Significant impacts of heterogeneous reactions on the chemical composition and mixing state of dust particles: A case study during dust events over northern China

Accepted Manuscript Significant impacts of heterogeneous reactions on the chemical composition and mixing state of dust particles: A case study during dust events over northern China Zhe Wang, Xiaole Pan, Itsushi Uno, Jie Li, Zifa Wang, Xueshun Chen, Pingqing Fu, Ting Yang, Hiroshi Kobayashi, Atsushi Shimizu, Nobuo Sugimoto, Shigekazu Yamamoto PII:

S1352-2310(17)30213-3

DOI:

10.1016/j.atmosenv.2017.03.044

Reference:

AEA 15255

To appear in:

Atmospheric Environment

Received Date: 31 December 2016 Revised Date:

22 March 2017

Accepted Date: 24 March 2017

Please cite this article as: Wang, Z., Pan, X., Uno, I., Li, J., Wang, Z., Chen, X., Fu, P., Yang, T., Kobayashi, H., Shimizu, A., Sugimoto, N., Yamamoto, S., Significant impacts of heterogeneous reactions on the chemical composition and mixing state of dust particles: A case study during dust events over northern China, Atmospheric Environment (2017), doi: 10.1016/j.atmosenv.2017.03.044. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Significant impacts of heterogeneous reactions on the chemical composition

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and mixing state of dust particles: a case study during dust events over

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northern China

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Zhe Wang1,2, Xiaole Pan1,2, Itsushi Uno2, Jie Li1, Zifa Wang1, Xueshun Chen1, Pingqing Fu1,

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Ting Yang1, Hiroshi Kobayashi3, Atsushi Shimizu4, Nobuo Sugimoto4, Shigekazu

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Yamamoto5

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Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences (CAS), Beijing, China

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Research Institute for Applied Mechanics (RIAM), Kyushu University, Fukuoka, Japan

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University of Yamanashi, Yamanashi, Japan

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National Institute for Environmental Studies (NIES), Tsukuba, Ibaraki, Japan

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Fukuoka Institute of Health and Environmental Sciences, Fukuoka, Japan

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Corresponding author: Zhe Wang, ([email protected])

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Keywords: Dust aerosols; anthropogenic pollutants; heterogeneous reaction; mixing state.

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Abstract

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The impact of heterogeneous reactions on the chemical components and mixing state of dust

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particles are investigated by observations and an air quality model over northern China between

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March 27, 2015 and April 2, 2015. Synergetic observations were conducted using a polarization

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optical particle counter (POPC), a depolarized two-wavelength Lidar and filter samples in

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Beijing. During this period, dust plume passed through Beijing on March 28, and flew back on

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March 29 because of synoptic weather changes. Mineral dust mixed with anthropogenic

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pollutants was simulated using the Nested Air Quality Prediction Modeling System (NAQPMS)

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to examine the role of heterogeneous processes on the dust. A comparison of observations shows

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that the NAQPMS successfully reproduces the time series of the vertical profile, particulate

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matter concentration, and chemical components of fine mode (diameter ≤ 2.5 µm) and coarse

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mode (2.5 µm < diameter ≤ 10 µm) particles. After considering the heterogeneous reactions, the

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simulated nitrate, ammonium, and sulfate are in better agreement with the observed values

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during this period. The modeling results with observations show that heterogeneous reactions are

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the major mechanisms producing nitrate reaching 19 µg/m3, and sulfate reaching 7 µg/m3, on

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coarse mode dust particles, which were almost 100% of the coarse mode nitrate and sulfate. The

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heterogeneous reactions are also important for fine mode secondary aerosols, for producing 17%

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of nitrate and 11% of sulfate on fine mode dust particles, with maximum mass concentrations of

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6 µg/m3 and 4 µg/m3. In contrast, due to uptake of acid gases (e.g. HNO3 and SO2) by dust

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particles, the fine mode anthropogenic ammonium nitrate and ammonium sulfate decreased. As a

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result, the total fine mode nitrate decreased with a maximum of 14 µg/m3, while the total fine

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mode sulfate increased with a maximum of 2 µg/m3. Because of heterogeneous reactions, 15% of

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fine mode secondary inorganic aerosols and the entire coarse mode nitrate and sulfate were

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internally mixed with dust particles. The significant alterations of the chemical composition and

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mixing state of particles due to heterogeneous reactions are important for the direct and indirect

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climate effects of dust and anthropogenic aerosols.

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1. Introduction

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Mineral dust is a major component of atmospheric aerosols, and contributes 43%, 56%, and 22%

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of total aerosols in terms of emissions, column mass and optical depth (Satheesh and Krishna

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Moorthy, 2005), respectively, and 29% of global mean aerosol radiative forcing due to aerosol–

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radiation interaction (IPCC, 2013). Every year over desert regions in East Asia, one of the four

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most important dust source regions in the world, a large quantity of dust particles is suspended in

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the atmosphere by strong winds, and transported long distances (e.g., to the northern Pacific,

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North America, or even globally) (e.g., Uno et al., 2001, 2009, 2011), affecting the climate and

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environment on the regional and global scale.

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One important characteristic of Asian dust is that calcium (Ca) accounts for 39% of the total of

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seven major crustal elements, which is significantly higher than the proportion in Saharan dust

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(17%) (Krueger et al., 2004). During transportation, Ca-rich Asian dust particles readily react

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with anthropogenic acidic species (e.g., nitric acid), as well as sulfur dioxide (SO2) and nitrogen

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dioxide (NO2), through heterogeneous processes (He et al., 2014) leading to the formation of

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large quantities of water-soluble aerosols, such as Ca(NO3)2 (Li and Shao, 2009; Fairlie et al.,

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2010).

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As a result of internal mixing with water-soluble aerosols, the chemical components, shape and

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size of dust particles will be altered, and consequently their optical properties and direct effects

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on the climate will change (Bauer et al., 2007; Wang et al., 2013). The coated dust particles have

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a high hygroscopicity and can become more efficient cloud condensation nuclei (CCN),

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changing the indirect climate effects of dust aerosols (Kelly et al., 2007). Therefore, the mixing

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state of air pollutants and dust particles is a critical issue for understanding the environmental

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and climate effects of ‘polluted dust’ in Asia.

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Many previous modeling studies have studied the impacts of heterogeneous reactions of dust on

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tropospheric chemistry. Zhang et al. (1994, 1999) found that nitrate and sulfate, 0.9–2.1 and 0.3–

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10 µg m-3, respectively, are formed on dust particles based on a box model under conditions

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representative of dust events in East Asia. Underwood et al. (2001) also quantified the

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importance of heterogeneous reactions involving NO2 and HNO3 on mineral oxide and mineral

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aerosol surfaces by combining a box model and laboratory measurements. Dentener et al. (1996)

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indicated that the heterogeneous reactions can indeed take place by using a global model, but a

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rigorous evaluation is not possible at that time due to a lack of measurements. Simulation results

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of three dimensional regional models (Phadnis et al., 2000; Tang et al., 2004) showed that the

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presence of mineral aerosol can greatly impact sulfate and nitrate distributions. However,

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detailed observations over Northern China were unavailable during these studies, which are

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important for evaluating the model results since large uncertainties exist in heterogeneous

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reactions (Dentener et al. 1996; Fairlie et al., 2010).

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Beijing, located in northern China, which has a larger emission intensity of anthropogenic

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pollutants than other regions of China, as well as other regions worldwide, is also close to the

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dust source region (about 200 km away). However, dust particles do not usually mix well with

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anthropogenic pollutants over the Beijing area during severe dust periods, because the strong

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northwest wind during severe dust events usually carries dust particles through Beijing quickly,

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while the concentration of anthropogenic pollutants also decreases dramatically due to the strong

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wind. Therefore, transmission electron microscopy (TEM) observations have indicated that only

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5% of dust particles are covered by visible coatings during dust periods (Li and Shao, 2009);

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dust nitrate is only formed in small quantities and almost no dust sulfate is formed in Beijing

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(Zhang and Iwasaka, 1999). As a result, dust particles usually mix with anthropogenic pollutants

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over the downstream region of their sources (Itahashi et al., 2010), when their concentrations

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have already become significantly lower than over source regions, which limits heterogeneous

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reactions on dust particles.

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However, between March 27, 2015 and April 2, 2015, high concentrations of mineral dust and

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anthropogenic pollutants were unusually trapped and circulated over northern China, providing a

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good opportunity to research the interactions between anthropogenic pollutants and dust particles

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in high concentration conditions. In this study, we focused on the formation mechanism of this

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atypical dust event and the impacts of heterogeneous processes on the chemical components and

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mixing state of dust particles, based on synergetic observations and modeling results of a

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regional chemical transport model.

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2. Methods

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2.1. Observations

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Online observations of the light scattering and polarization property of each particle were

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performed using a polarization optical particle counter (POPC) at the Institute of Atmospheric

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Physics (IAP: 116.4˚E, 39.9˚N), Beijing in the downstream region of dust sources (about 200 km

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away) (Figure 1). A POPC observes the intensity of the forward scattering signal, as well as the

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perpendicular and parallel depolarized components of the backward scattering signal of single

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particles by using a linearly polarized laser (Kobayashi et al., 2014). The size of each particle is

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determined based on the forward scattering intensity. The depolarization ratio (DR) is defined as

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the ratio of the perpendicular component to the total backward signal. Larger DR values indicate

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more non-spherical particles. Usually, dust particles have diameters larger than 3 µm and DR

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values larger than 0.1, while most anthropogenic aerosols are smaller than 1 µm, with DR values

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smaller than 0.2 (Pan et al., 2016). POPCs have previously been used successfully to investigate

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the mixing state of dust and anthropogenic particles in Japan (Kobayashi et al., 2014; Pan et al.,

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2015; Pan et al., 2016) and Korea (Sugimoto et al., 2015); however, this was the first time this

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method has been used in China.

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Figure 1. Modeling domain and spatial distributions of dust emissions (unit: ton/km2) during the

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period from March 27, 2015 to April 1, 2015. The white square indicates the location of Beijing.

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PM2.5 and PM10 were collected on Teflon filters at the IAP, at 12 h intervals, from March 27,

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2015 to April 12, 2015. For the extraction of water-soluble components from the aerosol filter

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sample (diameter of 2 cm), 10 mL of ultra-pure water was added to the sample vial, and then

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ion-chromatography (ICS-1600 for anions and ICS-1100 for cations; Thermo Fisher Scientific,

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Waltham, MA, USA) was used to analyze the aerosol ion concentration.

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The vertical distribution of the aerosol extinction coefficient was observed continuously using a

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depolarized two-wavelength Lidar at the Institute of Atmospheric Physics (IAP) (Yang et al.,

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2010). The contributions of anthropogenic and dust aerosols to the extinction coefficient were

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estimated according to the DR, assuming that anthropogenic (DR = 0.02) and dust (DR = 0.35)

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aerosols were externally mixed (Sugimoto et al., 2003). In this paper, the anthropogenic and dust

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extinction coefficients were used to evaluate model results and detect pollution and dust events.

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2.2. Numerical Models

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In this study, the Nested Air Quality Prediction Modeling System (NAQPMS) was used to

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simulate dust and air pollution processes (Wang et al., 2001; Li et al., 2012). NAQPMS was

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applied using a horizontal resolution of 45 km over East Asia (Figure 1), and with 20 vertical

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layers in a sigma coordinate. The meteorological field as an input of NAQPMS was provided by

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the Weather Research and Forecasting model (WRF; ver. 3.7.1) using the same domain and

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horizontal resolution. The boundary and initial conditions of the WRF were based on final

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analysis (FNL) data from the National Centers for Environmental Prediction (NCEP) in the

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USA. The gas phase chemistry module of the NAQPMS was the Carbon-Bond Mechanism Z

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(CBM-Z; Zaveri and Peters, 1999), and the aerosol thermodynamic module was ISORROPIA

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version 1.7 (Nenes et al., 1998). The aqueous chemistry and wet deposition module was based on

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the RADM mechanism used in the CMAQ version 4.6 (http://www.cmascenter.org). Dry

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deposition was modeled using the scheme described by Wesley (1989).

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The anthropogenic emissions (e.g., SO2, NOx, NH3, CO, BC, OC, and VOCs) were from the

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MIX inventory (http://meicmodel.org/dataset-mix.html) with a base year of 2010 (Li, M. et al.,

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2015). Dust emissions were computed online using the following equation (Wang et al., 2000;

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Li et al., 2012) :

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F=C ∙

∙ E ∙ u∗

1+

∗ ∗

∙ 1−

∗ ∗

∙ 1−

(1)

where F is the dust flux (kg m−2 s−1). The constant C1 is set to 1.0 × 10−5, and

(kg m−3) and g

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(m2 s−2) are the air density and acceleration due to gravity, respectively. The dust source

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function (E) represents the uplifting capability of the land surface, and reflects the impact of

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land use categories, vegetation fractions, and snow/ice cover on dust fluxes.

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friction and threshold friction velocities. RH and RH0 represent relative humidity and its 7

∗

and

∗

are the

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threshold value, respectively. Dust particles were separated into four bins: two fine mode bins

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covering the range of 0.43–1 µm and 1–2.5 µm, and two coarse mode bins (2.5–5 µm and 5–10

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µm). The spatial distribution of dust emissions during the period from March 27 to April 2,

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2015 was shown in Figure 1.

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To simulate the interactions of particles with pollutant gases, 28 heterogeneous reactions on

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sulfate, soot, dust and sea salt particles were included based on previous studies (Li et al., 2012;

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Li et al., 2015). Among the 28 heterogeneous reactions, 12 reactions on dust particles, together

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with their uptake coefficients, are listed in Table 1. These uptake coefficients were selected

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based on previous researches on China losses and Gobi desert to reduce the uncertainties due to

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dust mineralogy (Li et al., 2012). More details of the physical and chemical processes in

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NAQPMS can be found in previous paper (Li et al., 2012).

R1

Heterogeneous Reaction O3→products

(unitless) 2.7 × 10

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Reference (Zhu et al., 2010)

HNO3+CO32-→NO +H2O+CO2

R3

NO2→0.5HONO+0.5HNO3

2.1 × 10-6

(Zhu et al., 2010)

R4

NO3→HNO3

1.0 × 10-3

(Martin et al., 2003)

R5

N2O5→2HNO3

3.0 × 10-2

(Zhu et al., 2010)

R6

OH→products

R7 R8

R10 R11 R12

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R2

× !" × 0.018 ( = 8) (1 − !") × (1 − (1 − ) × !")

R9

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Table 1. Heterogeneous reactions in dust particles and reactive uptake coefficients

(Vlasenko et al., 2006; Wei, 2010)

1.0 × 10-1

(Zhu et al., 2010)

HO2→0.5H2O2

2.0 × 10-1

(Zhu et al., 2010)

H2O2→products

12 × RH-2 - 5.95 × RH + 4.08

(Pradhan et al., 2010)

SO2→SO+*

1.0 × 10-4

(Phadnis and Carmichael, 2000)

CH3COOH→products

1.0 × 10-3

(Zhu et al., 2010)

CH3OH→products

1.0 × 10-5

(Zhu et al., 2010)

HCHO→products

1.0 × 10-5

(Zhu et al., 2010)

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RH: relative humidity (unitless) in [0, 1].

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In order to quantify the importance of heterogeneous reaction, two simulations were conducted

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for sensitivity analysis: one was the control simulation with heterogeneous reactions and the

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other was the sensitivity simulation without heterogeneous reactions. The difference of the two

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simulations can be considered as the impacts of heterogeneous reactions. Although sensitivity

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analysis may be affected by nonlinear effects, it was widely applied in the previous modeling

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studies of dust heterogeneous reactions (Phadnis et al., 2000; Tang et al., 2004; Fairlie et al.,

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2010). To show the transport of the dust plume, trajectories were calculated using the Hybrid

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Single-particle Lagrangian Integrated Trajectory model (HYSPLIT; ver. 4) (Stein et al., 2015),

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based on NCEP Global Data Assimilation System (GDAS) data, with 0.5-degree resolution

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every 3 hours.

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3. Results and Discussion

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3.1. Air Pollution and Dust Episodes

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Figures 2a and 2c show the Lidar-derived vertical distributions of spherical (mostly

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anthropogenic) particles and non-spherical (mostly mineral) dust aerosol extinction coefficients

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from March 27, 2015 to April 2, 2015. The NAQPMS-simulated results, based on the Mie theory

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and an assumption of external mixing, are also shown in Figures 2b and 2d. The modeling results

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reproduced the Lidar retrieval well for both the time variation and vertical distribution of high

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extinction coefficient processes, indicating that the model successfully captured the dust and/or

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anthropogenic pollution episodes during this period.

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Figure 2. Lidar-observed (a, c) and model-simulated (b, d) time-height indications of

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anthropogenic (a, b) and dust (c, d) extinction coefficients in Beijing.

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Based on the extinction coefficients of anthropogenic and dust aerosols, three high extinction

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coefficient episodes were selected. Episode A on March 27 was an air pollution episode, in

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which the pollution extinction coefficient was high (> 0.5 km-1), while the dust extinction

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coefficient was relatively low (< 0.2 km-1). Episode B on March 28 was a pure dust episode, in

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which the dust extinction coefficient was very high (> 0.5 km-1), while the pollution extinction

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coefficient was quite low (< 0.1 km-1). Episode C, from March 29 to March 31, was an episode

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during which mineral dust was mixed with anthropogenic pollutants. During Episode C, the

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observed pollution extinction coefficient increased from 0.2 km-1 to 0.5 km-1, while the dust

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extinction coefficient decreased from 0.5 km-1 to 0.1 km-1.

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The vertical distributions of aerosols during the three episodes were different. In Episode A, the

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high spherical aerosol extinction coefficient (> 0.3 km-1) reached heights of less than 1,500 m,

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because anthropogenic aerosols were trapped and transported within the atmospheric boundary

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layer. The high extinction coefficient reached as high as 4 km when the dust extinction

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coefficient was dominant during Episode B. The elevated dust layer was produced by updrafts

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within the warm sector of a low-pressure system (Hara et al., 2009). However, during Episode C,

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both high dust and anthropogenic aerosol extinction coefficients reached about 2 km, and they

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were mixed in the atmospheric boundary layer.

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Observed and simulated meteorological parameters (wind vector, wind speed, and RH) and

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particulate matter (PM) concentrations are shown in Figure 3. PM2.5 was observed using a

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tapered element oscillating microbalance (TEOM) method, while observed PM with a diameter

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larger than 2.5 µm but less than 5 µm (PM2.5-5), and PM with a diameter larger than 5 µm but less

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than 10 µm (PM5-10), were measured using the POPC. The simulated weather conditions showed

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good consistency with observations as well as PM concentrations. According to the comparison

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between the simulated total (anthropogenic + dust) and dust particle concentrations in Figure 3

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d–f, it can be clearly seen that the PM2.5 mostly consisted of anthropogenic aerosols, while

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almost all the PM2.5-5 and PM5-10 were mineral dust particles.

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Figure 3. Observed and simulated meteorological parameters (a, wind vector; b, wind speed; c,

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relative humidity) and particulate matter (PM) (d, PM2.5; e, PM2.5-5; f, PM5-10) concentrations in

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Beijing. The black dashed lines in (d–f) indicate simulated mineral dust PM, while the blue lines

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indicate total PM (= anthropogenic + dust).

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During Episode A, the wind speed was low (< 5 m/s), and the wind direction was southerly

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(Figure 3a and 3b), which is the typical meteorological condition associated with heavy air

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pollution in Beijing. Fine particles (PM2.5) were dominant, with a concentration greater than 100

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µg/m3 (Figure 3d), while the PM2.5-5 and PM5-10 concentrations were both less than 50 µg/m3

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(Figures 3e and 3f). On the morning of March 28, heavy dust lasted 5 hours, with a strong

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northwest wind at a speed above 10 m/s. During this episode, the PM2.5 concentration was less

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than 103 µg/m3; however, the PM2.5-5 and PM5-10 concentrations increased significantly, with

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maximum values of 382 µg/m3 and 314 µg/m3, respectively.

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During Episode C, the PM2.5-5 and PM5-10 concentrations decreased gradually from 200 µg/m3

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and 150 µg/m3 to less than 50 µg/m3, while the PM2.5 concentration increased gradually to a

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maximum of approximately 200 µg/m3, indicating that mineral dust mixed gradually with

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pollutants. When the dust concentration was higher than 400µg/m3 on March 29, there was a

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weak south wind of less than 5 m/s, different from the typical dust events in which strong north

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winds prevailed.

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To identify the dust source of the atypical dust event (Episode C), the dust transport patterns and

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trajectories were plotted, and are shown in Figure 4. Dust particles were mainly emitted from

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Mongolia and the Inner Mongolia province of China (Figure 1) and arrived in Beijing rapidly

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(within half a day) due to the strong northwest wind (> 10 m/s) on March 28 (Figures 4a and 4c),

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where they remained for only 5 hours (Episode B). Then, the dust was quickly transported to the

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southeast of Beijing (i.e., Hebei and Shandong province). However, after the dust cloud moved

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out, it was transported back again due to the south wind over north China on March 29 (Figures

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4b and 4d). A high concentration of dust and pollutants then persisted in Beijing for 2 days

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(Episode C) (Figure 3), which provided an opportunity to research the impacts of heterogeneous

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processes on dust particles.

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Figure 4. Horizontal distribution of the total dust concentration (unit: µg/m3) and wind vector

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(unit: m/s) on March 28 03:00 UTC (a) and March 29 12:00 UTC (b). (c) and (d) represent the

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backward (red) and forward (blue) trajectories from Beijing at the same times as in (a) and (b).

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Similar dust backflow event on March 20, 2011 was reported by Xu et al., (2014). They also

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found that the sand-dust transported through Beijing, and flew back to Beijing again, and

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indicated that the backflow process had significant impact on PM concentrations and were

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important for air quality forecast in Beijing. We calculated the backward trajectories when dust

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weather happened based on the weather report of Beijing meteorology station during 2011 and

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2015, and found that this kind of dust backflow event happened every year, and twice per year in

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2011 and 2013.

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3.2. Impacts of Heterogeneous Reactions on Chemical Composition

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Figure 5 shows the observed fine and coarse mode aerosol chemical composition and gaseous

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nitric acid in Beijing, as well as the simulated results with and without heterogeneous reactions.

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Model-simulated dust nitrate, dust sulfate and water-soluble calcium (e.g. Ca(NO3)2) were

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produced by heterogeneous processes on dust particles, while anthropogenic sulfate, ammonium

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and nitrate were produced by gas phase and aqueous phase chemistries as well as thermodynamic

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equilibrium processes.

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It can be seen that dust nitrate and dust sulfate did not increase on March 28 (Episode B) when

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high dust concentrations appeared, which was due to low gas-phase precursors (Figure 5g) and

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RH (Figure 3c). During Episode C, after considering the heterogeneous reactions, the simulated

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fine mode sulfate increased with a maximum of 2 µg/m3, while nitrate and ammonium decreased

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with a maximum of 14 µg/m3 and 6 µg/m3, and maximum values of simulated fine mode

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concentrations of nitrate, ammonium, and sulfate were 61 µg/m3, 27 µg/m3, and 44 µg/m3,

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respectively. During this period, the simulation results without heterogeneous reactions

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underestimated sulfate and overestimated nitrate and ammonium, therefore, including

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heterogeneous reactions improved the model performance. However, models may have

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short-term random bias due to inaccurate meteorology or sudden change of emission, in such a

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case, the model performance may not be improved. A comparison between simulations with and

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without heterogeneous reactions showed that gaseous nitric acid decreased, with a maximum of

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15 µg/m3, due to heterogeneous reaction with CO32- (CaCO3, MgCO3, et al.) on dust particles

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(R2 in Table 1).

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Figure 5. Observed and simulated fine and coarse mode ions and gaseous nitric acid in Beijing.

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DNO3, DSO4, and DCa represent dust nitrate, sulfate, and water-soluble calcium produced by

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heterogeneous processes on dust particles, respectively. ANO3, ANH4, ASO4 represent

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anthropogenic sulfate, ammonium, and nitrate produced by gas phase and aqueous phase

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chemistries as well as thermodynamic equilibrium processes, respectively. Simulation results

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with and without heterogeneous reactions (w HR and w/o HR, respectively) are shown. 16

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Due to uptake of HNO3 and SO2 by dust particles through heterogeneous reactions, the

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precursors of anthropogenic nitrate and sulfate decreased. As a result, fine mode anthropogenic

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nitrate decreased, with a maximum of 19 µg/m3, and fine mode anthropogenic sulfate decreased

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by less than 2 µg/m3; meanwhile, dust nitrate and sulfate were produced in the fine mode dust

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particles. The fine mode dust nitrate concentration (maximum: 6 µg/m3) was lower than the

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decrease in anthropogenic nitrate, but the fine mode dust sulfate concentration (maximum: 4

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µg/m3) was higher than the decrease in anthropogenic sulfate. Therefore, the total simulated fine

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mode nitrate decreased with a maximum of 14 µg/m3, and sulfate increased, with a maximum of

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2 µg/m3, after considering heterogeneous reactions. As a result of the decrease in fine mode

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anthropogenic nitrate and sulfate, the fine mode anthropogenic ammonium also decreased with a

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maximum of 6 µg/m3. After considering heterogeneous reactions, among the simulated total fine

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mode nitrate and sulfate, 83% of nitrate and 89% of sulfate were still due to anthropogenic

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pollutants, indicating that anthropogenic aerosols were dominant in the fine mode. However, fine

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mode dust nitrate and dust sulfate were also important, with maximum values of 6 µg/m3 (17%

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of total fine mode nitrate) and 4 µg/m3 (11% of total fine mode sulfate), respectively.

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The simulated coarse mode concentrations of nitrate, sulfate, and water-soluble calcium, as

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products of heterogeneous reactions on dust, had maximum values of 19 µg/m3, 7 µg/m3 and 9

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µg/m3, respectively, and were also in good agreement with observed coarse mode concentrations.

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The observed coarse mode ammonium concentration was relatively low, with a mean value of

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0.8 µg/m3, which was less important in this period since it was significantly lower than the

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concentrations of coarse mode nitrate, sulfate, and calcium. Furthermore, both observation and

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model results showed that the coarse mode concentrations of nitrate, sulfate and calcium ions

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were low (1~5 µg/m3) during the air pollution episode (A), but significantly higher (7~19 µg/m3)

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during the dust mixing episode (C). These results confirmed that heterogeneous reactions were

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major sources of coarse mode NO

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3.3. Impacts of Heterogeneous Reactions on the Mixing State

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Since dust nitrate and sulfate were produced by heterogeneous processes on the surface of dust

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particles, they were mixed internally with dust particles. To identify the aerosol mixing state of

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the different episodes using POPC observations, volume concentrations of different sizes and

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DRs for anthropogenic aerosols (Episode A), mineral dust (Episode B), and mixed particles

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(Episode C) were plotted in Figures 6a, 6b, and 6c, respectively. It is clearly shown that the

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anthropogenic aerosols with DR values < 0.1 in the submicron range were dominant during

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Episode A (Figure 6a), and the volume concentration of larger particles (Dp > 3 µm) was

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relatively low. During Episode B (Figure 6b), the dominant aerosol was mineral dust particles

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with a diameter larger than 2 µm and DR values from 0.1 to 0.4. In addition, the volume

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concentration of anthropogenic aerosols was quite low. During Episode C (Figure 6c), both

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anthropogenic aerosols with small diameters and DR values and dust particles with large

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diameters and DR values were present in large volumes, indicating that they were mixed together

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during this period.

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Both the observations and model results indicated the concentrations of sulfate, ammonium,

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nitrate and calcium ions were higher than other water-soluble ions during air pollution and/or

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dust periods, and these four ions were defined as the principal water-soluble ions (PWIs) in this

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paper. The observed and simulated mass fraction of each PWI ,- in PM2.5 and PM2.5-10 during

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Episode C are shown as pie charts in Figure 6d. The mass fractions ,- were expressed as:

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in the presence of dust particles.

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and SO+*

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,- = ∑1

/23 ./

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-

where i (= 1 - 4) indicated sulfate, ammonium, nitrate, and calcium respectively, and

was the

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mass concentration of the ith ion. The simulated mass fraction of each ion agreed well with the

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observed values in both fine and coarse mode, with a bias of 1~8%, indicating that the model

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results were reasonable. According to the modeling results, 17% of nitrate, 11% of sulfate and

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100% of calcium in fine mode were produced by heterogeneous reactions, and they were

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internally mixed with dust particles since they were produced on dust particles. The simulated

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mass fraction of fine mode nitrate in fine mode PWIs was 48% (Figure 6d), and fraction of fine

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mode dust nitrate in fine mode nitrate was 17%, therefore, fine mode dust nitrate accounted for 8%

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(= 17% × 48%) of PWIs. Similarly, it can be seen that fine mode dust sulfate and calcium

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accounted for 3% and 4% of PWIs, respectively. As a result, 15% (=8%+3%+4%) of the total

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PWIs in the fine mode were internally mixed on the dust surface. Anthropogenic ammonium

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nitrate and ammonium sulfate were produced due to gas phase and aqueous phase chemistries as

341

well as thermodynamic equilibrium processes, and these processes were independent of dust

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particles, therefore, these anthropogenic ammonium nitrate and ammonium sulfate were

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externally mixed with dust particles. The internally mixed dust particles were then externally

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mixed with anthropogenic ammonium nitrate and ammonium sulfate (85% of PWIs = 1 - 15%)

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in the fine mode. For coarse mode particles, observations revealed that the concentration of

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coarse mode ammonium was quite low (only approximately 1% of PWIs), indicating that almost

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all of the sulfate and nitrate were related to dust, while the modeling results showed that all the

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nitrate and sulfate were produced by heterogeneous reactions and were internally mixed with

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dust particles.

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Figure 6. Mixing of anthropogenic and dust particles. Volume concentrations of different

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particle sizes and depolarization ratios (DRs) during (a) air pollution, (b) pure dust, and (c)

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mixed dust-pollution episodes in Beijing. (d) Observed (up) and simulated (bottom) mass

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fractions of sulfate (red), nitrate (blue), ammonium (green), and calcium (yellow) ions relative to

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the total principal water-soluble ions (PWIs) in PM2.5 (left) and PM2.5-10 (right) during the mixed

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dust episode.

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Due to internally mixed with anthropogenic particles, the DR of dust particles decreased

358

significantly during this period, especially for coarse mode dust particles. From Figure 6, it can

359

be seen that lots of coarse particles (diameter > 2.5 µm) existed in DR range 0.4-0.5 (between the

360

two black dash lines) in Episode B (Figure 6b), but not in Episode C (Figure 6c), which indicated

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DR decrease of coarse particles. The time series plot of coarse particles mass concentration and

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mean depolarization ratio also showed that depolarization ratio decreased significantly (Figure 7):

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the DR decreased by 0.04 (13%) from 0.30 (when fresh dust came firstly, marked as B in Figure

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7) to 0.26 (when the PM2.5-10 concentration was still as high as 150 µg/m3 during Episode C,

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marked as C in Figure 7). And at the end of Episode C (March 31), the DR decreased by 0.07

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(30%) to 0.23.

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Figure 7. Time series of coarse particles mass concentration and depolarization ratio

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Beside heterogeneous processes, several pathways exist for production of coarse mode aerosols

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(coagulation, condensation, and in-cloud processes); however, these processes are not

372

significant, based on the simulation results of Schutgens and Stier (2014). In this study, we also

373

performed a sensitivity simulation without aqueous chemistry, and the results indicated that

374

during this period, sulfate and nitrate produced by aqueous processes were less than 1.2 µg/m3

375

and 0.5 µg/m3, and almost all of them were in fine mode, which was consistent with Schutgens

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and Stier (2014) and indicated that aqueous processes were not as important as heterogeneous

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processes in this period.

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4. Conclusions

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To investigate the impacts of heterogeneous reactions on the chemico-physical properties of dust

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particles, we observed and simulated three dust and/or pollution episodes over northern China

382

during March 27, 2015 and April 2, 2015. During this period, an unusual dust episode circulating

383

over northern China because of synoptic weather changes was simulated using the NAQPMS to

384

examine the role of heterogeneous processes on the dust. A comparison of observations showed

385

that the NAQPMS model simulation successfully reproduced the time series of the observed

386

vertical profile, PM concentration, and chemical components of fine mode (diameter ≤ 2.5 µm)

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and coarse mode (2.5 µm < diameter ≤ 10 µm) particles. Our findings can be summarized as

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follows:

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1) Dust particles were transported from source regions to Beijing within half a day due to the

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strong northwest wind on March 28, and remained there for only 5 hours. After the dust

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cloud moved out, it was re-circulated back to Beijing regions due to the south wind over

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northern China on March 29, and then persisted in Beijing for 2 days, during which time

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significant heterogeneous reactions occurred on the dust particles.

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2) Simulated nitrate, ammonium, and sulfate, including the heterogeneous reactions, were in

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good agreement with the observed values. The modeling results clearly explained the

397

observations and confirmed that heterogeneous reactions were the major mechanisms

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producing coarse mode NO

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sulfate reaching 19 µg/m3 and 7 µg/m3, respectively.

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and SO+* , with the concentrations of coarse mode nitrate and

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3) Due to uptake processes of acid gases by dust particles, fine mode anthropogenic nitrate

401

decreased, with a maximum of 19 µg/m3, and fine mode anthropogenic sulfate decreased by

402

less than 2 µg/m3. Therefore, fine mode anthropogenic ammonium also decreased with a 22

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maximum of 6 µg/m3. In contrast, fine mode dust nitrate (maximum concentration: 6 µg/m3)

404

and fine mode dust sulfate (maximum concentration: 4 µg/m3) were produced in the dust

405

particles. As a result, total fine mode nitrate decreased, with a maximum of 14 µg/m3, and

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fine mode sulfate increased with a maximum of 2 µg/m3.

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4) Because of heterogeneous reactions, 17% of fine mode nitrate and 11% of fine mode sulfate

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were mixed internally on the surface of dust particles, and almost all coarse mode NO

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SO+*

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size, and shape of dust particles, due to heterogeneous processes and internal mixing, may

411

significantly change the optical characteristics and direct climate effect of the particles.

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Changes in the mixing state of dust particles also had impacts on CCN and cloud processes,

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which was not considered in this simulation but may be important for indirect radiative

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forcing characteristics and the regional climate in East Asia.

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were mixed internally with dust particles. Alterations of the chemical composition,

Acknowledgments

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and

This research was partly supported by a grant from National Natural Science Foundation

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of China (NNSF, 41505115), and the Japan Society for the Promotion of Science (JSPS,

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Grant-in-Aid for Scientific Research, 25220101).

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Highlights

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Interaction of dust and pollutants in a high concentration condition is reported.

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High level of dust and pollutants unusually circulated over northern China.

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The NAQPMS model successfully reproduced the observed dust and pollution episodes.

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Heterogeneous reactions significantly affect composition and mixing state of dust.

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