Paul A. Makar

Modelling gaseous dry deposition in AURAMS: A unified regional air-quality modelling system

An upgraded parameterization scheme for gaseous dry-deposition velocities has been developed for a new regional air-quality model with a 91-species gas-phase chemistry mechanism, of which 48 species are “transported” species. The well-known resistance analogy to dry deposition is adopted in the present scheme, with both O3 and SO2 taken as base species. Stomatal resistances are calculated for all dry-depositing species using a “sunlit/shaded big-leaf” canopy stomatal resistance submodel. Dry-ground, wet-ground, dry-cuticle, and wet-cuticle resistances for O3 and SO2, and parameters for calculating canopy stomatal resistance and aerodynamic resistance for these two base species are given as input parameters for each of the 15 land-use categories and/or five seasonal categories considered by the scheme. Dry-ground, wet-ground, dry-cuticle, and wet-cuticle resistances for the other 29 model species for which dry deposition is considered to be a significant process are scaled to the resistances of O3 and SO2 based on published measurements of their dry deposition and/or their aqueous solubility and oxidizing capacity. Mesophyll resistances are treated as dependent only on chemical species. Field experimental data have then been used to evaluate the schemes performance for O3 and SO2. Example sets of modelled dry-deposition velocities have also been calculated for all 31 dry-deposited species and 15 land-use categories for different environmental conditions. This new scheme incorporates updated information on dry-deposition measurements and is able to predict deposition velocities for 31 gaseous species for different land-use types, seasons, and meteorological conditions.

Chemical processing of biogenic hydrocarbons within and above a temperate deciduous forest

A one-dimensional canopy model was developed to study photochemical processes inside and above a mixed deciduous forest in southern Ontario. The Eulerian model made use of Lagrangian dispersion principles with a correction factor to incorporate the average ensemble time since emission to calculate atmospheric mixing; traditional diffusion methods were found to provide insufficient mixing to match the measurements. Neglecting chemical losses while making isoprene emission estimates was found to underestimate emission rates up to 40%. The ozone oxidation of biogenic and anthropogenic alkenes was found to be a potential source of hydroxyl (OH) and hydroperoxy (HO2) radicals during the morning and night. Reactions of HO2 with organic peroxy radicals formed by the OH oxidation of isoprene during the day were shown to be a significant source of organic peroxide formation above the canopy. The primary pathways of methacrolein and methylvinylketone formation were shown to be ozone oxidation of isoprene and hydroxyl radical oxidation of isoprene, respectively. Local ozone formation was shown to be limited by low mixing ratios of nitrogen oxides, despite high levels of isoprene present at the site.

Uptake of water by organic films: the dependence on the film oxidation state

We report quartz crystal microbalance (QCM) measurements of the room temperature uptake of water by thin (<1 μm) organic films. The mass of water taken up by films of dodecane, 1-octanol, octanoic acid, 1,5-pentanediol, 1,8-octanediol and malonic acid was measured as a function of the ambient relative humidity (RH). All compounds tested here displayed some sorption of water. Uptake by dodecane is probably due to surface adsorption; in the other films, water dissolves into the film material. Malonic acid and 1,8 octanediol show deliquescence-like behaviour at relative humitidies near 72% and 95%, respectively. In general, the higher the oxidation state of the film compound, the greater is the amount of water sorbed. An absorptive partitioning model, using UNIFAC calculations of activity coefficients, yields semiquantitative agreement with the experimental results at low relative humidities for all compounds except dodecane. Model estimates of water uptake were generally higher than measurements at low RH and lower than measurements at high RH. Model-measurement deviations displayed a similar nonlinear behaviour with changes in RH for all compounds. The differences between the modelled and measured uptakes yield insight into the limitations of currently available model parameters.

Mass-Conservation Issues in Modelling Regional Aerosols

The problem of using mass-inconsistent winds in tracer advection is relatively well recognized in the air quality modelling community. As pointed out by (1999), an inconsistency in the air-density and wind fields is equivalent to an additional source term in the tracer continuity equation, which will lead to mass-conservation violation in air quality models. There are a number of causes of mass inconsistency in the input air- density and wind fields for air quality models. For example, the continuity equation for air density is not always included as one of the prognostic equations in meteorological models, and air density is then a diagnostic variable; staggering of space and time grids in meteorological models for momentum and thermal variables can affect the accuracy of the solution of the continuity equation; furthermore, de-staggering in air quality (AQ) models can also introduce mass inconsistency. Various methods (both formal and ad hoc) have been used in practice to correct the error introduced by mass-inconsistent winds. This usually involves corrections to the winds to make them mass consistent (e.g., Odman and Russell, 1999) and adjustment to tracer fields through a correction to air density (e.g., Lu et al., 1997). (1999) also suggested a number of ways of correcting the mass-inconsistency error in air quality models based on a formal analysis of his continuity equation formulated in a generalised coordinate system.

Wintertime and Summertime Evaluation of the Regional Pm Air Quality Model Aurams

Three dimensional air quality models have established themselves as valuable tools for the development of emission control strategies. They give scientists the ability to investigate the consequences of multiple emission reduction scenarios as well as research optimum sets of pollutant and/or precursor emission reductions under various meteorological conditions. This capability is becoming crucial as advances in atmospheric science in the past decades have revealed numerous links between ground-level air pollution problems such as ozone, acid rain and particulate matter (PM) (Hidy et al., 1998). With the recognition of PM, especially, as a severe human health concern (Samet et al., 2000; Brook et al., 2002), efforts have lately focused on addressing this new issue without compromising any achievement in reducing ozone and acid rain. As demonstrated by (1997), decreasing VOCs, which are ozone precursors, could free nitrogen oxides and results, under certain conditions, in an increase in PM mass. The so-called ‘one atmosphere’ models are therefore desirable to study PM issues.