Source apportionment of visibility‐degrading aerosols in the lower fraser valley,B.C.

Abstract

The Lower Fraser Valley of British Columbia (LFV) is currently experiencing rapid population growth and is perceived to suffer from reduced air quality, specifically, elevated ozone concentrations and impaired visibility. It is necessary to identify the sources of visibility‐degrading aerosols in order to develop strategies to prevent further degradation and to institute measures to improve air quality in this region. Although chemical mass balance analyses have typically been used in such studies, herein, a P‐mode principal component analysis (PCA) is performed upon the ambient aerosol species at one polluted site in the LFV, and source profiles are inferred from the resulting component loadings. The loading coefficients on the five significant components (which account for over 70% of the variance in the fine aerosol speciation dataset) are used to assess the contribution of these sources to paniculate light scattering (b_sp) Automobile emissions and wood burning are found to contribute most to b_sp at this site.     

A Retrospective Analysis of Ozone Formation in the Lower Fraser Valley,British Columbia,Canada. Part I: Dynamical Model Evaluation

We conduct a retrospective study of ozone formation in the Lower Fraser Valley (LFV), using numerical models, observations, and emission inventories in order to understand relationships between reductions in local precursor emissions and episodic ozone concentrations. Because there appears to be little or no impact from precursor emissions upwind of the LFV during ozone episodes and because background concentrations of ozone and its precursors are generally from the North Pacific Ocean and quite low, summertime ozone formation in the LFV is mostly caused by local emissions. The observed change in behaviour of ozone formation must, therefore, arise from reductions in local precursor emissions. We exploit the observed changing precursor emission–ozone concentration relationship to perform a dynamical model evaluation. Complicating the analyses are an observed shift in the population patterns within the valley over the last 25 years and a small but documented change in the tropospheric background concentration of ozone. Ozone formation for four episodes, which capture the observed changes in ozone reduction and the different meteorological types that occur during LFV ozone events, are investigated using the Weather Research and Forecasting (WRF)-Sparse Matrix Operator Kernel Emissions (SMOKE)-Community Multiscale Air Quality (CMAQ) modelling system. In order to provide realistic simulations of past events, the SMOKE emission inventory is adjusted to account for temporal changes in the amount of emissions and locations of emission sources. Model output is compared with continuous observations, data collected from field campaigns, and previous modelling efforts. The WRF-SMOKE-CMAQ modelling framework is able to capture the changes in both the magnitude of the ozone concentrations and its spatial behaviour over the period of study. Many of the simulations show that the highest ozone concentrations occur outside the area sampled by the fixed monitoring network and within the LFV's numerous tributary valleys. Not all modelled episodes achieved the same agreement with observations and some of these discrepancies are likely related to shortcomings in the meteorological modelling. The model consistently overpredicts ozone at a number of stations within the City of Vancouver and underpredicts daytime NO_x concentrations there. Both results are consistent with a deficiency in NO_x emissions. The model shows a changing bias over time which also suggests uncertainties in the emission backcasting.

RÉSUMÉ?[Traduit par la rédaction] Nous menons une étude rétrospective de la formation de l'ozone dans le vallée du bas Fraser (VBF) à l'aide de modèles numériques, d'observations et d'inventaires d’émissions dans le but de comprendre les relations entre les réductions dans les émissions locales de polluants précurseurs et les concentrations épisodiques d'ozone. Parce qu'il semble n'y avoir que peu ou pas d'impact des émissions de polluants précurseurs en amont de la VBF durant les épisodes d'ozone et parce que les concentrations de fond de l'ozone et de ses précurseurs proviennent généralement du Pacifique Nord et sont très faibles, la formation d'ozone en été dans la VBF est principalement attribuable aux émissions locales. Le changement de comportement observé dans la formation d'ozone doit par conséquent résulter de réductions dans les émissions locales de précurseurs. Nous exploitons la relation changeante observée entre les émissions de précurseurs et la concentration de l'ozone pour effectuer une évaluation par modèle dynamique. Un déplacement observé dans les configurations de population à l'intérieur de la vallée au cours des 25 dernières années et un changement, petit mais documenté, dans la concentration troposphérique de fond de l'ozone viennent compliquer les analyses. Nous étudions la formation d'ozone lors de quatre épisodes qui capturent les changements observés dans la réduction d'ozone et les différents types météorologiques qui se produisent pendant les événements d'ozone dans la VBF en nous servant du système de modélisation SMOKE (Sparse Matrix Operator Kernel Emissions) – CMAQ (Community Multiscale Air Quality) du WRF (Weather Research and Forecasting). Afin de fournir des simulations réalistes des événements passés, nous avons ajusté l'inventaire d’émissions SMOKE pour tenir compte des changements au cours du temps dans la quantité d’émissions et dans la position des sources d’émissions. Nous comparons la sortie du modèle avec les observations continues, les données recueillies lors d’études sur le terrain et les efforts de modélisation précédents. Le cadre de modélisation WRF-SMOKE-CMAQ est capable de capturer les changements dans la grandeur des concentrations d'ozone ainsi que dans son comportement spatial durant la période de l’étude. Plusieurs des simulations montrent que les plus fortes concentrations d'ozone se produisent en dehors de la région échantillonnée par le réseau fixe de surveillance et à l'intérieur des nombreuses vallées affluentes de la VBF. Tous les épisodes modélisés n'ont pas exhibé le même accord avec les observations et certaines de ces divergences sont vraisemblablement dues à des lacunes dans la modélisation météorologique. Le modèle surprévoit constamment l'ozone à certaines stations dans la ville de Vancouver et sous-prévoit les concentrations de NO_x le jour à cet endroit. Les deux résultats sont cohérents avec un déficit dans les émissions de NO_x. Le modèle montre un biais qui change avec le temps, ce qui porte à croire à des incertitudes dans les émissions rétrospectives.     

A Retrospective Analysis of Ozone Formation in the Lower Fraser Valley,British Columbia,Canada. Part II: Influence of Emissions Reductions on Ozone Formation

A mechanistic exploration of how ozone formation in the Lower Fraser Valley (LFV) has changed over a 20-year (1985–2005) retrospective period was performed using numerical models, observations, and emissions data from four key episodes selected from the 20-year period. The motivation for this study was the observed differences in trends in summertime episodic ozone concentrations recorded at various monitoring stations within the valley; stations in the western part of the valley have generally shown a noticeable reduction in episodic ozone concentrations whereas stations in the eastern part of the valley have shown little or no improvement in their maximum 8-hour averaged ozone concentrations. Concurrent with these air quality changes, there has been a well-documented reduction in ozone precursor emissions along with an observed shift in the population patterns within the valley over the 20-year period. Ozone formation for four episodes, encompassing the different meteorological regimes that occur during LFV ozone events and spanning the retrospective period, were investigated using the Weather Research and Forecasting (WRF)-Sparse Matrix Operator Kernel Emission (SMOKE)-Community Multiscale Air Quality (CMAQ) modelling system. For each episode, two simulations, intended to isolate the effects of emission changes from meteorological changes, were performed: one with emissions set at the 1985 level and the other with emissions set at the 2005 level. Based on analysis of the model output, observational data, and precursor emission inventories, we find that the Port Moody station in the western LFV remains a volatile organic compound (VOC)-sensitive location; the central part of the LFV around the town of Chilliwack has generally changed from being VOC-limited to being NO_x-limited; the easternmost part of the valley around the town of Hope has been and remains NO_x-limited. Furthermore, based on the observational data and numerical model output, ozone production efficiency as a function of NO has increased noticeably at Chilliwack and likely in the other eastern parts of the valley. This efficiency increase has likely offset some of the benefits resulting from local NO_x emission reductions.

RÉSUMÉ?[Traduit par la rédaction] Nous avons effectué une exploration mécaniste de la façon dont la formation de l'ozone dans la vallée du bas Fraser (VBF) a changé au cours d'une période rétrospective de 20 ans (1985–2005) en nous servant de modèles numériques, d'observations et de données sur les émissions pour quatre épisodes clés choisis dans la période de 20 ans. Ce sont les différences observées entre les tendances dans les concentrations épisodiques d'ozone enregistrées à certaines stations de surveillance dans la vallée qui ont motivé cette étude : les stations dans la partie ouest de la vallée ont généralement affiché une réduction notable des concentrations épisodiques d'ozone alors que les stations dans la partie est de la vallée n'ont montré que peu ou pas d'amélioration dans les valeurs maximales des concentrations moyennes d'ozone sur 8 heures. Concurremment avec ces changements dans la qualité de l'air, il s'est produit une réduction bien documentée dans les émissions de précurseurs de l'ozone en même temps qu'un déplacement observé dans les configurations de population dans la vallée au cours de la période de 20 ans. Nous avons étudié la formation d'ozone au cours de quatre épisodes, englobant les différents régimes météorologiques survenus lors des événements d'ozone dans la VBF et couvrant la période rétrospective, à l'aide du système de modélisation SMOKE (Sparse Matrix Operator Kernel Emissions) – CMAQ (Community Multiscale Air Quality) du WRF (Weather Research and Forecasting). Pour chaque épisode, nous avons effectué deux simulations visant à isoler l'effet des changements dans les émissions de l'effet des changements météorologiques, l'une avec les émissions réglées au niveau de 1985 et l'autre avec les émissions réglées au niveau de 2005. En nous basant sur l'analyse de la sortie du modèle, les données d'observation et les inventaires des émissions de précurseurs, nous trouvons que la stations de Port Moody dans l'ouest de la VBF demeure un endroit sensible aux composés organiques volatiles (COV); la partie centrale de la VBF, autour de la ville de Chilliwack, a généralement changé de « limitée par les COV » à « limitée par les NO_x »; la partie la plus à l'est de la vallée, autour de la ville de Hope, était et est restée « limitée par les NO_x ». De plus, d'après les données d'observation et la sortie du modèle numérique, l'efficacité de la production d'ozone en fonction de NO a notablement augmenté à Chilliwack et vraisemblablement dans les autres parties de l'est de la vallée. Cette augmentation d'efficacité a probablement annulé certains gains provenant des réductions dans les émissions locales de NO_x.     

Source identification for fine aerosols in Mammoth Cave National Park

In this study, positive matrix factorization (PMF) was applied to the chemical composition data of the ambient PM_2.5 collected at the Mammoth Cave National Park, an IMPROVE site in Kentucky. Eight individual carbon fractions, four organic carbons (OCs), pyrolyzed organic carbon (OP) and three elemental carbons (ECs), were provided to the analysis. Nine sources including the well-distinguished gasoline emission and diesel emission were identified. Also, the back trajectories indicated the crustal factor in this study were likely caused by Saharan dust storms in the summer. The apportionment of nine sources was: gasoline emission (6.7%), diesel emission (3.1%), summer secondary sulfate (49.0%), winter secondary sulfate (0.6%), OP-rich secondary sulfate (16.2%), secondary nitrate (2.8%), Intercontinental dust plus soil (4.9%), wood smoke (13.6%), and aged sea salt (3.2%). The results of this study will help regularize the pollution control strategies in rural areas of Kentucky and upper mid-western US while demonstrating the feasibility of applying carbon fractions to the source apportionment of rural upper-Midwestern areas.     

On residual currents in the central strait of Georgia,B.C.: Research note

Abstract

Numerical modelling effort to understand low‐frequency circulation in the Strait of Georgia has been found to underestimate the strength of the circulation by roughly an order of magnitude. At least in part, this model defect may be due to the absence of statistical‐dynamical tendencies that result from eddy interactions (in reality). This defect is generic to ocean numerical models ranging from estuarine to global‐scale applications. A simple change to the formulation of eddy viscosity may help, making models somewhat “less wrong” if not yet “right”.     

Suburban-rural energy balance comparisons in summer for Vancouver,B.C.

Simultaneous energy balance observations at a rural and a suburban site in Vancouver, B.C. during the summer of 1983 are presented. The study is a follow-up to that conducted in 1980. Many of the 1980 results were unexpected and the present study seeks to assess their representativeness. The net radiant, turbulent sensible, and rural soil heat flux densities were measured directly. The suburban heat storage was parameterized and the turbulent latent heat flux densities were resolved as residuals in the energy balances. The 1983 average diurnal energy partitioning for both sites was typical of those quoted in the literature, suggesting that the 1980 results represent an extreme case. Suburban-rural differences showed the suburban area to have a 4% increase in net radiation, a 51% increase in turbulent sensible heat, and a 46% decrease in turbulent latent heat flux density. The values of the average daytime Bowen ratio were 0.46 and 1.28 for the rural and suburban areas, respectively. The sensible heat flux density exhibited relatively large values in the late afternoon and remained directed upward on many summer evenings. Large day-to-day variability in the relative magnitude of the suburban turbulent fluxes may have been due to synoptic influences. In this environment, the turbulent surface and mixed layers are closely coupled because of the low aerodynamic resistance over the rough surface.     

Meteorological and chemical controls on the vertical distribution of Beijing summer coarse and fine aerosols

Vertical profiles of fine and coarse aerosol particles were determined by cascade impactors at the me-teorological tower in Beijing for three days and one night, July 18-23, 1980. Coarse mode aerosols showed a maximum concentration at 47 m when there was an inversion at about 140 m height, and a rather uniform distribution when there was no inversion. This may indicate a two-component origin of coarse particles at the tower site, one being surface dust and the other being tall stack emissions. Fine mode aerosols showed more complex vertical profiles. Median particle size distributions of most metals were bimodal, indicating distinct coarse particle dispersion and fine accumulation mode processes. A chemical thermodynamic cal-culation indicates that fine mode Si can result from the reduction of silica to volatile SiO during coal combustion with limited air supply, a process which should release substantial amounts of carbon monoxide to the atmosphere.     

Suburban-rural energy balance comparisons for Vancouver,B.C.: An extreme case?

Synchronous observations of the energy balances of a suburban and a rural area in the Vancouver region are used to investigate the impact of urbanization on energy exchange. Net radiation and rural soil heat flux density were directly measured, suburban heat storage was parameterized, and the turbulent heat flux densities were evaluated using the Bowen ratio-energy balance method. Most comparisons were conducted during a period of drying following an unusually wet early summer. These conditions produced atypical but very interesting results. With cloudless skies and high radiant input, suburban-rural differences of both net radiation and evapotranspiration were contrary to previous results and intuition. In both cases, suburban values were greater than their rural counterparts. In most respects the rural site behaved as expected, and the explanation for these findings is thought to be related to advective assistance of evapotranspiration from the suburban area. Under lesser radiant forcing the suburban budget acted in greater conformity with past experience and suburban-rural differences were similarly more in agreement with expectation.     

Tide,Wind, and River Forcing of the Surface Currents in the Fraser River Plume

A long-term record of surface currents from a high-frequency radar system, along with near-surface hydrographic transects, moored current meter records, and satellite imagery, are analyzed to determine the relative importance of river discharge, wind, and tides in driving the surface flow in the Fraser River plume. The observations show a great deal of oceanographic and instrumental variability. However, averaged quantities yielded robust results. The effect of river flow, which determines buoyancy and inertia near the river mouth, was found by taking a long-term average. The resulting flow field was dominated by a jet with two asymmetric gyres; the anticyclonic gyre to the north had flow speeds consistent with geostrophy. The mean flow speed near the river mouth was 14.3?cm?s^–1, while the flow further afield was 5?cm?s^–1 or less. Wind stress and surface currents were highly coherent in the subtidal frequency band. Northwesterly winds drive a surface flow to the southeast at speeds of nearly 30?cm?s^–1. Southeasterly winds drive a surface flow to the northwest at speeds reaching 20?cm?s^–1; however, there is more spatial variability in speed and direction relative to the northwesterly wind case. A harmonic analysis was used to extract the tidally driven flows. Ellipse parameters for the major tidal constituents varied considerably in both alignment and aspect ratio over the radar domain, in direct contrast to a barotropic model which predicted rectilinear flow along the Strait of Georgia. This is a result of water filling and draining the shallow mud flats north of the Fraser's main channel. The M₂ velocities at the surface were also weaker than their barotropic counterparts. However, the shallow water constituent MK₃ was enhanced at the surface and nearly as strong as the mean flow, implying that non-linear interactions are important to surface dynamics.     

On the role of the unresolved eddies in a model of the residual currents in the central strait of Georgia,B.C.: Research Note

Abstract

A depth‐independent numerical model of the Juan de Fuca/Strait of Georgia system reproduces the broad structure of the observed depth‐averaged residual circulation in the Central Strait of Georgia but underestimates its magnitude (Marinone and Fyfe, 1992). Here we present some new calculations based on a re‐parameterization of the unresolved eddies in terms of “statistical dynamical tendencies” instead of the previous eddy‐viscosity treatment. With the new parameterization, the simulated time‐mean flow is closer to the observed circulation both in structure and magnitude. While not specifically designed to do so, the new parameterization also leads to a modest improvement in the low‐pass filtered component of the flow. Based on these results, the depth‐averaged residual currents in the region are conjectured to involve a four‐way balance between the hitherto ignored effect of “statistical dynamical tendencies” and conventional tidal, atmospheric and buoyancy forcing.     

Trace elements in the atmospheric aerosols at Delhi,North India

The total suspended particulate (TSP) levels at Delhi (north India) were measured on 116 days between February and October 1980. The observations were stratified according to season and the values of cross-correlation of the TSP and its components were evaluated. High TSP (209 g m^-3) levels were found during the summer period associated with hot and dry weather in the region and low TSP (109 g m^-3) were found during the monsoon period. Most of the TSP mass was associated with natural soil elements, such as Fe, Al, Mn, Ca, and K. Only a fraction of the mass of the TSP was comprised of elements from anthropogenic sources, e.g., Pb, Ni, Cd, Sb, Cu, and Zn. The aerosols at Delhi were potentially basic in nature, unlike those in European countries which are acidic in nature and cause acid rainfall.     

Local foehn effects in the upper Isar Valley, part 1: Observations

Summary An unusually strong nocturnal downvalley wind can be regularly observed in the upper Isar Valley close to Mittenwald (Bavarian Alps) when a high-pressure system is located over Central Europe or when ambient southerly winds are present. Due to the structure of the local topography, this downvalley wind has foehn-like properties in the sense that the breakthrough of the flow into the valley is characterized by a strong increase in temperature and a decrease in relative humidity. Therefore the author called this flow Minifoehn. In fact, wind speeds are low in comparison to deep foehn, but gusts may reach values up to 20ms^–1, even under the influence of high pressure systems with weak atmospheric pressure gradients. To investigate the Minifoehn, surface stations have been installed for collecting temperature, humidity, wind and pressure data. Measurements have shown that the Minifoehn represents the upper part of one of the drainage currents which flows over a mountain ridge into the valley at Mittenwald. Nocturnally cooled air drains from a plateau south of Mittenwald through different valleys which merge again near Mittenwald. It seems that the forcing of the nocturnal currents is dominated by the temperature difference between this plateau and the free atmosphere above Mittenwald at the same level. Strong temperature differences are found during clear winter nights and in case of subsidence inversions. Moreover, the appearance of the Minifoehn in autumn and winter is so frequent that we even may find a climatic effect: the upper Isar Valley is usually free of fog during these seasons and nocturnal temperatures are often considerably higher than in other Bavarian Alpine valleys at comparable altitude.     

Amino acid nitrogen in atmospheric aerosols: Occurrence,sources and photochemical modification

The presence of amino acids in atmospheric precipitation and aerosols has been noted for many years, yet relatively little is known about these or other nitrogen containing organic compounds in the atmosphere. Marine and continental rainwater analyses indicate that atmospheric aerosols, and subsequently atmospheric precipitation, may contain substantial levels of free and combined amino acids. The most likely source of amino N in the remote marine atmosphere appears to be the injection of proteinaceous material through the action of bursting bubbles at the sea-air interface or the long range transport from terrestrial sources. The capacity of these substrates to undergo photooxidation and photodegradation in the atmosphere to simpler species, such as ammonium ions, carboxylic acids, and for the S containing amino acids, oxidized forms of sulfur, has received little attention from atmospheric chemists. The photochemistry of covalently bound amino groups, particularly as found in peptides and amino acids, is discussed here with the purpose of summarizing what is known of their occurrence and their possible importance to atmospheric chemistry.     

Local foehn effects in the Upper Isar Valley,Part 3: Additional investigations

Summary The present paper is the continuation of two recent studies investigating the foehn-like valley wind system around Mittenwald (Bavarian Isar Valley). We deal with the synoptic/mesoscale conditions causing the local foehn (“Minifoehn”), considering field campaigns from both the mesoscale and the climatological point of view. Furthermore, we describe the structure and further features of the local foehn at smaller scales, using both the results of the VERTIKATOR field campaign and numerical simulations. We obtain as a new result that the foehn-caused local warm air pool around Mittenwald induces slight nocturnal upvalley winds between an adjacent valley basin located some 8 km north of Mittenwald and the basin of Mittenwald. Furthermore, a weak northerly flow may also occur at Mittenwald prior to the onset of the Minifoehn. Numerical simulations indicate that the local pressure gradient responsible for this phenomenon is related to a gravity wave forming over the hill range southwest of Mittenwald. Observations within a five-year period indicate that Minifoehn frequently occurs when ambient winds coming from the southern sector are predominant, but, contrary to deep foehn, weather conditions with northerly synoptic-scale flows do not necessarily exclude the development of the local foehn which comes from the southwest. We also present further evidence that in the presence of southerly synoptic-scale winds, orographic gravity waves interact with the drainage flow. Another new result is that strong synoptic-scale westerly winds are able to suppress the occurrence of Minifoehn. In addition, the possible influence of the Inn Valley wind system as well as dynamical differences between the thermally driven up- and downvalley winds are briefly discussed.     

Local foehn effects in the upper Isar Valley, Part 2: numerical simulations

Summary The local wind system in the upper Isar Valley (Bavarian Alps) near Mittenwald has the peculiarity that regularly strong foehn-like nocturnal flows occur, mainly during clear nights in autumn and winter. We will refer to this phenomenon as “Minifoehn”, as its properties are similar to the classical deep foehn in the sense that its breakthrough into the Isar Valley usually brings a striking increase in temperature and a concomitant decrease in relative humidity. Numerical simulations with the MM5 model reveal that this phenomenon is related to a nocturnal drainage flow originating from a plateau south of Mittenwald. This flow is driven by the temperature difference between this plateau (1180 m) and the free atmosphere above Mittenwald (920 m, 15 km north of the plateau) at the same level. The air masses flow through two different valleys that merge again further downstream. The upper part of one of the two drainage currents goes over a small mountain ridge (1180 m) south-west of Mittenwald and then descends into the Isar Valley, leading to an advection of potentially warm air towards Mittenwald. This branch of the drainage current constitutes the Minifoehn. The remaining part of the drainage current flows through a narrow gap towards the Isar Valley and then joins the drainage flow of this valley. As these air masses are significantly cooler than the Minifoehn branch, large horizontal temperature gradients can be found around Mittenwald. The dynamical behaviour of the cold air flow turns out to be qualitatively consistent with shallow-water theory only in the absence of a forcing by large-scale winds. Otherwise, gravity-wave induced pressure perturbations interact with the drainage flow and modify the low-level flow field. The simulations show that the gravity waves are excited by the mountain range that separates the two valleys mentioned above. Moreover, the simulations indicate that the structure of this nocturnal wind system is not very sensitive to the direction of synoptic-scale winds as long as they come from the southern sector. On the other hand, ambient northerly winds are able to prevent the drainage flow and therefore the local foehn effects in the Isar Valley provided that synoptic winds are strong enough. The results of the MM5 simulations are in good agreement with the measurements and observations described in part 1 of this study.