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    Characteristics and Formation Mechanisms of Fine Particulate Nitrate in Typical Urban Areas in China

    Xinlei Ge 1,*, Yanan He 1, Yele Sun 2, Jianzhong Xu 3, Junfeng Wang 1, Yafei Shen 1 and Mindong Chen 1

    1 Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control (AEMPC), Collaborative Innovation Center of Atmospheric Environment and Equipment Technology (CIC-AEET), School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China; (Y.H.); (J.W.); (Y.S.); (M.C.)

    2 State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China;

    3 State Key Laboratory of Cryospheric Sciences, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000, China;

    * Correspondence:; Tel.: +86-25-5873-1394

    Academic Editor: Guey-Rong Sheu Received: 17 January 2017; Accepted: 20 March 2017; Published: 22 March 2017

    Abstract: Nitrate is a very important aerosol component, thus elucidation of its characteristics and formation mechanisms is essential and important for effective reduction of aerosol pollution. In this work, highly time-resolved submicron aerosol (PM1) data measured by Aerodyne aerosol mass spectrometers (AMS) in Nanjing, Beijing and Lanzhou during both summer and winter were integrated to investigate the nitrate behaviors in urban China air. Results showed that nitrate occupied 1/8–1/4 of PM1 mass, typically higher than those observed in rural/remote regions. Relative mass fractions of nitrate also varied significantly at different pollution levels. Nitrate mass fractions generally increased with the increase of PM1 loadings during summer, while the contributions during winter increased first and then decreased with the increase of pollution levels. We further propose that there are at least three mechanisms that likely govern the urban nitrate behaviors: Type I—thermodynamics driven, Type II—photochemistry driven, and Type III—planetary boundary layer (PBL) dynamics driven. Analyses of the ammonium-sulfate-nitrate data revealed that ammonium nitrate was able to form before sulfuric acid was fully neutralized in some urban areas. Our findings provide useful insights into the characterization and reduction of fine particulate nitrate pollution.

    Keywords: fine aerosols; nitrate; aerosol mass spectrometer; diurnal variation; formation mechanism

    1. Introduction

    Fine particulate matter (PM2.5) pollution is a very serious and pressing environmental issue in the densely populated areas of China [1–3]. PM2.5 also has profound effects on the visibility, air quality, human health, and the Earth’s climate, etc. Nitrate is one of the major components of PM2.5, which can account for 4.5%–25% of the PM2.5 mass, depending upon locations and environments [4–7]. Previous studies also showed that nitrate contributions to PM2.5 mass could increase with the increase of PM2.5 mass loadings [8]. Indeed, nitrate was found to be an very important contributor to the high PM2.5 concentrations during heavy haze events in China [9]. In this regard, elucidation of the atmospheric behaviors of nitrate is critical to reduce the PM2.5 pollution.

    Atmosphere 2017, 8, 62; doi:10.3390/atmos8030062

  • Atmosphere 2017, 8, 62 2 of 12

    Nitrate may associate with different cations in the particle phase. Under marine environments, it may be partially present as NaNO3 due to the reaction: HNO3 + NaCl = NaNO3 + HCl, but this reaction mainly affects the coarse particles [10] rather than fine particles (PM2.5). Typically, fine aerosol nitrate exists in the form of ammonium nitrate, which is generated by the neutralization reaction: HNO3 + NH3 = NH4NO3. The precursor nitric acid (HNO3) is overwhelmingly produced by secondary oxidation processes, and only a very small portion is emitted directly, for example, from volcano eruptions [11]. The secondary formation of HNO3 includes two major pathways. During daytime, HNO3 is mainly produced from OH-initiated oxidation of NO2 [12]. During nighttime, NO2 can be oxidized by O3 to form NO3 radical, which then reacts with NO2 to form N2O5 [13]; N2O5 then heterogeneously reacts with water vapor on the particle surface and produces HNO3 (this process is favored by low temperature and high relative humidity) [14,15]. Even when HNO3 is available, the neutralization of HNO3 with NH3 can only occur efficiently under ammonium-rich conditions, as NH3 prefers to react with sulfuric acid (H2SO4) first [16]. Some earlier studies [17,18] demonstrate that a molar ratio of NH4+/SO42− larger than 1.5 can be treated as an ammonium-rich condition, meaning that NH4NO3 can be produced effectively after the formation of (NH4)3H(SO4)2. Nevertheless, this boundary value (1.5) may change as both homogenous and heterogeneous formations of NH4NO3 are possible in the real atmosphere [19], and the dominant pathway may vary at different locations, or during different seasons in the same location [20].

    Traditionally, atmospheric particles are collected onto filters, and then nitrate concentration is determined by using ion chromatography (IC). Such nitrate data has low time resolution, as the filters are collected every few hours or days. From the year 2000, Aerodyne aerosol mass spectrometers (AMS) [21,22], which can determine the aerosol compositions quickly and in real-time, were used widely around the world. Due to the relatively simple fragmentation pattern in the AMS ionization scheme, ammonium nitrate is recommended as a standard mass calibration material for all versions of AMS, and the AMS is able to quantify nitrate relatively well. Given that the AMS also provides highly-time resolved data, it is particularly useful for investigating the temporal variations and diurnal cycles of nitrate. In this study, we integrated and analyzed the AMS data collected from a few typical cities in China (Nanjing, Beijing, and Lanzhou) during both summer and winter. The purpose of this work is to provide an overview of the characteristics of ambient aerosol nitrate in typical urban areas of China, and propose the dominant mechanisms that govern such nitrate characteristics under different atmospheric environments. Our findings can help researchers interpret the nitrate behaviors in future studies, and provide useful insights into the effective reduction of particulate nitrate pollution.

    2. Experiments

    In this work, we selected Nanjing, Beijing, and Lanzhou—three cities representing Yangtze River Delta (YRD), Beijing-Tianjin-Hebei (Jing-Jin-Ji), and northwestern part of China, respectively—for our sampling locations. The sampling locations, sampling periods, and the versions of Aerodyne AMS used for these field campaigns are shown in Figure 1. For Nanjing, a soot-particle AMS (SP-AMS) [23,24] was deployed in urban Nanjing (Jiangxin Island) from 11 August to 18 September 2014, and in the campus of Nanjing University of Information Science and Technology (NUIST) from 20 February to 23 March 2015 (details in [25]). For Beijing, an Aerosol Chemical Speciation Monitor [26] (ACSM, a simplified version of Aerodyne AMS) was deployed during summer (26 June–28 August 2011) and winter (21 November 2011–20 January 2012), both at the Institute of Atmospheric Physics (IAP) of Chinese Academy of Sciences (CAS). For Lanzhou, the measurements were performed by using a high resolution time-of-flight AMS (HR-ToF-AMS) [27] in the Cold and Arid Regions Environmental and Engineering Research Institute (CAREERI) during summer (11 July–7 August 2012) and at Lanzhou University during winter (10 January–4 February 2014) (the two sites are only about 500 meters apart). Note that the results of the Beijing summer data have been published in Sun et al. [28], and winter data was presented in Sun et al. [29,30], while results of Lanzhou summer and winter were presented in Xu et al. [31] and Xu et al. [32], respectively. The nitrate data from Beijing and Lanzhou were re-visited

  • Atmosphere 2017, 8, 62 3 of 12

    here, together with the new Nanjing data, to unravel the aerosol nitrate behaviors and governing factors in different representative urban areas of China.Atmosphere 2017, 8, 62 3 of 12

    Figure 1. Sampling periods, locations, and aerosol mass spectrometers (AMS) versions of the datasets used in this work.

    Due to the limitation of transmission efficiency of the AMS lens, AMS is able to measure particles mainly in the submicron meter range, and species that can vaporize at ~600 °C, thus the AMS-measured particles are referred to as non-refractory PM1 (NR-PM1) [21]. During the aforementioned field measurements, although different types of AMS were deployed, all of them (SP-AMS, ACSM, and HR-ToF-AMS) used ammonium nitrate to conduct the mass calibration, since it is easy to evaporate and has a very simple fragmentation pattern (mainly m/z 30—NO+, and m/z 46—NO2+ from nitrate). The AMS is thus powerful in quantifying nitrate unless there are large amounts of organic or metal nitrates, since they can affect NO+ and NO2+ signals significantly [33]. Quantification of other aerosol components, including sulfate, chloride, ammonium, and total organics, were all based on their relative ionization efficiencies to nitrate. Only SP-AMS can simultaneously determine the refractory black carbon (rBC) mass, since it is eq


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