Seasonal Characterization of Organic Nitrogen in Atmospheric

Nov 21, 2017 - State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy...
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Article Cite This: ACS Earth Space Chem. 2017, 1, 673−682

Seasonal Characterization of Organic Nitrogen in Atmospheric Aerosols Using High Resolution Aerosol Mass Spectrometry in Beijing, China Weiqi Xu,†,§ Yele Sun,*,†,‡,§ Qingqing Wang,† Wei Du,†,§ Jian Zhao,†,§ Xinlei Ge,∥ Tingting Han,† Yingjie Zhang,†,∥ Wei Zhou,†,§ Jie Li,† Pingqing Fu,† Zifa Wang,† and Douglas R. Worsnop⊥ †

State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, 40 Huayanli, Chaoyang District, Beijing 100029, China ‡ Center for Excellence in Regional Atmospheric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China § University of Chinese Academy of Sciences, Beijing 100049, China ∥ Nanjing University of Information Science & Technology, Nanjing 210044, China ⊥ Aerodyne Research Inc., Billerica, Massachusetts 01821, United States S Supporting Information *

ABSTRACT: Despite extensive efforts to characterize organic nitrogen (ON) in atmospheric aerosols, knowledge of the sources and processes of ON in the megacity of Beijing is still limited, mainly due to the complexity of ON species and the absence of highly time-resolved measurements. Here we demonstrate the applications of Aerodyne high-resolution timeof-flight aerosol mass spectrometer combined with positive matrix factorization in characterization of ON in submicron aerosols. Our results show that the average nitrogen-to-carbon ratios (N/C) vary from 0.021 to 0.028, and the average ON concentrations range from 0.26 to 0.59 μg m−3 during four seasons in Beijing. ON accounts for 7−10% of the total nitrogen (TN) on average, yet the sources vary differently across different seasons. We found that 56−65% of ON was secondary during three seasons except winter when 59−67% was related to primary emissions. Particularly, more oxidized secondary organic aerosol contributes the dominant fraction of ON (39−44%) in spring, summer and autumn, while biomass burning is a more important source of ON in winter (23−44%). These results are consistent with the better positive correlations between N/C and oxygen-to-carbon ratio, a surrogate of organic aerosol aging, during these three seasons than that in winter. N/C also shows a clear increase as a function of relative humidity during all seasons, suggesting that aqueous-phase processing likely played an important role in formation of nitrogen-containing compounds. In addition, the uncertainties and limitations in quantification of ON with aerosol mass spectrometry are illustrated, particularly, ON could be underestimated by ∼20−42% by ignoring the fragment contributions in NHx+ and NOx+. KEYWORDS: organic nitrogen, aerosol mass spectrometry, sources, aqueous-phase processing, Beijing formation, e.g., nitrate radical oxidation,17 gas-particle partitioning, and cloud processing18,19 are also important for specific regions. Recent studies have found that ON species, such as aliphatic amines, are important precursors of secondary aerosol formation20−23 and may play significant roles in new particle formation and growth.24 However, real-time characterization of ON is still challenging due to the extremely complex ON

1. INTRODUCTION Organic nitrogen (ON) compounds, which are ubiquitous in atmospheric aerosols, participate in the chemical and physical processing in atmospheric aqueous phase,1−4 affect the hygroscopic properties of atmospheric droplets5 and also play an important role in the nitrogen cycle due to its bioavailability.6−9 The fraction of water-soluble ON (WSON) can account for ∼10−68% of total aerosol nitrogen from continental to oceanic sites, highlighting the importance of ON in atmospheric aerosols.10−16 The sources of ON are very dynamic depending on source locations and atmospheric processes. While primary emissions, e.g., biomass burning and vehicle exhausts are important sources of ON,8 secondary © 2017 American Chemical Society

Received: Revised: Accepted: Published: 673

September 18, 2017 November 6, 2017 November 21, 2017 November 21, 2017 DOI: 10.1021/acsearthspacechem.7b00106 ACS Earth Space Chem. 2017, 1, 673−682

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ACS Earth and Space Chemistry

secondary inorganic species and K+, suggesting that biomass burning and secondary formation are the two most important sources of WSON in Xi’an. In Beijing, Zhang et al.47 observed significant day-night differences in ON concentrations, and found that relative humidity (RH) was likely an important factor in facilitating ON formation. However, the study of OA with HR-AMS in China only started from 2008, mostly in Beijing, Pearl River Delta (PRD), Yangtze River Delta, and Lanzhou.43 While the N/C ratios of OA and different OA factors were reported in these studies, the quantification of ON and investigation of its sources and processes are rarely characterized. In addition, most previous HR-AMS studies were limited to a single season, the seasonal variations of ON and the effects of meteorological parameters are poorly understood. In this study, we demonstrate the applications of HR-AMS in characterization of ON in atmospheric aerosols during four seasons in the megacity of Beijing. The mass concentrations, diurnal profiles, and seasonal variations of ON are characterized. The mass spectral patterns and source contributions of nitrogen-containing families (CxHyNp+ and CxHyOzNp+) in different OA factors, and the sources and processes of ON are investigated

species, for example, wide range of carbon numbers, function groups, and solubility.25−27 Previous studies widely use the chemical, thermal or photo catalytic oxidation approaches to investigate ON from filter samples.25 These methods often involve large uncertainties because of the incomplete conversion of ON to inorganic nitrogen (IN), potential sampling losses and artifacts and much lower concentrations of ON than IN. In addition, filter analysis with low time resolution ranging from hours to days are difficult to characterize the rapid evolution processes of ON in atmosphere, particularly in regions with dynamic sources, for example, Beijing.28 Semicontinuous measurements of ON using oxidation approach were developed recently.11 The results show different diurnal behaviors of ON and correlations with collocated species (NH4+, SO42− and organic carbon (OC)) in summer and winter in the forest environment, indicating a variety of sources and processes of ON. This approach might have large uncertainties for periods with low ON levels due to the relatively high detection limit (DL) (0.14 μg m−3 for 1 h DL). The High-Resolution Time-of-Flight Aerodyne Aerosol Mass Spectrometer (HR-AMS hereafter) has a capability in differentiation of different ion categories with the same nominal integer mass.29 The N-containing fragments from ON in the mass spectra of HR-AMS mainly present in the forms of CxHyNp+ and CxHyOzNp+ families, while some organic compounds can also have a larger fraction of ON in the forms of NHx+ and NOx+ (e.g., amines, amides, and organic nitrates).30 Although the ratio of NO+/NO2+ is used to differentiate organic nitrates from inorganic ammonium nitrate in regions with dominant biogenic sources31,32 or low concentration of inorganic nitrate,33 this approach is generally not suitable for urban sites with high concentration of inorganic nitrate. While quantification of organic contributions to NHx+ and NOx+ is challenging, ON in CxHyNp+ and CxHyOzNp+ families can be determined from the elemental compositions of organic aerosols (OA) including nitrogen-to-carbon (N/C) and organic mass-to-organic carbon (OM/OC) ratios.34,35 Previous studies also showed that the spectral patterns of CxHyNp+ and CxHyOzNp+ families can provide new insights into sources and processes of ON. For example, Aiken et al.36 identified a local OA factor which was highly correlated with two amine ions, that is, C3H8N+ (m/z 58) and C5H12N+ (m/z 86). Such a factor with high N/C ratio (0.06) was likely from the local industrial emissions. Similarly, a nitrogen-enriched OA (N/C = 0.053) that was tightly correlated with C2H4N+, C3H8N+, and C4H10N+ ions was observed in New York City.37 Further analysis showed that this factor played an important role in aerosol processing via acid−base chemistry.38 Struckmeier et al.39 found that cigarette smoke emissions can have a large impact on C5H10N+ peak (a nicotine fragment). In recent years, HR-AMS was used for offline analysis of filter samples40−42 and a factor with high N/C ratio was almost identified in each of the study, suggesting that ON was important in water-soluble OA. Although numerous HR-AMS studies have been conducted in various environments,43 most of them simply reported the N/C ratios in OA factors, the concentration and sources of ON are rarely characterized. WSON that can contribute 15−68% to total nitrogen (TN) in fine particles has been also extensively studied in China.10,40,44,45 Shi et al.46 found that ON (>70%) dominantly existed in particles with size smaller than 2.1 μm in Qingdao. Ho et al.10 found strong correlations between WSON and

2. EXPERIMENTAL PROCEDURES 2.1. Sampling and Instrumentation. The HR-AMS was deployed at Institute of Atmospheric Physics, Chinese Academy of Sciences, a representative urban site in Beijing.48 The sampling periods covered four seasons including spring (March 3−27, 2014), summer (June 3−July 11, 2014), autumn (October 14−November 12, 2014) and winter (December 16, 2013- January 17, 2014 and November 13−December 15, 2014). The set up and operations of the HR-AMS were described in detail in Xu et al.49 Briefly, aerosol particles less than 2.5 μm were drawn through a PM2.5 cyclone (model URG-2000− 30EN), dried by a diffusion silica-gel dryer, and then isokinetically sampled into the HR-AMS. The HR-AMS was operated every 2 min in high mass resolution W-mode (m/Δm = ∼5000) and mass-sensitive V-mode (m/Δm = ∼2000) alternately in the winter of 2013, and the time resolution was changed to 5 min for both modes during other seasons. During these studies, collocated instruments including a twowavelength Aethalometer (model AE22, Magee Scientific Corp.), a cavity attenuated phase shift NO2 monitor (CAPSNO2, Aerodyne Research Inc.) and various gas analyzers from Thermo Scientific were used to measure the black carbon (BC), NO2 and gaseous species (CO, O3, NO/NO2, and SO2). More details can be found in our previous studies.49,50 The meteorological parameters including wind, temperature (T) and RH were obtained from the Beijing 325 m meteorological tower nearby. During the winter campaign of 2014, volatile organic compounds (VOCs) were also measured by gas chromatograph system equipped with a mass spectrometer and a flame ionization detector (GC-MS/FID). The photochemical age was calculated following the approach used in previous studies:51,52 Δt = [X] [E]

⎛ [X] 1 [X] ⎞ × ⎜ln |t = 0 − ln ⎟ [OH](k x − kE) ⎝ [E] [E] ⎠

(1)

[X]

and [E] |t = 0 refer to the concentration ratios and initial

emission ratios of m+p-xylene and ethylbezene, respectively. Kx 674

DOI: 10.1021/acsearthspacechem.7b00106 ACS Earth Space Chem. 2017, 1, 673−682

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ACS Earth and Space Chemistry and kE represent the OH rate constants of m+p-xylene and ethylbezene. [OH], an averaged OH radical concentrations (0.72 × 106 molecules cm−3), is calculated by the empirical [X] equation suggested by Ehhalt and Rohrer53 The [E] |t = 0 with

where OC is the carbon content of OA; OM/OC and N/C are the elemental ratios determined from elemental analysis; NNO−3 and NNH+4 are the nitrogen concentrations in inorganic NO3 and NH4 measured by HR-AMS. The mass resolution (m/Δm) needed for the separation of CxHyN1+ and CxHyO1N1+ from adjacent hydrocarbon and oxygenated ions linearly increases as a function of m/z (Figure S1). Given that the average mass resolution of HR-AMS is ∼5000 in this study, accurate separation and quantification of N-containing ions at m/z < 60 is possible, while it is more difficult for m/z > 60. Figure S2 shows the cumulative mass fraction of CxHyNp+ and CxHyOzNp+ as a function of m/z during four seasons, of which 67−80% of CxHyNp+, and 54− 81% of CxHyOzNp+, respectively, are at m/z < 60. These results indicate that large m/z’s (>60), where the mass resolution is insufficient to resolve the N-containing ions from adjacent ions contribute a relatively small fraction of total ON. In addition, several N-containing ions (CH4N+, C2H6N+, C3H8N+, CHON+, CH2ON+, CH3ON+, and CH4ON+) with large exact mass differences from adjacent ions can be relatively well determined depending on their signals and the mass resolution of HR-AMS (see Figures S3 for several examples at m/z 30, 43, 44, 45, 46, and 58 during wintertime). CxHyN2+ and CxHyO2N1+ ions might be important for specific N-compounds; however, separation and quantification of these ions are more challenging and not involved in this study.

1.94 ppb/ppb is calculated by the ratio of the highest concentrations of m+p-xylene and ethylbezene, which is comparable to those reported in Beijing (2.0)54 and eastern China (2.2).52 2.2. HR-AMS Data Analysis. The HR-AMS data regarding the elemental ratios of OA, mass concentrations of nonrefractory submicron aerosols (NR-PM1) species and the ionspeciated composition were analyzed using high resolution data analysis software (PIKA V 1.15D). The ammonium nitrate particles were used to calibrate ionization efficiency (IE), following the standard protocols,55,56 and the default relative ionization efficiency (RIE) except ammonium determined from pure ammonium nitrate was used. Because the mass fraction of NH4NO3 was smaller than the threshold value (0.4) affecting the CE significantly and aerosol particles were dry and slightly acidic indicated by NH4+measured/NH4+predicted57 during all studies, a constant collection efficiency (CE) of 0.5 was applied to all data sets.58 The elemental ratios of OA including N/C, OM/OC and oxygen-to-carbon (O/C) ratios were determined with the recently updated method called Improved-Ambient (IA).59 Although the I-A method does not affect the N/C ratio compared with the previous parametrization named the AikenAmbient (A-A),34 it can cause approximately 9% increase in OM/OC, leading to a corresponding decrease in ON concentrations. Positive matrix factorization (PMF)60 was performed to high-resolution mass spectra (HRMS) to determine OA factors during four seasons. Because of the limited mass resolution, the m/z’s larger than 120 were partly excluded during the PMF analysis (only major fragment ions were included). The isotopic ions contributing a small fraction of the total OA loading (∼2−3%) were also excluded. A five-factor solution at fpeak = 0 can be well interpreted in winter of 2014, which included the fossil OA (FOA), cooking-related OA (COA), biomass burning OA (BBOA), more oxidized oxygenated OA (MO-OOA), and less oxidized oxygenated OA (LO-OOA). In spring of 2014, we were able to resolve 8 factors (fpeak = 0) including three COA (COA1, COA2, and COA3), hydrocarbon-like OA (HOA), BBOA, coal combustion OA (CCOA), LO-OOA, and MO-OOA. The detailed PMF analysis and results in other three seasons, that is, summer (HOA, COA1, COA2, LO-OOA, MO-OOA), autumn (HOA, BBOA, COA1, COA2, LO-OOA, MO-OOA) and winter of 2013 (BBOA, HOA, COA, CCOA, LO-OOA, MO-OOA) have been detailed in Xu et al.61 and Sun et al.50 In this study, different COA factors were combined as one COA factor for easy comparisons and discussions. The sum of LO-OOA and MO-OOA was used as a surrogate of secondary OA (SOA), and the sum of FOA, COA, HOA, BBOA, and CCOA was regarded as primary OA (POA). The ON and TN in submicron organic aerosols can be determined as OC = organics/(OM/OC)

(2)

ON = OC × N/C × (14/12)

(3)

TN = ON + NNO3− + NNH4+

(4)

3. RESULTS AND DISCUSSION 3.1. Mass Concentrations and Mass Spectral Features of ON. The average ON level in submicron aerosols ranged from 0.26 to 0.59 μg m−3 during four seasons (Table 1). The Table 1. Summary of Average Mass Concentrations of ON (μg m−3) in Different OA Factors and the Total OA during Four Seasons LO-OOA MO-OOA BBOA CCOA HOA COA FOA total OA

spring

summer

autumn

2013 winter

2014 winter

0.066 0.190 0.013 0.030 0.040 0.092

0.057 0.110

0.100 0.230 0.035

0.056 0.130 0.250

0.040 0.047

0.120 0.100

0.074 0.100 0.098 0.053 0.077 0.023

0.43

0.26

0.59

0.43

0.063 0.067 0.57

ON presented the lowest concentration in summer, which was nearly twice lower than those observed in the other three seasons. This can be explained by the more precipitation and higher boundary layer height in summer, and also additional sources of ON in other seasons, e.g., biomass burning and coal combustion emissions. The ON concentrations determined by HR-AMS in Beijing were much higher than those observed at other sites, for example, 0.24 μg m−3 in Mexico City,36 0.09 μg m−3 in Fresno,62 and 0.02 μg m−3 in Rocky Mountain National Park,63 but typically less than those observed in PM2.5 in China, for example, 2.86−3.16 μg m−3 in Beijing44,47 and 1.5 μg m−3 in Changzhou.40 The contribution of ON to TN was relatively constant across different seasons (7−11% on average), indicating that ON is an important contributor to the total nitrogen budget during all seasons. Note that the HR-AMS ON contribution was ∼20−30% lower than those reported in PM2.5 675

DOI: 10.1021/acsearthspacechem.7b00106 ACS Earth Space Chem. 2017, 1, 673−682

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Figure 1. Mass spectra of ion categories of CxHyNp+ and CxHyOzNp+ in SOA, POA, and OA in (a) spring, (b) summer, (c) autumn, and winters of (d) 2013 and (e) 2014. The pie charts show the average contributions of OA factors to ON during four seasons.

from offline filter analysis.44,46,47 This is consistent with the relatively lower N/C ratios reported by HR-AMS (0.021− 0.028) than those in PM2.5 and fog waters (0.036−0.35).10,11,64 One of the reasons is that a large fraction of ON exists in the particles with diameter 1−2.5 μm.65 Another reason is the HRAMS ON could miss a large fraction of ON by ignoring its contributions in NOx+ and NHx+. Figure 1 shows the average mass spectra of N-ion categories (CxHyNp+ and CxHyOzNp+) in OA (m/z < 60) during four seasons, which are all dominated by CxHyNP+ (74.1%−77.8%). This is consistent with the dominance of CxHyNp+ in N-ion families in previous studies.62,66,67 As shown in Figure 1, the CxHyNp+ families were all characterized by prominent CHN+ (m/z 27), CH4N+ (m/z 30), C2H3N+ (m/z 41), and C2H4N+ (m/z 42) peaks during all seasons, but the spectral patterns were different among different seasons. For example, the signals of C1HyN+ families were much higher than those of C2HyN+ in summer and autumn, while they were comparable in other seasons. These results indicate different nitrogen sources in different seasons. Indeed, the three major CnH2n+2N+ ions (CH4N+, C2H6N+, and C3H8N+) that can be well separated and quantified present different slopes in different seasons (CH4N+/C2H6N+ = 2.7−4.1 and CH4N+/C3H8N+ = 2.5−5.1) although they are well correlated during all seasons (r2 = 0.72− 0.98 for CH4N+ vs C2H6N+, and r2 = 0.56−0.93 for CH4N+ vs C3H8N+). We noticed that the slopes of CH4N+/C2H6N+ and CH4N+/C3H8N+ in summer were higher and the correlations were worse than those in other three seasons (Figure S4), indicating multiple influences on the sources of ON in summer. The mass spectra of CxHyOzNp+ were dominated by two ions of CHON+(m/z 43) and CH2ON+(m/z 44) that were highly correlated during all seasons (r2 = 0.94−0.98), suggesting the presence of amides or amino acids.68 The CxHyNp+ and CxHyOzNp+ showed different spectral patterns between POA and SOA (Figure 1), indicating that ON species from primary emissions and secondary formation could be different. In fact, the N/C ratios in SOA (0.026−0.043) were overall more than twice those in POA (0.011−0.022) except the winter of 2014 when SOA and POA showed

comparable N/C ratios (∼0.030). This is also consistent with the dominant contributions of CxHyNp+ and CxHyOzNp+ in SOA (Table S1), highlighting the important source contributions of SOA to ON. Further support is from the better correlations between SOA and major N-containing ions during all seasons than most of POA factors except BBOA (Figure S5), which was different from that in NYC, where MO-OOA showed a weak correlation with CxHyNp+.37 In addition, LOOOA showed lower N/C than MO-OOA during all seasons (Figure S6), consistent with those observed in Sichuan69 Beijing70 and Kaiping,71 but different from those in Changdao72 and Hong Kong,73 implying that ON can be generated in different secondary reaction ways. The N/C ratios of COA varied from 0.010−0.015 except the winter of 2013 (0.004), which is overall consistent with those (0.008−0.018) from different cooking emissions.74 Comparatively, the N/C ratios of BBOA showed large variations (0.004−0.082), likely suggesting different biomass burning in different seasons. Such large variations were also observed in previous studies, for example, 0.03 as O/C increased from ∼0.2 to >0.6, suggesting the formation of more nitrogencontaining compounds during the aging of OA. Similar increases in ON and OC in the afternoon due to the enhanced photochemistry were also observed in California67 and the Southeastern U.S.11 The diurnal cycles of ON in autumn and winter were relatively similar, which were both characterized by nearly twice higher concentrations at nighttime than daytime. Similarly, higher ON/TN ratios were observed at nighttime, but the influences of cooking OA on ON/TN were much less than those in spring and summer. ON shows tighter correlations with SOA and CO2+ than POA and C4H9+ in autumn, indicating a dominant source of secondary ON (56%). In contrast, the correlations of ON with SOA and POA were comparable during the winter of 2013 (r2 = ∼0.8) and 2014 (r2 = ∼0.9), suggesting the ON sources from both primary emissions and secondary formation. This result is consistent with the observations in Xi’an that water-soluble ON was correlated with both secondary inorganic species and K+.10 In fact, as shown in Figures 3d and 3e, N/C was positively correlated with O/C in winter, and ON concentration increased from 0.08 to 1.25 μg m−3 as photochemical age increased from 10 h to 22h (Figure 3f). Also, N-ion families (CnH2n+2N+ and CHyON+) show increasing trends as a function of photochemical age (Figure S8), indicating that photochemical production still played a role in the formation of ON in winter. 3.3. RH Effects on ON. The variations of ON and N/C as a function of RH during four seasons are presented in Figure 4. At low RH levels (RH < 60%), ON showed an increase as the increase in RH during all seasons, yet the increasing rates were different with the highest values in winter (0.23 and 0.26 μg m−3/10% RH in 2013 and 2014, respectively) and the lowest one in summer (0.05 μg m−3/10% RH). Because aerosol liquid 678

DOI: 10.1021/acsearthspacechem.7b00106 ACS Earth Space Chem. 2017, 1, 673−682

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carrier gas illustrated the most prominent CH2N+ peak in CxHyNp+ families.79 It is estimated that the current N/C ratio is underestimated by ∼20% on average by ignoring the ON in CH2N+ at m/z 28, and NHx+ and NOx+ families, and the underestimation can be up to ∼42% if organic nitrates are dominant in OA.30 Fourthly, accurate determination of N/C ratio strongly depends on the mass resolution (m/Δm) because of much lower signals of CxHyNp+ and CxHyOzNp+ than CxHy+ and CxHyOz+. The V-mode of HR-AMS that has a typical mass resolution of ∼2000, which is difficult to separate and quantify most of nitrogen-containing ions above m/z 50 (Figure S1). While the W-mode with nearly twice higher mass resolution (∼4000−5000) is better for separating nitrogen ion fragments, it also has larger uncertainties in high resolution peak fitting due to much lower signals. Thus, an optimized peak shape tuning before sample analysis is necessary and of great importance for a better quantification of ON. The recently developed long time-of-flight AMS with a much improved mass resolution (∼8000) yet similar sensitivity as that of V-mode of the HR-AMS will greatly help the future ON characterization. Fifthly, uncertainties in elemental analysis of OA can also influence the ON quantification. For example, the recently proposed I-A method produces ∼9% higher OM/OC ratio than that of A-A method,59 which would lead to a decrease in OC, and then ON concentration according to eqs 2 and (3).

whereas those of ON can be affected by both primary and secondary OA factors. Consistent with our conclusion, higher concentrations of ON were also observed at higher RH levels in summer and winter in Beijing47 and higher WSON in wet days than dry days in southern Scotland.77 Figure 4 compares the average mass spectra of N-ion families (CxHyNp+ and CxHyOzNp+) between low and high RH levels. While the average N/C ratios at RH > 60% (0.027−0.035) were ubiquitously higher than those at RH < 60% (0.020− 0.025), the mass spectra also have some differences. Generally, CxHyNp+ families showed 9%−30% higher fraction at high RH levels than those at low RH values, and ∼8%−44% for CxHyOzNp+, consistent with the increases in N/C and ON at higher RH levels. The different spectra between high and low RH (Figure S13) were characterized by prominent peaks of CH4N+ (m/z 30), C2H6N+(m/z 44) and C3H8N+(m/z 58) during four seasons, and CnH2n+2N+ was correspondingly higher by 47%−60% at high RH levels. This result indicates that aqueous-phase processing appears to produce a considerable amount of amine-related nitrogen compounds. The ratios in Figure S13a also showed pronounced CH5N+ (m/z 31) and C2H7N+ (m/z 45), further supporting the formation of amines at high RH levels. Also note that the ratios of C1HxN+ between high and low RH levels were higher than those of C2HxN+ and C3HxN+, suggesting that aqueous-phase processing have different impacts on nitrogen-containing compounds with different molecular weights. We also observed enrichments of CHON+(m/z 43) and CH2ON+ (m/z 44) at high RH levels except summer, likely indicating the formation of amide-related compounds (R−C(O)-NH2) in aqueous-phase.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsearthspacechem.7b00106. Average relative contributions of ion categories, mass resolution, cumulative mass fraction, examples of high resolution mass spectra analysis, scatter plots, correlation coefficients, average mass spectra, diurnal profiles of correlation coefficients, variations of mass concentrations, wind speed, and LWC, ratios of OA species and ON to HOA as a function, and difference and ratio of average mass spectra (PDF)

4. LIMITATIONS In this study, we have demonstrated the capability of HR-AMS in quantification and characterization of ON in atmospheric aerosols. The highly time-resolved ON combined with PMF analysis provides more insights into the sources and processes of ON. However, limitations in quantification of ON with HRAMS are still remained. First, HR-AMS only measures ON in submicron aerosols due to the transmission efficiency of aerodynamic lens while a considerable fraction of ON often exists in particles with diameter larger than 1 μm. For example, the average concentration of WSON was 5.5 N mol m−3 in PM1.3−10, which is nearly 50% of that in PM1.3 at a remote marine location in the Eastern Mediterranean.65 Therefore, the HR-AMS equipped with the newly developed aerodynamic lens that can transmit ∼90% of PM2.578 should to be deployed further to characterize ON in PM1−2.5. Second, HR-AMS with thermal vaporization (∼600 °C) only detect nonrefractory ON, but may underestimate ON by missing the fraction in refractory materials, e.g., dust. Third, the N/C ratio is a key factor affecting the accuracy in ON quantification, which depends on the separation and quantification of N-containing ions (CxHyNp+, CxHyOzNp+, NHx+, NOx+, and minor CxHyOzNpSq+). The current N/C ratio is overall underestimated because: 1) the contribution of ON in NOx+ and NHx+ families (e.g., from fragmentation of organic nitrates and some amino compounds) cannot be separated. For example, the NO+/NO2+ ratio in this study is close to that of ammonium nitrate (=2.28), and quantification of ON in NOx+ and NHx+ is impossible; 2) AMS elemental analysis excludes the most important nitrogen-containing ion, CH2N+ (m/z 28) due to the overwhelming interference of adjacent N2+ ion. Recent HR-AMS offline analysis of filter samples with argon as

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-10-82021255. ORCID

Yele Sun: 0000-0003-2354-0221 Xinlei Ge: 0000-0001-9531-6478 Pingqing Fu: 0000-0001-6249-2280 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Project of Basic Research (2014CB447900) and the National Natural Science Foundation of China (41575120, 41571130034).

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REFERENCES

(1) Laskin, A.; Smith, J. S.; Laskin, J. Molecular characterization of nitrogen-containing organic compounds in biomass burning aerosols using high-resolution mass spectrometry. Environ. Sci. Technol. 2009, 43 (10), 3764−3771.

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DOI: 10.1021/acsearthspacechem.7b00106 ACS Earth Space Chem. 2017, 1, 673−682

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ACS Earth and Space Chemistry

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