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The Lower Fraser Valley (LFV, Figure 1) spanning the Canada/USA border at 49° N is a roughly triangular valley with its westward end being the shoreline of the Strait of Georgia. The LFV contains the city of Vancouver, and its satellite communities with a total population of two million persons, mostly in the Canadian part of the valley. In the last two decades, the LFV has experienced several air pollution episodes. Primary as well as secondary pollutant concentrations are unusually high for such relatively small population. Several experimental campaigns (Pacific93, Pacific2001, Steyn et al. 1997), have been organized to investigate the physical mechanisms responsible for such elevated air pollutant levels. It has been hypothesized that the particularly low boundary layer height in the valley and the recirculation processes, induced by mesoscale circulations, play an important role.

Our objective is to estimate the excess mortalities resulting from ambient present- day concentrations of O3 and PM2.5 in the United States. We use the U.S. Environmental Protection Agency’s Community Multi-scale Air Quality (CMAQ) version 4.3 model to simulate present levels of air pollution. We then remove anthropogenic emissions and repeat the simulations. Using epidemiological doseresponse functions we use the increase in pollution levels between the two simulations to estimate total mortalities resulting from exposure to O3 and PM2.5. We estimate that in 1996 107,000 and 162,000 additional mortalities occurred in the United States due to increased exposure to ambient O3 and PM2.5 concentrations respectively. The total mortalities incurred, 269,000 deaths, is more than ten times larger than the 23,000 mortalities that are predicted to be averted in 2010 from implementation of the 1990 CAAA. Our analysis indicates that tens of thousands of lives could be saved by substantial further improvements in U.S. air quality.

Acid deposition has been an environmental and policy concern in North America since the 1970s. To date there have been two national initiatives in Canada to address the acid deposition problem through SO2 emission reductions: (a) the 1985 between the Canadian federal government and the seven eastern provinces (e.g., Environment Canada, 1994); and (b) the , signed in 1998 between the federal government and all 12 Canadian provinces and territories (e.g., Federal/Provincial/Territorial Ministers of Environment and Energy, 1998; Environment Canada, 2002). In addition, in 1991 the Canadian and U.S. federal governments signed the , Annex 1 of which committed each country to specific SO2 and NOx emission reductions (Government of Canada and Government of the United States of America, 1991, 2002).

The objective of this study is to examine the source-receptor relationships and health impacts of air pollution resulting from emissions from individual states in the continental United States. Air pollution is a widespread problem of spatially and temporally varying magnitude. Over 115 million U.S. individuals are exposed to air pollution levels in excess of one or more health-based ambient standards in 1996 (EPA, 1996). Policy decisions can benefit from a quantification of source-receptor relationships and resulting health and environmental damages. The damage or environmental assessment processes, however, often suffer from a lack of scientific knowledge and tools.

During the summer 2003 a POVA Intensive Observation Period (IOP) aimed at determining the sources of airborne pollutants and monitoring their concentrations in two French Alpine valleys: the Chamonix and the Maurienne valleys (see figure 1 for geographic location). The Pollution of Alpine Valleys (POVA) program was launched in 2000 after the traffic interruption under the Mont-Blanc that followed the tragic accident in the tunnel. The Mont-Blanc tunnel was reopened at the end of 2002 and caused the high duty vehicle traffic (about 1100 trucks per day) to be back in the Chamonix valley. The summer 2003 IOP took place from 5 to 12 July in the Chamonix valley. A high ozone event occurred from 5 to 14 July at regional scales and was well characterised by measurements at rural monitoring sites. To better understand the particular atmospheric circulation, and to study the chemical reactions of airborne pollutants within the valleys, mesoscale modelling is applied. For meteorological calculation, the fifth generation PSU/NCAR Mesoscale Model (MM5) was used at scales ranging from 27 to 1 km. MM5 was coupled with the Chemistry Transport Model (CTM) CHIMERE at regional scales and with the CTM TAPOM at a one-kilometre resolution. Simulations were performed for the period 5-12 July 2003 with different emission sets aiming at studying the impact of the international road traffic in the valley on airborne pollutant concentrations.

Effective formulation of control strategies requires knowledge of the responsiveness of ozone to emissions of its two main precursors, nitrogen oxides (NOx) and volatile organic compounds (VOC). While responsiveness depends nonlinearly on an array of spatially and temporally variable factors, a large body of research has sought to classify ozone formation into categories of chemical regime (Sillman, 1999). In NOx-limited regimes, ozone increases with increasing NOx and exhibits only slight sensitivity to VOC; in VOC-limited (or NOx-saturated) regimes, ozone increases with VOC and exhibits slight or even negative sensitivity to NOx. Transitional conditions of dual sensitivity also occur. Classification of ozone production regime helps determine whether NOx or VOC emissions should be targeted more aggressively in strategies to reduce ozone.

Long-term simulations of atmospheric pollutants containing aerosols as an essential component may conveniently be used to support the assessment of the role of suspended particulate matter for the atmospheric environment on various temporal and spatial scales. They can help to bridge gaps of our knowledge and thus to broaden the range of applications of observations in various respects. This holds, among others, for the distribution of aerosols and its variability in space and time, their generation, emission and resulting source-receptor relationships, their spectral distribution and possible composition. For instance, the finest particle fraction cannot yet efficiently be measured by existing observational networks. Regarding the possibility that this component is mainly responsible for adverse impacts of aerosols on human health, simulations of monitored components – usually total suspended particulates (TSP), PM10 or PM2.5 – may be employed to establish useful quantitative relationships between these parameters and the finer particle mode of the aerosol spectrum and apply it to health impact studies.

A general understanding of the interface between meteorological and dispersion models is that the latter ones should assimilate the information coming from meteorological model with minor internal processing, mainly oriented at computation of some surrogate indices like stability parameters or mixing layer height. However, the situation is rarely that simple. The dispersion model (DM) as a client of the numerical weather prediction (NWP) meteorological model (MM) requires significantly higher spatial resolution that that of NWP and, possibly, it should also vary in space and time because concentrations of atmospheric pollutants are usually much more irregular than meteorological variables. This brings about the problems of multi-scale nested modelling, boundary conditions, agreement between the scales and several other issues sometimes referred as a downscaling problem and its influence on accuracy of the DM at various scales.

The Western Mediterranean Basin (WMB) is surrounded by high coastal mountains and is influenced by the Mediterranean sea. In summer it becomes isolated from the traveling lows and their frontal systems. The meteorology and the origin of the air masses arriving at the Iberian Peninsula are highly influenced by the Azores high pressure system which is located over the Atlantic Ocean and that intensifies during the warm season inducing very weak pressure gradient conditions all over the region. A number of studies have shown that during this period, layering and accumulation of pollutants such as ozone and aerosols were taking place along the northeastern Iberian Peninsula (NEIP) (Millán ., 1992, 1997).

The chemistry of ozone (O3) and its two main precursors, nitrogen oxides (NOx) and volatile organic compounds (VOCs) represents one of the major fields of uncertainty in atmospheric chemistry. The ozone weekend effect refers to a tendency in some areas for ozone concentrations to be higher on weekends compared to weekdays, despite emissions of VOCs and NOx that are typically lower on weekends due to different anthropogenic activity. This phenomenon was first reported in the United States in the 1970s (Cleveland , 1974; Lebron, 1975) and has been since reported mainly in the U.S. and Europe. Higher weekend ozone tends to be found in urban centers, while lower weekend ozone is found in downwind areas. Altshuler (1995) have suggested that the weekend effect is related to whether ozone formation is VOC- or NOx-sensitive, with higher weekend ozone occurring is VOC-sensitive areas. Despite this, there is a high uncertainty in the causes of the weekend effect, and six hypothesis have been set (CARB, 2003): (1) NOx reduction; (2) NOx timing; (3) carryover near the ground; (4) carryover aloft; (5) increased weekend emissions; (6) increased sunlight caused by decreased soot emissions.