Letter pubs.acs.org/journal/estlcu
High N2O5 Concentrations Observed in Urban Beijing: Implications of a Large Nitrate Formation Pathway Haichao Wang,† Keding Lu,*,† Xiaorui Chen,† Qindan Zhu,† Qi Chen,† Song Guo,† Meiqing Jiang,† Xin Li,† Dongjie Shang,† Zhaofeng Tan,† Yusheng Wu,† Zhijun Wu,† Qi Zou,† Yan Zheng,† Limin Zeng,† Tong Zhu,† Min Hu,† and Yuanhang Zhang†,‡ †
State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing, China ‡ CAS Center for Excellence in Regional Atmospheric Environment, Chinese Academy of Science, Xiamen, China S Supporting Information *
ABSTRACT: The heterogeneous hydrolysis of dinitrogen pentoxide (N2O5) is important to understanding the formation of particulate nitrate (pNO3−). Measurements of N2O5 in the surface layer taken at an urban site in Beijing are presented here. N2O5 was observed with large day-to-day variability. High N2O5 concentrations were determined during pollution episodes with the co-presence of large aerosol loads. The maximum value was 1.3 ppbv (5 s average), associated with an air mass characterized by a high level of O3. N2O5 uptake coefficients were estimated to be in the range of 0.025−0.072 using the steadystate lifetime method. As a consequence, the nocturnal pNO3− formation potential by N2O5 heterogeneous uptake was calculated to be 24−85 μg m−3 per night and, on average, 57 μg m−3 during days with pollution. This was comparable to or even higher than that formed by the partitioning of HNO3. The results highlight that N2O5 heterogeneous hydrolysis is vital in pNO3− formation in Beijing.
■
■
INTRODUCTION N2O5 is an important reactive nitrogen species with regard to nighttime chemistry.1 The heterogeneous reaction of N2O5 on ambient aerosols forms particulate nitrate (pNO3−) and ClNO2 (R1) on chloride-containing aerosols, which play an important role in global NOx removal2,3 and subsequent daytime photochemistry through ClNO2 photolysis.4,5 N2O5 + (H 2O or Cl−) → (2 − ϕ)NO3− + ϕClNO2
METHODS Site Description and Instrumentation. Measurements were conducted on the campus of Peking University (39°99′ N, 116°30′ E) from September 11 to October 4, 2016. The campus is located northeast of the Beijing city center and near the fourthring road and a six-lane traffic road. The two roads are often congested. The site represented a typical urban and polluted area with large amounts of fresh, anthropogenic emissions. The measurements were taken on the roof of a five-story building (approximately 20 m above the ground), and most of the instruments were placed in a top-floor room, except for the cavity-enhanced absorption spectrometer (CEAS), which was placed on the roof and housed in a shelter with a short sampling line (1 m). A number of trace gas species and the properties of particles were monitored during this campaign. The details of the instrumentation setup are listed in Table S1. N2O5 was measured using a CEAS system with one heated channel, which was described recently and successfully deployed in both winter and summer campaigns in a suburban part of Beijing.15 The limit of detection (LOD) was estimated to be 2.7 pptv (1σ) with an
(I)
Field measurements showed secondary particulate formation dominated the mass concentration of PM2.5 in Beijing throughout the year. Despite the particulate organic compounds, nitrate dominated the secondary inorganic composition of PM because of the efficient reduction in SO2 emissions in China.6,7 Previous studies showed the nocturnal pNO3− enhancement events were observed frequently in Beijing8,9 and suggested those may be attributed to the heterogeneous reaction of N2O5.10−12 Until now, few studies have examined N2O5 chemistry based on direct field observations of N2O5 in China. Elevated mixing ratios of N2O5 were reported in the residual layer in Hong Kong and suburban Beijing,13−15 and low N2O5 concentrations were observed in urban Jinan and in Wangdu.14,16 In this work, we present direct measurements of N2O5 and related species in a typical urban area in Beijing, check its variation characteristics, and investigate its role in pNO3− formation. © XXXX American Chemical Society
Received: Revised: Accepted: Published: A
August August August August
11, 16, 16, 16,
2017 2017 2017 2017 DOI: 10.1021/acs.estlett.7b00341 Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX
Letter
Environmental Science & Technology Letters
Figure 1. Time series of the observed N2O5 concentrations, particle nitrate, the relevant parameters (NO2, O3, PM2.5, temperature, and relative humidity) and the experimentally determined production rates of NO3. The time resolution of N2O5 was 5 s, and that of the other parameters was 10 min.
NO2, O3, and N2O5 and the reaction rate constant of NO2 + O3 (kNO2+O3) (eq 1, derived in Text S2).
uncertainty of 19%. The dry-state number size distributions were measured to be from 0.01 to 0.7 μm with a scanning mobility particle sizer (SMPS) (TSI Inc. 3010 CPC and TSI Inc. 3081 DMA). The aerosol surface concentrations were calculated on the basis of the dry-state number size distributions assuming spherical particles and corrected to ambient (wet) aerosol surface concentration (Sa) by an empirical function of the hygroscopic growth factors determined at this site.17 The uncertainty of the ambient Sa was estimated to be approximately 30%, associated with the measurement error of the dry surface areas by the SMPS (20%) and the error of the applied hygroscopic growth factor (20%). NO/NO2 and O3 were measured by NOx chemiluminescence (Thermo Scientific, TE-42i-TR) and an ultraviolet photometric O3 analyzer (Thermo Scientific, TE-49i), respectively. The uncertainties are 20, 20, and 5% for NO, NO2, and O3, respectively. A laser-induced fluorescence (LIF) instrument, with an uncertainty of 11%, was used to measure the OH radical.18 A gas and aerosol collector combined with ion chromatography (GAC−IC), with an uncertainty of 20%, was used to measure HNO3, HCl, and NH3.19 A time-of-flight aerosol chemical speciation monitor (ToF-ACSM) with a capture vaporizer and a PM2.5 aerodynamic lens was used to measure the nonrefractory chemical composition of fine particles (NR-PM2.5),20−22 which has an uncertainty of ∼30%. A TEOM 1400A analyzer was used to measure the dry-state mass concentration of PM2.5. The relative humidity (RH) and temperature (T) were measured by a weather station (Met One Instruments Inc., 064 and 083E for RH and T, respectively). N2O5 Uptake Coefficients (γN2O5) Estimation. Assuming the total losses of NO3 and N2O5 were kept in balance with the production in the air mass after a certain period of time following sunset, γN2O5 was estimated using a steady-state method.23 The validity of the steady-state method was confirmed using a box model (Text S1 and Table S2). The model uses Regional Atmospheric Chemical Mechanism version 2 (RACM)24 and included the N2O5 heterogeneous hydrolysis reaction.25 The steady-state lifetime of N2O5 was calculated from observations of
τ(N2O5) =
[N2O5] k NO2 + O3[NO2 ][O3]
(1)
γN2O5 was then estimated by the linear regression of τ(N2O5)−1Keq[NO2] and 0.25C × Sa × Keq[NO2] (eq 2, derived in Text S2) τ(N2O5)−1 × Keq[NO2 ] = k NO3 + 0.25γN O C × Sa × Keq[NO2 ] 2 5
(2)
where C denotes the mean molecular speed of N2O5 and Keq represents the thermal equilibrium constant between NO3 and N2O5. The fitted slope denoted γN2O5, and the fitted intercept denoted the NO3 reactivity (kNO3), where kNO3 is the total firstorder rate coefficient for the loss of NO3 and mainly attributed to the reaction of NO3 with volatile organic compounds (VOCs) and NO.23
■
RESULTS AND DISCUSSION Overview of the Observations. The time series of N2O5 and other relevant parameters are shown in Figure 1. Meteorological conditions during the measurement were characterized by high temperatures (average of 21 ± 4 °C) and high RH (average of 67 ± 19%). The average pNO3− mass concentration in PM2.5 for the measurement period was 14.2 μg m−3 and accounted for 25% of the dry-state PM2.5 mass concentration. High O3 concentrations were often observed in the presence of high PM concentrations. The daily O3 peaks over 93 ppbv (Chinese national air quality standard) occurred on 10 of 24 days, with a nocturnal average of 19 ± 22 ppbv, and depicted a high level of photochemical pollution during the campaign. Attacked by strong and continuous fresh emissions, the near surface O3 decayed fast after sunset and was totally titrated by NO before midnight most days. NO2, converted from B
DOI: 10.1021/acs.estlett.7b00341 Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX
Letter
Environmental Science & Technology Letters
model were generally
