Sensitivity of continental boundary layer chemistry to a new isoprene oxidation mechanism Jingqiu Mao (Harvard), Fabien Paulot (Caltech), Daniel Jacob (Harvard), Paul Wennberg (Caltech), Ronald Cohen (UC Berkeley) and funding from NASA ACMAP

Isoprene Emissions Affect Atmospheric Composition and Climate Air Quality ozone, aerosols temperature, radiation, land use isoprene emission Biomass burning Lightning Human activity climate

Intercontinental Chemical Transport Experiment –North America Phase A (INTEX-A) GEOS-Chem chemical transport model - GEOS-5 assimilated met field - 1 year spin up at 2x2.5 degree - New lightning vertical distribution based on Ott et al. (2010). - Rescaled CO emission based on Kopacz et al. (2010). We use a new isoprene oxidation mechanism to test our understanding of isoprene chemistry, mainly based on two recent papers - Paulot et al., ACP, 2009 Paulot et al., Science, 2009 Better understand underestimated OH in boundary layer (Ren et al., 2008) Better understand the discrepancy between OMI and MEGAN isoprene emissions over US (Millet et al., 2008) From July 1st to August 15th of 2004 Millet et al. (2008)

Detailed description of secondary products from the first generation of isoprene nitrates isoprene nitrates degrade to secondary products in a order of hours (k(d-isopn)=9.5e-11); secondary products have a longer lifetime, particularly MVKN (5.4e-12) and PROPNN (1.0e-15)

SOA precursor

(Dillion et al., 2008) (Paulot et al., 2009) (Crounse et al., 2010, in prep)

Median vertical profiles in INTEX-A (Observation vs. model) ozone difference in free trop is due to propagation. The lightning vertical distribution seems worse than Hudman et al. (2007).

Observed OH/ modeled OH in continental boundary layer PROPHET INTEXA Ren et al. (2008) The underestimated OH at high isoprene condition is reproduced in default chemistry. This is remarkably improved in new isoprene chemistry.

Observational constraints on isoprene nitrates Observation HCHO vs. Alkyl Nitrates Model with new isoprene chemistry Slope=0.119 Slope=0.114 The slope slightly increases to 0.13 when increasing IN+O3 rates to Lockwood et al. (2010), which is ISOPND+O3 (5.3e-17) and ISOPB+O3(1.06e-16) The slope (Isoprene Nitrates vs. HCHO) in the observation is well captured by the model. This slope is fairly robust in the new isoprene chemistry with various IN+O3 rates Perring et al. (2009)

Speciation of isoprene nitrates in the model The offset may be from other organic nitrates. The majority of isoprene nitrates are from the second generation products (PROPNN, MVKN etc.). The vertical profile is insensitive to the deposition velocity of the first generation products (mainly driven by chemical loss). The vertical profile is insensitive to the rates of IN+O3, only changing the relative distribution of these organic nitrates. increase of ozone+IN will slightly increase total IN, more nitrogen is stored in secondary products. τ (OH) τ (ozone) δ-ISOPN 1hr 5hr β-ISOPN 7hr 2hr

HCHO yield at different NOx conditions (new isoprene chemistry vs. default GEOS-Chem mechanism) Default GEOS-Chem mechanism is mainly from Horowitz et al. (1998)!!!! Two mechanisms show similar HCHO yield at NOx=1ppb and 0.1ppb, since HCHO is mainly produced through β-hydroxyl channel. Prompt HCHO formation, important for deriving isoprene emission from satellite observations. Difference at NOx=0.01ppb is mainly due to the yield from ISOPOO+HO2. Computed in a photochemical box model. Initialized with 1ppb isoprene. O3 (40ppb), CO (100ppb), and NOx are held constant.

overestimate of ozone Too much lightning NOx? (6 Tg/yr) new isoprene chemistry – default chemistry 0-2 km 4-6 km ppb 2-4 km 6-8 km NOx emissions from lightning, anthro sources? Or isoprene emissions? While all the new mechanism increases OH recycling, we all need to look at the constraint from global mean OH. Too much lightning NOx? (6 Tg/yr) Too much isoprene emissions? (MEGAN) Halogen chemistry?

Extra slides

Heff (moles L-1 atm-1 ) ΔH/R (K) Reference HCOOH 167,000 (pH = 5) -6100 Ito et al., 2007 CH3COOH 11,400 (pH = 5) -6300 MOBA 23,000 GLYC 41,000 -4600 GLYX 360,000 -7200 Schweitzer et al., 1998 MGLY 3,700 -7500 δ-ISOPN 17,000 -9200 β-ISOPN MACRN MVKN PROPNN 1000 R. Sander (NITROOXYACETONE) RIP 83,000 -7400 use H2O2 IEPOX MAP 840 (f0 =1, reactive) -5300 R. Sander HNO3 210,000 -8700 Deposition for new tracers

OH in July of 2004(average between 10am-2pm) Global impact on OH OH in July of 2004(average between 10am-2pm) Impact on OH is more significant in tropics. Global annual mean OH increase by 10-20%.

model Obs Consistent with underestimated HO2 in the box model, which does not have OH recycling from RO2+HO2 . Ren et al. (2008)

GEOS-Chem (chemical transport model) GEOS-5 assimilated met field 1 year spin up at 2x2.5 degree New lightning vertical distribution based on Ott et al. (2010) Rescaled CO emission based on Kopacz et al. (2010) Updated reaction rates with JPL06 and IUPAC06 Updated photolysis cross sections and quantum yield with Fast-JX Non local PBL mixing LINOZ cross tropopause ozone flux

OH budget in continental boundary layer (new isoprene chemistry vs. default GEOS-Chem mechanism) default chemistry new isoprene chemistry Major difference is from HO2+NO (mainly from HO2). RO2+HO2 does not contribute much, since it cannot compete with NO when NO is about 100 ppt. Why is HO2 higher in new isoprene chemistry?

HOx budget in continental boundary layer kHOx=L(HOx)/HOx default chem new isoprene new treatment of isoprene nitrates reduce HOx sink as well, but relatively minor OH recycling from HO2+RO2 increases HOx and thus OH (through HO2+NO).