Characteristics of Urban Ozone Formation During CAREBEIJING-2007 Experiment
Zhen Liu04/21/09
2. Observed O3 precursors and their implications
3. PAN simulation with aromatics + OH methylglyoxal
4. Characteristics of ozone formation
5. Summary
Outline
1. Introduction and methods
Campaigns of Air Quality Research in Beijing and surounding Region
Adapted from Zhu’s Overview of CAREBEIJING study
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.
O3-NOx-hydrocarbon chemistry200 reactions involving 120 species
Transport24 tracers, 35 vertical layers
WRFMeteorology fields
In situ observationand
3-D REAM output
Observed O3 precursors – NOy species
Average Daytime NOy Budget for 10 - 28 August 2007Total NOy: 28.2ppbv
NO17.3%
NO229.6%
HNO323.3%
PAN13.6%
HONO3.8%
HNO40.06%
N2O50.02% Residual
12.2%
NO NO2 HNO3 PAN HONO HNO4 N2O5 Residual
Observed O3 precursors – VOC species
Average VOC compositionTotal VOC: 303.5 ppbC
Alkanes31%
Alkenes5%
Aromatics
64%
Alkanes Alkenes Aromatics
SSC* fromEmission Observation
% %Toluene 67.4 47.5
Ethyl Benzene 7.7 20.2
M,p-Xylene4.9
19.17.9
O-xylene 7.2 7.51,3,5-trimethyl benzene 1.6 2.41,2,3-trimethyl benzene 2.1 1.41,2,4-trimethyl benzene 1.3 1.9
Table 1. Comparison of the emission inventory based SSC and observed mixing ratios for aromatic species*steady state concentration
PAN simulation with aromatics + OH methylglyoxal
K(molec-
1cm3s-1) for OH
oxidation
MGLY formati
on yield
Toluene 6.00E-12 16.70%Ethyl Benzene 7.50E-12 /
M,p-Xylene2.10E-11
25%1.30E-11
O-xylene 1.30E-11 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%
Table 2 Methylglyoxal yields and related reaction rate constants used in the photochemical model
Ozone formation rate and HOx in different scenarios
S1: neither PAN nor HONO is constrainedS2: PAN constrained with observationS3: HONO constrained with observationS4: both PAN and HONO constrained with observationS5: S2+aromaticsMGLYS6: S4+aromaticsMGLY
Ozone formation rate sensitivity to NOx levels – only standard scenario
Typical VOC limited regime
Daytime evolution
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.
Thanks a lot!