Seasonal Characteristics of Black Carbon Aerosols over an Urban City in India: Source Analysis Using Concentration Weighted Trajectories

In this study year-round (2013) measurements of surface black carbon (BC) mass concentration recorded using a seven-channel Aethalometer (AE-31) over a tropical semi-urban site in Nagpur, in central India are presented. The annual mean BC during the studied period was 3.64 ± 1.81 μg m−3, which is comparable to that observed at few Indian and international sites. Seasonal variability in BC concentration with maximum values during the winter (4.86 ± 2.3 μgm-3) and minimum in the monsoon (2.3 ± 0.7 μg m−3) season and moderate (~3.7 μg m−3) during the pre and post monsoon seasons was observed. The diurnal variation of BC concentration shows two peaks, one in the morning (07:00–08:00 h local time (LT)) and other prominent peak in the evening hours (19:00–21:00 h LT), with a minimum peak in the afternoon (~16:00 h LT), which could be associated with diurnal variation in local sources, boundary-layer dynamics and synoptic meteorology. The relationship between measured BC concentration and the meteorological parameters for the study location reveals a positive (negative) correlation with the mean diurnal range of air temperature (relative humidity, wind speed and rainfall). The estimated monthly mean absorption Angstrom exponent values range from 0.87 to 1.13 suggesting the change in the sources from fossil-fuel burning in the monsoon and post-monsoon periods to biomass burning in the winter and pre-monsoon. The concentration weighted trajectories analysis suggests that the potential source regions contributing to the observed BC concentration change seasonally and are positioned in the north, northwest of the observational site and in central India.