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2.8 r

(1/?)-( +)-a-Pinene

4.8s

(lS)-( — Va-Pinene

4.6«

3-Carene

5.6«

Myrcene

6.6«

p-Cymene

2.1«

Methyl vinyl ketone

2.0«

''Atkinson and Aschmann (1985), corrected to k(C\ + «-butane) = 2.18 x 10"10 cm' molecule -1 s"1.

' Kaiser and Wallington (1998), based on reported values of k{) and k d Kaiser and Wallington (1996).

' Bedjanian et al. (1998); this is the value of kx.

'' Ragains and Finlayson-Pitts (1997).

''Atkinson and Aschmann (1985), corrected to k(C\ + «-butane) = 2.18 x 10"10 cm' molecule -1 s"1.

' Kaiser and Wallington (1998), based on reported values of k{) and k d Kaiser and Wallington (1996).

' Bedjanian et al. (1998); this is the value of kx.

'' Ragains and Finlayson-Pitts (1997).

all very fast, approaching the collision-controlled regime.

The reaction proceeds primarily by addition to a double bond to form a chlorine-containing alkyl radical, which then adds 02 to ultimately generate characteristic chlorine-containing oxygenated products. For example, l-chloro-3-methyl-3-buten-2-one is formed in the absence of NO from the addition of chlorine atoms to the 4-position of isoprene, followed by secondary reactions of the alkyl radical with 02 etc. This product might therefore be expected to be a reaction product in the low-NOx environment of the marine boundary layer (Nordmeyer et al., 1997).

A small portion of the reaction proceeds by what appears to be abstraction of the allylic hydrogen atom from the weaker C-H bond (e.g., Lee and Rowland, 1977). In the case of the isoprene reaction, for example, about 15% of the overall reaction at 1 atm pressure and room temperature proceeds by abstraction (Ragains and Finlayson-Pitts, 1997). It should be noted, however, that this net hydrogen abstraction may not be a simple abstraction per se but rather proceed via an addition-elimination reaction (e.g., Kaiser and Wallington, 1996b; Ragains and Finlayson-Pitts, 1997).

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