Sources and processes of iron aerosols in a megacity of Eastern China

Iron (Fe) in aerosol particles is a major external source of micronutrients for marine ecosystems, and poses a potential threat to human health. To understand these impacts of aerosol Fe, it is essential to quantify the sources of dissolved and total Fe. In this study, we applied a receptor modelling for the first time to apportion the sources of dissolved and total Fe in fine particles collected under five different weather conditions in Hangzhou megacity of Eastern China, which is upwind of East Asian outflow. Results showed that Fe solubility (dissolved to total Fe) was the largest in fog days (6.3 ± 2.6 %), followed by haze (4.6 ± 1.9 %), dust (2.1 ± 0.7 %), clear (1.7 ± 0.6 %), and rain (0.8 ± 0.3 %) days. Positive Matrix Factorisation (PMF) analysis suggested that industrial and traffic emissions were the two dominant primary sources of dissolved and total Fe during haze and fog days, but dust was the dominant source in dust days. About 15 % of dissolved Fe was associated with secondary sources during haze and fog days, although it was less than 5 % during dust and clear days. Transmission electron microscopy analysis of individual particles showed that approximately 76 % and 87 % of Fe-containing particles were internally mixed with acidic sulfates and nitrates in haze and fog days, respectively. Our results indicated that aqueous surface of aerosol particles promotes heterogeneous reactions between acidic species and Fe aerosol, contributing to higher Fe solubility during fog and haze days.

Recommendations for HCHO and SO2 Retrieval Settings from MAX-DOAS Observations under Different Meteorological Conditions

Recently, the occurrence of fog and haze over China has increased. The retrieval of trace gases from the multi-axis differential optical absorption spectroscopy (MAX-DOAS) is challenging under these conditions. In this study, various reported retrieval settings for formaldehyde (HCHO) and sulfur dioxide (SO2) are compared to evaluate the performance of these settings under different meteorological conditions (clear day, haze, and fog). The dataset from 1st December 2019 to 31st March 2020 over Nanjing, China, is used in this study. The results indicated that for HCHO, the optimal settings were in the 324.5–359 nm wavelength window with a polynomial order of five. At these settings, the fitting and root mean squared (RMS) errors for column density were considerably improved for haze and fog conditions, and the differential slant column densities (DSCDs) showed more accurate values compared to the DSCDs between 336.5 and 359 nm. For SO2, the optimal settings for retrieval were found to be at 307–328 nm with a polynomial order of five. Here, root mean square (RMS) and fitting errors were significantly lower under all conditions. The observed HCHO and SO2 vertical column densities were significantly lower on fog days compared to clear days, reflecting a decreased chemical production of HCHO and aqueous phase oxidation of SO2 in fog droplets.

The Combined Effects of ENSO and Arctic Oscillation on Wintertime Fog Days in Eastern China

The data of fog days from weather observation stations in China and the NCEP/NCAR re-analysis data from 1954 to 2007 are used to investigate the combined effects of El Niño and Southern Oscillation (ENSO) and Arctic Oscillation (AO) on the number of winter fog days in eastern China. The results show that during El Niño the enhanced low-level southwesterly warm-moist airflow could lead to the temperature rise and humidity increase in eastern China. Note that the rate of humidity increase is faster than the rate of temperature rise, which makes the air in eastern China easy to be saturated. Besides, in winter the North China is dominated by the sinking airflow, so a large-value area of fog days appears in eastern China with the center in North China. While in La Niña years, the atmospheric circulation and its influence on the fog days are the opposite. During the positive AO period, the East Asian trough weakens and the low-level westerly jet moves northward, preventing northwesterly cold air from moving southward. The warming and humidification of North China and the slight temperature drop in South China would cause more fog days in North China and fewer fog days in South China. The effect of negative AO is opposite to that of positive AO. The combined effects of ENSO and AO are far greater than the sum of their individual effects. Under El Niño and positive AO, the number of fog days would increase significantly in North China during the whole winter. Besides, ENSO has greater impacts than AO during early winter and vice versa during later winter.

A Fog Climatology at Abu Dhabi International Airport

Fog has a significant effect on aviation and road transport networks around the world. The International Airport in Abu Dhabi, United Arab Emirates, experiences dense fog during winter months that affect operations at the airport. We describe the fog climatology at the airport using 36 years of aviation routine weather reports (METAR), an important long-term data source, and report on the number of fog days per year, the seasonal cycle, the diurnal cycle, and the duration of fog events. Fog days per year vary from 8 to 51, with a mean of ~23.91 days (standard deviation of 9.83). Events are most frequent from September until March, with December and January being the most active months. November, unexpectedly, has a low number of fog days, which appears to be due to a decrease in aerosol loading in the atmosphere. The most fog days experienced in one month is 13 (March 2004). Fog occurs any time from 1900 to 1100 local time, and the frequency increases as night progresses, peaking around sunrise. Fog events most frequently last 1 h or less. Events of 9 h or more were recorded in January and December, with the longest event lasting 16 h. Events are strongly dependent on the land–sea breeze and seldom form when the wind is blowing from the Arabian Gulf. The thickness of the nocturnal inversion layer increases up to about 500 m AGL on fog days as compared with 273 m AGL on clear-sky days. This study is the first to use the 36-yr dataset to characterize fog climatology at Abu Dhabi Airport.

Anomalous Atmospheric Circulation Associated with the Extremely Persistent Dense Fog Events over Eastern China in the Late Autumn of 2018

Under a declining trend of fog days in China, the duration of fog events since the 1990s reached a significant peak in the late autumn of 2018 over Eastern China. The average anomalous fog days were 4.74 d in November 2018 over Jiangsu Province in Eastern China, with a 1.73 standard deviation departure from climatology. Those fogs can thus be identified as a significantly abnormal climatic event with long duration, strong intensity, and extensive coverage. Based on the daily evolutions and correlations of atmospheric parameters, the dense fogs are revealed to be well configured by favorable metrological conditions such as weak dynamic progress, strong inversion in the lower troposphere and saturated air near the surface. If not disturbed, the intensification or duration of these conditions will further promote and maintain the development of fogs. The anomalous atmospheric background associated with those favorable meteorological conditions is revealed by composing the standardized anomalies of circulation fields during the fog days. Over the fog areas, vortex activities or cold air invasion is effectively hampered and the atmosphere inclines to be stable, due to the anomalous circulation pattern composed of the broadened jet stream, weakened jet core over Eastern China, undermined East Asian trough, declined East Asian winter monsoon, and enhanced anomalous southerly flows that transport abnormal warm and wet air to Eastern China. The vapor supplement is intensified by both sustained anomalous northward wind at the lower troposphere and anomalous westward wind in the near-surface. Overall, the numbers of standardized anomalies of 1000–200-hPa height, temperature, wind, and moisture fields during these fog days all significantly depart from climatology for that locale and time of the season, further demonstrating that the persistent dense fogs over Eastern China in the late autumn of 2018 is an unusual weather event with extreme synoptic-scale departures from normal.

The vertical distribution of PM2.5 and boundary-layer structure during winter haze in Nanjing

<p>At the end of November 2018, a heavy air pollution event was recorded by many meteorological stations in the Yangtze River Delta (YRD), China. The local PM2.5 concentration exceeding to 200 µg m<sup>-3</sup>. This is the heaviest, longest and most widespread heavy-polluted weather in Jiangsu Province since 2018. Meanwhile, there has been severe foggy weather in Jiangsu Province, with visibility less than 200 meters in most parts of the province. In order to study the interaction between PM2.5 concentration and boundary layer height in the haze event, and the effect of fog on pollutant aggregation, the boundary layer structure of the continuous haze process was analyzed by using the SORPES Observation of Nanjing University's Xianlin Campus. The results of the analysis show that:<br>1, The PM2.5 concentration in the boundary layer is inversely correlated with the boundary layer height, the higher the PM2.5 concentration, the lower the boundary layer height during the day. By absorbing and scattering solar radiation, atmospheric aerosols affect the balance of surface energy and reduce the sensitive heat flux, thereby inhibiting the development of the boundary layer. While inhibited development of the boundary layer will limit the diffusion of atmospheric aerosols, thereby increasing the concentration of atmospheric aerosols in the boundary layer. In addition, nocturnal atmospheric aerosols absorb heat, leading to strong grounding inversion temperature the next day, further inhibiting the development of the daytime boundary layer. <br>2, The fog-top inversion is very strong, far stronger than the inversion caused by atmospheric aerosols. Therefore, the heights of the boundary layer of fog days are much lower than that of non-fog days under the same pollution conditions.<br>3, During the fog, the PM2.5 concentration significantly reduced. And after the fog dissipated, due to the sun, the air moisture evaporation, PM2.5 concentration quickly reverted to the pre-fog state. Fog has limited wet removal of PM2.5.<br>4, Fog can inhibit the development of the boundary layer, with the continuation of the fog process, the pollution in the boundary layer continues to increase. At the same time, due to the inhibition of the development of the boundary layer, the diffusion of water vapor in the air is also affected, resulting in the boundary layer water vapor content is always in a high state, thus promoting the production of fog.</p>

Influence of Arctic Oscillation on Frequency of Wintertime Fog Days in Eastern China

The influence of Arctic Oscillation (AO) on the frequency of wintertime fog days in eastern China is studied based on the winter AO index, the wintertime fog-day data of national stations in China, and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data from 1954 to 2007. The results show that heavy fog and light fog are more likely to occur during winter in eastern China with the strong interannual variability. During the winter with the positive-phase AO, there are more days of heavy fog in North China but less in South China, while light fog days become more in the whole of eastern China. It is mainly because that when AO is in the positive phase, the pressure in the polar region decreases at 500 hPa; the pressure in East Asia increases anomalously; the East Asian trough decreases; and the low-level westerly jet moves northward, preventing the northwesterly cold air from moving southward. Therefore, the whole eastern China gets warmer and wetter air, and there are more light fog days with the enhanced water vapor. However, the atmosphere merely becomes more towards unstable in South China, where the precipitation increases but the heavy fog days decreases. Nevertheless, heavy fog days increase with the water vapor in North China because of moving towards a stable atmosphere, which is formed by the anomalous downdrafts north of the precipitation center in South China. When AO is in the negative phase, the situation is basically opposite to that in the positive phase, but the variations of the corresponding fog days and circulations are weaker than those in the AO-positive-phase winter, which may be related to the nonlinear effect of AO on climate.