Mainz Isoprene Mechanism 2 (MIM2): an isoprene oxidation mechanism for regional and global atmospheric modelling

Abstract

Abstract. We present an oxidation mechanism of intermediate size for isoprene (2-methyl-1,3-butadiene) suitable for simulations in regional and global atmospheric chemistry models, which we call MIM2. It is a reduction of the corresponding detailed mechanism in the Master Chemical Mechanism (MCM v3.1) and intended as the second version of the well-established Mainz Isoprene Mechanism (MIM). Our aim is to improve the representation of tropospheric chemistry in regional and global models under all NOx regimes. We evaluate MIM2 and re-evaluate MIM through comparisons with MCM v3.1. We find that MIM and MIM2 compute similar O3, OH and isoprene mixing ratios. Unlike MIM, MIM2 produces small relative biases for NOx and organic nitrogen-containing species due to a good representation of the alkyl and peroxy acyl nitrates (RONO2 and RC(O)OONO2). Moreover, MIM2 computes only small relative biases with respect to hydrogen peroxide (H2O2), methyl peroxide (CH3OOH), methanol (CH3OH), formaldehyde (HCHO), peroxy acetyl nitrate (PAN), and formic and acetic acids (HCOOH and CH3C(O)OH), being always below ≈6% in all NOx scenarios studied. Most of the isoprene oxidation products are represented explicitly, including methyl vinyl ketone (MVK), methacrolein (MACR), hydroxyacetone and methyl glyoxal. MIM2 is mass-conserving with respect to carbon, including CO2 as well. Therefore, it is suitable for studies assessing carbon monoxide (CO) from biogenic sources, as well as for studies focused on the carbon cycle. Compared to MIM, MIM2 considers new species like acetaldehyde (CH3CHO), propene (CH2=CHCH3) and glyoxal (CHOCHO) with global chemical production rates for the year 2005 of 7.3, 9.5 and 33.8 Tg/yr, respectively. Our new mechanism is expected to substantially improve the results of atmospheric chemistry models by representing many more intermediates, that are transported and deposited, which allows us to test model results with many more new measurements. MIM2 allows regional and global models to easily incorporate new experimental results on the chemistry of organic species.

Improved global modelling of HO<sub>x</sub> recycling in isoprene oxidation: evaluation against the GABRIEL and INTEX-A aircraft campaign measurements

“…The MIM2 version used here includes the modifications discussed in Taraborrelli et al (2009), and in particular, the OHforming channel in the reactions of HO 2 with acyl peroxy radicals and with 2-oxo peroxy radicals, with branching ratios of 0.5 and 0.15, respectively, based on laboratory investigations Jenkin et al, 2008). Following Butler et al (2008) (also Pugh et al, 2010a), we reduce by 50% the effective rate constant of isoprene + OH, relative to the IUPAC recommended rate (Atkinson et al, 2006).…”

“…1. The reactions of ISOPBO2 and ISOPDO2 with NO and HO 2 are obtained from the MIM2 mechanism (Taraborrelli et al, 2009). The cross reactions of the isoprene peroxy radicals are also taken into account, based on experimental self-reaction rates for structurally similar peroxy radicals (Peeters and Müller, 2010).…”

“…Our basis mechanism is a modified version of the MIM2 mechanism (Taraborrelli et al, 2009), itself a reduction of the detailed Master Chemical Mechanism (MCMv3.1, Jenkin et al, 1997;Saunders et al, 2003). MIM2 and MCM consider OH addition to the terminal carbons, while neglecting the minor addition to the inner carbons.…”

“…2, we describe the isoprene + OH reaction schemes and how they are implemented in the global model. We start with the Mainz Isoprene Mechanism (MIM2, Taraborrelli et al, 2009), which serves as the base scheme for our simulations, as well as the MIM2+ scheme , and we continue with the epoxide formation mechanism (Paulot et al, 2009b) and the Leuven Isoprene Mechanism (LIM0, Peeters et al, 2009) including the rate and mechanism adjustments proposed in Peeters and Müller (2010) (GABRIEL, Lelieveld et al, 2008) and over the Eastern United States (INTEX-A, Ren et al, 2008). Conclusions are drawn in Sect.…”

“…The model results for HO x , isoprene, NO, and ozone are evaluated against air-based observations from the GABRIEL campaign, conducted over the Guyanas in October 2005, and from the INTEX-A campaign over the Eastern US in summer 2004. The version 2 of the Mainz Isoprene Mechanism (MIM2, Taraborrelli et al, 2009) used as reference mechanism in our simulations, has been modified to test (i) the artificial OH recycling proposed by , (ii) the epoxide formation mechanism proposed by Paulot et al (2009b), and finally (iii) the HO x regeneration of the Leuven Isoprene Mechanism (LIM0) proposed by Peeters et al (2009) ;Peeters and Müller (2010). The simulations show that the LIM0 scheme holds by far the largest potential impact on HO x concentrations over densely vegetated areas in the Tropics as well as at mid-latitudes.…”