Analysis of the formation of fog and haze in North China Plain (NCP)

Abstract

North China Plain (NCP) is one of the most populated and polluted regions in China. During the recent years, haze and fog occur frequently and cause severely low visibility in this region. In order to better understand the impact of aerosol particles on the formation of haze and fog, a long-term record of haze and fog occurrences in the past 56 years (from 1954–2009) over NCP is analyzed. The results show that there are rapid changes in the occurrences of haze and fog over NCP. The occurrences of haze and fog were low during 1970–1980, and reached a maximum during 1981–1998. After 1999, the occurrences of haze and fog slightly decreased. There was a nonlinear relationship between the occurrences of haze and fog. When the occurrence of haze was lower than 40 days yr<sup>−1</sup>, the occurrence of fog was strongly proportional to the occurrence of haze. However, when the occurrence of haze was high (larger than 75 days yr<sup>−1</sup>), the occurrence of fog was not sensitive to the occurrence of haze. In order to better understand the relationship between the occurrences of haze and fog as well as the effect of aerosol particles on the formation of haze and fog, an in-situ field experiment was conducted during a period with a mixed occurrence of haze and fog. The analysis of the experiment suggests that there were considerably high aerosol concentrations during the measurement period, with an averaged aerosol number concentration of 24 000 cm<sup>−3</sup>. The measurement also shows that a large amount of aerosol particles can act as condensation nuclei to enhance the formation of fog droplets. As a result, a large amount of fog droplets (>1000 cm<sup>−3</sup>) with small size in radius (5–6 μm) were observed during the fog period, resulting in extremely low visibility (less than 100 m)

What Causes Observed Fog Trends: Air Quality or Climate Change?

“…They argue that the decrease of fog occurred mainly during the winter (heating period) and that an increase of air quality since 2003 is the main cause for the decrease of fog. A similar pattern with increasing fog in the 1970s and 1980s and a decreasing trend after 1999 was observed by Quan et al (2011). Ding and Liu (2014) argue for an observed decrease of fog in China after 1990 that it is a decrease of rH that causes that trend.…”

“…In this study, we analyze rather specific situations of fog that may form at rH < 100%, when deliquescence of particles is intense enough to reduce the horizontal visibility to less than 1000 m. The conditions we study are somewhat at the verge between haze and fog. Haze describes conditions in the atmosphere with a reduction of the horizontal visibility to less than a few km, but not to less than 1 km, by dry particles or by deliquesced particles or both, and during conditions of high relative humidity, though clearly below 100% (Quan et al, 2011). In a given air mass, the conditions may change from haze to fog when the relative humidity increases to near 100% and above.…”

Aerosol Optical Properties of a Haze Episode in Wuhan Based on Ground-Based and Satellite Observations

“…At present, there is no globally consistent definition of haze. The Chinese Meteorological Administration (CMA) defines haze as conditions with visibility <10 km, whereas some studies have adopted a visibility criterion of <5 km [46,47]. For all previous studies, visibility has been adopted as the basic index; however, because both haze and fog result in reduced visibility, on 1 June 2010, the CMA issued a standard for haze observation and forecasting in China.…”

Climatological study of a new air stagnation index (ASI) for China and its relationship with air pollution

“…Light winds lack the ability to disperse air pollutants or transport them far away Rigby and Toumi, 2008;Tai et al, 2010), and 5 those pollutants blown by winds could directly contaminate the downwind zone (Wehner and Wiedensohler, 2003;Elminir, 2005). In addition, high relative humidity usually results in low visibilities through aerosol hygroscopic growth Quan et al, 2011).…”