Abstract
Severe haze events and their radiation feedbacks exert a profound impact on the weather and tropospheric chemistry. Using the on-line-coupled Weather Research and Forecasting with Chemistry (WRF-Chem) model, this study investigates the impacts of direct aerosol-radiation feedbacks on local air quality (i.e. particulate matter and ozone photochemistry) during a severe autumn haze episode in Nanjing megacity, eastern China. Pronounced radiation feedbacks are found for the predictions of meteorological and chemical variables. In response to the negative radiative forcing of scattering-dominant anthropogenic haze aerosols, the instantaneous irradiance and temperature at the surface lower by 130 W m−2 and 1.1–1.4 °C, respectively, leading to a reduction of boundary layer height by 103.2–232.6 m (11–38%) and vertical wind speed by 0.1–0.8 mm s−1 (2–30% at mid-day) during this haze event. Such a stable atmosphere favours the accumulation of fine particles (30.5 μg m−3, 28.7%) and NO2 (6.0 ppb, 23.7%) in the urban pollution plume. The weaker turbulent mixing and photochemical activity associated with the enhanced titration loss, and reduced downward radiation and photolysis rate result in a 0.1−5.0 ppb (12.0%) reduction of near-surface ozone. The simulations highlight that the aerosol-radiation feedbacks play an important role in the atmospheric transport and chemistry of large urban pollution plumes.
Highlights
Atmospheric aerosols with diameters larger than 50–100 nm could affect the earth’s radiation balance by directly scattering or absorbing solar radiation (‘direct radiative effect’) (Charlson et al, 1992), and by indirectly altering the cloud optical properties and lifetime through acting as cloud condensation nuclei (‘indirect radiative effect’) (Twomey, 1974).The aerosol–radiation interactions have substantial effects on meteorology and air quality, which has caused wide-spread concern recently
WRF is known to over-predict wind speed in particular at low to moderate values, which can be partly attributed to the unresolved terrain features by the default surface drag parameterization (Jimenez and Dudhia, 2012)
Wang et al (2015) designed similar experiments and found that atmospheric aerosols cooled the planetary boundary layer (PBL) but warmed the atmosphere above it in northern China, leading to a more stable atmospheric stratification and a decrease in PBL height by about 33%. Such weakening of PBL development is the result of lower air entrainment, as the vertical wind speed is reduced by 0.1– 0.8 mm s−1 (2–30% at mid-day) when the feedback of aerosols is taken into account (Fig. S4a), which is comparable to Pere et al (2014) who reported a reduction in vertical wind speed (5–80%) by the Russian wildfire
Summary
Atmospheric aerosols with diameters larger than 50–100 nm could affect the earth’s radiation balance by directly scattering (e.g. sulphate and nitrate) or absorbing (e.g. dust and black carbon) solar radiation (‘direct radiative effect’) (Charlson et al, 1992), and by indirectly altering the cloud optical properties and lifetime through acting as cloud condensation nuclei (‘indirect radiative effect’) (Twomey, 1974). Jacobson (1997) was first to develop a fully-coupled on-line model to account for the radiation feedback of size-resolved aerosols on photolysis and temperature profiles. They found that aerosols reduced solar radiation and ozone mixing ratio in Los Angeles by 6.4 and 2%, respectively (Jacobson, 1997, 1998). This study attempts to assess the direct aerosol-radiation feedbacks on local air quality for a severe haze event (15–17 October 2015) in Nanjing megacity, using the on-line-coupled WRF-Chem model.
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