Environ. Sci. Technol. 2009, 43, 3061–3066
Particulate Nitrate Formation in a Highly Polluted Urban Area: A Case Study by Single-Particle Mass Spectrometry in Shanghai XIAOFEI WANG, YAPING ZHANG, HONG CHEN, XIN YANG,* AND JIANMIN CHEN* Department of Environmental Science & Engineering, Fudan University, 220 Handan Road, Shanghai 200433, China FUHAI GENG Shanghai Meteorological Bureau, 166 Puxi Road, Shanghai 200030, China
Received July 20, 2008. Revised manuscript received February 5, 2009. Accepted February 27, 2009.
An aerosol time-of-flight mass spectrometer was deployed in August 2007 to characterize the 0.1-2.0 µm diameter particles in Shanghai to examine nitrate-containing particles. About 39% of the mass spectra of single particles contained nitrate ion peaks. The relative intensity of nitrate signals showed a pronounced diurnal profile, peaking in the late night or early morning during highly polluted days, and is closely correlated with the ambient relative humidity (RH). However, during the sampling days with good air quality, the diurnal pattern of nitrate changed by showing much lower signal intensity of nitrate with irregular variation. Poor correlation between the signals of ammonium and nitrate in the mass spectra excluded the possibility of NH4NO3 as a major form of particulate nitrate, whose formation is favored by high RH and low temperature. The peak intensities of nitrate during the nighttime and high concentrations of O3 and NO2 strongly suggest that the heterogeneous reactions of N2O5 and NO3 on the aerosol surface dominated the particulate nitrate formation on polluted days.
1. Introduction Nitrate is one of the major inorganic components of urban aerosol particles. It is mainly formed from gas-to-particle partitioning in the atmosphere and is usually identified as a secondary aerosol component (1, 2). Gaseous HNO3 and N2O5 (and NO3) are major precursors of particulate nitrate (3). Gaseous HNO3 is produced via the oxidation of nitrogen dioxide (NO2) by hydroxyl radical (OH): NO2 + OH + M f HNO3 + M
(R1)
where M represents a third body (mainly N2 and O2). This reaction is primarily a daytime reaction because most OH sources are photolytic in nature. Gaseous HNO3 readily adsorbs to surfaces including those of aerosol particles, particularly if the surface is wet. Different surfaces have different effects on nitric acid uptake. HNO3 uptake on a * Address correspondence to either author. Phone: 86-2155665272; fax: 86-21-65642080; E-mail:
[email protected] (X.Y.);
[email protected]. 10.1021/es8020155 CCC: $40.75
Published on Web 03/26/2009
2009 American Chemical Society
soot surface is mostly reversible and does not release gasphase products such as HONO, NO3, NO2, or N2O5 (4). However, HNO3 irreversibly adsorbs to a dust aerosol surface due to the neutralization reaction with some cations (e.g., Fe3+, Ca2+, and Mg2+) (5). For wet surfaces, gas-to-particle partitioning is mainly governed by Henry’s constant of nitric acid in aqueous solutions, which, in turn, is dependent on the temperature and chemical compositions (6). HNO3 can also react with NH3 to form ammonium nitrate (NH4NO3) aerosol. HNO3, NH3, and NH4NO3 reach an equilibrium between the gas phase and solid or aqueous phase of aerosol particles: NH4NO3(s) h HNO3(g) + NH3(g)
(R2)
NO3-(aq) + NH4+(aq) h HNO3(g) + NH3(g)
(R3)
Many studies have shown that the above equilibria were key factors in determining the concentration of particulate nitrate (7-9). However, this is not the case for an area with insufficient NH3. NH3 tends to react with H2SO4 first. Only if there is excess NH3 can it react with HNO3 to form NH4NO3 (10). For example, measurable NH4NO3 aerosol was not present in a field study conducted in Great Smoky Mountains National Park of the United States, where sulfate was not completely neutralized by NH3 (11). During the nighttime, nitrate radical NO3 formed by the oxidation of NO2 can react with NO2 to form N2O5: NO2 + O3 f NO3 + O2
(R4)
NO3 + NO2 + M h N2O5 + M
(R5)
Reaction R5 is the only source of N2O5 in the atmosphere. Because NO3 is only present at significant concentrations at night due to its rapid photolysis, reaction R5 is restricted to the dark. N2O5 hardly reacts with water vapor (3), but it is easily hydrolyzed on a wet particle surface. NO3 radical can also be hydrolyzed on wet surfaces. However, it is difficult to distinguish the hydrolysis of NO3 radical from that of N2O5: N2O5(g) + H2O(l) f 2HNO3(aq)
(R6)
NO3(aq) + H2O(l) f HNO3(aq) + OH(aq)
(R7)
Hydrolysis of NO3 and N2O5 are the major formation channels of particulate nitrate during the nighttime. Modeling works on a global scale have revealed that the uptake of N2O5 and NO3 into aerosols has tremendous effects on the tropospheric budgets of NOx and O3 (12-14). In much of the northern hemisphere, more than half of the nitric acid formed is presumed to occur via reactions R6 and R7. In an urban atmosphere, the hydrolysis of N2O5 and NO3 can be even more important as a source of HNO3 due to their higher concentrations in the more polluted regions (15, 16). Here we present the real-time single-particle analysis of nitrate-containing aerosol particles in Shanghai by deploying an aerosol time-of-flight mass spectrometer (ATOFMS). Shanghai is the biggest city in China with frequent, heavy air pollution. The purpose of this work is to investigate how particulate nitrate was formed in the urban area of Shanghai and the effects of gas-phase pollutant (such as NO2 and O3) VOL. 43, NO. 9, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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levels and meteorological conditions. We also aim to characterize the predominant types of nitrate-containing particles.
2. Experimental Section 2.1. ATOFMS. The ATOFMS (TSI-3800) has been described in detail in previous publications (17, 18). Briefly, air is introduced into a vacuum through an aerodynamic focus lens (AFL) (19) that is designed to effectively transmit particles in the size range from 100 nm to 3 µm. The lens and the following differential pumping region with skimmers create a narrow particle beam. Each particle is accelerated to a terminal velocity depending on its aerodynamic size. Particles then pass through two orthogonally oriented continuouswave diode laser beams. Scattered light signals from each particle are collected by photomultiplier tubes. The particle size is determined by measuring the transit time between the two laser beams. The firing of a pulsed ultraviolet laser (frequency-quadrupled Nd:YAG laser, 266 nm) is triggered on the basis of the particle size to desorb/ionize chemical species from the particle. Both positive and negative ions generated from laser ablation are analyzed simultaneously by time-of-flight mass spectrometry. In this work, polystyrene latex spheres (Nanosphere Size Standards, Duke Scientific Corp., Palo Alto) of 0.22-2.00 µm diameter were suspended using an atomizer (TSI-3076) to create monodisperse aerosols used for size calibration. The power density of the desorption/ ionization laser was kept at about 1.0 × 108 W/cm. 2.2. Ambient Measurements. The ATOFMS is located in the building of the Department of Environmental Science and Engineering at Fudan University in Shanghai. This site is close to residential, traffic, and construction emissions sources and is representative of urban areas. Ambient aerosols were transferred to the ATOFMS through a 4 m long copper tube with a 1/2 in. diameter. A cyclone was used on the inlet and pulled at 10 L/min to minimize the particle residence time in the sampling line and interaction with the tube wall. The inlet of the sampling tube was about 5 m above the ground and 0.5 m above the roof of the building. The ATOFMS was operated for 7 days (almost 24 h per day) during Aug 1-5 and Aug 8-9 in 2007. The local meteorological data including temperature, relative humidity (RH), atmospheric pressure, wind speed, and direction were provided by the Shanghai Meteorological Bureau. The daily average concentrations of PM10, SO2, and NO2 were obtained from the Air Pollution Index (API) report by the Shanghai Environment Protection Bureau (http://www.envir.gov.cn/). The hourly average concentrations of ambient O3 were provided by the Shanghai Meteorological Bureau. 2.3. Data Analysis. A total of 380 282 particles with both positive and negative mass spectra were collected during the experiment, that is, approximately 15% of the sized particles. Most of the particles were located in the size range of 0.1-2.0 µm in diameter with the average value at 0.55 µm. All single-particle mass spectra acquired were converted to a list of peaks at each m/z by setting a minimum signal threshold with TSI MS-Analyze software. The resulting peak lists together with other ATOFMS data were imported into YAADA (version 2.0, www.yaada.org), a software toolkit for single-particle data analysis written in Matlab (version 7.0). In this study we use the relative intensity, the ion fraction of each species relative to the total ion current in the mass spectrum, to represent the relative amount of the species in particles, because the relative intensity is more reproducible than the absolute intensity due to the spectrum to spectrum differences in the interaction of the particle with an ionization laser beam (20, 21). ATOFMS markers for nitrate include NO+ (m/z 30), Na2NO3+ (m/z 108), NO2- (m/z 46), and NO3(m/z 62). Here we only use two strong anion fragments, NO2or NO3- (with absolute area no smaller than 1000), to search 3062
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nitrate-containing particles. ATOFMS markers for other major chemical species (as shown in Table S1, Supporting Information) and the rules for particle classification used in this work are similar to those of Liu et al. (22).
3. Results and Discussion 3.1. Meteorological Conditions and Air Pollution. As shown in Figure S1, Supporting Information, both the temperature and RH exhibited strong diurnal variation patterns during the entire sampling period. The temperature peaked shortly after midday, while the RH peaked at midnight or in the early morning. Judging from the daily average concentrations of three major pollutants (PM10, SO2, and NO2), as shown in Figure S2, Supporting Information, Aug 1-5 can be viewed as highly polluted days, while the air quality during Aug 8-9 was much better. Notably, on Aug 3, the average concentrations of PM10, SO2, and NO2 reached 0.174 mg/m3, 0.098 mg/ m3 (34 ppb), and 0.104 mg/m3 (51 ppb), respectively. Figure S3, Supporting Information, shows the 24 h air parcel trajectory arriving at the sampling site at 12:00 a.m. each day during the sampling period. The heavy pollution during Aug 1-5 was mainly due to stable meteorological conditions with prevailing winds from the south to slightly southeast. The average wind speed was 2.2 m/s. Air parcels from Fujian and Zhejiang provinces passed through the southern suburbs and city center of Shanghai. Starting from Aug 8, the winds were mainly from the east with a higher wind speed of 6.3 m/s. Air parcels sampled during this period were mainly from the East China Sea and were relatively clean. 3.2. Diurnal Variation Patterns and Formation Mechanism of Particulate Nitrate. A total of 148 150 nitratecontaining particles were obtained, accounting for 39.0% of the total particles. Figure 1a shows the temporal profiles for the relative intensity of nitrate and the RH in 1 h resolution. Two different variation patterns of nitrate can be clearly identified. During Aug 1-5, nitrate showed a pronounced diurnal pattern that peaked in the late night or early morning and reached the lowest value in the afternoon. The diurnal pattern of nitrate correlated closely to the ambient RH variation during this period. However, during Aug 8-9, the nitrate intensity decreased significantly and showed much smaller variation compared to that of the previous five days while the temporal profile of the RH did not change very much. The diurnal pattern for nitrate was irregular for these two days. On Aug 9, the nitrate intensity peaked at noon. The substantial change in the temporal profile of nitrate may suggest different formation mechanisms of particulate nitrate in these two sampling periods. The correlation between the RH and particulate nitrate concentration has been reported in previous field studies (mostly in the United States) (23-25) and attributed to the gas-to-particle partitioning of NH4NO3 precursors (gaseous HNO3 and NH3), which is favored by lower temperature and higher RH (26). The equilibrium formation (or dissociation) constant of NH4NO3 was used in an attempt to investigate the diurnal cycles of the particulate nitrate mass. However, in most of these experiments, the equilibrium constant curve did not fit the observed diurnal variation of nitrate very well. Usually, the nitrate concentration remained high till 9:00 a.m., while the formation constant of nitrate and the ambient RH started to decline after 6:30 a.m. (23-25). High photochemical production of HNO3 might be responsible for maintaining the nitrate level during this gap of time. This sustained high level of particulate nitrate was not observed in our experiment. As shown in Figure 1a, the relative intensity of nitrate tightly matched the RH variation during Aug 1-5, revealing a nitrate formation process driven by the water content in aerosol. As the only gaseous base present in the atmosphere in significant amounts, NH3 undergoes acid/base reaction with
FIGURE 1. Temporal profiles for (a) the relative intensity of nitrate signals in mass spectra and the ambient RH and (b) the relative intensities of ammonium and sulfate signals in mass spectra. acids in the ambient air, mostly sulfuric and nitric acids, forming the respective ammonium salts in the particulate phase. Due to the high ionization thresholds of ammonium sulfate and ammonium bisulfate, they are difficult to detect at the wavelength of the desorption ionization laser used in ATOFMS (266 nm) (27). However, this makes ATOFMS a good method to detect the presence of NH4NO3. For example, during the 1999 Atlanta supersite experiment, the ATOFMS temporal trend for ammonium-containing particles tracked the profile for nitrate particles in submicrometer mode and nitrate was typically coupled with ammonium in the same spectra, indicating the abundance of NH4NO3 (22). In our experiment, 4864 ammonium particles were identified as containing both nitrate and ammonium, which only accounted for 3.3% of the total number of nitratecontaining particles. Such a low ratio suggests that NH4NO3 was not the major component of nitrate particles during our sampling period. Figure 1b shows the temporal profiles of the relative intensities of ammonium and sulfate in 1 h resolution. Ammonium showed a clear diurnal variation pattern during Aug 1-5 peaking around midday, which was quite similar to that of sulfate. Particulate sulfate is formed predominantly from SO2 oxidation in the cloud-free atmosphere and more commonly in clouds and fogs. The high number concentration of sulfate-containing particles (295 161, 77.6% of the total particles) in our measurement is consistent with the heavy SO2 pollution in this area. The correlation between the relative intensities of ammonium and sulfate suggests that ammonium sulfate or bisulfate was probably the main form of particulate ammonium. The low intensities of the ammonium signals were due to the high ionization
threshold of ammonium sulfate and bisulfate. Our observation agrees well with previous research on the ion chemistry of aerosols in Shanghai by Wang et al. (28). In a two year field measurement, they found that SO42- was incompletely neutralized by NH4+ in the aerosols in Shanghai and both NH4HSO4 and (NH4)2SO4 were formed. No abundant NH4NO3 was observed because of insufficient NH3 in this area. Thus, the diurnal variation pattern of nitrate during Aug 1-5 cannot be explained by the gas/particle partitioning of NH4NO3. As described in the Introduction, the other possible sources of particulate nitrate include the uptake of gaseous HNO3 produced from the daytime reaction R1 and the nighttime heterogeneous reactions R6 and R7. Given the sticky nature of gaseous HNO3, it can adsorb to or react with many kinds of particles in the atmosphere. Peak concentrations of particulate nitrate had been observed during the daytime when production of HNO3 was mainly through the oxidation of NO2 by OH (reaction R1) (23, 29). However, we did not observe any daytime peak in the temporal file of nitrate during Aug 1-5. (The small wiggle around 6:00 p.m. on Aug 2 was due to the RH oscillation.) Thus, the nighttime heterogeneous reactions (R6, R7) must have played a dominant role in the production of HNO3 in this period. Hydrolysis of NO3 and N2O5 starts with the gas-phase reactions R4 and R5. The nocturnal NO3 and N2O5 are produced in significant amounts only if both NO2 and O3 concentrations are high. Figure 2 shows the hourly average concentration of O3 and daily averaged concentration of NO2 in urban areas of Shanghai during the sampling period. The concentrations of NO2 and O3 on the polluted days (Aug 1-5) were much higher than on the clean days (Aug 8-9). VOL. 43, NO. 9, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Hourly averaged concentrations of O3 and daily averaged concentrations of NO2 in the urban area of Shanghai during the sampling period. We note that, during Aug 1-5, O3 remained at a high level after sunset when the particulate nitrate signal intensities started to increase. The heterogeneous uptake of gaseous N2O5 by aerosols has been studied extensively. Ambient conditions have great effects on the values of the reactive uptake coefficients (γ). γ on a water surface has a negative temperature dependence to a substantial degree (30). Recent laboratory studies suggest that γ on sulfate aerosols increases steadily with the RH and the water content of the aerosol controls N2O5 uptake (31). Thus, the nighttime heterogeneous reactions R6 and R7, which are favored by high RH and low temperature, resulted in the pronounced peak concentrations of particulate nitrate in Figure 1a. After midnight, the O3 concentration decreased due to the reactions with NOx such as reaction R4, which would decrease the production rates of NO3 and N2O5. However, with the increasing RH, the produced nitrate was retained in the particle phase when the water content in the aerosols increased. The concentration of particulate nitrate remained high until early morning. During the daytime, gaseous N2O5 and NO3 are quickly photolyzed. The decrease of water content in aerosols and the sharp increase of particulate sulfate (as shown in Figure 1b), which was not fully neutralized by NH4+, led to higher particle acidity and excluded nitrate from the particle phase. On the clean days, although there was no significant change in the diurnal variation of the ambient RH, the low level of gaseous pollutants such as NO2 and O3 would suppress both the daytime and nighttime reactions from producing HNO3, compared to polluted days. Thus, the nitrate intensity decreased considerably and showed irregular variation on those two days. 3.3. Classifications of the Nitrate Particles. According to the ATOFMS markers listed in Table S1, Supporting Information, all the nitrate-containing particles over the duration of the study are classified into three major aerosol types in the order of dust, sodium-containing, and carbonaceous aerosols. The rest are regarded as only containing secondary species such as sulfate, nitrate, or ammonium. Table 1 lists the particle numbers for each aerosol type. Figure 3 shows the average mass spectrum of nitratecontaining particles from each aerosol type. Nitrate-containing dust particles (Figure 3a) usually had positive ion peaks at m/z 7, 23, 24, 27, 39, 40, 55, and 56, representing Li+, Na+, Mg+, Al+, K+, Ca+, Mn+, and Fe+/CaO+, respectively. The negative ion spectra showed the presence of carbon (m/z -12, -24, -36, and -48), nitrate (m/z -46 and -62), sulfate (m/z -80, -96, and -97), chloride (m/z -35 and -37), silicates (m/z -60 and -76), and phosphate (m/z -60 and -79). The presence of silicates and phosphate suggested that 3064
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TABLE 1. Summary of Nitrate-Containing Particle Counts for Each Aerosol Type particle type
particle count
percentage (%)
dust sodium-containing carbonaceous pure secondary total
23 660 20 976 66 263 37 251 148 150
16.0 14.2 44.7 25.1 100
the main sources of dust aerosol particles were soil or sand lifted by the wind or other physical mechanisms (2). The common occurrence of carbon indicated that dust aerosols aged in the atmosphere and mixed with carbonaceous compounds. The major sources of sodium-containing particles were ocean spray and biomass burning. We note that sodiumrich dust particles are excluded from this group. Nitratecontaining sodium particles (Figure 3b) showed strong positive ion signals at m/z 23 [Na+] and 39 [K+] and some carbonaceous ions signals at m/z 36 [C3+], 48 [C4+], and 60 [C5+]. In the negative mass spectrum, strong signals for CN(m/z -26) and carbon clusters (m/z -24, -36, -48, -60, and -72) confirmed the presence of biomass burning particles. The low intensity of chlorine signal indicated that most chlorides in sea salt particles were replaced by nitrate via heterogeneous reaction (32). Both positive and negative mass spectra of nitratecontaining carbonaceous particles (Figure 3c) showed typical carbon cluster peaks at m/z ( 12n. Fragments of organic carbon (OC) with m/z at 12n + 1/2/3 (n ) 1, 2, 3,...) were also observed. Strong sulfate signals were present in most of the negative mass spectra. The intense CN- peak in the negative mass spectrum indicated the presence of organic nitrogen compounds in the carbonaceous particles. The last group of nitrate-containing particles was characterized by positive ions with relatively high mass (up to 400 Da), as shown in Figure 3d. Such a mass spectral pattern suggests that they were oligomer-containing particles derived from secondary organic aerosol (SOA) (33-35). The signature of an oligomer with a progression of peaks separated by 14-16 Da was not very clear in Figure 3d, probably due to the presence of multiple oligomeric precursors. Other OC markers included C2H3+, C2H3O+, CN-, and CNO-/C2H2O-. The negative mass spectra were dominated by strong sulfate and nitrate signals. 3.4. Impact of the Aerosol Composition on the Particulate Nitrate Formation. Figure 4shows the temporal profiles of the relative intensity of nitrate for each aerosol
FIGURE 3. Average mass spectra of nitrate-containing particles for each aerosol type: (a) dust, (b) sodium-containing, (c) carbonaceous, (d) pure secondary.
FIGURE 4. Temporal profiles of the relative intensity of nitrate on each aerosol type. type. During the polluted days, each aerosol type exhibited a similar diurnal variation pattern. However, the RH dependence on the relative intensity of nitrate in pure secondary particles was much stronger than in the other types. Surprisingly, its strong diurnal pattern extended to the clean days while other types showed weaker variation. We note that this group had the highest relative intensity of sulfate
among all the aerosol types. One possible explanation for its high RH dependence is that the sulfate content together with the oxidized organics in this type of particles may absorb more water at high RH, which favors the particulate nitrate formation. Lee et al. (23) reported that relatively high concentrations of nitrate were retained on mineral particles even at low RH VOL. 43, NO. 9, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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because of the neutralization reactions of alkali or alkaline earth cations. In our experiment, however, the RH dependence of nitrate intensity in dust particles was quite strong, suggesting that the neutralization reactions may not be important here because the cations in dust aerosols would be neutralized quickly by the abundant sulfate.
Acknowledgments We gratefully acknowledge Dr. Song Gao for valuable discussions. This work was supported by the 973 Program (Grant No. 2008CB417205) from The Ministry of Science and Technology of China, The National Natural Science Foundation of China (Grant No. 40875074), and the Pujiang Project (Grant No. 07pj14018) from The Science and Technology Commission of Shanghai Municipality.
Supporting Information Available Table of ATOFMS markers used to search for the major components, figures of hourly temperature and relative humidity, daily average concentrations of PM10, SO2, and NO2, and 24 h air parcel trajectories. This information is available free of charge via the Internet at http://pubs.acs.org.
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