Simulated nutrient dissolution of Asian aerosols in various atmospheric waters: Potential links to marine primary productivity

Atmospheric Environment 164 (2017) 224e238

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Simulated nutrient dissolution of Asian aerosols in various atmospheric waters: Potential links to marine primary productivity Lingyan Wang a, Yanfeng Bi a, Guosen Zhang b, Sumei Liu a, c, *, Jing Zhang b, Zhaomeng Xu a, Jingling Ren a, c, Guiling Zhang a, c a

Key Laboratory of Marine Chemistry Theory and Technology MOE, Ocean University of China/Qingdao Collaborative Innovation Center of Marine Science and Technology, Qingdao 266100, PR China State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, PR China c Laboratory for Marine Ecology and Environmental Science, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266100, PR China b

h i g h l i g h t s
Acid processing promotes the release of NHþ 4 in Asian dust.
P and Si in aerosols have similar dissolution pattern but different solubility.
P and Si dissolution of Asian aerosols in AW fit first-order kinetic model.
Source, acid/particle ratio and Pc co-control the P and Si solubility in aerosols.
Dissolved P and Si of Asian aerosols in AAW far exceed that in non-AAW.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 January 2017 Received in revised form 31 May 2017 Accepted 3 June 2017 Available online 5 June 2017

To probe the bioavailability and environmental mobility of aerosol nutrient elements (N, P, Si) in atmospheric water (rainwater, cloud and fog droplets), ten total suspended particulate (TSP) samples were collected at Fulong Mountain, Qingdao from prevailing air mass trajectory sources during four seasons. Then, a high time-resolution leaching experiment with simulated non-acidic atmospheric water (nonAAW, Milli-Q water, pH 5.5) and subsequently acidic atmospheric water (AAW, hydrochloric acid solution, pH 2) was performed. We found that regardless of the season or source, a monotonous decreasing pattern was observed in the dissolution of N, P and Si compounds in aerosols reacted with non-AAW, and  the accumulated dissolved curves of P and Si fit a first-order kinetic model. No additional NO 3 þ NO2 þ dissolved out, while a small amount of NH4 in Asian dust (AD) samples was released in AAW. The similar dissolution behaviour of P and Si from non-AAW to AAW can be explained by the Transition State Theory. The sources of aerosols related to various minerals were the natural reasons that affected the amounts of bioavailable phosphorus and silicon in aerosols (i.e., solubility), which can be explained by the dissolution rate constant of P and Si in non-AAW with lower values in mineral aerosols. The acid/particle ratio and particle/liquid ratio also have a large effect on the solubility of P and Si, which was implied by Pearson correlation analysis. Acid processing of aerosols may have great significance for marine areas with limited P and Si and post-acidification release increases of 1.1e10-fold for phosphorus and 1.2e29fold for silicon. The decreasing mole ratio of P and Si in AAW indicates the possibility of shifting from a Silimit to a P-limit in aerosols in the ocean, which promotes the growth of diatoms prior to other algal species. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Nutrient dissolution Asian aerosol Solubility Atmospheric water Yellow Sea

1. Introduction * Corresponding author. Key Laboratory of Marine Chemistry Theory and Technology MOE, Ocean University of China/Qingdao Collaborative Innovation Center of Marine Science and Technology, Qingdao 266100, PR China. E-mail address: [email protected] (S. Liu). http://dx.doi.org/10.1016/j.atmosenv.2017.06.005 1352-2310/© 2017 Elsevier Ltd. All rights reserved.

Atmospheric deposition is an important source of macronutrients (N, P, Si) and a micronutrient (Fe) in the upper water column of the ocean, supporting the growth of phytoplankton (Duce et al.,

L. Wang et al. / Atmospheric Environment 164 (2017) 224e238

2008; Jickells et al., 2005; Paytan and McLaughlin, 2007; Treguer and De La Rocha, 2013). Compared with N and Fe, the contents of bioavailable P and Si in atmospheric deposition are usually deficient, assuming the Redfield ratio (C: N: Si: P ¼ 106:16:16:1), which is required by phytoplankton and nitrogen fixation (N: P ¼ 40 and P: Fe ¼ 8e150) for diazotrophs (Baker et al., 2007), due to the more insoluble phase in aerosols. Due to the pH and ionic strength, the solubility of P and Si in aerosols in seawater may be lower than those in Milli-Q water (Chen et al., 2006; Kocak, 2015). Therefore, the bioavailability (or solubility) of P and Si in aerosols before deposition into the ocean plays a crucial role in estimating new production because the soluble fractions deposited in the ocean can be instantly utilized by phytoplankton. The solubility of nutrients in aerosols is usually operationally defined as the percentage of the dissoluble content accounting for the total loading, which is variable and is affected by the source, chemical composition and acidity of aerosols, as well as the leaching procedure (Mackey et al., 2012, and references therein). The main sources of P and Si are mineral components with low solubility, such as apatite and silicate (Engelbrecht and Derbyshire, 2010; Filippelli, 2008). Some other sources, such as fuel combustion, biomass burning, volcanic ash and primary biogenic aerosols
and Querol, 2010; Jones and Gislason, 2008; for P and Si (Giere Mahowald et al., 2005; Wang et al., 2014), and lithophytes and biogenic silica for Si (Conley, 2002; Wu et al., 2015), may contribute more soluble fractions. Atmospheric aerosols undergo various physical and chemical processes, including aggregation and deposition, condensation and evaporation cycles, wet scavenging and photochemical processes, prior to deposition into the ocean, which may affect the relative proportion of soluble and insoluble components of P and Si. Recently, variations of P and Si solubility were linked to acid processing in some studies (Aghnatios et al., 2014; Anderson et al., 2010; Baker et al., 2006a, 2006b; Nenes et al., 2011). The mechanisms influencing the dissolution of P and Si in aerosols are not well known. In general, the chemical composition of aerosols directly influences the pH in atmospheric water because of the role that condensation nuclei play in the formation of cloud and fog droplets, which in turn affects the fractions of soluble components in aerosols and subsequently changes the stoichiometric ratio of bioavailable nutrients deposited into the ocean, as well as the composition of phytoplankton species. The worldwide acidification of atmospheric water, including rainwater, cloud droplets and sea fog, is also reported frequently, and the pH of atmospheric water in some coastal areas is even below 3 (Sasakawa and Uematsu, 2005; Wang et al., 2012). Therefore, the evolution of P and Si in the transport of aerosols through the atmosphere deserves great attention from scientific researchers. However, simulations of many atmospheric models are hampered by the uncertainty of the solubility of P and Si due to a lack of observational data (Krishnamurthy et al., 2010). As one of the largest dust sources in the world, Asian dust aerosols usually fertilize marginal seas in China, the North Pacific and even the Atlantic Ocean as well as areas of North America through long distance transport (Uematsu et al., 2003; Uno et al., 2009). In terms of the Yellow Sea, mineral dust accompanied by precipitation tends to stimulate the growth of phytoplankton in the ocean and even cause the occurrence of blooms (Jo et al., 2007; Shi et al., 2012). Studies show that Asian aerosols, in particular dust aerosols, often adsorb large amounts of anthropogenic gases (e.g., NOx, SO2, NH3) and support a hotbed of heterogeneous reactions for them during transport through the atmosphere (Geng et al., 2009), and Asian mineral dust particles can be acidified to pH < 2 (Meskhidze et al., 2003). Furthermore, the amounts of absorbed acid gases in aerosols increase with the increase of the relative air humidity offshore (Vlasenko et al., 2006). All of these factors

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indicate that enhanced dissolution of key nutrient elements (P, Si) can be expected through atmospheric water chemistry reacting with acidic species, especially in a coastal atmospheric environment with sufficient atmospheric water and pollution sources. Then, the understanding of the interaction between Asian aerosol particles and atmospheric water with the knowledge of the dramatic increase in anthropogenic acidic gas emissions has a very profound biogeochemical significance. Fitting dissolution kinetic characteristics is a good tool to explore the dissolution mechanism of nutrients in aerosols. Previous kinetic studies have focused on the dissolution kinetic characteristics of enriched-P or Si pure end-member mineral substances, such as apatite, aluminosilicate, and biogenic silica, as well as trace metals, such as Fe, Al, and Ca, in aerosols or aerosol analogues, i.e., screened soil (Desboeufs et al., 1999; Guidry and Mackenzie, 2003; Moriceau et al., 2009; Shi et al., 2011; Wu et al., 2015). However, the atmospheric water chemistry of N, P and Si in actual aerosol samples is more representative than soil or mineral dust from the original area because the physicochemical properties of aerosols are greatly modified during transport through the atmosphere. In this study, an open flow-through leaching experiment was carried out using coastal aerosols with the chemical characteristics of different seasons and sources, including two Asian dust samples from different atmospheric water sources over a short time scale. The aim was to explore (i) the dissolution kinetic process of nutrients (especially refractory elements P and Si) in Asian aerosols under simulated atmospheric water environments (pH 5.5 and pH 2); (ii) factors influencing the solubility of nutrients in aerosols; (iii) potential effect of acid processing between aerosols and atmospheric water on marine primary production.

2. Materials and methods 2.1. Study site and sampling Fulong Mountain (36 040 N, 120 200 E) is located in the northwest coastal city of Qingdao of the South Yellow Sea, which is the transitional zone between the continent and ocean, and the average elevation is approximately 76 m (Fig. 1). Due to the northwest (NW) monsoon, in winter and spring, local aerosols come from dust from the Taklimakan Desert and Loess Plateau passing through highly developed industrial cities. The southeast (SE) monsoon prevails in summer, and local aerosols mainly come from sea-salt particles, while a small percentage of aerosols come from southwest (SW) urban pollutants; the other seasons may have a mixture of various aerosol sources (Zhang et al., 2007). In addition, local aerosols are heavily affected by anthropogenic activities, including fossil fuel combustion, industrial production and transportation. Therefore, Fulong Mountain is an ideal location to measure Asian-outflow materials or marine substances and is suitable for us to study the acid processing of aerosols before deposition into the ocean. A total of 64 total suspended particulate (TSP) samples were collected using a KC-1000 high-volume TSP sampler with polycarbonate filters (effective area 18.2 cm  23 cm, pore size 0.4 mm) and an approximate 1.0 m3/min flow rate from February 2004 to December 2005. The sampling frequency was 2e5 samples every month. Before sampling, the filters were treated by pH 2 high purity hydrochloride acid diluted with Milli-Q water (18.2 MU). After collection, the sample filters were stored in a refrigerator at 20  C until analysis. The filters were weighed carefully before and after sampling and conditioned in a desiccator for at least 48 h before weighing.

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Fig. 1. The sampling site (Fulong Mountain) is marked with a filled triangle, which is located in the northwest part of the Yellow Sea. The right of the figure is a magnified view. The local aerosols are deeply influenced by the monsoon, and the grey arrows represent the northwest monsoon, while the black arrows represent the southeast monsoon.

2.2. Leaching experiment Ten TSP samples were chosen to conduct the leaching experiment, whose sources conformed with the prevailing wind directions (NW, SW, SE for spring and autumn expressed as spr-NW, spr-SW, spr-SE and aut-NW, aut-SW, aut-SE; SW and SE for summer expressed as sum-SW and sum-SE; NW and SW for winter expressed as win-NW and win-SW) during the four seasons over the study period (Fig. 2). The aerosol sources were identified by air mass archive trajectory analysis (NOAA Air Resources Laboratory, HYSPLIT trajectory model) (Draxler and Rolph, 2003). Two Asian dust (AD) samples (i.e., NW for spring and autumn) were used to evaluate the difference between sand dust sources and non-sand dust sources. The characteristics of the samples are listed in Table 1 and Table S1. The leaching experiment was conducted using an open-flow leaching system modified from Eyckmans et al. (2001) (Fig. 3). Sampled filters were cut into 47-mm diameter subsample filters and rolled into a mini column, with particles toward the inside. The leaching solutions were introduced into this device by a peristaltic pump and passed through the surface of the subsample filters. Ultimately, leachates containing nutrients were filtered by the 0.45-mm pore size micro polyethersulfone (PES) filters and collected. Here, we set the flow rate as 2.4 mL/min, and the leaching time was 62.5 min, with 25 min for Milli-Q water (pH 5.5) and subsequently 37.5 min for the hydrochloric acid solution (pH 2). Leachates of 50 aliquots, each for 75 s, were collected, including 20 Milli-Q water-soluble samples and 30 acid-soluble samples. The sample for SW in autumn, which contained large amounts of mineral silicate, was found to be not dissolved completely in the pH 2 acid solution after determination of the output solution, so we extended the acid leaching time to 48.75 min. The entire leaching time (62.5 min or 73.75 min) was also within the reevaporation time of cloud droplets (Warneck, 1989) and the deposition time (~2 h) of sea fog, assuming a 50-mm droplet diameter and a 500 m height (Sasakawa et al., 2003). We chose a relatively high flow rate (2.4 mL/min) on the premise that the pressure within the system was steady enough that the leaching solutions did not leak and filters did not collapse, which blocked the flow (Eyckmans et al., 2001). The particle masses of the ten aerosol samples used in this

study ranged from 1.16 mg to 7.22 mg. The total volume of the leaching solution was 150 mL, with an exception of 177 mL for SW in autumn. Therefore, the particles/liquid ratio (Pc) of 2.7e32.7 mg/ L were within the range of most natural rain water (<40 mg/L) (Zhang et al., 2005), and 50% of the concentrations were higher than the particulate concentrations in cloud droplets (<20 mg/L) (Desboeufs et al., 2001). Nutrients in this leaching device were maintained in an unsaturated condition, which avoided the reabsorption or secondary precipitation loss of nutrients on particles found in batch dissolution experiments (Mackey et al., 2012; Shi et al., 2011). Furthermore, the high time-resolution measurements can provide us with more information. The experiment was carried out at room temperature (~298 K) and pressure. Two scenarios were considered, as follows: (1) One non-acidic atmospheric water sample equilibrated with CO2 (pH 5.5, Milli-Q water) was encountered with aerosols. (2) Subsequent dissolution due to the adsorption of anthropogenic acid gas (e.g., NOx, SO2) in an extreme environment with a pH 2 hydrochloric acid solution, which can be encountered in some cloud and fog droplets (Sasakawa and Uematsu, 2005; Wang et al., 2012). Though previous work in our laboratory suggested that the pH (3.7e7.1) of rain water west of the Yellow Sea may be higher (Han et al., 2013), it is still valuable to study acid rain. A steady-state condition should be reached for each aerosol sample in both the water solution or acid solution, which implied that the dissolution process had finished according to the operational leaching solution. However, there was no unified criterion to define the steady-state conditions in the open-flow experiment
n et al., 2008) because the virtual steady-state may not be (Rozale €hler et al., 2005). In reached in some dissolution experiments (Ko our study, the concentrations in the last few leachates were below the detection limit, which indicated that the nutrients dissolved completely and a steady-state condition was reached. The concentrations of nutrients below the detection limit had great analytical errors, which were removed from subsequent calculations and analysis.

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Fig. 2. Three-day air mass back trajectories were computed from (a) the northwest (NW) desert source over Siberia and the Gobi Desert, Taklimakan Desert and Loess Plateau; (b) the southwest (SW) source over eastern China and southern China; (c) the southeast (SE) marine source over the East China Sea and the Yellow Sea. Triangles represent the sampling site (Fulong Mountain).

2.3. Sample analysis  þ 3 2 The nutrients (NO 3 þ NO2 , NH4 , PO4 , SiO3 ) in the collected output solutions for each leaching experiment were determined by a Skalar SANPlUS autoanalyser (Zhang et al., 2007). The nutrients extracted via the batch ultrasonic method (Zhang et al., 2007) were also determined. The relative standard deviations for the determination of nutrients were <3%. The detection limit was  þ 0.06 mmol L1 and 0.09 mmol L1 for NO 3 þ NO2 and NH4 and 2 0.05 mmol L1 and 0.07 mmol L1 for PO3 and SiO , respectively. 4 3 The leaching experiment was conducted once for every filter sample. However, the previous pre-experiment showed that the relative standard deviations for the five replicate experiments were <10% for all nutrients (Zhang, 2004). Total phosphorus (TP) in aerosol samples was measured by the high temperature ashing method (Carbo et al., 2005). Twenty-

millilitre crucibles, each with a 47-mm diameter subsample filter, were placed in a muffle furnace at 550  C for 2 h and then cooled. The inorganic phosphorus in burning ashes was extracted in an ultrasound for 1 h by 20 mL of 1 M HCl, then filtered using a 0.45mm cellulose acetate membrane and measured by spectrophotometry. Total silicon (TSi) in aerosol samples was measured by the NaOH fusion method (Georg et al., 2006). A 20 cm2 subsample filter and ~300 mg soild NaOH flux were placed in a 30 mL Ag crucible. Then the crucibles were placed in a muffle furnace at 730  C for 10 min. After cooling, the crucibles with 20 mL Milli-Q water were heated in the electric hot plate until the residue was dissolved in water completely. The contents in crucibles were transferred to 50 mL centrifuge tubes completely after rinsing sidewall of crucibles several times using Milli-Q water. The final solutions were diluted to 50 mL weakly acidified solutions by 10 N HCl and Milli-Q water. The dissolved silicate was measured by spectrophotometry.

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dissolution rate (mmol g1 min1); t is the leaching time of each leachate, 1.25 min. To model the dissolution process of P and Si from aerosols, the two independent first-order kinetic models in Eq. (3a-b) were used to fit the accumulated dissolution curves related to time, which were used in some analogous dissolution kinetic processes (Moriceau et al., 2009; Shi et al., 2011). The theoretical basis of the first-order kinetic model is that the concentration variation gradient of related components against the time in solution (dCsolution ) dt is a negative linear function of the solid concentration (Csolid ) (see Eqs. (1)e(3) in Truesdale et al. (2005)). The slope (k) is the dissolution rate constant. The first order kinetics

Qt ¼ Q1  Q1  ek1 t

0  t < t25

Qt ¼ Q2  Q2  ek2 ðt  ts Þ Fig. 3. A simple system for the open-flow experiment. The structure diagram is modified from Eyckmans et al. (2001).

For the total metal, every 20 cm2 subsample PC filter was decomposed in a PTEF vial by 2 mL ultra-pure 30% aqueous ammonia overnight (Okubo et al., 2013), then heated to nearly dryness in the electric hot plate, and finally digested with the mixed acid (3 mL HNO3 þ 3 mL HClO4 þ 1 mL HF) at 180  C for 72 h. The concentrations of total metal (TM) elements, including Al, Fe, K, Ca, Na, Mg, Cu, Zn, Ba, V, Pb, Cr, Ti, Ni, Mn and Co, were measured by Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES, iCAP6300, Thermo Fisher Scientific Company, USA). The analytical precisions of TP, TSi and TM were all <5%. The chemical morphology of the particles in aerosol samples were analysed by a field emission scanning electron microscopy (SEM, Hitachi S-4800, Japan) with an accelerating voltage of 10 kV. The elemental composition on the surface of particles was measured by the energy-dispersive X-ray spectrometer (EDS) attached with SEM running at 15 kV accelerating voltage. The resolution and dwell time for SEM-EDS were 1.0 nm/15 kV and 70 s, respectively. 2.4. Data processing method The raw concentrations of nutrients in leachates during the leaching experiments were measured in mmol L1, while the data were normalized to the particulate mass concentrations Cparticle (i.e., the content of dissoluble nutrients in unit mass particle, mmol g1) calculated by Eq. (1). Then, the corresponding dissolution rate R (mmol g1 min1) was obtained by Eq. (2):

Cparticle ¼

R ¼

ðCout  Cin Þ  Vleach  Stotal Ssub  mparticle  1000

Cparticle t

(1)

(2)

where Cparticle is the particulate mass concentration (mmol g1); Cin and Cout (mmol L1) are the concentrations of nutrients in the input solution and output solution, respectively. Here, Cin is regarded as zero for phosphate and dissolved silicate in Milli-Q water or a hydrochloric acid solution because the value measured was under the detection limit. Vleach is the volume of leachates, 3 mL; Stotal and Ssub are the effective area of the aerosol filter and area of the subsample filter in the mini column, 373.1 cm2 and 17.35 cm2, respectively. Mparticle is the particulate mass on the aerosol filter. R is the

t  ts

(3a) (3b)

where Qt is the accumulated amount of dissolved nutrients at time t normalized to the concentration in unit mass (unit: mmol g1). Q1 and Q2 represent the initial contents of the pool1 and pool2 phases in mmol g1, respectively. k1 and k2 are the dissolution rate constants of each pool in min1, and t is the time in min. This model is composed of two independent exponential formulas that differ from the sum of exponential formulas used by Shi et al. (2011), which indicates that the nutrients studied are in two phases and released one after the other due to changes in environmental parameters (here, pH) rather than a simultaneous release in the same solution (Moriceau et al., 2009). Eq. (3a) and Eq. (3b) were used to fit the dissolution scenarios in non-acidic and acidic water, respectively. t25 represents the time when the Milli-Q water leaching solution is substituted by acid solution. ts represents the transitional time at which the dissolution rates decrease in the acid solution (i.e., the time at which the maximum concentrations of nutrients are reached in the acid solution). The curves of the dissolution rate that increased in the first several minutes in the acid solution can be explained by the Transition State Theory and are controlled by the thickness and nature of the transition layer (Desboeufs et al., 1999). The first order kinetic equation is not applicable in the transitional period, while other kinetic equations are beyond the scope of this study due to a lack of other relative experimental results. Aerosol enrichment factors (EF) were used to evaluate the enriched or depleted extent of a given element (X) relative to crustal material, calculated by Eq. (4).




EFcrust ¼ ðX AlÞaerosol ðX AlÞcrust

(4)

Where (X/Al)aerosol is the mass concentration ratio of element (X) to Al in aerosol samples. (X/Al)crust is the average abundance ratio of element (X) to Al in upper continent crust (Taylor and Mclennan, 1995). The accumulated solubility of P and Si (SolX, X ¼ P, Si) were calculated according to Eq. (5).

SolX ð%Þ ¼ 100 

t X

Ct =CT

(5)

0

Where Ct is the soluble concentrations (mmol g1) of nutrients at time t (min), t ¼ 1.25, 2.5, … … 62.5 or 73.75. CT is the total concentration (mmol g1) of nutrient elements in the aerosols (i.e., TP or TSi). The accumulated solubility at 1.25 min, 25 min (water solution), and 62.5 min or 73.75 min (water plus acid solution) were temporarily called instantaneous solubility, water-soluble solubility and acid-soluble solubility, respectively.

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To facilitate our analysis of the effect of acid processing on new production in the ocean, we defined an acidified factor parameter F to evaluate the effect of acidic constituents on the bioavailability of nutrients (Eq. (6)).

F ¼ Macid =Mwater . X X X ¼ Ca;i  Va Ca;i  Va Cb;j  Vb þ

(6)

where Macid and Mwater are the integral molar quantities of nutrients dissolved in the acidic solution and water solution, respectively. Note that we make an assumption that the components dissolved in the water solution can also dissolve out in the acidic solution. Ca,i and Cb,j are the nutrient concentrations (unit: mmol L1) of the ith and jth sub-samples, respectively, which were leached by Milli-Q water and a pH 2 hydrochloric acid solution. Va and Vb were the volumes of every leached sub-sample. In this study, i ¼ 1, 2, … … 20, j ¼ 21, 22, … … 59, and Va ¼ Vb ¼ 0.003 L. The larger value of F indicated the greater proportion of acid-soluble nutrients. 2.5. Backtrajectory cluster analysis To evaluate the potential contribution of aerosol acid processing on marine primary production during various seasons, we should know the proportion of each source arriving at the sampling station. A 72 h backtrajectory cluster analysis was computed using TrajStat, which is a plugin of the GIS-based software, MetoInfo (Wang et al., 2009). All backtrajectories were calculated within 6 h intervals (00:00, 06:00, 12:00, 18:00 UTC) at a starting height of 500 m above ground level for each day during the two-year sampling period (2004e2005). The NCEP/NCAR Reanalysis meteorological data provided by NOAA/ARL (National Oceanic and Atmospheric Administration/Air Resource Laboratory ftp://arlftp. arlhq.noaa.gov/pub/archives/rea-nalysis) were employed in the trajectory analysis. 3. Results 3.1. Trajectory analysis and general characteristics of the original aerosols Three-day air mass back trajectories were computed for aerosol samples, except a two-day air mass back trajectory was used for aut-NW because the distance that the air mass travelled was long enough to trace the original area along with the high wind speed (Fig. 2aec). All ten air masses were classified into three main sources: (1) NW mainly came from the deserts in northwest and northern China or the Gobi Desert in Mongolia, passing through the Loess Plateau or the highly industrialized Beijing-Tianjin-Tangshan area prior to arriving in Qingdao (Fig. 2a). Affected by the agrotype in northern China (i.e., alkaline calcisols or aeolian sandy soil or grey desert soil and cinnamon brown soil), aerosols in the original area are primarily composed of dust-derived aerosols rich in Ca, Al, and Si, which are ~50e90% of the total particle mass, while other contributors from artificial acidic components (e.g., sulphate, nitrate) are from coal combustion, vehicle exhaust, agriculture activities and so on (Zhang et al., 2002, 2003). (2) SW (Fig. 2b) originated from some inland and coastal cities in southern China, which were subjected to the effects of industrial processes and coal combustion (66%) in inland cities and more soil dust, building dust and vehicle exhaust (66%) in coastal cities (Qiu et al., 2001; Xie et al., 2005). A deficiency of alkali metals, such as Ca and Mg, and

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more acidic purplish soil or paddy soil or red-yellow soil rich in Fe and Al may lead to aerosols from the SW that are more acidic than those from the NW. (3) SE (Fig. 2c) crossed over the ocean and may have more sea salt particles, including soluble inorganic phosphate and dissolved silicate. In our study, the aerosol samples used in the leaching experiment had different chemical compositions (Table S1). The maximum concentrations of Al appeared in the samples from NW sources. The elements in aerosol samples from NW and SW sources had comparable concentrations, including Si, P, Fe, Mn, Mg, Ca, Ti and Ba. While the aerosol samples in winter had maximum concentrations of anthropogenic metal elements such as Cu, Pb and Zn, and the aerosol samples from SW sources in other seasons took the second place. We can also obtain some information relative to the source of aerosols used in our study according to the concentrations of TSP and nutrients (Table 1). Except for spr-SW, the TSP concentrations of the spr-NW (429.8 mg m3) and aut-NW (303.5 mg m3) aerosol samples were 2e7-fold higher than those of other aerosol  þ samples, while the mass concentrations of NO 3 þ NO2 and NH4 were 2e9-fold and 3e31-fold lower than in other samples, respectively. The high particle loading and low concentrations of  þ NO 3 þ NO2 and NH4 in spr-NW and aut-NW indicated that they came from Asian dust (AD) sources and were less affected by anthropogenic activity. However, win-NW was heavily polluted by 1 1  þ NO 3 þ NO2 (749 mmol g ) and NH4 (1 104 mmol g ) produced by industrial and livestock activity. The extraordinary high TSP in sprSW may result from urban construction and soil dust, similar to the above analysis of original aerosols. 3.2. Particle morphology analysis of aerosols from various sources The aerosol samples from NW sources contained large amounts of irregular mineral particles enriched with Al and Si, along with Mg, K, Ca and Fe (Figs. S1aef and S2a-b). Soot aggregates mixed with mineral particles were pervasive in win-NW samples, while few in AD samples (i.e., spr-NW and aut-NW), which indicated that aerosols in winter were heavily polluted by the product from coal combustion. Many smoothly spherical particles like fly ash particles embedded with mineral particles was the most obvious evidence of the interaction between mineral dust with anthropogenic pollutants (Fig. S1e). The spectrogram of EDS manifested that the spherical particles contained large amounts of N elements, which were mainly from acid precursors (e.g. NOx). The aerosol samples from SW sources contained many elongated particles enriched with S, Pb and angular irregular particles enriched with S, Zn (Figs. S1g-n and S2c-d). These particles were mainly produced by metal smelter and steel factory (Xie et al., 2005). Affected by monsoon climate, the aerosol samples for sprNW and aut-NW contained some mineral components, while the sum-SW sample had some organisms attached to the rich-Pb particles. The win-SW sample had similar particle morphology with win-NW sample. The aerosol samples from SE sources contained many sea salt particles along with some small secondary particles which may be sulfates and nitrates (Li et al., 2016) (Fig. S1o-t). As with the aerosol samples for SW sources, the aerosols had seasonal characteristics with more minerals like aluminosilicate in spr-SE (Fig. S1p) and more clay minerals like goethite (Fig. S1s). 3.3. Dissolution of nutrients Regardless of the source and season, the concentrations of nutrients all decreased sharply from the highest concentrations within the first 5 min and then remained at an extremely low level

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Table 1 Mass Concentrations (mg m3) of total suspended particulates (TSP), accumulated water-soluble and concentrations (mmol g1) of nutrients and concentrations (mmol g1) of total phosphorus (TP) and total silicon (TSi) in ten aerosol samples from various sources. Source

Sampling datea

TSP

 NO 3 þ NO2

NHþ 4

PO34

SiO23

TP

TSi

spr-NW spr-SW spr-SE sum-SW sum-SE aut-NW aut-SW aut-SE win-NW win-SW

10/03/04 27/04/05 12/04/04 04/07/05 22/07/05 06/11/05 17/09/04 03/09/04 11/12/05 15/12/04

429.8 518.6 65.7 199.5 69.7 303.5 135.2 132.9 143.8 193.5

224 977 1136 1060 2201 289 1060 955 749 841

236 1072 3025 1567 7615 303 1630 3910 1104 1645

2.28 10.29 7.23 7.57 19.41 3.78 12.13 15.04 5.14 10.11

1.24 1.28 1.83 0.58 7.12 0.53 7.89 2.44 0.82 1.08

24.53 36.36 14.43 25.80 21.66 23.20 28.31 26.63 32.27 45.44

2108 2607 2586 2129 1755 2554 2376 1987 2809 2178

a

Sampling date given as day/month/year.

after 10e20 min with Milli-Q water (pH 5.5), showing high released rates of nutrients in non-acidic atmospheric water (Fig. 4). The initial maximum concentrations of water-soluble phosphate and silicate were 0.76e12.8 mmol g1 and 0.17e5.86 mmol g1, respectively, with the low values from NW samples. These concentrations were converted to dissolution rates within the range of 1 0.61e10.2 mmol g1 min1 for PO3 min1 4 and 0.14e4.69 mmol g þ   for SiO2 . The highest concentrations of NH and NO þ NO 3 4 3 2 were 2 two orders of magnitude larger than those of PO3 4 and SiO3 . After 25 min, the Milli-Q water was replaced with pH 2 dilute  þ hydrochloric acid, and the concentrations of NO 3 þ NO2 and NH4 almost remained constant except for two AD events (Fig. 4a and b), and NHþ 4 had further dissolved, which lasted for approximately 5 min. Phosphate and dissolved silicate had similar dissolution kinetic curves that differed from the inorganic nitrogen species in the acidic solution; the concentrations of acid-soluble leachates rapidly increased to a peak value within 5e10 min and subsequently dropped to the lowest concentrations within 10e30 min (Fig. 4cef). The maximum concentrations of acid-soluble phosphate and silicate were 0.13e5.81 mmol g1 and 0.49e11.5 mmol g1, respectively, with high values from NW and SW samples with corresponding dissolution rates of 0.10e4.65 mmol g1 min1 for PO3 and 4 0.39e9.2 mmol g1 min1 for SiO2 3 . 3.4. Dissolution kinetics of Asian aerosol P and Si The first-order kinetic model fit the curves well, the R2 values of almost all of the fitting curves were >0.95, and the Reduced Chi-Sqr (0e1.22), a parameter that judges curve fitting, was very small (Tables 2 and 3). The dissolution rate constant for water-soluble components ranged from 0.13 to 0.80 min1 for P, with a minimum value for aut-NW and maximum value for sum-SE, while it was 0.14e0.91 min1 for Si, with a minimum value for spr-NW and maximum value for aut-SW. On the whole, there were distinct differences in the dissolution rate constants in sources for watersoluble P (SE: 0.27e0.80 min1 > SW: 0.18e0.65 min1 > NW: 0.13e0.32 min1), but this was not the case for water-soluble Si (SE: 0.54e0.68 min1, SW: 0.27e0.91 min1, NW: 0.14e0.90 min1). In the acidic solution, two dissolution patterns for both P and Si occurred: the dissolution rate increased before ts and decreased after ts. The substitution times (ts) ranged from 26.25 min to 32.5 min, with a transitional period lasting 2.5 mine7.5 min (Fig. 4cef). There were almost the same substitution times for P and Si in the same sample, except that the substitution times of Si in aut-SW and win-NW were 1.7-fold and 3-fold those of P, respectively. The dissolution rate constant of all samples had no significant variations after ts (average value ± standard deviation: 0.23 ± 0.06 min1 for P and 0.17 ± 0.07 min1 for Si), which

indicated that the similar P pool compounds or Si pool compounds dissolved out after ts. 4. Discussion 4.1. Dissolution mechanism of nutrients in Asian aerosols In general, nitrogen species in aerosols are mainly water-soluble components (e.g., NH4NO3, (NH4)2SO4, NH4Cl, NaNO3, and Ca(NO3)2) (Ottley and Harrison, 1992). Therefore, the dissolution of nitrogen species in water solution is inclined to be simple diffusion dissolution or ion exchange with hydrogen and hydroxyl ions, from which we deduce that on contact with atmospheric water, NHþ 4 and  NO 3 þ NO2 will be released rapidly and completely. Actually, a ratio of acid-soluble and water-soluble components F < 1.01 for  þ NO 3 þ NO2 and an average F of 1.03 for NH4 explain the predominance of the water-soluble component well. The further release of NHþ 4 in the AD samples during acid processing may result from the hydrolyzation of part of water-insoluble reduced organic nitrogen compounds (e.g., humic compounds, amino acid and urea) in the acid solution, which was derived from primary soil organic matter or formed by gas-to-particle transformations and absorption to pre-existing AD aerosols during the transport process (Cornell et al., 2003; Neff et al., 2002; Russell et al., 2003). The dissolved amounts of NHþ 4 in the acid solution account for 10%e20% of the total amounts of the water-soluble and acid-soluble components, showing the strong neutralization ability of acidic species and the potential of nitrogen fertilization in AD aerosols. In our study, the dissolution curves of P and Si in non-acidic atmospheric water showed a consistent pattern with that of N species, with high initial dissolution rates and subsequently continually decreasing dissolution rates, while the dissolution curves of P and Si in the acid solution showed larger fluctuations compared to N (Fig. 4cef). In contrast to nitrogen species, phosphorus and silicon species in aerosols may have more complexity due to the linkage with various mineral phases. Therefore, the dissolution pattern may result from the dissolution of the labile surface-adsorbed P and Si in the water solution and collapse of the crystal structure of internal refractory P and Si compounds under the attack of protons in the acid solution. Fig. 5 shows the relationship of aerosol samples from various sources, accumulated water and acid soluble concentrations of P and Si as well as respective dissolution rate constant with total digest concentrations of metals regarded as environmental parameters. The dataset participating in the canonical correspondence analyses (CCA) did not contain the exceptionally high water and acid soluble silicon concentrations in aut-SW (see detail explain in section 4.2). Elements including Si, Mg, Fe and K had

L. Wang et al. / Atmospheric Environment 164 (2017) 224e238

231

  Fig. 4. Variations in concentrations (mmol g1) of nutrients along with the leaching time (min) for NHþ 4 in (a) Asian dust (AD) samples, i.e., NW in spring and autumn; NO3 þ NO2 in 2 (b) NW in autumn; PO3 4 and SiO3 in (c) spring, (d) summer, (e) autumn, (f) winter. NW ¼ northwest, SW ¼ southwest, SE ¼ southeast. The open points are the values under the detection limit and are discarded through the calculations.

average crustal EF values of <1.7, except for Mn with the average crustal EF values of 2.9, indicating that they had similar sources with Al from mineral aerosols (Fig. S3). Elements (Co, Na, V, Ni, Ba, Ti, P and Ca) with the average crustal EF values of 3e10 were dominated by the mixed sources. While the four remaining elements of Cr, Zn, Pb and Cu were mainly produced by anthropogenic activities with the average enrichment factors of >10. The

distribution of aerosol samples in Fig. 5 was closely related to the concentrations of total metals from different sources, which was consistent with the results of SEM-EDS in section 3.2. The concentrations of water soluble P and Si (WSP and WSSi) as well as respective dissolution rate constant in non-acidic water solution (Pk1 and Si-k1) are distributed in the quadrant II and III, which have a close affinity with the elements from anthropogenic sources and

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Table 2 Kinetic parameters of phosphorus, dissolution rate constants k (min-1), the dissolved amounts (Q1 and Q2) in the corresponding pool (mmol g-1), correlation coefficients (R2) and Reduced Chi-Sqr (RCS). Sample source

Milli-Q water (pH 5.5)

Hydrochloric acid (pH 2)

k1

Q1

R2

RCS

k2

Q2

R2

RCS

spr-NW spr-SW spr-SE sum-SW sum-SE aut-NW aut-SW aut-SE win-NW win-SW

0.32 0.18 0.48 0.65 0.80 0.13 0.21 0.27 0.14 0.47

2.28 10.29 7.23 7.57 19.41 3.78 12.13 15.04 5.14 10.11

0.998 0.986 0.995 0.996 0.997 0.955 0.936 0.998 0.995 0.972

0.001 0.11 0.03 0.03 0.13 0.04 0.55 0.05 0.01 0.21

0.21 0.20 0.20

11.21 5.17 3.95

0.988 0.972 0.971

0.11 0.08 0.05

0.25 0.17 0.35 0.16 0.22

11.72 5.44 6.79 7.98 3.58

0.995 0.996 0.981 0.997 0.997

0.05 0.01 0.12 0.01 0.003

Table 3 Kinetic parameters of silicon, dissolution rate constant k (min1), dissolved amounts (Q1 and Q2) in the corresponding pool (mmol g1), correlation coefficients (R2) and Reduced Chi-Sqr (RCS). Sample source

spr-NW spr-SW spr-SE sum-SW sum-SE aut-NW aut-SW aut-SE win-NW win-SW

Milli-Q water (pH 5.5)

Hydrochloric acid (pH 2)

k1

Q1

R2

RCS

k2

Q2

R2

RCS

0.14 0.27 0.55 0.75 0.68 0.90 0.91 0.54 0.32 0.72

1.24 1.28 1.83 0.58 7.12 0.53 7.89 2.44 0.82 1.08

0.959 0.993 0.988 0.991 0.998 0.998 0.999 0.994 0.990 0.999

0.006 0.001 0.006 0.001 0.01 0 0.03 0.005 0.001 0

0.10 0.14 0.19

12.26 5.94 4.50

0.978 0.995 0.966

0.21 0.01 0.08

0.16 0.10 0.31 0.18 0.14

14.96 79.88 2.59 1.82 3.12

0.992 0.997 0.985 0.983 0.994

0.13 1.22 0.01 0.006 0.005

Fig. 5. Canonical correspondence analysis (CCA) plot showing the distribution of the mass concentrations (mmol g1) of water and acid soluble P and Si (WSP, WSSi, ASP, ASSi), as well as the respective dissolution rate constant in water and acid solution (Pk1, Si-k1, P-k2, Si-k2) and sample filters along with CCA1 and CCA2 axes in relation to the environmental factors (the concentrations (mmol g1) of total elements in aerosol samples, including Al, Si, Mg, Mn, Fe, K, Ca, Na, Ba, V, Ti, Co, Ni, P, Cr, Pb, Cu and Zn).

mixed sources. Affected by anthropogenic sources, sum-SW, autSW, win-SW and aut-SE are distributed in the same quadrant II with WSP, which suggests that P from anthropogenic sources had labile phosphorus pool. This phenomenon is consistent with the P acidified factor F values of 1.5e1.7 lower than that in NW (2.9e10) from dust sources (i.e., the aerosol samples in the quadrant II have more WSP) (Fig. 6). Furutani et al. (2010) reported that the water soluble P fractions and anthropogenic P fractions in aerosols over Western North Pacific had similar contributions to total phosphorus, which also indicates the anthropogenic aerosols have labile phosphorus. The strongly contrast relationship between the concentrations of total Si and total Na in aerosol samples indicates that the decreasing of mineral components and the increasing of sea salt components may happen during the transport process of aerosols to the ocean. The positive correlation of WSSi with the concentrations of total Na in aerosols and the rapid decline in dissolution rate in water indicate the preferential dissolution of amorphous silicon (phytoliths or biogenic silica) weathered on the surface of aerosols as reported by Desboeufs et al. (1999). In contrast to the parameters relative to water solution, the concentrations of acid soluble P and Si (ASP and ASSi) as well as respective dissolution rate constant in acid solution (P-k2 and Si-k2) were distributed in the quadrant I and IV, which have a close relation with the concentrations of total mineral elements in the aerosols. In general, the refractory P compounds are in the form of apatite [Ca5(PO4)3-(F, Cl, OH)] or tightly bound to Fe, Al oxides/hydroxides and CaCO3 in soil and mineral aerosols (Bergametti et al., 1992; Paytan and McLaughlin, 2007; Ridame and Guieu, 2002). For silicon, a crustal element, mainly exists in mineral crystal lattices such as aluminosilicate or quartz. Therefore, the phosphorus and silicon dissolved in acid solution may come from the dissolution of refractory mineral components, which are consistent with the larger F values for aerosols from NW sources than others (Fig. 6). However, the concentrations of total Ca in aerosols were present in the quadrant I, which indicates that pollutants (e.g., acid components) may increase the acidity of aerosols and induce the dissolution of more labile Ca-P relative to Fe, Al-P in aerosols before arriving in Qingdao. It seems reasonable that high concentrations of nitrate are in aerosol samples used in our study except for AD samples (Table 1). From the above, we hold the opinion that non-acid atmospheric water chemistry of aerosols dissolve P and Si species that are weakly absorbed or weathered on the surface of aerosols and that acidification will strengthen the dissolution of refractory Ca, Fe, AlP and increase the bioavailability of phosphorus and silicon. In this study, the average F value of 2.8 (1.1e10) for PO3 4 suggests that there are large amounts of refractory P compounds in actual aerosols (Fig. 6), comparable with the results (2e9-fold) of 72 h sea water leaching aerosols linking the additional P with mineral phase dissolution (Mackey et al., 2012). The average F value of 7.9 (1.2e29) for SiO2 3 is larger than that for P, which indicates a greater potential for the release of Si in acidic atmospheric water. Our aerosol leaching curves of P and Si are similar to the mineral dissolution results of Wollast and Chou (1985) and Desboeufs et al. (1999). The Transition State Theory can be used to explain the behaviour of P and Si in actual aerosols from non-acidic atmospheric water (pH 5.5) to acid atmospheric water (pH 2). On contact with atmospheric water, aged fine particles on the surface of aerosols begin to dissolve rapidly or quick ion exchange occurs until a residual layer whose composition differs from the initial compound composition forms with a gradually reduced dissolution rate. When acidic gases are mixed with aerosols, P and Si species are adjusted rapidly to adapt to the new solution (e.g., protonation rates are maximized quickly), and a new residual layer forms.

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Fig. 6. F-values of phosphate and dissolved silicate from various sources. The error bars present the accumulated relative standard deviation caused by the calculation of Eq. (6).

Fig. 7. Accumulated solubility (%) of (left) P and (right) Si from various sources. They share the same legend as the top figure.

4.2. Solubility of P and Si and their major control factors The water-soluble P solubility (WSPS) was 37% ± 27% (9.3e90%) (Fig. 7), which was comparable to values from over the Atlantic ocean reported by Graham and Duce (1982) (36%) and Baker et al. (2006b) (32%). The large standard deviation indicated the significant difference in the datasets (n ¼ 10), which can be attributed to the effects of the sources. Aerosols of dust-derived sources (NW) had lower water-soluble phosphorus solubility (16% ± 7%) than anthropogenic-dominated aerosols (32% ± 9%) and marine aerosols (65% ± 21%). Compared to Saharan dust, Asian dust (AD) had similar labile phosphorus in terms of solubility: 8% for Saharan dust (Baker et al., 2006a) and 13% for AD (i.e., spr-NW and aut-NW). The high instantaneous P solubility (3.1e59%, mean: 17%) was 39% (17e66%) of the water-soluble P solubility, suggesting that the majority of reactive phosphate (orthophosphate PO3 4 ) can be released immediately on contact with atmospheric water. Our study found that reactive phosphate can dissolve out completely within 10e15 min. The dissolution of insoluble phosphorus in non-acidic atmospheric

water may need a longer dissolution time, which also depends on the competitive effect with the velocity of deposition. However, atmospheric acid processing (i.e., the reaction between aerosols and acidic atmospheric water (pH 2)) increased the solubility of P to 35%e99% within half an hour, indicating that atmospheric acid processing can quickly increase the bioavailability of phosphorus in aerosols. Then, we attempted to find some associations between the water-soluble phosphorus solubility and concentration of acid components in aerosols, but unfortunately, there was no correlation between the water-soluble phosphorus solubility and air volume concentration of nitrate (mmol/m3) (Table 4). This phenomenon was also found in the study of Baker et al. (2006a) despite an obvious increase in P solubility in a westeeast Saharan dust plume transect. Hsu et al. (2013) and Meskhidze et al. (2005) emphasized that the acid/dust ratio and SO2/dust ratio were more effective parameters to explain the effect of acid processing on the solubility of trace elements, such as Fe. In our study, a significant positive correlation (r ¼ 0.928, p < 0.01) between the water-soluble

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Table 4 Person correlation coefficients (r) for water-soluble P solubility (WAPS, %), acid-soluble P Solubility (ASPS, %), water-soluble Si Solubility (WSSiS, %), acid-soluble Si solubility 1 (ASSiS, %), dissolution rate constants (k, min1) of P and Si, the particle/liquid ratio (Pc, mg L1), mass concentrations of nitrate (Mc- NO 3 , nmol g ), volume concentrations of nitrate (Vc-NO-3, nmol m3), mass concentrations of total phosphorus (TP, nmol g1) and total silicon (TSi, nmol g1).

WSPS ASPS WSSiS ASSiS P-k1 P-k2 Si-k1 Si-k2 Pc Mc-NO-3 Vc-NO-3 TP TSi a b

WSPS

ASPS

WSSiS

ASSiS

P-k1

P-k2

Si-k1

Si-k2

Pc

Mc-NO-3

Vc-NO-3

TP

TSi

1 0.469 0.861a -0.301 0.646b 0.393 0.276 0.654 -0.840a 0.928a -0.063 -0.403 -0.313

1 0.507 0.425 0.060 0.343 -0.311 0.363 -0.472 0.226 -0.370 -0.685b 0.078

1 0.103 0.666 0.683 0.160 0.711 -0.635 0.851a -0.103 -0.237 -0.489

1 -0.171 -0.069 -0.258 -0.633 0.385 -0.364 -0.223 -0.128 -0.130

1 0.064 0.214 0.029 -0.588 0.715b -0.142 -0.242 -0.625

1 0.123 0.755b -0.143 -0.067 -0.086 -0.117 -0.427

1 0.005 -0.138 0.241 -0.173 -0.084 -0.232

1 -0.449 0.295 -0.141 -0.203 -0.055

1 -0.744b 0.358 0.448 0.258

1 0.123 -0.180 -0.326

1 0.440 0.160

1 -0.305

1

Significant at 0.01. Significant at 0.05.

phosphorus solubility and acid/particle ratio (i.e., the mass concentration of nitrate in unit of mmol/g, an analogue to acid/dust ratio and SO2/dust ratio) was found. Though there was a lack of other acid species (e.g., sulfuric acid and organic acids) in our study, the high concentrations of sulphate and nitrate usually co-occurred in the same sample (Meskhidze et al., 2005; Vet et al., 2014), and the hydration reaction of nitrogen oxides to form nitric acid was more rapid than heterogeneous reactions of SO2, which can preferentially act on insoluble phosphorus (Sasakawa et al., 2003). In particular, the warm and humid environment increased the rate of conversion from acid precursors (SO2, NOX) to acid (Vlasenko et al., 2006; Yuan et al., 2014), which may contribute to the relative higher k values of phosphorus in aerosols from sea sources in summer though more acid precursors were generally considered to be emitted into the atmosphere due to coal combustion in winter. In our study, the acid/particle ratios of aerosols in warm and humid environments were higher than others under cold and dry conditions (Table 1). In addition, the particles/liquid ratio (Pc) was also an important factor affecting the solubility of phosphorus (r ¼ 0.840, p < 0.01), but was inferior to the acid/particle ratio. The acid-soluble P solubility was closely associated with the mass concentration of TP (r ¼ 0.685, p < 0.05), and the degree of influence of acid processing depended on the source, with different values of the acidified factor F (average value: NW 5.7 > SW 1.6 > SE 1.5) (Fig. 6). The solubility pattern of Si is slightly different from that of P (Fig. 7). The water-soluble silicon solubility of Asian aerosols was 0.16% ± 0.23% (0.02e0.74%), comparable to that of Saharan dust (0.2%) and one order of magnitude lower than the values used in an ocean biogeochemical model (7.5%) (Baker et al., 2006a; Krishnamurthy et al., 2010). Therefore, 7.5% solubility of Si used in the ocean biogeochemical model is not appropriate neither for Saharan dust or Asian dust. In addition, using 30.8% weight percentage of dust or Si/Al ratio in the upper crust (Taylor and Mclennan, 1995) to calculate the Si content in dust aerosols may also cause an overestimate of 19%e54% in our study. Because the crustal enrichment factor EFcrust of Si was 0.68 ± 0.19 in our study (Fig. S3), indicating that silicon loss of different degrees occurred during the transport process of dust to the ocean. Most of the water-soluble silicon solubility in aerosols was less than 0.1%, except the sum-SE and aut-SW aerosol samples, which had 0.41% and 0.74% solubility of Si, respectively. The extra high solubility of Si in aut-SW may be caused by the air mass trajectory from the East China Sea passing over a developed coastal city and mixing with more building dust, moist air and acid species (Qiu et al., 2001). This was proved by the acid/particle ratio (i.e., mass concentrations of

nitrate, nmol g1) of aut-SW, which was higher than that of other terrestrial sources (Table 1). The instantaneous solubility of Si ranged from 0.01% to 0.25%, with an average of 0.07%. Though the solubility of Si in the acid solution was still small, ranging from 0.05% to 5.4%, the values were an order of magnitude larger than those in the water solution. The impact of sources on acid-soluble silicon solubility was more obvious than for P (F average value: NW 16 > SW 5.5 > SE 2.4). To reduce the effect of the exceptional high solubility value for aut-SW on the Pearson correlation analysis, the dataset of Si solubility used to analyse the affecting factors did not contain aut-SW (Table 4). In contrast to total water-soluble P solubility, the watersoluble Si solubility (WSSiS) had only linear correlation with the acid/particle ratio (r ¼ 0.851, p < 0.01). However, we found that the power law was more appropriate to describe the relationship of water-soluble Si and P solubility with particle/liquid ratio (WSSiS ¼ 0:582  ðPcÞ0:851 , r2 ¼ 0.706; WSPS ¼ 194  ðPcÞ0:721 , r2 ¼ 0.752), which is also fit for the description of the P dissolution in Saharan dust (Ridame and Guieu, 2002). 4.3. Potential impacts on the marine primary productivity of the Yellow Sea In general, the amounts of nutrients in aerosols dissolved in the ultrapure water or filtered surface seawater are regarded as bioavailable fractions. Here, our study explored the significance of atmospheric water acidification (artificial acid composition added into the atmospheric aerosols which reacted with rain water or cloud and fog droplets) before aerosols enter the ocean. In the leaching experiment, an acidic solution led to greater P and Si dissolution, indicating that more inorganic phosphorus and silicon in atmospheric aerosols will be available to phytoplankton if there were sufficient acidic materials added to aerosols. These nutrients can be utilized by phytoplankton as soon as they are deposited in the ocean and can contribute to primary productivity immediately. Therefore, previous estimations on bioavailable P and Si fluxes of atmospheric aerosols should be minimal values (Bartoli et al., 2005), and the potential contribution of atmospheric deposition to marine primary productivity can be evaluated.  þ 3 In our aerosol samples, the DIN (NO 3 þ NO2 þ NH4 )/PO4 2 (143e580) and DIN/SiO3 (153e3634) mole ratios were far greater than the Redfield ratio (N: Si: P ¼ 16: 16: 1) required by phytoplankton (Table 5), though there were some differences among algae species according to in situ enrichment incubation experiments (Zou et al., 2001). Therefore, the amounts of bioavailable P

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Table 5 Stoichiometric mole ratio of N/P, N/Si and P/Si in ten TSP samples before and after acidification in this study. Sample source

spr-NW spr-SW spr-SE sum-SW sum-SE aut-NW aut-SW aut-SE win-NW win-SW

N/P

N/Si

P/Si

Before acidification

After acidification

Before acidification

After acidification

Before acidification

After acidification

202 199 575 143 506 157 230 323 580 246

22 92 322 132 459 40 137 194 296 158

371 1601 2274 1410 1379 1117 153 1994 3634 2302

27 227 543 759 1170 40 21 1031 738 437

1.8 8.0 4.0 9.8 2.7 7.1 0.7 6.2 6.3 9.4

1.3 2.5 1.7 5.7 2.5 1.0 0.2 5.3 2.5 2.8

and Si are the determining factors of new production triggered by atmospheric inputs. The central Yellow Sea was chosen as the evaluated area, where there is minimal effect from river inputs. In terms of the central Yellow Sea, DIN/SiO2 3 in the surface water often exceeds 1 in spring (Zhang et al., 2005), and a phosphate limiting condition in the surface water in the Yellow Sea was also reported (Liu et al., 2003). The contributions of new production triggered by phosphate and dissolved silicate in aerosols over the Yellow Sea before and after acidification during four seasons were evaluated in Table 6. The seasonal dry deposition fluxes of phosphate and dissolved silicate adopt the results from 64 aerosols sampled during the period 2004e2005, and primary production (PP) and new production (NP) during all four seasons come from the data integrated by Zhang et al. (2005) and references therein. It is hypothesized that nutrients are assimilated by phytoplankton in terms of the Redfield ratio (106:16:1 ¼ C:Si:P). To more accurately assess the contributions of phosphate and dissolved silicate after acidification, a mix acidified factor Fmix was calculated by Eq. (7). Fmix ¼ Fdust*Adust þ Fanthro*Aanthro þ Fmarine*Amarine

(7)

where dust, anthro and marine sources were roughly denoted by NW, SW þ NE and SE, respectively. A was the percentage contributions of each source (NW, SW þ NE, SE) for spring (MarcheMay), summer (JuneeAugust), autumn (SeptembereNovember) and winter (DecembereFebruary) obtained by backtrajectory cluster analyses (Fig. 8). Fdust, Fanthro and Fmarine were calculated by the average F value of NW, SW and SE for the four seasons, respectively (Fig. 6). Before acidification, the NP values triggered by phosphate in aerosols were 2.31e5.76 mg C m2 d1, with the maximum in spring, when large amounts of dust aerosols flowed out of the northern areas in China (Table 6). The maximum contributions to

NP occurred in summer when stratification is obvious and the nutrients from upwelling are intercepted, which was up to 77%, with the other seasons less than 7.9%. In the sand dust seasons (i.e., spring and autumn), the contributions decrease to 0.9% and 1.3%. The evaluated contributions in our study are 1e3 orders of magnitude higher than the average contributions (0.12%) of phosphate in aerosols to the global export production modelled by Krishnamurthy et al. (2010). After acidification, the contribution (85%) in summer changed little and the contributions from other seasons increased 4%e23%. When calculating the contributions of NP triggered by dissolved silicate, we found that it can be ignored except for summer (3%), and the range was 0.03%e0.05% for the other seasons, far less than the 0.21% found by Krishnamurthy et al. (2010). Even so, the contributions after acidification for spring, autumn and winter were one order of magnitude more than before acidification. The aerosols were Si-limited versus P in terms of the contributions to NP. However, we found that the stoichiometric molar ratios of P and Si in Fig. 9 from various sources all showed a declining tendency in non-acidic atmospheric water over time, except for spr-NW, indicating that the contents of dissolved phosphate decreased relative to dissolved silicate over time, though the ratios were still above the Redfield ratio (P: Si ¼ 0.0625). In the acid solution, this trend was particularly obvious and the ratios decreased to 0.0625. Table 5 shows the dissolved P/Si of aerosols before and after acidification. The mole ratio of P to Si was, on average, 5.6 (0.7e9.8), which was 2-fold more than the ratios (average 2.6, range: 0.2e5.7) after acidification. This dissolution pattern may change the Si limitation to a P limitation in aerosols and contribute to the preferential growth of diatoms in the central Yellow Sea, excluding the effects of riverine input (Zhang et al., 2007). Therefore, the high P/Si in aerosols can rapidly provide phosphate for phytoplankton growth in P-limited areas, but the interaction

Table 6 Estimated seasonal contributions of new production (NP) triggered by phosphate and dissolved silicate in aerosols over the Yellow Sea before and after acidification. Nutrient Season

PPa (mgC m2 NPa (mgC m2 Fmixb Before acidification d1) d1) Flux (mmol m2 Triggered NP (mgC d1) m2 d1)

Contribution Flux (mmol m2 Triggered NP (mgC (%) d1) m2 d1)

Contribution (%)

After acidification

P

Spring Summer Autumn Winter

808 655 737 345

302 3 305 73

7.7 1.1 3.2 2.9

2.16 1.81 3.01 4.53

2.75 2.31 3.82 5.76

0.9 77 1.3 7.9

16.63 1.99 9.63 13.14

21.18 2.54 12.22 16.70

7 85 4 23

Si

Spring Summer Autumn Winter

808 655 737 345

302 3 305 73

12 1.3 20 4.9

1.18 1.07 1.73 1.61

0.09 0.09 0.14 0.13

0.03 3 0.05 0.04

14.16 1.39 34.6 7.89

1.08 0.12 2.8 0.64

0.4 4 0.9 0.9

a b

Primary Production (PP) and New Production (NP) were integrated over depth by Zhang et al. (2005). Seasonal mix acidified factors (Fmix) for P and Si were calculated by Eq. (7).

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Fig. 8. Three-day backward trajectories at Fulong Mountain for spring, summer, autumn, and winter during 2004e2005. The colour lines show the clustering trajectories; blue lines represent NW, pink lines represent SW, green lines represent SE, and red lines represent NE. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

between aerosols and atmospheric acidic water can lower the molar ratio of P and Si, which may relieve Si-limited conditions due to the more larger contents of Si than P in aerosols since Si is a major constituent in the earth's crust and stimulates preferential growth of diatoms relative to other algal species. 5. Conclusions and implications The leaching experiment of aerosols indicated that the inorganic nitrogen species were primarily composed of water-soluble com  ponents (NHþ 4 , NO3 and NO2 ) that can be released rapidly and become available for phytoplankton. Acid processing of AD aerosols in atmospheric water was an important way to convert some water-insoluble organic nitrogen species into bioavailable ammonium and contributed 10%e20% of the total ammonium amount in the water þ acid solution. This discovery was conducive to improving the understanding of the bioavailability of waterinsoluble organic nitrogen components, which may compose a relatively large proportion of total nitrogen in aerosols compared to water-soluble nitrogen components (Russell et al., 2003), though we had no conclusive evidence of specific relative organic compounds. The SEM-EDS and CCA analysis indicated that phosphorus and silicon in aerosols from land-based sources were major constituents in combination with refractory mineral components such as apatite, Fe, Al-P and aluminosilicate, while the marine sources had

more water-soluble components. The phosphorus in aerosols from anthropogenic sources was bioavailable phosphorus which was labile in non-acidic atmospheric water. The dissolution patterns of P and Si were similar and can be explained by the Transition State Theory. The satisfactory fits of the first-order kinetic models for the accumulated dissolution curves of P to Si can give important reference values to the atmospheric model. The dissolution rate constants of P and Si in non-acidic atmospheric water were 0.13e0.80 min1 and 0.14e0.91 min1, respectively. In the acidic solution, the relative fixed dissolution rate constant can be accepted to be 0.23 min1 for P and 0.17 min1 for Si. The water-soluble solubility of P and Si in Asian aerosols were 37% ± 27% (9.3e90%) and 0.16% ± 0.23% (0.02e0.74%), respectively. The control factors of the water-soluble P and Si solubility were dominated by the acid/particle ratio (nmol g1) and the particle/ liquid ratio (mg L1). Under the interference of human activities, the reactions of aerosols with acidic atmospheric water would release more bioavailable phosphorus and silicon, 1.1e10-fold and 1.2e29-fold more than the contents released in non-acidic atmospheric water. The contributions of new primary productivity triggered by dissolved phosphate in aerosols were maximized (77%) in summer because of water stratification and were less than 10% in other seasons, while the contributions caused by dissolved silicate were less than 0.1% for spring, autumn and winter and 3% for summer. Acid processing of aerosols can increase the contributions up to one order of magnitude more than before

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the source area, physicochemical processes during entrainment and transport and mixing with other aerosol or gases, nutrients in local aerosols may have specificity and complexity, which require the incorporation of more complicated models and technical analyses, such as scanning electron microscopy with transmission electron microscopy energy-dispersive X-ray spectrometer (TEMEDX), to deeply interpret the dissolution kinetic process and specific phase in which nutrient elements are present. Acknowledgements This study was supported by the National Science Foundation of China (41521064 and 41376086), Aoshan Talents Program Supported by Qingdao National Laboratory for Marine Science and Technology (No. 2015ASTP-OS08), and the Taishan Scholars Program of Shandong Province. We sincerely thank the crew of the surface-based observing station of the Qingdao Meteorological Administration for their help in sample collection. We are grateful to the editor and referees for their constructive suggestions and comments. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.atmosenv.2017.06.005. References

Fig. 9. Molar ratio of phosphate and dissolved silicate over time: (a) NW; (b) SW; (c) SE.

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