate from traffic-related air pollution¹.<br><br> <br> ¹ Abbreviations: ESCAPE, European Study of Cohort for Air Pollution Effects; TRANSPHORM, Transport related Air Pollution and Health impacts – Integrated Methodologies for Assessing Particulate Matter; EC/OC, elemental/organic carbon; PAH, polycyclic aromatic hydrocarbons; B[a]P, benzo[a]pyrene, GIS, Geographic Information Systems; LUR, Land Use Regression; NO_x, nitrogen oxides; NO₂, nitrogen dioxide; PM_2.5, mass concentration of particles less than 2.5 μm in size; PM_2.5 absorbance, measurement of the blackness of PM_2.5 filters, this is a proxy for elemental carbon, which is the dominant light absorbing substance; PM₁₀, mass concentration of particles less than 10 μm in size; RB, regional background; S, Street; EPA, United States Environmental Protection Agency; LUR, Land Use Regression; RMSE, Root Mean Squared Error.

• Chemical composition and sources of coastal marine aerosol particles during the 2008 VOCALS-REx campaign

  The chemical composition of aerosol particles (<i>D</i>_p &le; 1.5 μm) was measured over the southeast Pacific Ocean during the VAMOS (Variability of the American Monsoon Systems) Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-Rex) between 16 October and 15 November 2008 using the US Department of Energy (DOE) G-1 aircraft. The objective of these flights was to gain an understanding of the sources and evolution of these aerosols, and of how they interact with the marine stratus cloud layer that prevails in this region of the globe. Our measurements showed that the marine boundary layer (MBL) aerosol mass was dominated by non-sea-salt SO₄^2&minus;, followed by Na⁺, Cl^&minus;, Org (total organics), NH₄⁺, and NO₃^&minus;, in decreasing order of importance; CH₃SO₃^&minus; (MSA), Ca²⁺, and K⁺ rarely exceeded their limits of detection. Aerosols were strongly acidic with a NH₄⁺ to SO₄^2&minus; equivalents ratio typically < 0.3. Sea-salt aerosol (SSA) particles, represented by NaCl, exhibited Cl^&minus; deficits caused by both HNO₃ and H₂SO₄, but for the most part were externally mixed with particles, mainly SO₄^2&minus;. SSA contributed only a small fraction of the total accumulation mode particle number concentration. It was inferred that all aerosol species (except SSA) were of predominantly continental origin because of their strong land-to-sea concentration gradient. Comparison of relative changes in median values suggests that (1) an oceanic source of NH₃ is present between 72° W and 76° W, (2) additional organic aerosols from biomass burns or biogenic precursors were emitted from coastal regions south of 31° S, with possible cloud processing, and (3) free tropospheric (FT) contributions to MBL gas and aerosol concentrations were negligible. The very low levels of CH₃SO₃^&minus; observed as well as the correlation between SO₄^2&minus; and NO₃^&minus; (which is thought primarily anthropogenic) suggest a limited contribution of DMS to SO₄^2&minus; aerosol production during VOCALS.