1. Characteristics of Urban Ozone Formation During CAREBEIJING-2007 Experiment
Zhen Liu
04/21/09

2. Outline 1. Introduction and methods
2. Observed O3 precursors and their implications
3. PAN simulation with aromatics + OH methylglyoxal
4. Characteristics of ozone formation
5. Summary

3. Campaigns of Air Quality Research in Beijing and surounding Region
Adapted from Zhu’s Overview of CAREBEIJING study

4. A 1-D photochemical model
The 1-D photochemical model (e.g. Wang et al., 2008) driven by WRF assimilated meteorology fields adopts the photochemical and dry and wet deposition modules from the GEOS-CHEM model (Bey et al., 2001). The model includes a detailed photochemical mechanism with about 200 reactions and 120 concentration-varying chemical species; 24 tracers (family or species) are transported using the scheme by Walcek (2000) to describe O3-NOx–hydrocarbon chemistry. The in situ measurement data and 3D-REAM output were used to constrain the model.
In situ observation
and
3-D REAM output
O3-NOx-hydrocarbon chemistry
200 reactions involving 120 species
Transport
24 tracers, 35 vertical layers
WRF
Meteorology fields

5. Observed O3 precursors – NOy species
Average Daytime NOy Budget for August 2007
Total NOy: 28.2ppbv NO
17.3%
NO2
29.6%
HNO3
23.3%
PAN
13.6%
HONO
3.8%
HNO4
0.06%
N2O5
0.02%
Residual
12.2%

6. Observed O3 precursors – VOC species
Average VOC composition
Total VOC: ppbC
Alkanes
31%
Alkenes 5%
Aromatics
64%
Table 1. Comparison of the emission inventory based SSC and observed
mixing ratios for aromatic species
*steady state concentration
SSC* from
Emission
Observation %
Toluene
67.4
47.5
Ethyl Benzene
7.7
20.2
M,p-Xylene
4.9
19.1
7.9
O-xylene
7.2
7.5
1,3,5-trimethyl benzene
1.6
2.4
1,2,3-trimethyl benzene
2.1
1.4
1,2,4-trimethyl benzene
1.3
1.9

7. K(molec-1cm3s-1) for OH oxidation
PAN simulation with aromatics + OH methylglyoxal
Table 2 Methylglyoxal yields and related reaction rate
constants used in the photochemical model
K(molec-1cm3s-1) for OH oxidation
MGLY formation yield
Toluene
6.00E-12
16.70%
Ethyl Benzene
7.50E-12 /
M,p-Xylene
2.10E-11
25%
1.30E-11
O-xylene
21.70%
1,3,5-trimethyl benzene
4.09E-11
90%
1,2,3-trimethyl benzene
3.13E-11
20%
1,2,4-trimethyl benzene
3.33E-11
44%

8. Ozone formation rate and HOx in different scenarios
S1: neither PAN nor HONO is constrained
S2: PAN constrained with observation
S3: HONO constrained with observation
S4: both PAN and HONO constrained with observation
S5: S2+aromaticsMGLY
S6: S4+aromaticsMGLY
We did six different simulations to investigate the influences of oxidant levels on ozone and radicals.

9. Ozone formation rate sensitivity to NOx levels – only standard scenario
Typical VOC limited regime
What is the ozone formation regime in Beijing? VOC limited or NOx limited? Based on our simulations with 100%, 90% and 70% observed NOx, there is an anti-correlation between ozone formation rate and NOx levels. Beijing is in the VOC-limited regime.
Daytime evolution

10. Summary
A 1-D photochemical model was applied to analyze a full suite of atmospheric pollutants data during CAREBEIJING-2007 Experiment (August 2007).
Observed oxidants (PAN and HNO2) levels are much higher than predicted from the standard model, which almost increase by 100% the ozone formation rate, as well as the HOx (OH and HO2) concentration.
The abundance of aromatic VOC (~60% of total VOC in ppbC) leading to the high level of methylglyoxal appears to play a key role in PAN formation.
Tests for ozone sensitivity to NOx levels indicate the typical VOC-limited mechanism of ozone formation in urban Beijing.

11. Thanks a lot!