Comparison of wet-only and bulk deposition at Chiang Mai (Thailand) based on rainwater chemical composition

Abstract

The chemical composition of 122 rainwater samples collected daily from bulk and wet-only collectors in a sub-urban area of Chiang Mai (Thailand) during August 2005–July 2006 has been analyzed and compared to assess usability of a cheaper and less complex bulk collector over a sophisticated wet-only collector. Statistical analysis was performed on log-transformed daily rain amount and depositions of major ions for each collector type. The analysis of variance (ANOVA) test revealed that the amount of rainfall collected from a rain gauge, bulk collector and wet-only collector showed no significant difference (∝=0.05). The volume weight mean electro-conductivity (EC) values of bulk and wet-only samples were 0.69 and 0.65 mS/m, respectively. The average pH of the samples from both types of collectors was 5.5. Scatter plots between log-transformed depositions of specific ions obtained from bulk and wet-only samples showed high correlation (r>0.91). Means of log-transformed bulk deposition were 14% (Na⁺ and K⁺), 13% (Mg²⁺), 7% (Ca²⁺), 4% (NO₃⁻), 3% (SO₄²⁻ and Cl⁻) and 2% (NH₄⁺) higher than that of wet-only deposition. However, multivariate analysis of variance (MANOVA) revealed that ion depositions obtained from bulk and wet-only collectors were not significantly different (∝=0.05). Therefore, it was concluded that a bulk collector can be used instead of a wet-only collector in a sub-urban area.

Introduction

Presently, emission of air pollutants is rapidly increasing mainly from anthropogenic activities due to large population, a rapidly growing economy, high-energy consumption, agricultural production and industrialization. The two major groups of air pollutants are SO_x and NO_x. Their oxidations result in particulate sulfate and particulate nitrate which ultimately determine the lifetime of those pollutants in the atmosphere (Bayraktar and Turalioglu, 2005).

Acid deposition, the major removal pathway for secondary pollutants, can take place in two forms wet and dry deposition, which together is referred to as bulk deposition. Wet deposition is defined as the process by which secondary pollutants are incorporated into cloud and deposited back to the earth's surface in the form of rain, snow and mist. In theory, wet deposition is best measured by using a wet-only collector which has a removable lid that covers a collecting bucket to exclude dry deposition during a dry period and opens whenever precipitation is detected by a precipitation sensor (Dämmgen et al., 2005; Staelens et al., 2005; Plaisance et al., 1998). However, it has the drawback of being expensive, requiring a power supply and sensors. Therefore, the use of bulk collectors to collect wet precipitation allows for the cost-effective operation of large networks (Dämmgen et al., 2005).

The difference in chemical composition between bulk and wet-only deposition depends on deposition of dry gases and particles which could have been resuspended by wind and deposited into the bulk collector during the dry period. Dry deposition may contaminate wet deposition where bulk collectors are used (Galloway and Likens, 1976; Fowler and Cape, 1984; Tanner, 1999; Akkoyunlu and Tayanc, 2003; Staelens et al., 2005). However, this contamination is not considered as a serious problem in rural locations distant from polluting sources (RGAR, 1987). Normally, Ca²⁺, Mg²⁺ and K⁺ concentrations are often higher in bulk collectors than in wet-only collectors (Thimonier, 1998), while differences for NO₃⁻, SO₄²⁻ and NH₄⁺ are smaller, but local or regional sources of emissions can significantly influence the composition of bulk collectors (Stedman et al., 1990). Although Galloway and Likens (1976) recommended that only wet-only collectors be used to collect rain precipitation for chemical analysis; however, there is some evidence from the literature that suggests bulk collectors may be used instead of wet-only collectors at high altitudes in winter (Peden and Skowron, 1978; Reddy and Claassen, 1985; Sisterson et al., 1989).

Many previous studies concerning the composition of precipitation have been carried out (Ahmed et al., 1990; Saxena et al., 1996; Bravo et al., 2000; Thepanondh et al., 2005). However, only few studies (Stedman et al., 1990; Ranalli et al., 1997; Staelens et al., 2005) have been done on differences in the chemical composition of precipitation collected by bulk and wet-only collectors. In Thailand, only one work from Granut et al. (1996) was reported by using the scatter plot and percent difference between ion concentrations from both bulk and wet-only collectors. The study also revealed that bulk deposition was 10–30% higher than wet deposition. Mg²⁺ showed the greatest difference between both depositions, and the result also revealed that Mg²⁺ was a predominantly neutralized component.

The aims of this study are therefore: (i) to analyze and compare the chemical compositions in precipitations collected from bulk and wet-only collectors, and (ii) to assess the usability of a bulk collector in comparison to a wet-only collector.