Investigation of trace metals in air particulate matter collected from Colombo

Abstract

The suspended particulate matter (SPM) in air is a complex multi-phase mixture of all airborne chemicals.Among the airborne inorganic chemicals, compounds associated with heavy metals have subjected tonumerous chemical investigations due their known health effects. Uniqueness of the heavy metalshas attributed its use in identifying or tracing the pollutant sources through source apportionmentstudies and in understanding the distribution of pollutants through computer modeling.

In general, airborne metals exist in trace quantities. The suspended particulate matter may contain themetallic species as fine particles or they may have adsorbed onto other particulate matter originatedfrom natural or human activities. Agglomerated heavier particle settles due to gravitation and pose alesser risk to human health. Finer particles with aerodynamic diameter less than 2.5 urn (PM2) arebelieved to posses the biggest health threat through respiration

Central Environmental Authority (CEA) has been involved in air quality monitoring at the ColomboFort Railway Station. Using their high volume air sampler, particles having aerodynamic diameter lessthan I0 ~L1n(PM 10) were collected onto glass fiber filters daily during the period of November 2003 toNovember 2005 November. They were gravimetrically analyzed by CEA. This study was focused tofurther investigate randomly selected filters (two filters per month) to differentiate the levels of selectedmetals during the period. The filters were acid digested to recover the airborne metallic species.Levels of copper (Cu), nickel (Ni), cadmium (Cd), lead (Pb) and manganese (Mn) were analyzed byusing atom ic absorption spectrophotometer at the Department of Chemistry, University of Colombo

The average concentrations found during the study were Cu = 0.29 ± 0.25 ng/rri', N i = 0.21 ± 0.1 0 ng/m', Pb = 0.59 ± 0.48 ng/rn", Cd = 0.03 ± 0.03 ng/rn ', Mn = 0.43 ± 0.22 ng/rn '. During the study periodPb levels were found to be the highest and Cd levels were found to be the lowest. The maximumairborne levels were 1.24 ng/rn ' for ClI in April 2005,0.56 ng/rn ' for Ni in January 2005,2.37 ng/m 'for Pb in February 2004, 0.09 ng/m ' for Cd in February 2004 and 0.78 ng/rn ' for Ni in May 2005.

On average the minimum airborne levels were found in the period of May to August. This is attributedto the settling of particles with rainfall as well as the wind patterns which drifts the pollutants awayfrom the sampler during the season. From November to Apri I all the five metals have showed relati velyhigher concentrations compared to any other months in each year. This may be due to poor dispersionpatterns in the Colombo air which results in stagnating pollutants. In general, wind speed is relativelyhigher for the period of May to August compared to the period of November to April

The most prevailing metal in the Colombo air is Pb while the least prevailing metal is Cd. The levelsofPb have demonstrated a pronounced decline in concentration starting from April 2005 whrch coincideswith the introductiorr of un-leaded petrol to Sri Lankan market in January 2004.