Model of Volatile Hydrocarbon Emission

Higher plants are known to emit volatile hydrocarbons such as isoprene and monoterpenes into the atmosphere. The World wide emission rate of these natural hydrocarbons has been estimated to be 1.8-8.3 * 10¹¹ kg y^-1 which exceeds that of non methane hydrocarbons originating from human sources. Natural hydrocarbons have been suggested to be responsible for the blue haze found around forested areas on sunny days and the high rural ozone concentration in summer. The study of natural hydrocarbon emissions from plants is therefore of key importance to our understanding of the global effects of atmospherically born hydrocarbons.

An important plant emitted hydrocarbon is isoprene and a simple model of leaf isoprene emission as proposed by Guenther, in the paper detailed in the acknowledgment below, is formulated in Wimovac.

The leaf isoprene model produces simple empiracle responses of isoprene emission in response to key leaf microclimate properties such as light intensity, temperature and relative humidity. A typical example of the output from the leaf isoprene emission module is given below.

Figure above. Leaf isoprene emission response to incident light intensity

Figure above. Leaf isoprene emission response to leaf temperature

Wimovac then uses this leaf isoprene emission module in conjunction with its physically based models of canopy structure, light interception, energy budget and water status to scale up from the leaf to a canopy level. In turn the canopy level module is used in a long term run dialog which numerically integrates the isoprene emission rates over long periods of time to give long term total emissions.

The following equation provides an isoprene emissions model which can be incorporated into simple plant model systems.

where E₁ is isoprene emission rate, S₁ is the mean isoprene emission rate at 301K, 1000umol/m²/s, 40% relative humidity and 330ppm CO₂ (70.7 nmol/m²/s). H is the correction factor for relative humidity, C is the correction factor for CO₂ concentration, L is the correction factor for irradiance and T is the correction factor for leaf temperature

where RH is the relative humidity (percent) and H1 (=0.00236) and H2 (-0.8495). A relative humidity increase of 10% results in only a small (2.36%) increase in isoprene emission rate.

Isoprene emission rates were constant between CO₂ concentrations of 50 and 600 and decreased at lower concentrations and at higher concentrations. This can be described by:

where[CO₂] is the carbon dioxide mixing ratio (ppm) and C₁ and C₂ are empirical coefficients according to the following relationship:

If [CO₂] 100 Then

        C1 = .00195

        C2 = .805

ElseIf [CO₂] >= 100 And [CO₂] <= 600 Then

        C1 = 0

        C2 =1

ElseIf [CO₂] > 600 Then

    C1 = -.00041

    C2 = 1.28

End If

The divergent behavior displayed by isoprene emission rates and CO₂ assimilation rates with changes in CO₂ concentration suggests that the link between these two processes is an indirect one.

The equation for L is based on the electron transport algorithm of Farquhar et al, (1980).

where

f is the fraction of light absorbed by chloroplasts (=0.385), I is the irradiance (umol/m2/s) and L₁ (=105.6) and L2 (=6.12) are empirical coefficients. L₁ and L₂ were determined using a non linear best fit to isoprene emission data.

The temperature dependence of isoprene emission increases exponentially to a maximum between 35C and 40C. This is characteristic of the effect of temperature on enzyme activation and can be described by the following equation.

where T_L is leaf temperature (Kelvins), T_s is the normalizing temperature (=301K), R is the real gas constant (8.314 J K^-1 Mol^-1) and T₁ (=95100 J/mol), T₂ (=231000 J/mol) and T₃ (=311.83 K ) are empirical coefficients.

The first isoprene emissions module gives facilities for the prediction of expected isoprene emission rates for a given set of physical conditions. This may be useful for lab based experimental work where a leaf is being measured under a controlled set of light, temperature and CO₂ conditions, the test module works by calling the calculation module with the correct parameters supplied.

The second module allows the calculation of IE for a simple plant canopy over a time span ranging from 1hr to 1yr. The module does this be calling many of the same functions as the canopy assimilation routine, including the macro and micro meteorological routines, stomatal conductance and leaf nitrogen routines. The canopy is split into sunlit and shaded leaf types and a LAI for each is calculated based on sun/earth geometry. Further details of this are given in the Wimovac help file which may be downloaded along with the rest of the program from the homepage download option.

The different light, temperature and CO₂ conditions at sunlit and shaded leaves are calculated and the IE rate calculated for each leaf type based on these. The canopy IE rate is then taken to be the sum of these. At a later date a multiple layer IE model will be formulated and currently work is progressing at Essex that attempts a mechanistic understanding of isoprene emission.

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Last modified: August 19, 1997