Characterization of black carbon concentrations of haze with different intensities in Shanghai by a three-year field measurement

Abstract

Relationships between black carbon (BC) aerosols and haze with different intensities are analysed by using hourly averaged data of BC mass concentration (CBC) and meteorological parameters from April 2007 to March 2010 in Shanghai. The results show that the number of cumulative haze hours accounted for 59.6% of the all valid hours during the three-year test period, while slight, mild, moderate and severe haze contributed 64.4%, 24.3%, 8.8% and 2.5% of the haze hours, respectively. Seasonal haze hour percentages in all haze hours were 19.8, 19.5, 24.5 and 36.2 for spring, summer, fall and winter, respectively. The annual averaged CBC for haze hours and non-haze hours were 5.05 μg/m3 and 2.04 μg/m3 during the three test years, respectively. The observation results show a positive correlation between monthly averaged CBC and haze intensity, i.e., the higher the haze intensity, the higher the CBC. According to the test data, four ranges of CBC were introduced to show the relationships between BC aerosols and haze hour percentage, and found that the higher the CBC, the higher the percentage of haze hours in the corresponding range. When CBC was higher than 4.5 μg/m3, the haze hours accounted for 95.0% of the valid test hours of the three year time span, and it can be used as a sufficient condition of haze occurrence. In addition, a set of 96-h backward trajectories indicate that the air masses originated from north China were the main non-local sources of haze in spring, fall and winter in Shanghai, and emissions from south regions of Shanghai were the primary non-local sources of the haze in summer.