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Local air pollution is strongly affected by synoptic weather systems, such as fronts, troughs, low-altitude vortices, or high-altitude ridges. Nevertheless, few studies have analyzed the meteorological properties of cold or warm air masses associated to these systems and their impact on local air quality. In this study, hourly observations of fine particulate matter (diameter of up to 2.5 µm, i.e., PM2.5), wind (V), temperature (T), pressure (P), and precipitation (R), acquired in Hangzhou in 2014–2020, were analyzed. From this analysis, weather patterns were categorized into 27 types; 89 and 94 cases illustrating the passage of warm and cold air masses over Hangzhou were identified, respectively; the influence of air mass temperature, wind speed, and wind direction on PM2.5 concentrations and local accumulation or removal was quantified. The main results are as follows. (1) Pollution events occurred more frequently for cold than for warm air masses, but average pollutant concentration was lower for cold air masses; (2) 48% of the cold air mass cases corresponded to PM2.5 decreases and 52% to PM2.5 increases, with strong cold air masses (ΔT24h > 4°C; ∣V∣average > 4 m s−1) markedly reducing local pollution, but weak cold air masses (ΔT24h < 2°C; ∣V∣average < 2 m s−1) primarily inducing pollutant transport and accumulation; (3) for warm air masses, PM2.5 accumulation or removal occurred in 60% and 40% of the cases, respectively: warm air masses (ΔT24h > 4°C) reduced the PM2.5 concentration whereas weaker winds (∣V∣average < 2 m s−1) increased it; and (4) PM2.5 concentration decreased sharply within 4 h after the passage of strong cold air masses, but more gradually within 14 h after the passage of strong warm air masses. These results considerably improve the current understanding of the influence of cold and warm air masses on local pollution patterns. [ABSTRACT FROM AUTHOR]

Long‐range transport of PM2.5 from the North China Plain (NCP) by cold frontal passage is considered to be an important haze formation mechanism in the Yangtze River Delta (YRD). However, detailed 3D distributions of PM2.5 and meteorological fields during the front moving through are not fully understood. Herein, in situ observations showed that most haze events in autumn and winter in the YRD were associated with cold frontal passages. A typical cold front episode on November 2–3, 2017, was investigated with unmanned aerial vehicle sounding and model simulations, which both revealed that PM2.5 in the upper air (∼1.0 km) reached the YRD earlier than that moving at the ground level. When the cold front reached the NCP, air pollutants ahead of the front was lifted from the ground level to the upper air. Subsequently, increased PM2.5 concentrations and higher wind speed in the upper air resulted in stronger PM2.5 outflow from the NCP to YRD compared to that at ground level. The maximum PM2.5 mass contribution from the NCP to YRD was 15.6 μg m−3 (accounted for 31.5% of the total PM2.5 in the YRD), which appeared at ∼1.0 km altitude. Compared to the noncold front period, the PM2.5 mass transported from the NCP was 10–15 times higher during the cold front episode. Results indicate that the combination of multisite sounding and elaborate simulation can effectively explain the impact of synoptic processes on the transport, distribution, and evolution of air pollution and provide precise pollution control directives. Key Points: Most haze events in autumn and winter in the Yangtze River Delta were associated with cold frontal passagesBoth unmanned aerial vehicle sounding and model simulation revealed a stronger and faster upper‐level transport of PM2.5 than at ground levelThe PM2.5 mass transported from the North China Plain was 10–15 times higher during a cold front episode [ABSTRACT FROM AUTHOR]

Visibility is an indicator of atmospheric transparency and is widely used in many research fields, including air pollution, climate change, ground transportation, and aviation. Although efforts have been made to improve the performance of visibility meters, a significant error exists in measured visibility data. This study conducts a well-designed simulation calibration of visibility meters, which proves that current methods of visibility measurement include a false assumption, which leads to the long-term neglect of an important source of visibility error caused by erroneous values of Ångström exponents. This error has two characteristics. (1) Independence, in which the magnitude of the error is independent of the performance of the visibility meter. It is impossible to reduce this error by improving the performance of visibility meters. (2) Uncertainty, in which the magnitude of the error does not show a clear change pattern but can be substantially larger than the measurement error of visibility meters. It is impossible to accurately estimate the magnitude of this error nor its influence on visibility measurements. Our simulations indicate that, as errors in visibility caused by erroneous values of Ångström exponents are inevitable using current methods of visibility measurement, reliable visibility data cannot be obtained without major adjustments to current visibility measurement methods. [ABSTRACT FROM AUTHOR]