Aerosols cut winter ozone but raise it in summer
Aerosol haze over China changes ozone in opposite directions in January and July by altering mixing and sunlight.
Source
Impacts of aerosol direct effects on tropospheric ozone through changes in atmospheric dynamics and photolysis rates
Inspect source passages
Aerosols cut winter ozone but raise it in summer
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Ozone was compared with observations from 74 cities across China.
In this study, the model performance in the simulation of O 3 will be evaluated through the comparison with observations from 74 cities across China from the China National Urban Air Quality Real-time Publishing Platform ( http://113.108.142.147:20035/emcpublish/ ).
Winter combined aerosol effects reduced daily-max ozone by up to 39 μg m−3.
The combined effect of both ∆Dynamics and ∆Photolysis results in an overall reduction in DM1O 3 as evident across the JJJ and SCH regions with monthly-average reductions of up to 39 μg m −3 .
What they did
Three WRF-CMAQ runs for January and July 2013 turned aerosol radiation feedbacks on or off. Differences isolated photolysis versus dynamical pathways. Process rates and ozone were checked against observations from 74 Chinese cities.
What they found
Including aerosol direct effects reduced high winter ozone bias and low summer bias. In January, combined effects cut daily-max ozone by up to 39 μg m−3; in July they raised it by up to 4 μg m−3, mainly via more stable, precursor-rich boundary layers. Photolysis cuts ozone in winter but can raise noon ozone in summer because unused morning precursors react later.
The limits
What it doesn't show
Heterogeneous N2O5 chemistry is not quantified. Two months in 2013 cannot represent all years. Urban monitors may not represent rural ozone, and PM2.5–ozone correlations mix chemistry with shared sources.
Key terms
- Aerosol direct effects (ADEs)
- Scattering and absorption of sunlight by particles, which change temperature, mixing, clouds, and photolysis.
- DM1O3
- Daily 1-hour maximum surface ozone.
- VOC-limited versus NOx-limited
- Winter northern China ozone falls if NOx rises (titration); summer NOx-limited ozone rises if NOx is concentrated by a stable PBL.
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In January, combined aerosol direct effects on Chinese daily-max ozone were:
Common questions
Why can dimming photolysis raise summer afternoon ozone?
Slower morning chemistry leaves extra NOx/VOC that then produce more ozone at noon, outweighing weaker sunlight.
Do aerosols always suppress ozone?
No. Winter combined ADEs reduce ozone; summer dynamics increase it across much of China.
Why does a stable PBL change ozone sign by season?
The same extra NOx titrates ozone in VOC-limited winter but fuels ozone in NOx-limited summer.
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