1. 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

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

3. 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)

5. 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)

6. SOA precursor

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

8. 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).

9. 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.

10. 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)

11. 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

12. 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.

13. 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?

14. Extra slides

15. 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

16. 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%.

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

18. 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

19. 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?

20. 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).