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!