A New Explicit Model of Isoprene Oxidation EffectsonNOx,Oxidant,andVOCBudgets

KelvinH.Bates*&theCaltechisopreneteam:JohnCrounse,LeahDodson,ReneeMcVay,LauraMertens,TranNguyen,EricPraske,BeckySchwantes,MaHSmarte,JasonStClair,AlexTeng,PaulWennberg,JohnSeinfeld

Abstract

A nearly complete gas-phase oxidaNon mechanismof isoprene and its major products is developed,incorporaNngresultsfromrecentlaboratorystudiesaswellas insights from theoreNcal approaches. The explicitmechanism,made available in KPP format for easy use inbox modeling, is compiled with the aim of providingaccurateatmosphericsimulaNonsoftheimpactofisopreneemissionsonHOx,NOx,andproductsknowntobeinvolvedincondensedphaseprocesses.AreliablerepresentaNonofthe role of isoprene photochemistry in these chemicalprocesses is crucial to quanNfying its influence on ozoneformaNon and regional climate. AddiNonally, we offer asimplified mechanism appropriate for implementaNon inchemical transport models that retains the essenNalchemistry required to accurately simulate this chemistryunderthetypicalcondiNonswhereisopreneisemiHedandoxidized in the atmosphere. Here, we show preliminaryresultsfromtheincorporaNonofthissimplifiedmechanismintoGEOS-Chem,aglobalchemicaltransportmodel.

Outline

Thefollowingisalistoftopicscoveredinourreview,andthemajorsecNonsofourexplicitisoprenemechanism.Topics in bold (the “greatest hits”) are described on thisposter,butfeelfreetoaskaboutanyoftheothers!

1.MechanismDevelopment•RO2+NOreac@ons•RO2+HO2reac@ons•Hydrogenshi`s(isomerizaNon)

2.Isoprene+OH•LocaNonofOHaddiNon•O2addi@ontoallylicradicals-peroxyradicaldynamics•Reac@onsofISOPOO(NO,HO2,RO2,&isomeriza@on)

3.Isoprene+O34.Isoprene+NO35.Isoprene+Cl6.PhotochemistryofmajoroxidaNonproducts

•Methylvinylketone(MVK)•Methacrolein(MACR)•Hydroxyhydroperoxides(ISOPOOH)•Epoxydiols(IEPOX)•Nitrates(hydroxy-,carbonyl-,andhydroperoxynitrates)•Methacryloylperoxynitrate(MPAN)•Hydroperoxyaldehydes(HPALD)•Hydroxymethylhydroperoxide(HMHP)

7.Preliminarymodeling8.RemainingChallenges

PeroxyRadicalDynamics

•WeuseaschememodifiedfromTengetal.,“IsoprenePeroxyRadicalDynamics,”J.Am.Chem.Soc.,2017.

•Itiscrucialtorepresenttheallylicandperoxyradicalsderivedfromisopreneasadynamicsystem,astheirisomer(andproduct)distribuNoncanvarywithpressure,temperature,andbimolecularlifeNme.

•Theexplicitmechanismindividuallyrepresentsall10allylicandperoxyradicalsthatfollowOHaddiNonatC1andC4; the simplified mechanism reduces this to two species, represenNng the steady-state peroxy radicaldistribuNons. These simplificaNons provide isomer and reacNon pathway branching raNos within 5% of theexplicitmodelforatmosphericcondiNons(P=0.5-1atm,T>280K,kbimolecular<1s-1).

RO2+NO

•We implement a newparameterizaNon for the branchingraNosofnitrateformaNon(α)fromRO2+NOreacNons.

•DependenceofαonmolecularsizeisderivedfromTeng,etal.,2015,wheren=#ofnon-Hatoms(forn>4):

α0=[0.045×n]–0.2•DependenceofαonTandPisderivedfromArey,etal.,

2001;thesegraphsshowαvsT&Pforn=6and10:

•αisscaled×1.25/×0.75forterNary/primaryperoxyradicals.•αisfurtheradjustedforfuncNonalizaNon(β-/γ-carbonyl,

nitrate,hydroxy,andhydroperoxygroups).

RO2+HO2

• We formulate RO2 + HO2 rate coefficientsbased onOrlando,et al., 2012, a`er addingdata and refizng for n = # of non-H atoms(excludingtheperoxymoiety):

• We parameterize the radical propagaNng

branchingraNo(RO2+HO2àRO+OH+O2)similarlytotheRO2+NOànitratebranchingraNo, based on funcNonalizaNon: terNarykjhor

PreliminaryModeling

•Apreliminaryversionofthesimplifiedmechanism(~150reacNonsand50species)wasincorporatedintoGEOS-Chem,achemicaltransportmodel.

•Comparisonsshownbelowarebetweenoursimplifiedmechanismandthebasicchemicalmechanismthatcomeswithv.10-01ofGEOS-Chem.

• SimulaNons were run for the year 2014 following an 18-month spinup,usingGEOS-FPmeteorologyona4˚x5˚gridwith72verNcallevels.

•MapsshowannualaveragesovertheboHom1kmofthetroposphere.

Ozone:prodê2.3%,loadingê4.1%

NO:prodê0.9%,loadingê1.4% OH:prodé2.7%,loadingê0.8%

-20% +20%0 -40% +40%0

-10% +10%0

OH

NO3

O3

RIO2

NOHO2

isom. RO2

7.5% 3.2%

89.3%

41.7% 31.3%

4.8%22.2%

é

ê

éé é

ê ê

Globallyaveragedreac@vepathwaybranching

(1,4)-E-δ

(1,2)-β

(1,4)-Z-δ(4,1)-E-δ

(4,3)-β

(4,1)-Z-δ

1.4%

59.8%

2.9%0.6%

17.2%

18.0%

n = 6

280 290 300 310Temperature (K)

0.2

0.4

0.6

0.8

1

Pres

sure

(atm

)

0.06

0.08

0.1

0.12

0.14

0.16

0.18

n = 10

280 290 300 310Temperature (K)

0.2

0.4

0.6

0.8

1 0.2

0.25

0.3

0.35

0.4

Thenewmechanismresultsin:•IncreasedNOxtransportfrom

highernitrateyields• Increased OH recycling from

isomerizaNonpathways•LoweroverallO3producNon

10-4 10-2 100 102

bimolecular reactivity (s-1)

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

(full-

sim

ple)

diff

eren

ce

1-OH trans delta1-OH beta1-OH cis delta4-OH trans delta4-OH beta4-OH cis delta

10-4 10-2 100 102

bimolecular reactivity (s-1)

-0.2

-0.1

0

0.1

0.2

(full-

sim

ple)

diff

eren

ce

beta isomer isomerizationdelta isomer isomerizationbeta isomer bimoleculardelta isomer bimolecular

10-4 10-2 100 1020

0.1

0.2

0.3

0.4

0.5

0.6

0.7

fract

ion

reac

ting

via

each

isom

er

ISOMER BRANCHING

10-4 10-2 100 1020

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

fract

ion

reac

ting

via

each

pat

hway

PATHWAY BRANCHING

Lookforourupcomingpaper,“TheGas-PhaseReac@onsofIsopreneanditsMajorOxida@onProducts”,inChem.Rev.

ISOP(OOH)2(*2)

-10% +10%

FULLMODEL

10-4 10-2 100 102 104

bimolecular reactivity (s-1)

0

0.2

0.4

0.6

0.8

1

fract

ion

of b

imol

ecul

ar re

actio

ns th

roug

h ea

ch p

athw

ay

FULL MODEL: fraction reacting via each pathway

T = 280 KT = 290 KT = 300 KT = 310 K

FULLMODEL

(FULL–SIMPLE)DIFFERENCE

(FULL–SIMPLE)DIFFERENCE

typicaltroposphericPvs.Tprofile

α α

Le1:isopreneperoxyradicaldynamicsfollowingtheaddiNonofOH.Above:dependenceofperoxyradical isomer (le`)andreacNvepathway

(right)branchingraNosonbimolecularlifeNme(xaxis)andtemperature(linestyle) in thenewexplicitmechanism(top),and thedifferencebetweentheexplicitandsimplifiedmodels(boHom).

> secondary >primary, withincreasedradicalpropagaNon forβ-/γ-carbonyl,nitrate, hydroxy,and hydroper-oxygroups.

0 5 10 15n = # of C (old) or C+O (new)

0.5

1

1.5

2

k*10

11 c

m3 m

olec

-1 s

-1

RO2 + HO2 rate fitting

new: n = #C+Oold: n = #C

Globallyaveragedperoxyradicalisomerpathwaybranching

é(1,2):(4,3)-βraNoàéMVK:MACRraNo(×2)

SOAprecursors:globalproducNonscaledto500Tgy-1isoprene

OOHHO

Smallorganics:changefromstandardv.10-01mechanism

OONO2

O

OHOOH

OH

HOO

1pptv 100

0.5pptv 20

0.5pptv 5

2pptv 800

IEPOX178Tgy-1

MPAN32.3Tgy-1

Dihydroxy-dihydroperoxides29.2Tgy-1

Dihydroxy-dinitrates6.9Tgy-1 OH

ONO2

OH

O2NO

COproducNonê0.7%,loadingé1.2%

-15% +15%

HCHOproducNonê1.1%,loadingé3.9%

-20% +20%

FormicAcidproducNonê16%,loadingé1.1%

-20% +20%

Ace@cAcidproducNonê14%,loadingé4.5%

k=2.82×10-13×e(1300/T)×[1–e(-0.231×n)]cm3molec-1s-1RO2+HO2