Kinetic analysis
According to the above discussed PESs, the formation of isoprene adduct
radical leads to the formation of SOAs through subsequent reactions. The
rate constants for the reaction between isoprene and Cl radicals are
carried out over the temperature range of 278K-348K with an interval of
10K at 1 atm pressure. The CVT/SCT is used to calculate the rate
constant for the formation of Cl isoprene adduct radicals
(K1a, K1b, K1c, &
K1d) at M06-2X/6-311+G* level of theory. The rate
constant for the reaction of isoprene with Cl radical to C1, C3, C4, and
C5 positions is found to be 4.89⨯10-11,
6.91⨯10-10, 1.63⨯10-10 and
8.12⨯10-10 cm3/molecule/sec
respectively at 278K. At high-temperature 350K, the rate constant for
the reaction of isoprene with Cl radical to C1, C3, C4, and C5 positions
is found to be 6.38⨯10-11,
8.12⨯10-10, 7.52⨯10-10 and
8.83⨯10-10cm3/molecule/sec. These
obtained rate constants K1a, K1b,
K1c, and K1d are listed in(Table 6) and the obtained reaction rate constants are
increased with increasing temperature which is displayed inFigure 12 . In this study, the calculated rate constants for the
primary Cl-isoprene adduct radical is feasible with the experimentally
reported values of 5.55⨯10-10,4.48⨯10-10cm3molecule-1s-1[42,51]. This result reveals that the reaction
between Cl radical and isoprene is kinetically and thermodynamically
more favorable in the marine boundary.
The branching ratios are calculated for K1a,
K1b, K1c, and K1d at the
temperature range of 278K to 348K and its calculated values are listed
in (Table 7). The branching ratio plot for rate constant
associated with the formation of I1a, I1b, I1c and I1d are shown inFigure 13 . Hence, the calculated total kinetic rate constant is
used to calculate the atmospheric lifetime. The atmospheric lifetime of
Cl-isoprene adduct radical can be calculated using the following given
expression
\begin{equation}
\tau=\frac{1}{K\left[\text{Cl}\right]}\nonumber \\
\end{equation}Where K is the total rate constant obtained from the above kinetic
studies and Cl is the atmospheric concentration of the Cl atom. Under
the atmospheric condition, the Cl concentration is
1⨯105 molecules per cm3[52]. The troposphere lifetime (\(\tau\)) of
isoprene is estimated from the total rate coefficient value of primary
Cl-isoprene adduct radical reaction at the temperature range of 298K.
The lifetime of Cl-isoprene adduct radical is estimated to be 6.49 hours
at the temperature of 298K using the average atmospheric concentration
of Cl atom. The lifetime reported in this study is in good agreement
with the Gaia Fantechi et al estimated values of 5
hours[53].