Comparison of Tree Leaf-washing and Four-stage Filter Pack Sampling Methods for Analysis of Atmospheric Particulates Deposition

Abstract

Atmospheric particulates are deposited to the earth surface via wet and dry deposition. Particulate deposition can be collected by four-stage filter pack method, but it is quite expensive due to complexity of deployment and electricity requirement. Leaf-washing method is an alternative sampling method, which is easy, low operating cost, no electricity requirement and can be applied in wider areas. This study aims to compare particulate deposition from those two methods to test their sampling efficiencies and to compare seasonal variation of the depositions. The sampling has been conducted for 9 months from July 2012 to March 2013 at Mae Hai Research Center, Chiang Mai University, Chiang Mai Province, where the acid deposition monitoring station is located. In case of four-stage filter pack, samples have been collected every 10 days for the whole sampling period. For leaf-washing method, jambolan plum tree (Syzygium cumini) was selected because it is an evergreen tree and has an appropriate leaf size. It is higher than 3 meters (according to filter pack sampling height) and it is located near the station, which is suitable for comparison study. The sampling was carried out monthly in wet season and every 10 days in dry season. The tree canopy was divided into 8 directions and 3 leaves were randomly selected from each direction. A selected leaf was washed by 100 ml of deionized water. Sample solution was divided for pH and electro-conductivity measurement, and analysis of ions (Na+, NH4+, K+, Ca2+, Mg2+, Cl-, NO3- and SO42-) by ion chromatography. It was found that sampling directions (tree leaf position) of the tree canopy affected to amount of particulate deposition. The directions near pollutant sources and stay upwind tended to have higher deposition of particulates. In terms of seasonal

variation, both methods provided higher ion deposition in dry season than wet season due to wash out effect from precipitation. Ion deposition collected by four-stage filter pack method was approximately 2-4 times higher than that collected by the leaf-washing method both in wet and dry seasons. It is mainly because the leaf-washing method was affected by meteorological conditions particularly precipitation, free flow of particulate deposition on leaf surface, which depends on wind speed and direction. Moreover, leaf characteristics such as surface, shape and size also affected to particulate deposition on leaves. Therefore, ion depositions obtained from the two methods were significantly different (p < 0.05). However, the first three major ions from both methods were identical. Sulfate and nitrate were the dominant anions, while calcium was the major cations detected. In addition, correlations between pairs of the same ions were relatively high (r = 0.250-0.932) for both methods in dry season. Therefore, leaf-washing method showed high potential to be used as a sampling method for atmospheric particulates deposition in dry season in the area where appropriate trees exist.