Characterizing nighttime vertical profiles of atmospheric particulate matter and ozone in a megacity of south China using unmanned aerial vehicle measurements.

Daytime atmospheric pollution has received wide attention, while the vertical structures of atmospheric pollutants at night play a crucial role in the photochemical process on the following day, which is still less reported. Focusing on Guangzhou, a megacity of South China, we established an unmanned aerial vehicle (UAV) equipped with micro detectors to collect consecutive high-resolution samples of fine particle (PM2.5), submicron particle (PM1.0), black carbon (BC) and ozone (O3) concentrations in the atmosphere, as well as the air temperature (AT) and relative humidity (RH) within a 500 m altitude during nighttime from Oct. 24th to Nov. 6th, 2018. The measurements showed that PM2.5, PM1.0, and BC decreased with altitude and were influenced by the nighttime shallow planetary boundary layer (PBL) where BC was more accumulated and fluctuated. In contrast, O3 was positively correlated with altitude. Backward trajectory clustering and Pasquill stability classification showed that advection and convection significantly influenced the vertical distribution of all pollutants, particularly particulate matter. External air masses carrying high concentrations of pollutants increased PM1.0 and PM2.5 levels by 145% and 455%, respectively, compared to unaffected periods. The ratio of BC to PM2.5 indicated that local emissions had a minor role in nighttime particulate matter. Vertical transport caused by atmospheric instability reduced the differences in pollutant concentrations at various heights. Geodetector and generalized additive model showed that RH and BC accumulation in the PBL were significant factors influencing vertical changes of the secondary aerosol intensity as indicated by the ratio of PM1.0 to PM2.5. The joint explanation of RH and atmospheric stability with other variables such as BC is essential to understand the generation of secondary aerosols. These findings provide insights into regional and local measures to prevent and control night-time particulate matter pollution.

Characterization of the vertical evolution of urban nocturnal boundary layer by UAV measurements: Insights into relations to cloud radiative effect.

The complex structure of the nocturnal boundary layer (NBL) and its impact on air pollution remain poorly understood. In this study, we present in-situ nocturnal flight measurements onboard an unmanned aerial vehicle (UAV) during the wintertime of 2022 at an urban site in Hefei, China. Besides, co-located measurements of radiation intensity and total amount of cloud were conducted. The vertical distribution of temperature, particle number concentration, and relative humidity were obtained to study the structure of NBL and the key factors driving the evolution of the NBL. A multi-layer inversion boundary layer was observed during haze and fog episodes, which affects the vertical diffusion of particles near the surface and leads to a vertical gradient of particle number concentrations. The particle size distribution demonstrates a drastic vertical variation over different sections of the nocturnal boundary layer: homogeneously mixed in the SBL and the RL layer, sharply reduced in the IL. It is found that the temperature and particle number concentration differences between near-surface and at 500 m are highly related to variations of the radiation intensity and the amount of cloud. The decreased cloud cover enhances the surface cooling, creating a shallow NBL with multiple inversion layers, which reinforces the suppression of vertical diffusions and consequently promotes the accumulation of aerosols within the NBL. This reveals an important mechanism for the impact of NBL evolution modulated by cloud radiative effect on the formation of urban haze.

Vertically increased NO3 radical in the nocturnal boundary layer.

In the nocturnal boundary layer, nitrate radical (NO3) has an important contribution to atmospheric chemistry through oxidation of nitrogen oxides and hydrocarbons. Vertical distributions of NO2, O3 and NO3 were measured by four differential optical absorption spectroscopy instruments at meteorological tower in Beijing from June 1 to July 22, 2019. The results show the mean diurnal variations of NO2, O3, and NO3 display a single peak (up to 65.0 ppbv, 196.8 ppbv and 317.5 pptv, respectively) in time. O3 and NO3 mixing ratios generally increased against heights, which is opposite to NO2, suggesting the contribution of O3 to NO3 production at higher altitude. According to the correlation coefficients between NO3 production rates (PNO3) and NO2 or O3 levels, PNO3 was sensitive to NO2 mixing ratio at higher altitude but to O3 near the ground. Averaged NO3 lifetimes (τNO3) of lowest, middle, upper and highest layer intervals were 104, 118, 164 and 213 s, respectively, which indicates τNO3 increase against height and explains why NO3 mixing ratios are larger at higher altitude to some extent. Main control factors of NO3 removal changed from gas-phase reactions to N2O5 hydrolysis with height increase. When relative humidity (RH) exceeded 70% or PM2.5 level exceeded 50 µg·m-3, τNO3 was almost less than 300 s with mixing ratio lower than 70 pptv. The clear negative dependence of τNO3 on RH and PM2.5 reveals the influencing factors on indirect loss. Under polluted conditions, vertical profiles of NO2, O3 and NO3 varied drastically. Stable atmosphere (low nocturnal boundary layer height and thermal inversion), RH level and RH gradient are the main reason for the evident difference in NO3 gradient. Vertically increased NO3 radicals may imply the formation of nitrate aerosols and further increase the nitrate content in high- altitude particulate matter.

Early Warning Signals for Regime Transition in the Stable Boundary Layer: A Model Study.

The evening transition is investigated in an idealized model for the nocturnal boundary layer. From earlier studies it is known that the nocturnal boundary layer may manifest itself in two distinct regimes, depending on the ambient synoptic conditions: strong-wind or overcast conditions typically lead to weakly stable, turbulent nights; clear-sky and weak-wind conditions, on the other hand, lead to very stable, weakly turbulent conditions. Previously, the dynamical behaviour near the transition between these regimes was investigated in an idealized setting, relying on Monin-Obukhov (MO) similarity to describe turbulent transport. Here, we investigate a similar set-up, using direct numerical simulation; in contrast to MO-based models, this type of simulation does not need to rely on turbulence closure assumptions. We show that previous predictions are verified, but now independent of turbulence parametrizations. Also, it appears that a regime shift to the very stable state is signaled in advance by specific changes in the dynamics of the turbulent boundary layer. Here, we show how these changes may be used to infer a quantitative estimate of the transition point from the weakly stable boundary layer to the very stable boundary layer. In addition, it is shown that the idealized, nocturnal boundary-layer system shares important similarities with generic non-linear dynamical systems that exhibit critical transitions. Therefore, the presence of other, generic early warning signals is tested as well. Indeed, indications are found that such signals are present in stably stratified turbulent flows.