Introduction
The reactions of non-methane hydrocarbons (NMHCs) with atmospheric
oxidizing species (such as hydroxyl (OH) radical, nitrate
(NO3) radical, ozone (O3) molecule and
chlorine radical (Cl˙)) are most important in the earth’s troposphere
and these contribute to the formation of secondary organic aerosols
(SOAs)[1–4]. Among such non-methane hydrocarbons,
isoprene (2-methyl-1, 3-butadiene
(C5H8)) is one of the most prominent and
abundant non-methane hydrocarbon existing in the lower level of the
troposphere which is released into the atmosphere through various
biogenic and anthropogenic processes [5,6]. Like
methane, the excretion of isoprene in the world is around 500-600 Tgc
year-1 due to the unfinished combustion of diesel
fuels, gasoline, automobile exhausts, petroleum refining, plastic, or
rubber industries and other biogenic volatile organic compounds (BVOCs)[7–9]. Due to this excessive emission, isoprene
plays a central role in the tropospheric chemistry related to air
quality and climate change. The abundant isoprene in the atmosphere
undergoes several chemical processes, which is leading to their
potential removal from the atmosphere and transformation processes in
the atmosphere. Especially, oxidation processes can reduce the
atmospheric lifetime of biogenic isoprene in the daytime and it
contributes to the formation of secondary organic aerosols
(SOAs)[10].
For the last few decades, there are several experimental and theoretical
studies have been reported on the kinetics of reactions of isoprene with
atmospheric oxidants. M.L. Ragains and B.J. Finloyson Pitts have
experimentally studied the kinetics and mechanism of the reaction of Cl
atom with isoprene at 760 Torr and 298K using relative rate techniques[11]. Another important experimental study on
isoprene was done byYuri Bedjanian et al., where they studied the
kinetics of the reaction of isoprene+Cl and obtained the overall rate
coefficient over the temperature range of 233-320 K at 1 Torr[12]. Inseon Suh and Renyi Zhang have investigated
the degradation mechanism of Cl initiated isoprene and measured the rate
constant between Cl atom and isoprene is (4.0±0.3)
⨯10-10 cm3molecule-1 S-1 at 5 to 10 Torr and
298K [13]. Subsequently, Inseon Suh et al. both
experimental and theoretically study the reaction of
isoprene+NO3 and they measured the rate coefficient in
the pressure range of 5-7 Torr at 298±2K using chemical ionization mass
spectrometry (CIMS) detection [14]. Recently,
Victoria P. Barber et al reported the formation of four carbon criegee
intermediate from ozonolysis of isoprene [15].
Earlier this year, a theoretical investigation on isoprene was done by
Xirui Guo et al., where they studied the reaction of isoprene with Cl
radical and the reaction rate constants for tight and loose transition
states were calculated by RRKM theory and canonical variational
transition state theory (CVT) respectively [16].
From the earlier literature, the OH radicals and O atoms are well known
most important oxidants in the troposphere, which are responsible for
the removal of VOCs in the atmosphere. Meanwhile, Cl radicals are also
highly reactive in the coastal and marine boundaries. This highly
reactive Cl radical is produced in the marine boundary as a consequence
of both direct emission and chemical processes. Generally, particle
bound chlorine released into the gas phase through heterogeneous
processes on particulate surfaces or sea salt aerosols. Recently, Luo et
al reported that the emission of Cl radicals from seawater is estimated
to be 1792.6 Tg Cl yr-1 [17] and
it is comparably higher than that of other sources (such as dust 15 Tg
Cl yr-1 and volcanic eruptions 2 Tg Cl
yr-1). Therefore, in the present study aimed to
investigate the degradation of isoprene with Cl radical along with their
intermediates, transition states and product radicals in the presence of
O2, NO and H2O molecules. The earlier
investigation mainly focused on primary reaction of isoprene with Cl
radical which hinders the understanding of secondary reactions from Cl
radical initiated isoprene and formation of SOAs from Cl radical
initiated isoprene. A detailed theoretical study on the kinetics of Cl
initiated isoprene and the secondary reaction is yet to be known.
Therefore, in this work, a quantum chemical study is used to understand
the initial and subsequent reaction of isoprene+Cl. The canonical
variational transition (CVT) state theory [18]including small curvature tunneling (SCT)[19] is
used to calculate the relative rate coefficient for the reaction of
isoprene+Cl. The obtained rate coefficients are used to determine the
lifetime of isoprene for the removal of Cl radicals. In addition to
that, the reaction force analysis is also calculated to find out the
structural fate of the reaction and their results are correlated with
the relative energy profile in the present work.