Aerosol-radiation interaction (ARI) includes direct scattering and/or absorption of incoming solar radiation by atmospheric aerosols and induced adjustments to the surface energy budget, thermodynamic profile and cloudiness.

ARI has been confirmed to cool the surface but heat the air aloft. It also enhances the atmospheric stability, accumulation and formation of fine particulate matter (PM_2.5) in the planetary boundary layer (PBL), and eventually deterioratesair quality during haze events.

However,modification of photolysis in the atmosphere caused by aerosols absorbing or scattering solar radiation (i.e., the aerosol-photolysis interaction, or API) ultimately changes ozone (O3) formation and atmospheric oxidizing capability (AOC), further influencing secondary aerosol formation and offsetting ARI effects on PM_2.5 pollution.

Recently, a research team led by Prof. LI Guohui from the Institute of Earth Environment (IEE) of the Chinese Academy of Sciences quantitatively assessedhow much PM_2.5 could be attributed to the combination of ARI and API during a persistent heavy haze episode in the North China Plain in winter.

The study was conducted from a modeling perspective with a combination of measurements. Itwas published in PNAS on Apr. 14.

Based on observations, the researchers found that secondary aerosols constituted a major fraction of PM_2.5 in Beijing, and weredetermined to a large extent by atmospheric oxidants affected by API.

The results of the study indicated that API caused the daytime NO₂ photolysis rate constant and O₃ concentrations to decrease by 22.6% and 18.6%, respectively.

“Such a pronounced decrease of AOC will inevitably impede secondary aerosol formation. Actually, the API effect on secondary aerosol formation could be observed indirectly from analyses of measurements through its impact on O₃,” said Prof. LI.

The researchers found that ARI contributed to a 7.8% increase in near-surface PM_2.5. However, API suppressedsecondary aerosol formation. As a result, the combination of ARI and API resulted in only a 4.8% net increase in PM_2.5, with almost 60% of the PM_2.5 enhancement due to ARI only.

“The total aerosol effect on radiation, or the synergetic effect of both ARI and API, does not constitute an important factor in driving heavy haze formation, except for extremely severe haze,” said Prof. LI.

Contact: LI Gguohui, State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, 710061 Xi’an, China. Email: ligh@ieecas.cn