Multivariate directional analysis of precipitation content in the Quebec region

Abstract

International audienceThe problem of acidic precipitation has become an important environmental concern ; related damages can affect atmospheric, terrestrial and aquatic components of our ecosystem. This paper focuses on wind-driven atmospheric transport of contaminants, notably nitrates and sulphates. Conventional wisdom in eastern Canada holds that winds originating from the east bring precipitation less loaded with acidic components than winds originating from the southwest; the latter carry contaminants generated by industries located in the American Midwest. An experimental confirmation of this phenomenon is presented here. SPERBER (1987) showed that hourly series of precipitation content and wind direction, measured at a reception site, are adequate to represent the lagrangian history of precipitating systems (New York City region). Following this result, we suppose that the north-eastern continental atmospheric system is homogeneous enough so that winds measured at tour reception site (Québec City region) are representative of the whole system. Thus, our experiment is performed in eulerian coordinates. The data bank used in the statistical analyses contains 10 time series: the weekly concentrations of 9 compounds found in the precipitation (H, Ca, Cl, K, Mg, Na, NO3, NH4 and SO4) gathered at the local Montmorency site and a series of weekly prevailing winds measured at the nearby Quebec City airport. The time series contain 312 observations covering a full six years period (December 1981 to December 1987). As the original data bank of concentrations is episodic, i.e. an observation is available for each day with a significant precipitation event, volumes and loadings are used to derive the average weekly values of concentrations. In contrast, hourly series of direction (projected in 36 directions) and velocity of the prevailing winds are used to build, via a vectorial addition, a weekly series of wind directions projected on a 12 points wind rose where directions correspond to the wind origin and not its destination. Classic statistical methods are used to deal with this data bank. Principal component analysis studies relationships between series of concentrations in the precipitations, while correspondence analysis highlights the relationships between the series of precipitation content and the series of wind direction. The final statistical method, analysis of variance, is used to test the significance of relationships highlighted by the correspondence analysis. The principal component analysis showed that all variables were positively correlated with the first component, which reflects the fact that a highly loaded precipitation event will show high concentrations for each of the nine variables. The second component discriminates two groups of variables: one includes NO3, SO4 and NH4, the other Ca, Mg, and Cl. The acidity variable, H, is nearer to the acid ion group (NO3, SO4, NH4) than the other ion group (Ca, Mg and Cl). The correspondence analysis shows that high concentrations of acidic compounds (NO3, SO4 and NH4) are highly related to winds from the W and WSW directions; conversely lower concentrations of the same compounds are associated with winds from the E and ENE directions. The elements Ca, Mg and Cl, reputed to be an oceanic origin, show high functional relationships between high concentrations and E, ENE winds and also between low concentrations and W, WSW winds. These results support the popular belief initially presented that acid precipitation is largely associated with winds from the southwest. The analyses of variance show that precipitation concentrations (NO3 and SO4) are significantly different according to the wind directions. Nitrate and sulphate concentrations associated with winds originating from the west are respectively 0,33 mg.l-1 and 1,73mg.l-1 compared to 0,24 mg.l-1 and 1,48 mg.l-1 for winds originating from the east. The analyses of variance also indicate a significant seasonal effect, where mean monthly concentrations in nitrates are highest for winter months and early spring, whereas sulphate concentrations are highest for the summer months