Effects of the sea-land breeze on coastal ozone pollution in the Yangtze River Delta, China.

Aim at the effects of the coastal characteristic on ozone pollution in the Yangtze River Delta (YRD), a campaign was launched at the Ningbo, China in the summer of 2020, which mainly covered the monitoring of the vertical profiles of ozone (O3) concentration, three-dimensional wind field, temperature and humidity profiles and parameters of boundary layer dynamic-thermodynamic structure. At the coastal research station, a sea-land breeze (SLB) circulation accompanied by a concurrent coastal low-level jets (CLLJ) structure was observed and identified during 11-12 May 2020. The sea breeze first formed at 10:00 LT on 11 May, turned to land breeze at night, and returned to sea breeze again at 10:00 LT the next morning, prevailing at altitudes of 0-0.5 km and 0-0.3 km respectively. Land breeze at night carries O3 from the inland to the sea forming high ozone levels over the sea, and the shift of the sea breeze during daytime further blew pollution back to the land. Additionally, the conversion of SLB contributed to the occurrence of CLLJ at the altitudes of ~0.3-0.7 km by 02:00 and 06:00 LT, of which the center of wind speed reached ~13 m s-1. The CLLJ-induced turbulent activity decoupled the residual layer (RL) and stable boundary layer, leading to a reduction of RL-O3 levels and an increase of ~50 µg m-3 in surface-O3 concentration. The YRD's unique coastal characteristics make O3 pollution causes in coastal areas more complicated.

The dynamic multi-box algorithm of atmospheric environmental capacity.

It is very important for air pollution prevention and control to accurately quantify atmospheric environment capacity (AEC) in the planetary boundary layer (PBL). This study developed a high temporal-resolution dynamic multi-box algorithm to estimate PM2.5 AEC with a PBL ceilometer and Doppler wind profile lidar in Beijing City. Compared with the traditional A-value method, two primary improvements are introducing the time coefficient and vertical multi-box assumption into the original box model. The algorithm can accurately calculate the PM2.5 AEC under different circulation patterns and predict the short-time dynamic change of AEC. The results show that the time coefficient effectively reduced the estimation errors when the initial PM2.5 concentration, horizontal wind speed and PBL heights change greatly with time, such situation is consistent with most circulation patterns. And the improvement of multi-box model is much more remarkable when the PM2.5 concentration and horizontal wind change greatly in the vertical direction, such as A, NE and W type circulations. The ideal AEC under polluted circulation patterns won't increase infinitely with wind speed and PBL height, generally less than 30 t/h. The horizontal advection has a much greater effect on expanding the capacity of PM2.5 than the vertical diffusion under clean circulation patterns, and the maximum value of ideal AEC can reach 50 t/h. The positive residual AEC under clean circulations indicates surplus capacity for PM2.5 because of vigorous turbulences, while weak diffusion and ventilation conditions under polluted circulations lead to negative residual AEC and insufficient capacity of atmosphere.

How do aerosols above the residual layer affect the planetary boundary layer height?

We revealed that the absorption aerosol lying below or above the morning residual layer (MRL) promotes (stove effect, heating the MRL layer) or strongly inhibits (dome effect, heating the temperature inversion layer) the development of planetary boundary layer (PBL) after sunrise, while scattering aerosol exhibits similar suppression (surface or aloft umbrella effect) on the PBL regardless of its vertical location. However, the role of different type of aerosols (i.e., strong absorption aerosol and purely scattering aerosol) present from MRL to upper atmosphere remains lacking and therefore, needs to be further explored. Utilizing a large-eddy simulation model constrained by the in-situ observations in urban Beijing, we observed that the dome inhibition of absorption aerosols on PBL development becomes weaker as elevating the aerosol layer, and the effect (virtual dome effect) remains no change beyond a certain height, which is defined as the dome effective height z. This height z is highly related to the surface sensible heat flux. By comparison, the altitude of light-scattering aerosols relative to the MRL was less important. The scattering aerosols exhibit similar inhibition from MRL to upper atmosphere (aloft umbrella effect), but to a weaker extent than the virtual dome effect. The virtual dome effect and aloft umbrella effect play a leading role during some extremely polluted scenarios with deep aerosol layer, such as sandstorms and volcanic eruptions. Aerosol dome, virtual dome, and aloft umbrella effects, together with aerosol stove and surface umbrella effects, further advance the understanding on aerosol-PBL interactions, which is, more broadly, applied to interpret the impact of aerosol on PBL over other ecosystems as well as exoplanet atmospheres.