First long-term and near real-time measurement of trace elements in China's urban atmosphere: temporal variability, source apportionment and precipitation effect

Atmospheric trace elements, especially metal species, are an emerging environmental and health concern with insufficient understanding of their levels and sources in Shanghai, the most important industrial megacity in China. Here we continuously performed a 1 year (from March 2016 to February 2017) and hourly resolved measurement of 18 elements in fine particles (PM2.5) at the Shanghai urban center with an Xact multi-metals monitor and several collocated instruments. Mass concentrations (mean +/- 1 sigma; ng m(-3)) determined by Xact ranged from detection limits (nominally 0.1 to 20 ng m(-3)) to 15 mu g m(-3). Element-related oxidized species comprised an appreciable fraction of PM2.5 during all seasons, accounting for 8.3% on average. As a comparison, the atmospheric elements concentration level in Shanghai was comparable with that in other industrialized cities in East Asia but 1 or 2 orders of magnitude higher than at sites in North America and Europe. Positive matrix factorization (PMF) was applied to identify and apportion the sources of the elements in the PM2.5 mass. Five different factors were resolved (notable elements and relative contribution in parentheses): traffic-related (Ca, Fe, Ba, Si; 46 %), shipping (V, Ni; 6 %), nonferrous metal smelting (Ag, Cd, Au; 15 %), coal combustion (As, Se, Hg, Pb; 18 %) and ferrous metal smelting (Cr, Mn, Zn; 15 %). The contribution from the exhaust and non-exhaust vehicle emissions, i.e., the traffic-related factor shows a strong bimodal diurnal profile with average concentration over 2 times higher during the rush hour than during nighttime. The shipping factor was firmly identified because V and Ni, two recognized tracers of shipping emissions, are almost exclusively transported from the East China Sea and their ratio (around 3.2) falls within the variation range of V / Ni ratios in particles emitted from heavy oil combustion. Interestingly, nearly half of the K was derived from coal combustion with high mineral affinity (elements associated with aluminosilicates, carbonates and other minerals in coal ash). The contributions of nonferrous metal smelting to the trace elements are consistent with a newly developed emission inventory. Although the precipitation scavenging effect on the mass concentration of the trace elements varied among different species and sources, precipitation could effectively lower the concentration of the traffic- and coal combustion-related trace elements. Therefore, water spray to simulate natural types of precipitation could be one of the abatement strategies to facilitate the reduction of ambient PM2.5 trace elements in the urban atmosphere. Collectively, our findings in this study provide baseline levels and sources of trace elements with high detail, which are needed for developing effective control strategies to reduce the high risk of acute exposure to atmospheric trace elements in China's megacities.