Atmospheric wet and dry deposition of trace elements at 10 sites in Northern China

Y P Pan,Y S Wang

doi:10.5194/acp-15-951-2015

Y P Pan, Y S Wang

Open Access

https://doi.org/10.5194/acp-15-951-2015

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Abstract

Abstract. Atmospheric deposition is considered to be a major process that removes pollutants from the atmosphere and an important source of nutrients and contaminants for ecosystems. Trace elements (TEs), especially toxic metals deposited on plants and into soil or water, can cause substantial damage to the environment and human health due to their transfer and accumulation in food chains. Despite public concerns, quantitative knowledge of metal deposition from the atmosphere to ecosystems remains scarce. To advance our understanding of the spatiotemporal variations in the magnitudes, pathways, compositions and impacts of atmospherically deposited TEs, precipitation (rain and snow) and dry-deposited particles were collected simultaneously at 10 sites in Northern China from December 2007 to November 2010. The measurements showed that the wet and dry depositions of TEs in the target areas were orders of magnitude higher than previous observations within and outside China, generating great concern over the potential risks. The spatial distribution of the total (wet plus dry) deposition flux was consistent with that of the dry deposition, with a significant decrease from industrial and urban areas to suburban, agricultural and rural sites, while the wet deposition exhibited less spatial variation. In addition, the seasonal variation of wet deposition was also different from that of dry deposition, although they were both governed by the precipitation and emission patterns. For the majority of TEs that exist as coarse particles, dry deposition dominated the total flux at each site. This was not the case for potassium, nickel, arsenic, lead, zinc, cadmium, selenium, silver and thallium, for which the relative importance between wet and dry deposition fluxes varied by site. Whether wet deposition is the major atmospheric cleansing mechanism for the TEs depends on the size distribution of the particles. We found that atmospheric inputs of copper, lead, zinc, cadmium, arsenic and selenium were of the same magnitude as their increases in the topsoil of agricultural systems. At a background forest site in Northern China, the total deposition flux of lead observed in this study (14.1 mg m−2 yr−1) was twice that of the critical load calculated for temperate forest ecosystems in Europe. These findings provide baseline data needed for future targeting policies to protect various ecosystems from long-term heavy metal input via atmospheric deposition.

Highlights

Air pollution is generally considered an accumulation in the atmosphere of substances that, in sufficient concentrations resulting from excessive anthropogenic emissions and natural sources, endanger human health and the environment
When the 25 Trace elements (TEs) at each site were roughly identified using enrichment factors (EFs) relative to the average crustal composition with Al as a reference (Duce et al, 1975; Mason and Morre, 1982), only Pb, Zn, Cd, As, Se, Ag and Sb had EFs above 10, suggesting that the fluxes of these TEs were substantially affected by human activities
This study provides the first long-term direct measurements of atmospheric wet and dry deposition fluxes of crustal and anthropogenic metals on a regional scale across China

Summary

Introduction

Air pollution is generally considered an accumulation in the atmosphere of substances that, in sufficient concentrations resulting from excessive anthropogenic emissions and natural sources, endanger human health and the environment. In addition to mitigation measures taken by local governments, two primary natural processes have been recognized as participating in the reduction of air pollutants: dry and wet deposition. The removal of pollutants from the atmosphere by wet deposition is often considered an important natural mediating factor in cleansing the atmosphere (Yang et al, 2012). In contrast to the episodic nature of wet deposition, dry deposition is a continuous and dependable process involved in atmospheric cleansing (Grantz et al, 2003). In regions with low precipitation, such as the Mediterranean climate area (Muezzinoglu and Cizmecioglu, 2006), dry deposition as a cleansing mechanism is more important than wet deposition on an annual basis. The relative importance of wet versus dry deposition may depend on the efficiencies of these two mechanisms but it varies with the local availability of precipitation (Muezzinoglu and Cizmecioglu, 2006). In the absence of simultaneous measurements of these two processes, their relative and combined contributions to the total deposition remain unclear, and debate remains over whether dry deposition is the major cleansing mechanism

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