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Abstract
Abstract. The Border Air Quality and Meteorology Study (BAQS-Met) was an intensive, collaborative field campaign during the summer of 2007 that investigated the effects of transboundary pollution, local pollution, and local meteorology on air quality in southwestern Ontario. This analysis focuses on the measurements of the inorganic constituents of particulate matter with diameter of less than 1 μm (PM1), with a specific emphasis on nitrate. We evaluate the ability of AURAMS, Environment Canada's chemical transport model, to represent regional air pollution in SW Ontario by comparing modelled aerosol inorganic chemical composition with measurements from Aerosol Mass Spectrometers (AMS) onboard the National Research Council (NRC) of Canada Twin Otter aircraft and at a ground site in Harrow, ON. The agreement between modelled and measured pNO3− at the ground site (observed mean (Mobs) = 0.50 μg m−3; modelled mean (Mmod) = 0.58 μg m−3; root mean square error (RSME) = 1.27 μg m−3) was better than aloft (Mobs = 0.32 μg m−3; Mmod = 0.09 μg m−3; RSME = 0.48 μg m−3). Possible reasons for discrepancies include errors in (i) emission inventories, (ii) atmospheric chemistry, (iii) predicted meteorological parameters, or (iv) gas/particle thermodynamics in the model framework. Using the inorganic thermodynamics model, ISORROPIA, in an offline mode, we find that the assumption of thermodynamic equilibrium is consistent with observations of gas and particle composition at Harrow. We develop a framework to assess the sensitivity of PM1 nitrate to meteorological and chemical parameters and find that errors in both the predictions of relative humidity and free ammonia (FA ≡ NH3(g) + pNH4+ − 2 · pSO42-) are responsible for the poor agreement between modelled and measured values.
Highlights
Atmospheric PM1 refers to liquid and solid particles suspended in air with aerodynamic diameters of less than 1 μm
A Unified Regional Airquality Modeling System (AURAMS)-modelled PM1 sulphate and Aerosol Mass Spectrometers (AMS) sulphate measurements in Harrow (Fig. 2a) were reasonably well correlated (Table 1) throughout the campaign with three major exceptions that occurred during 19–20 June, 24–29 June and 10–11 July periods
In order to assess the variables that control the ability of AURAMS to accurately predict PM1 nitrate, we examine the parameters governing the mass loading of particulate nitrate by ating these maps, we can identify regimes in which the formation of particulate nitrate is favourable for a given pair of T and Free ammonia (FA) values
Summary
Atmospheric PM1 refers to liquid and solid particles suspended in air with aerodynamic diameters of less than 1 μm. Recent studies have linked PM1 to pulmonary disease and reduced lung function (Oberdorster, 2001; Gauderman et al, 2004), cardiac arrest (Dockery, 2001; Peters et al, 2001; Dominici et al, 2006) and in some cases even premature death (Burnett et al, 2000; Pope et al, 2002) For these reasons understanding the processes that govern the sources, sinks and chemical composition of PM1 is of utmost importance. In Eastern North America, more than 50% of the composition of atmospheric fine particulate matter is inorganic in nature, containing mostly sulphate (SO24−), ammonium (NH+4 ) and nitrate (NO−3 ) (Jacob, 1999; Pinder et al, 2007) These constituents of inorganic PM1 are secondary in nature and are formed by physical processes (e.g. condensation) or chemical reactions of precursor gases such as H2SO4(g), HNO3(g)
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