While the elevated ambient levels of particulate matters with aerodynamic diameter of 2.5 µm or less (PM_2.5) are alleviated largely with the implementation of effective emission control measures, an opposite trend with a rapid increase has been seen in surface ozone (O₃) in the North China Plain (NCP) region over the past several years. It is critical to determine the real culprit causing such a large increase in surface O₃. In this study, 7-year surface observations and satellite retrieval data are analyzed to determine the long-term change in surface O₃ as well as driving factors. Results indicate that anthropogenic emission control strategies and changes in aerosol concentrations as well as aerosol optical properties such as single-scattering albedo (SSA) are the most important factors driving such a large increase in surface O₃. Numerical simulations with the National Center for Atmospheric Research (NCAR) Master Mechanism (MM) model suggest that reduction of O₃ precursor emissions and aerosol radiative effect accounted for 45 % and 23 % of the total change in surface O₃ in summertime during 2013–2019, respectively. Planetary boundary layer (PBL) height with an increase of 0.21 km and surface air temperature with an increase of 2.1 ^∘C contributed 18 % and 12 % to the total change in surface O₃, respectively. The combined effect of these factors was responsible for the rest of the change. Decrease in SSA or strengthened absorption property of aerosols may offset the impact of aerosol optical depth (AOD) reduction on surface O₃ substantially. While the MM model enables quantification of an individual factor's percentage contributions, it requires further refinement with aerosol chemistry included in the future investigation. The study indicates an important role of aerosol radiative effect in development of more effective emission control strategies on reduction of ambient levels of O₃ as well as alleviation of national air quality standard exceedance events.

The atmospheric carbon dioxide (CO₂) mixing ratio and its carbon isotope (δ¹³C-CO₂) composition contain important CO₂ sink and source information spanning from ecosystem to global scales. The observation and simulation for both CO₂ and δ¹³C-CO₂ can be used to constrain regional emissions and better understand the anthropogenic and natural mechanisms that control δ¹³C-CO₂ variations. Such work remains rare for urban environments, especially megacities. Here, we used near-continuous CO₂ and δ¹³C-CO₂ measurements, from September 2013 to August 2015, and inverse modeling to constrain the CO₂ budget and investigate the main factors that dominated δ¹³C-CO₂ variations for the Yangtze River delta (YRD) region, one of the largest anthropogenic CO₂ hotspots and densely populated regions in China. We used the WRF-STILT model framework with category-specified EDGAR v4.3.2 CO₂ inventories to simulate hourly CO₂ mixing ratios and δ¹³C-CO₂, evaluated these simulations with observations, and constrained the total anthropogenic CO₂ emission. We show that (1) top-down and bottom-up estimates of anthropogenic CO₂ emissions agreed well (bias < 6 %) on an annual basis, (2) the WRF-STILT model can generally reproduce the observed diel and seasonal atmospheric δ¹³C-CO₂ variations, and (3) anthropogenic CO₂ emissions played a much larger role than ecosystems in controlling the δ¹³C-CO₂ seasonality. When excluding ecosystem respiration and photosynthetic discrimination in the YRD area, δ¹³C-CO₂ seasonality increased from 1.53 ‰ to 1.66 ‰. (4) Atmospheric transport processes in summer amplified the cement CO₂ enhancement proportions in the YRD area, which dominated monthly δ_s (the mixture of δ¹³C-CO₂ from all regional end-members) variations. These findings show that the combination of long-term atmospheric carbon isotope observations and inverse modeling can provide a powerful constraint on the carbon cycle of these complex megacities.

Entrainment is critical to the development of the atmospheric convective boundary layer (CBL), but little is known about how entrainment is impacted by the aerosol radiative effect. An aerosol radiation transfer model is used in conjunction with large-eddy simulation (LES) to quantify the impact of aerosol shortwave radiative heating on entrainment and thermodynamics of an idealized dry CBL under aerosol-loading conditions. An entrainment equation is derived within the framework of a zero-order model (ZOM) with the aerosol radiative heating effect included; the equation is then examined against the LES outputs for varying aerosol optical depths (AODs) and free-atmosphere stratification scenarios. The results show that the heat flux profiles become more nonlinear in shape as compared to the case of the clean (no aerosol pollution) CBL, with the degree of nonlinearity being highly dependent on the AOD of the layer for the given type of radiation-absorbing aerosols. As AOD increases, less solar radiation reaches the surface and thus the surface heat flux becomes smaller, and both actual (LES) and ZOM-derived entrainment flux ratios decrease. This trend is opposite to the clean CBL where the LES-predicted flux ratios show an increasing trend with diminishing surface heat flux, while the ZOM-calculated flux ratio remains constant. The modified dimensionless entrainment rate closely follows the −1 power law with a modified Richardson number. The study suggests that including the aerosol radiative effect may improve numerical air quality predictions for heavy-air-pollution events.

Entrainment is critical to the development of the atmospheric convective boundary layers (CBLs) and for the exchange of energy and substances between the boundary layer and free atmosphere. In this study, entrainment relationships are systematically evaluated from large‐eddy simulations (LESs) of shear‐free and sheared CBLs under a broad variety of atmospheric conditions. A total of 36 LES runs are conducted for varying free‐atmosphere stratifications, surface heat fluxes, and height‐constant geostrophic winds. It has been found that entrainment flux ratios have a common value of 0.2 regardless of the stratification of free atmosphere and the surface heat‐flux value when analysed within the zero‐order‐model (ZOM) framework for the shear‐free CBLs, whereas the actual (LES) entrainment ratios are not only stratification‐dependent, but also surface heat flux‐dependent. For the sheared CBLs, the entrainment‐flux ratios are substantially increased when the geostrophic winds are strong while they have a slight reduction under weak wind conditions. In the free‐encroachment regime, the dimensionless entrainment rates have been found to be approximately constant and surface heat‐flux independent, with the value in the range from 0.4 to 0.7 depending on the method used to evaluate the CBL depth.