*2.1. Study Area and Sample Collection*

Nanjing (118◦22"and 119◦14" E, 31◦14" and 32◦37" N) is the capital of Jiangsu province, the second largest city in the Yangtze River Delta. With rapid development in recent decades, vehicle exhaust, coal-fired power generation, and industrial activities have become the main anthropogenic sources of PM in Nanjing. The Gulou District is the urban center of Nanjing with huge population and heavy traffic. So, we chose the Gulou campus of Nanjing University and Zifeng Tower in this district to be the sampling sites. One of the sampling sites, Zifeng Tower, is a 450-meter skyscraper completed in 2010 in Nanjing, China. It is the tallest building in Jiangsu province, and the 10th tallest building in the world (as of August 2016). The observation instruments were located on the top platform at 380 m. Another set of observation instruments was located on the roof of a 20 m high building in Gulou campus, Nanjing University. The two observation points are close with a distance of less than 700 m. The locations of sampling sites are shown in Figure 1. The two sampling sites are represented as Gulou and Zifeng in the following text.

**Figure 1.** The locations of sampling sites.

The sampling campaign was carried out from 16 November to 12 December in 2016 at 20 m and 380 m height, except 28 November due to power failure at Gulou. During the experiment, size-fractionated ambient particulate matter (PM) were collected with an eight-stage Sierra-Andersen cascade impactor (Andersen Instruments, Inc., Atlanta, GA, USA), which can classify nine intervals in the following order: <0.43 μm, 0.43–0.65 μm, 0.65–1.1 μm, 1.1–2.1 μm, 2.1–3.3 μm, 3.3–4.7 μm, 4.7–5.8 μm, 5.8–9.0 μm, and 9.0–10 μm (aerodynamic diameter) [25,36]. Two parallel samplers with flow rate of 28.3 L min−<sup>1</sup> were placed at each height in order to obtain chemical compositions of particles with Teflonmembrane filters and quartz fiber filters (Diameter 81 mm). Forty-seven hours of sampling was performed every two days from 9 am to 8 am and on the third day to obtain adequate materials at high level.

#### *2.2. Chemical Analysis*

Elements (e.g., Al, As, Ba, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Ni, P, Pb, Sn, Sr, Ti, V, and Zn), ions (e.g., F−, CH3COO−, HCOO−, Cl−, NO3 <sup>−</sup>, SO4 <sup>2</sup>−, C2O4 <sup>2</sup><sup>−</sup>, Na+, NH4 +, K+, (CH3)2NH2 +, Ca2+, Mg2+), and carbonaceous materials (organic carbon (OC) and elemental carbon (EC)) were analyzed for each sample.

## 2.2.1. Analysis of Elements

Elements obtained from Teflon filters were analyzed by an inductively coupled plasma mass spectrometer (ICP-MS, Agilent Technologies, Inc., Model 7500a, Santa Clara, CA, USA). Calibration with reference material (Environmental Calibration Standard, Part 5183- 4688, Agilent Technologies) demonstrated good linearity and sensitivity for the instrument. The relative standard deviation for each measurement (repeated twice) was within 3%. The method detection limits (MDLs) were determined by adding 3 standard deviations of the blank readings to the average blank values [26].
