Aerosol Liquid Water Driven by Anthropogenic Inorganic Salts

Feb 12, 2018 - An estimation based on measurements of the chemical composition over Beijing showed that the average ALWC was the highest compared with...
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Letter

Aerosol Liquid Water Driven by Anthropogenic inorganic salts: Implying its key role in the haze formation over North China Plain Zhijun Wu, Yu WANG, Tianyi TAN, Yishu ZHU, Mengren Li, Dongjie SHANG, Haichao Wang, Keding Lu, Song Guo, Limin Zeng, and Yuanhang Zhang Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.8b00021 • Publication Date (Web): 12 Feb 2018 Downloaded from http://pubs.acs.org on February 13, 2018

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Aerosol Liquid Water Driven by Anthropogenic Inorganic Salts:

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Implying Its Key Role in the Haze Formation over North China Plain

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Zhijun Wu*, Yu Wang#, Tianyi Tan, Yishu Zhu, Mengren Li, Dongjie Shang, Haichao Wang, Keding Lu, Song Guo, Limin Zeng, Yuanhang Zhang State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China # Now at Centre for Atmospheric Sciences, School of Earth and Environmental Sciences, University of Manchester, Manchester M13 9PL, UK. 2

11

*

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Abstract

13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

This study reveals the aerosol liquid water (ALWC) in PM2.5 ranged from 2% up to 74% associating with the secondary inorganic fraction rose from 24% to 55% and ambient relative humidity (RH) increased from 15% to 83% in the atmosphere over Beijing. Unexpectedly, the secondary inorganic fraction in PM2.5 increased with an increase in the ambient RH, which is a meteorological parameter independent of anthropogenic activities, indicating the presence of a feedback mechanism driven by Henry’s law and thermodynamic equilibrium. During haze episodes, simultaneously elevated RH levels and anthropogenic secondary inorganic mass concentrations resulted in an abundant ALWC. The condensed water could act as an efficient medium for multiphase reactions, thereby facilitating the transformation of reactive gaseous pollutants into particles and accelerating the formation of heavy haze. The ALWC was well correlated with the mass concentrations of both nitrate and sulfate, indicating both nitrate and sulfate salts play key roles in determining the ALWC. Coincident with a significant reduction in SO2 emissions throughout China, nitrates will become a dominant anthropogenic inorganic salt driving the ALWC. Thus, the abundance of the ALWC and its effects on the aerosol chemistry and climate should be reconsidered.

Corresponding author: Zhijun Wu ([email protected])

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(Photo by: Zhijun Wu)

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1 Introduction

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Condensed water is an important component of atmospheric particles and

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significantly contributes to ambient aerosol mass1-3, optical properties4-5, and

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gas-particle transformations through complex physical and chemical mechanisms6,

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thereby playing profound roles on air quality degradation7. Water in the particulate

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phase has been proposed as a plasticizer since its presence results in changes to the

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particle phase state, which subsequently has a substantial effect on molecular

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diffusion on both particle and bulk surfaces8-11. Condensed water provides the

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medium

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Aqueous-phase reactions, including N2O5 hydrolysis, can irreversibly drive gas

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uptake14-15 and enhance the formation of secondary inorganic aerosols (SIAs)16, which

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are used to refer to sulfate, nitrate, and ammonium in this study. Recent field surveys,

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modeling results, and experimental studies also highlighted the role of aerosol liquid

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water in the formation of global secondary organic aerosols (SOAs)12,

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Condensed water facilitates the partitioning of water-soluble, polar, organic precursors

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into condensed phases25 and subsequent aqueous-phase reactions into more

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oxygenated organic components6. The aerosol liquid water content (ALWC) is

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determined according to the ambient relative humidity (RH), aerosol mass

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concentration, and chemical composition4. Sulfate and nitrate salts represent the most

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important components that drive the particle hygroscopicity and ALWC in areas with

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intense anthropogenic activities26-29. Since the ALWC attributed to anthropogenic

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SIAs may significantly influence the air quality5, 30 and regulate biogenic aqueous

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SOAs formation17,

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provide an explanation of atmospheric physicochemical processes.

for

the

multiphase

chemistry

throughout

the

atmosphere6,

12-13

.

17-24

.

31-33

, understanding the ALWC and its determinants is vital to

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Heavy haze events, which occur frequently throughout the atmosphere over the

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North China Plain (NCP), threaten the health of millions of people. Moreover, the

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outflow of Asian air pollution could have an impact on global weather patterns34.

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Secondary aerosols was considered a main source of the particulate pollution during

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such haze events35-36. Recently, the reactive nitrogen chemistry within aerosol liquid

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water was found to be a source of particulate sulfates during haze events over China16,

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37

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role on the additional formation of oxidized SOAs38. A recent study showed that the

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uptake coefficient of N2O5 could be high and that it was related to a high ALWC in

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Beijing, thereby increasing the formation of nitrates39. These findings highlight the

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importance of the ALWC in secondary aerosol formation, and thus, in the generation

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of heavy haze events. An estimation based on measurements of the chemical

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composition over Beijing showed that the average ALWC was the highest compared

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with other locations around the world3. However, a thorough understanding of the

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association of the ALWC with the ambient RH, chemical composition, and particulate

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matter pollution levels in the atmosphere over Beijing is still lacking.

. Observations from Beijing revealed that aqueous-phase processes play a dominant

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The ALWC was calculated in the present study on basis of long term data sets

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provided by aerosol mass spectrometer (AMS), hygroscopicity-tandem differential

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mobility analyzer (H-TDMA), and measurements from filter samples. Furthermore,

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the effects of particle chemical compositions and of the RH and air pollution levels on

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the ALWC were evaluated. We discovered that simultaneously elevated levels of the

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RH, inorganic fraction, and PM2.5 mass concentration resulted in an abundant ALWC

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associated with anthropogenic SIAs during haze episodes throughout the atmosphere

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over Beijing. This abundant ALWC may play a profound role in visibility degradation,

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aerosol mass loading, and secondary aerosol formation.

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2 Materials and Methods

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The measurements were conducted at the Peking University Atmosphere

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Environment Monitoring Station (PKUERS), which is located in the northwestern

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urban area of Beijing. At the sampling site, various meteorological parameters,

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including the wind speed, wind direction, ambient RH, and temperature, were

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measured by a weather station. The non-refractory particle chemical compositions

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were measured using an Aerodyne High-Resolution Time-of-Flight Aerosol Mass

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Spectrometer (HR-ToF-AMS). Data consisting of daily filter-based water-soluble ions

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in PM2.5 were obtained from filter samples in addition to laboratory ion

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chromatography analysis. An H-TDMA and a scanning mobility particle sizer (SMPS)

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and aerodynamic particle sizer (APS) system were respectively employed to measure

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the size-resolved hygroscopic growth factors (HGFs) and particle number size

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distribution (PNSD) from 3 nm to 10 µm. The dataset utilized for the calculation is

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listed in Table S1. A detailed description of the instrumentation and data validation

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can be found in the supporting information (SI).

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First, the ALWC was calculated using the ISORROPIA-II thermodynamic

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model40 with AMS particle mass concentrations of three inorganic species (NH4+,

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SO42−, and NO3−). The results were then compared with the ALWC derived from the

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HGF-PNSD. The measured ambient RH and temperature were set as the input

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parameters. ISORROPIA-II was operated while assuming that the particles are

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“metastable”, meaning that a solid formation was prevented in the model. The ALWC

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calculated during the reverse mode, in which the known quantities were the

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temperature, the RH, and the precursor concentrations in the aerosol phase40, was

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ultimately taken (see the SI). The ALWC contributed by the organic fraction was

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calculated using a method in the literature41 (see the SI).

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Second, the ALWC was calculated using the ISORROPIA-II thermodynamic

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model with four filter-based inorganic species (NH4+, Cl-, SO42−, and NO3−) and four

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cations (K+, Na+, Ca2+, and Mg2+). The organic fraction was not considered in the

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calculation of the ALWC, and therefore, the ALWC may have been underestimated.

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Finally, the ALWC was calculated from the particle hygroscopicity

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measurements. In this method, the volume of wet particles at the ambient RH was first

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calculated by combining the size-resolved HGFs measured via the H-TDMA with the

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PNSDs. Then, the ALWC was calculated by subtracting the volume of dry aerosol

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particles from that of wet particles. A brief introduction is given in the SI. A detailed

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description of this method can be found in Bian et al.42.

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A comparison (Fig. S1) was conducted between the ALWC calculated using

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ISORROPIA-II both with and without water contributed by organic compounds and

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the ALWC calculated using the HGFs and PNSDs (the data were collected in June ACS Paragon Plus Environment

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2014). Considering the uncertainties in both the ISORROPIA-II model and the

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HGF-PNSD method (see the explanation in the SI), we concluded that the ALWC

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calculated using ISORROPIA-II agreed well with that calculated from the

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size-resolved HGFs and PNSDs. The abovementioned comparisons mutually validate

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the HGF-PNSD method and the ISORROPIA-II model, and thus, their results can be

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used to calculate the ALWC within polluted atmospheric environments such as that

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over Beijing.

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The following analysis employed the ALWC results calculated using

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ISORROPIA-II with three-year filter-based ion concentrations in addition to the

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ALWC observations derived from nine-month size-resolved HGFs and PNSDs. The

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former (shown in Fig. 1 and 2) were used to gain insight into the relationship between

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the ALWC and the particle chemical composition, ambient RH, and PM2.5 mass

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concentrations. The latter (displayed in Fig. 3) provided a picture of the seasonal and

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diurnal variations in the ALWC.

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3 Results and Discussion

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Fig. 1 displays the time series of the PM2.5 chemical composition and the

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meteorological parameters during two typical heavy haze episodes (Episode I and II).

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The NOx and SO2 concentrations are displayed in Fig. S2. On 25 November 2015, a

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clean air mass was carried over the study area by strong northerlies, resulting in a

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relatively low PM2.5 mass concentration of 24.8 µg/m3. Over the following 6 days, the

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wind speed decreased to 1 m/s, indicating the occurrence of stagnant conditions. Such

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stagnant weather conditions are typically associated with air masses that move slowly

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from the south and southwest of Beijing and spend a greater amount of time over

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industrialized regions, thereby resulting in higher mass concentrations of particulate

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matter43-44. The observed PM2.5 mass concentrations increased up to 480.4 µg/m3 in a

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step-wise fashion. The NOx and SO2 concentrations increased to 172.5 ppbv and 19.3

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ppbv, respectively (refer to Fig. S2), indicating high gas precursor concentrations. The

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heavy haze episode ended with the appearance of a strong northerly. The second haze

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episode began on 3 December 2015 with very similar variations in the PM2.5 mass

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concentration, chemical composition, and meteorological parameters.

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Figure 1. Time series of the PM2.5 concentrations and chemical composition (left

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y-axis) and of the ambient relative humidity (right y-axis). The wind speed is

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represented by the size of the gray circles.

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The measured PM2.5 chemical composition (Fig. 1) revealed that the SIA mass

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concentration increased with an increase in the PM2.5 mass concentration and

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accounted for approximately 61% and 55% of the PM2.5 on 1 and 9 December,

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respectively, which represent the heaviest haze days. Markedly, the ambient RH

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continued to rise during the heavy haze episodes. For example, the RH increased from

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23% on 3 December 2015 to 68% on 9 December 2015 during Episode II. The ALWC

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was 211.5 and 103.6 µg/m3 on 1 and 9 December, respectively, and accounted for 44%

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and 48% of the PM2.5. These observations demonstrated that the RH, the inorganic

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fraction of PM2.5, and the PM2.5 mass concentration increased simultaneously during

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the heavy haze episodes and potentially brought about the elevated ALWC within the

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PM2.5.

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To confirm that the results observed during the individual air pollution episodes

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are generally applicable to the atmosphere over Beijing, a three-year filter-based

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dataset was employed to investigate the relationships among the ALWC, the inorganic

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fraction of PM2.5, the PM2.5 mass concentration, and the ambient RH. The SIA mass

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fraction, PM2.5 mass concentration, and aerosol liquid water mass concentration during

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the entire sampling period were grouped and averaged corresponding to an RH bin

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width of 10%. The SIA fraction (circles) plotted as a function of the RH is displayed in

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Fig. 2(a). The circles are colored according to the ALWC, and the sizes of the circles

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are scaled to the PM2.5 mass concentration. The data displayed in Fig. 2(a) were

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provided in the Table S2. As was the case for the individual pollution episodes (Fig. 1),

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the long-term measurements revealed that both the dry PM2.5 mass concentration and

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the SIA fraction increased with an elevated ambient RH. However, the PM2.5 mass

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concentration decreased (as indicated by the circle sizes) as the RH exceeded 80%

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because wet deposition, which typically occurred at higher RH conditions (>80%). On

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average, as ambient RH increased from 15% to 83%, the SIA mass fraction in PM2.5

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increased from 24% to 55%, and the ALWC increased from 2% to 74% (refer to

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Table S2).

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As displayed in Fig. 1(a), the SIA mass fraction was positively correlated with

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the ambient RH. This is unexpected, because inorganic compounds are related to

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anthropogenic activities and should not change with variations in the ambient RH

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without considering global climate change caused by human activities. Therefore, a

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driving force must exist for gaseous pollutants, such as SO2 and NOx, for the

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transformation of particulate SIAs. We propose a scenario as follows. If a highly

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concentrated aqueous solution of, e.g., 30% RH, absorbs more inorganic matter due to

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N2O5 hydrolysis or SO2 uptake and oxidation reactions, the solution should be forced

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to absorb more water to maintain an equilibrium with water vapor1, i.e., in this

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example, to maintain an aqueous solution water activity of 0.3. A subsequent increase

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in the RH would result in larger particles with greater water content and inorganic

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fraction compared with a particle that does not take up inorganic matter. Any gas

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would partition into an aqueous solution following Henry’s law. If a gas such as N2O5

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is removed from the air and converted into an inorganic species which would remain

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in a condensed phase, the species will be depleted from the gas and condensed phases.

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However, Henry’s law is applicable, suggesting that the abovementioned depletion

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from the gas phase via uptake should balance the depletion in the condensed phase via

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the aqueous phase chemistry. Alternatively, if this gas is constantly emitted or

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supplied from the Anthropocene, it will constantly partition into the condensed phase

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in preparation for a chemical reaction to occur, thus triggering the feedback

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mechanism. This means that Henry’s law become a driving force to reach equilibrium

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concentration, enhanced water uptake due to enhanced nitrate mass, but the

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thermodynamic equilibrium with water vapor will ensure that the aqueous solution

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water activity remains equal to the RH. To maintain thermodynamic equilibrium

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while adjusting for reactive uptake, the particles must increase their water and

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inorganic contents, thereby increasing the inorganic mass fraction. We should note

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that the SOA might participate such feedback process: The enhanced aerosol liquid

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water facilitate the aqueous phase SOA formation20-22, thereby, increased the SOA

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mass. Considering the lower hygroscopicity of organic aerosols compared to SIA45-47,

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the enhanced ALWC associated with aqueous SOA may be insignificant. While, the

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SOA may influence the feedback process in other ways. For example, the gas uptake

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(e.g. of N2O5) might be reduced when particles are covered by organic films48-49, thus,

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weaken the feedback loop. 1.0

1.0

(a) 3

20 10

= 99

]

0.6

0.6

0.4

0.4

0.2

0.2

0.0 0

20

SIA fraction > 0.6 0.5-0.6 0.4-0.5 0.3-0.4 0.2-0.3 < 0.2 All data points

0.8

ALWC

30

]

= 40

3

SIA Mass/PM2.5 Mass

40

PM2.5 [µg/m Water [µg/m

50

0.8

(b)

40 60 RH [%]

80

0.0 0

20

40 60 RH [%]

80

100

238 239

Figure 2. Panel (a): Inorganic fraction in PM2.5 (circles) as a function of the relative humidity. The

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circles are colored according to the aerosol liquid water concentration, and the sizes of the circles are

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scaled to the PM2.5 mass concentration. Panel (b): Aerosol liquid water content (ALWC) of condensed

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water with respect to the dry PM2.5 mass concentration. The ratios are grouped according to the

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inorganic fraction in PM2.5.

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Fig. 2 (b) displays the ALWC as a function of the ambient RH. The data points

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are grouped according to the inorganic fraction in PM2.5. The data in different groups

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were fitted by log(y)=a*x+b, as shown by the colored curves in Fig. 2 (b). The ALWC

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generally increased with an increase in the ambient RH, and the ALWC associated

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with higher inorganic fractions was much larger than that associated with lower

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inorganic fractions at a constant RH. In other words, an increase in the ALWC for a

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given RH increment was greater for particles with larger SIA mass fractions (refer to

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Fig. 2 (b)). As discussed above, the polluted days were typically associated with a

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higher inorganic fraction in PM2.5. In addition, our observations showed that the

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contribution of nitrates to the SIA mass concentration increased significantly with an

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increase in the PM2.5 mass concentration (Fig. S3). Nitrates accounted for 25% of the

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SIAs for PM2.5 concentrations lower than 50 µg/m3 and 45% for PM2.5 concentrations

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greater than 150 µg/m3. This means that nitrates become a dominant inorganic

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component during heavy haze events. It is well known that ammonium nitrate has a

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lower deliquescence RH (DRH=61.8%) than ammonium sulfate (DRH=79.9%)1.

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Thus, at even lower RH levels, the dominant nitrate particles containing other soluble

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components may absorb water and increase the ALWC28.

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Fig. 3 shows the detailed monthly and diurnal variations in the ALWC of PM2.5.

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During the observation period from May 2014 to January 2015, the highest ALWC

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was observed in July in association with the highest ambient RH. The ALWC was

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abundant during November 2014 and January 2015, during which haze episodes were

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frequent. As expected, the diurnal patterns showed a higher ALWC during the

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nighttime due to the higher ambient RH after sundown. The ALWC abundance at

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night may influence the nighttime chemistry, including N2O5 hydrolysis15 and the

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nitrate radical (NO3) oxidation of biogenic volatile organic compounds50. The

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nighttime formation of nitrates may drive greater inorganic fractions and accordingly

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greater ALWCs.

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20

100

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80

12

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ALWC (µ µg/m )

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4

0

0 May

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July

Sep Month

Jan

Nov 2014

2015

Figure 3: Monthly and daily average trends of the aerosol liquid water content (ALWC) from May 2014 to January 2015.

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The long-term observations in our study showed that the ALWC was well

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correlated with the nitrate and sulfate mass concentrations (see Fig. S4), indicating

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that both nitrate and sulfate salts play key roles in determining the ALWC. This

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suggests that a reduction in both the sulfate and the nitrate fraction could efficiently

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reduce the ALWC over the NCP. Differently, a substantial fraction of the particulate

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ALWC over the Eastern U.S. could be attributed to anthropogenic sulfate

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components19. The ALWC over the Italian Po Valley is driven by locally formed

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anthropogenic nitrates17. Since 2007, the emission of anthropogenic SO2 throughout

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China has declined by 75%51. This is not true for the emission of nitrogen oxides,

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however. As a consequence, the nitrate concentration is expected to become more

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important than that of sulfates. This implies that emission controls on nitrogen oxides

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will constitute an efficient method for reducing the particle mass loading and ALWC

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over the NCP in the future.

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The elevated ALWC on haze days trends toward an increasing particle mass

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concentration, thereby increasing the light scattering properties of submicron particles

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and leading a degradation of the visibility4, 52. Most importantly, the enhanced ALWC

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increases several trace gases (such as N2O5 and HO2) uptake coefficients by diluting

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the absolute concentration of solute, regulating the acidity or acting as a reactant in

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the bulk14, 53-54. Furthermore, the increased aerosol surface area due to hygroscopic

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aerosol growth leads to the increasing of heterogeneous reaction rate53. Previous

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studies reported that a high ALWC could speed up the uptake coefficient of N2O515, 55,

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thereby enhancing the formation of particulate nitrates. In addition, the ALWC could

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serve as a reactor for the transformation of SO2 to sulfate in the aqueous phase16, 37, 56.

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Furthermore, the ALWC could facilitate the formation of SOAs through the aqueous

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phase chemistry and photochemistry12, 18, 22, 31. Thus, an increase in the ALWC during

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pollution episodes over Beijing would significantly increase the reaction rates and

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probabilities of adsorbed species in ambient aerosols and therefore increase the uptake

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coefficients of many trace gas compounds. An elevation in the ALWC due to increased

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ambient RH levels, water-soluble inorganic fractions, and aerosol mass loading may

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lead to a positive feedback loop wherein aqueous particles readily uptake pollutants

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that could react to form additional anthropogenic inorganic components; these

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components could then uptake additional water, further accelerating the aerosol mass

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accumulation over time periods of pollution57.

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The NCP is China’s largest steel production and cement manufacturing region;

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these industries are strongly dependent upon coal. Sulfur- and nitrogen-containing

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gaseous precursors such as SO2 and NOx that are emitted from the combustion of

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fossil fuels are transferred into hygroscopic particle constituents (i.e., sulfate and

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nitrate salts) via atmospheric chemistry processes. As a result, particulate matter in the

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atmosphere over the NCP carries an abundance of SIA components58, leading to high

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SIA-derived ALWCs at elevated RH levels during haze episodes due to the triggered

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positive feedback loop. Such a mechanism may also be an effective pathway for

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producing the ALWCs in other environments, especially regions in Southeast Asia that

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exhibit an increased fossil fuel consumption59-60. The SIAs associated with condensed

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water particles may provide the relevant media through which anthropogenic SIAs may

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modulate the formation and properties of biogenic SOAs. Previous field observations

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have already provided evidence for the influence of the SIA-derived ALWC on the

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formation of SOAs17, 19, 22. An increase in the water uptake ability due to SIAs may

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affect climate change at a regional scale by altering cloud microphysics and optical

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properties. We must note that changes in the particulate matter constituents that drive ACS Paragon Plus Environment

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the ALWC and its effects on the aerosol chemistry coincident with significant

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reductions in global SO2 emissions, especially in China, should be reconsidered.

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Notes

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The authors declare no competing financial interest.

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Acknowledgement

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This work is supported by the following projects: National Natural Science

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Foundation of China (41475127, 41571130021) and National Key R&D Program of

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China (2016YFC0202800: Task 1). The authors would like to greatly thank Min Hu,

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Yuxuan Bian, Ying Chen, and Hongyu Guo for useful discussion and thank three

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anonymous reviewers for their hints and suggestions.

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References

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