First long-term and near real-time measurement of trace elements in China's urban atmosphere: temporal variability, source apportionment and precipitation effect
Authors: Chang Yunhua , Huang Kan , Xie Mingjie , Deng Congrui , Zou Zhong , Liu Shoudong , Zhang Yanlin 
Published: August 2018

Abstract. 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 ± 1σ; ng m−3) determined by Xact ranged from detection limits
(nominally 0.1 to 20 ng m−3) to 15 µg 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.