Seasonal Characterization of Organic Nitrogen in Atmospheric

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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 ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.7b00106 • Publication Date (Web): 21 Nov 2017 Downloaded from http://pubs.acs.org on November 24, 2017

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ACS Earth and Space Chemistry

1

Seasonal Characterization of Organic Nitrogen in Atmospheric Aerosols

2

Using High Resolution Aerosol Mass Spectrometry in Beijing, China

3 4

Weiqi Xu1,3, Yele Sun1,2,3*, Qingqing Wang1, Wei Du1,3, Jian Zhao1,3, Xinlei Ge4, Tingting

5

Han1, Yingjie Zhang1,4, Wei Zhou1,3, Jie Li1, Pingqing Fu1, Zifa Wang1, Douglas R.

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Worsnop5

7 8

1

State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric

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Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing

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100029, China

11

2

Center for Excellence in Regional Atmospheric Environment, Institute of Urban

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Environment, Chinese Academy of Sciences, Xiamen 361021, China

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3

14

4

University of Chinese Academy of Sciences, Beijing 100049, China

Nanjing University of Information Science & Technology, Nanjing 210044, China 5

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Aerodyne Research Inc., Billerica, Massachusetts 01821, USA

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17 18

*

To whom correspondence should be addressed: Yele Sun ([email protected]) 40 Huayanli, Chaoyang District, Beijing 100029, China. Tel.: +86-10-82021255

19

1

ACS Paragon Plus Environment


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ABSTRACT

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Despite extensive efforts to characterize organic nitrogen (ON) in atmospheric

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aerosols, knowledge of the sources and processes of ON in the megacity of Beijing is

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still limited, mainly due to the complexity of ON species and the absence of highly

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time-resolved measurements. Here we demonstrate the applications of Aerodyne

25

high-resolution time-of-flight aerosol mass spectrometer combined with positive

26

matrix factorization in characterization of ON in submicron aerosols. Our results

27

show that the average nitrogen-to-carbon ratios (N/C) vary from 0.021 to 0.028, and

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the average ON concentrations range from 0.26 to 0.59 μg m-3 during four seasons in

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Beijing. ON accounts for 7-10% of the total nitrogen (TN) on average, yet the sources

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vary differently across different seasons. We found that 56-65% of ON was secondary

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during three seasons except winter when 59-67% was related to primary emissions.

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Particularly, more oxidized secondary organic aerosol contributes the dominant

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fraction of ON (39-44%) in spring, summer and autumn, while biomass burning is a

34

more important source of ON in winter (23-44%). These results are consistent with

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the better positive correlations between N/C and oxygen-to-carbon ratio, a surrogate

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of organic aerosol aging, during these three seasons than that in winter. N/C also

37

shows a clear increase as a function of relative humidity during all seasons,

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suggesting that aqueous-phase processing likely played an important role in

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formation of nitrogen-containing compounds. In addition, the uncertainties and

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limitations in quantification of ON with aerosol mass spectrometry are illustrated,

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particularly, ON could be underestimated by ~20 – 42% by ignoring the fragment 2


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42

contributions in NHx+ and NOx+.

43 44

Keywords: Organic nitrogen, aerosol mass spectrometry, sources, aqueous-phase

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processing, Beijing

46 47 48

1. INTRODUCTION Organic nitrogen (ON) compounds, which are ubiquitous in atmospheric aerosols,

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participate in the chemical and physical processing in atmospheric aqueous phase,1-4

50

affect the hygroscopic properties of atmospheric droplets5 and also play an

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important role in the nitrogen cycle due to its bioavailability.6-9 The fraction of

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water-soluble ON (WSON) can account for ~10-68% of total aerosol nitrogen from

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continental to oceanic sites, highlighting the importance of ON in atmospheric

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aerosols.10-16 The sources of ON are very dynamic depending on source locations and

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atmospheric processes. While primary emissions, e.g., biomass burning and vehicle

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exhausts are important sources of ON,8 secondary formation, e.g., nitrate radical

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oxidation,17 gas-particle partitioning, and cloud processing18, 19 are also important for

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specific regions. Recent studies have found that ON species such as aliphatic amines

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are important precursors of secondary aerosol formation20-23 and may play significant

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roles in new particle formation and growth.24 However, real-time characterization of

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ON is still challenging due to the extremely complex ON species e.g., wide range of

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carbon numbers, function groups, and solubility.25-27

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Previous studies widely use the chemical, thermal or photo catalytic oxidation 3



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approaches to investigate ON from filter samples.25 These methods often involve

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large uncertainties due to the incomplete conversion of ON to inorganic nitrogen (IN),

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potential sampling losses and artifacts and much lower concentrations of ON than IN.

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In addition, filter analysis with low time resolution ranging from hours to days are

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difficult to characterize the rapid evolution processes of ON in atmosphere,

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particularly in regions with dynamic sources, e.g., Beijing.28 Semi-continuous

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measurements of ON using oxidation approach were developed recently.11 The

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results show different diurnal behaviors of ON and correlations with collocated

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species (NH4+, SO42- and organic carbon (OC)) in summer and winter in the forest

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environment, indicating a variety of sources and processes of ON. This approach

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might have large uncertainties for periods with low ON levels due to the relatively

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high detection limit (DL) (0.14 µg m-3 for 1 h DL).

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The High-Resolution Time-of-Flight Aerodyne Aerosol Mass Spectrometer

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(HR-AMS hereafter) has a capability in differentiation of different ion categories with

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the same nominal integer mass.29 The N-containing fragments from ON in the mass

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spectra of HR-AMS mainly present in the forms of CxHyNp+ and CxHyOzNp+ families,

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while some organic compounds can also have a larger fraction of ON in the forms of

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NHx+ and NOx+ (e.g., amines, amides, and organic nitrates).30 Although the ratio of

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NO+/NO2+ is used to differentiate organic nitrates from inorganic ammonium nitrate

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in regions with dominant biogenic sources

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nitrate,33 this approach is generally not suitable for urban sites with high

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concentration of inorganic nitrate. While quantification of organic contributions to

31, 32

or low concentration of inorganic

4


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NHx+ and NOx+ is challenging, ON in CxHyNp+ and CxHyOzNp+ families can be

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determined from the elemental compositions of organic aerosols (OA) including

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nitrogen-to-carbon (N/C) and organic mass-to-organic carbon (OM/OC) ratios.34, 35

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Previous studies also showed that the spectral patterns of CxHyNp+ and CxHyOzNp+

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families can provide new insights into sources and processes of ON. For example,

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Aiken et al. 36 identified a local OA factor which was highly correlated with two amine

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ions, i.e., C3H8N+ (m/z 58) and C5H12N+ (m/z 86). Such a factor with high N/C ratio

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(0.06) was likely from the local industrial emissions. Similarly, a nitrogen-enriched OA

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(N/C=0.053) that was tightly correlated with C2H4N+, C3H8N+, and C4H10N+ ions was

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observed in New York City.37 Further analysis showed that this factor played an

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important role in aerosol processing via acid-base chemistry.38 Struckmeier et al.

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found that cigarette smoke emissions can have a large impact on C5H10N+ peak (a

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nicotine fragment). In recent years, HR-AMS was used for offline analysis of filter

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samples

40-42

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and a factor with high N/C ratio was almost identified in each of the

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study, suggesting that ON was important in water-soluble OA. Although numerous

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HR-AMS studies have been conducted in various environments,43 most of them

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simply reported the N/C ratios in OA factors, the concentration and sources of ON

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are rarely characterized.

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WSON that can contribute 15-68% to total nitrogen (TN) in fine particles has

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been also extensively studied in China.10, 40, 44, 45 Shi et al.

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dominantly existed in particles with size smaller than 2.1 μm in Qingdao. Ho et al. 10

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found strong correlations between WSON and secondary inorganic species and K+, 5


46

found that ON (>70%)


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suggesting that biomass burning and secondary formation are the two most

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important sources of WSON in Xi'an. In Beijing, Zhang et al.

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day-night differences in ON concentrations, and found that relative humidity (RH)

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was likely an important factor in facilitating ON formation. However, the study of OA

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with HR-AMS in China only started from 2008, mostly in Beijing, Pearl River Delta

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(PRD), Yangtze River Delta, and Lanzhou.43 While the N/C ratios of OA and different

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OA factors were reported in these studies, the quantification of ON and investigation

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of its sources and processes are rarely characterized. In addition, most previous

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HR-AMS studies were limited to a single season, the seasonal variations of ON and

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the effects of meteorological parameters are poorly understood.

47

observed significant

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In this study, we demonstrate the applications of HR-AMS in characterization of

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ON in atmospheric aerosols during four seasons in the megacity of Beijing. The mass

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concentrations, diurnal profiles, and seasonal variations of ON are characterized. The

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mass spectral patterns and source contributions of nitrogen-containing families

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(CxHyNp+ and CxHyOzNp+) in different OA factors, and the sources and processes of ON

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are investigated

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2. EXPERIMENTAL PROCEDURES

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Sampling and instrumentation

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The HR-AMS was deployed at Institute of Atmospheric Physics, Chinese Academy

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of Sciences, a representative urban site in Beijing.48 The sampling periods covered

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four seasons including spring (March 3-27, 2014), summer (June 3-July 11, 2014),

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autumn (October 14- November 12, 2014) and winter (December 16, 2013- January 6


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17, 2014 and November 13 - December 15, 2014).

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The set up and operations of the HR-AMS were described in detail in Xu et al. 49

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Briefly, aerosol particles less than 2.5 μm were drawn through a PM2.5 cyclone

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(model URG-2000-30EN), dried by a diffusion silica-gel dryer, and then isokinetically

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sampled into the HR-AMS. The HR-AMS was operated every 2 min in high mass

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resolution W-mode (m/Δm = ~5000) and mass-sensitive V-mode (m/Δm = ~2000)

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alternately in the winter of 2013, and the time resolution was changed to 5 min for

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both modes during other seasons.

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During these studies, collocated instruments including a two-wavelength

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Aethalometer (model AE22, Magee Scientific Corp.), a cavity attenuated phase shift

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NO2 monitor (CAPS-NO2, Aerodyne Research Inc.) and various gas analyzers from

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Thermo Scientific were used to measure the black carbon (BC), NO2 and gaseous

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species (CO, O3, NO/NO2, and SO2). More details can be found in our previous studies.

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49, 50

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obtained from the Beijing 325 m meteorological tower nearby.

The meteorological parameters including wind, temperature (T) and RH were

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During the winter campaign of 2014, volatile organic compounds (VOCs) were

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also measured by gas chromatograph system equipped with a mass spectrometer

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and a flame ionization detector (GC-MS/FID). The photochemical age was calculated

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following the approach used in previous studies:51, 52 ሾXሿ

1

x E

150 151

[X]

∆t= ሾOHሿ൫k -k ൯ ×(In ሾEሿ | -In [E] )

149

Where

ሾଡ଼ሿ ሾ୉ሿ

and

[X]

|

[E] t=0

(1)

t=0

refer to the concentration ratios and initial emission

ratios of m+p-xylene and ethylbezene, respectively. Kx and kE represent the OH rate 7



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constants of m+p-xylene and ethylbezene. [OH], an averaged OH radical

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concentrations (0.72×106 molecule cm-3), is calculated by the empirical equation

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suggested by Ehhalt and Rohrer

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the ratio of the highest concentrations of m+p-xylene and ethylbezene, which is

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comparable to those reported in Beijing (2.0) 54 and eastern China (2.2) .52

157

HR-AMS data analysis

53

[X]

The

|

[E] t=0

with 1.94 ppb/ppb is calculated by

158

The HR-AMS data regarding the elemental ratios of OA, mass concentrations of

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non-refractory submicron aerosols (NR-PM1) species and the ion-speciated

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composition were analyzed using high resolution data analysis software (PIKA V

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1.15D). The ammonium nitrate particles were used to calibrate ionization efficiency

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(IE) following the standard protocols,55,

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efficiency (RIE) except ammonium determined from pure ammonium nitrate was

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used. Because the mass fraction of NH4NO3 was smaller than the threshold value (0.4)

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affecting the CE significantly and aerosol particles were dry and slightly acidic

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indicated by NH4+measured/NH4+predicted

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efficiency (CE) of 0.5 was applied to all datasets.58 The elemental ratios of OA

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including N/C, OM/OC and oxygen-to-carbon (O/C) ratios were determined with the

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recently updated method called Improved-Ambient (I-A).59 Although the I-A method

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does not affect the N/C ratio compared with the previous parameterization named

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the Aiken-Ambient (A-A),34 it can cause approximately 9% increase in OM/OC,

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leading to a corresponding decrease in ON concentrations .

173

57

Positive matrix factorization (PMF)

56

and the default relative ionization

during all studies, a constant collection

60

was performed to high-resolution mass

8


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spectra (HRMS) to determine OA factors during four seasons. Because of the limited

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mass resolution, the m/z’s larger than 120 were partly excluded during the PMF

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analysis (only major fragment ions were included). The isotopic ions contributing a

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small fraction of the total OA loading (~2-3%) were also excluded. A five-factor

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solution at fpeak=0 can be well interpreted in winter of 2014, which included the

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fossil OA (FOA), cooking-related OA (COA), biomass burning OA (BBOA), more

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oxidized oxygenated OA (MO-OOA), and less oxidized oxygenated OA (LO-OOA). In

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spring of 2014, we were able to resolve 8 factors (fpeak =0) including three COA

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(COA1, COA2 and COA3), hydrocarbon-like OA (HOA), BBOA, coal combustion OA

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(CCOA), LO-OOA and MO-OOA. The detailed PMF analysis and results in other three

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seasons, i.e., summer (HOA, COA1, COA2, LO-OOA, MO-OOA), autumn (HOA, BBOA,

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COA1, COA2, LO-OOA, MO-OOA) and winter of 2013 (BBOA, HOA, COA, CCOA,

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LO-OOA, MO-OOA) have been detailed in Xu et al.

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different COA factors were combined as one COA factor for easy comparisons and

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discussions. The sum of LO-OOA and MO-OOA was used as a surrogate of secondary

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OA (SOA), and the sum of FOA, COA, HOA, BBOA and CCOA was regarded as primary

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OA (POA).

191

61

and Sun et al. 50. In this study,

The ON and TN in submicron organic aerosols can be determined as:

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OC = Organics / (OM/OC)

(2)

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ON = OC × N/C × (14/12)

(3)

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TN=ON+NNO - +NNH4+

195

3

(4)

Where OC is the carbon content of OA; OM/OC and N/C are the elemental ratios 9



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determined from elemental analysis; NNO

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concentrations in inorganic NO3 and NH4 measured by HR-AMS.

3

-

and NNH4 + are the nitrogen

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The mass resolution (m/Δm) needed for the separation of CxHyN1+ and CxHyO1N1+

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from adjacent hydrocarbon and oxygenated ions linearly increases as a function of

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m/z (Figure S1). Given that the average mass resolution of HR-AMS is ~5000 in this

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study, accurate separation and quantification of N-containing ions at m/z < 60 is

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possible while it’s more difficult for m/z > 60. Figure S2 shows the cumulative mass

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fraction of CxHyNp+ and CxHyOzNp+ as a function of m/z during four seasons, of which

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67-80% of CxHyNp+, and 54-81% of CxHyOzNp+, respectively are at m/z < 60. These

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results indicate that large m/z’s (> 60) where the mass resolution is insufficient to

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resolve the N-containing ions from adjacent ions contribute a relatively small fraction

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of total ON. In addition, several N-containing ions (CH4N+, C2H6N+, C3H8N+, CHON+,

208

CH2ON+, CH3ON+ and CH4ON+) with large exact mass differences from adjacent ions

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can be relatively well determined depending on their signals and the mass resolution

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of HR-AMS (see Figures S3 for several examples at m/z 30, 43, 44, 45, 46 and 58

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during wintertime). CxHyN2+ and CxHyO2N1+ ions might be important for specific

212

N-compounds, however separation and quantification of these ions are more

213

challenging and not involved in this study.

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3. RESULTS AND DISCUSSION

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Mass concentrations and mass spectral features of ON

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The average ON level in submicron aerosols ranged from 0.26 to 0.59 μg m-3

217

during four seasons (Table 1). The ON presented the lowest concentration in summer, 10


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which was nearly twice lower than those observed in the other three seasons. This

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can be explained by the more precipitation and higher boundary layer height in

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summer, and also additional sources of ON in other seasons, e.g., biomass burning

221

and coal combustion emissions. The ON concentrations determined by HR-AMS in

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Beijing were much higher than those observed at other sites, e.g., 0.24 μg m-3 in

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Mexico City,36 0.09μg m-3 in Fresno,

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Park,63 but typically less than those observed in PM2.5 in China, e.g., 2.86-3.16μg m-3

225

in Beijing

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relatively constant across different seasons (7-11% on average), indicating that ON is

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an important contributor to the total nitrogen budget during all seasons. Note that

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the HR-AMS ON contribution was ~20-30% lower than those reported in PM2.5 from

229

offline filter analysis.44, 46, 47 This is consistent with the relatively lower N/C ratios

230

reported by HR-AMS (0.021-0.028) than those in PM2.5 and fog waters (0.036-0.35).10,

231

11, 64

232

diameter 1-2.5 μm.65 Another reason is the HR-AMS ON could miss a large fraction of

233

ON by ignoring its contributions in NOx+ and NHx+.

44, 47

62

and 0.02 μg m-3 in Rocky Mountain National

and 1.5 μg m-3 in Changzhou.40 The contribution of ON to TN was

One of the reasons is that a large fraction of ON exists in the particles with

234

Figure 1 showed the average mass spectra of N-ion categories (CxHyNp+ and

235

CxHyOzNp+) in OA (m/z < 60) during four seasons, which was all dominated by CxHyNP+

236

(74.1%-77.8%). This is consistent with the dominance of CxHyNp+ in N-ion families in

237

previous studies.62,

238

characterized by prominent CHN+ (m/z 27), CH4N+ (m/z 30), C2H3N+ (m/z 41), and

239

C2H4N+ (m/z 42) peaks during all seasons, but the spectral patterns were different

66, 67

As shown in Figure 1, the CxHyNp+ families were all

11



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240

among different seasons. For example, the signals of C1HyN+ families were much

241

higher than those of C2HxN+ in summer and autumn, while they were comparable in

242

other seasons. These results indicate different nitrogen sources in different seasons.

243

Indeed, the three major CnH2n+2N+ ions (CH4N+, C2H6N+, and C3H8N+ ) that can be well

244

separated and quantified present different slopes in different seasons (CH4N+/C2H6N+

245

= 2.7-4.1 and CH4N+/C3H8N+ = 2.5-5.1) although they are well correlated during all

246

seasons (r2 = 0.72-0.98 for CH4N+ vs. C2H6N+, and r2 = 0.56-0.93 for CH4N+ vs. C3H8N+).

247

We noticed that the slopes of CH4N+/C2H6N+ and CH4N+/C3H8N+ in summer were

248

higher and the correlations were worse than those in other three seasons (Figure S4),

249

indicating multiple influences on the sources of ON in summer. The mass spectra of

250

CxHyOzNp+ were dominated by two ions of CHON+(m/z 43) and CH2ON+(m/z 44) that

251

were highly correlated during all seasons (r2 = 0.94-0.98), suggesting the presence of

252

amides and/or amino acids.68

253

The CxHyNp+ and CxHyOzNp+ showed different spectral patterns between POA and

254

SOA (Figure 1), indicating that ON species from primary emissions and secondary

255

formation could be different. In fact, the N/C ratios in SOA (0.026-0.043) were overall

256

more than twice those in POA (0.011-0.022) except the winter of 2014 when SOA

257

and POA showed comparable N/C ratios (~0.030). This is also consistent with the

258

dominant contributions of CxHyNp+ and CxHyOzNp+ in SOA (Table S1), highlighting the

259

important source contributions of SOA to ON. Further support is from the better

260

correlations between SOA and major N-containing ions during all seasons than most

261

of POA factors except BBOA (Figure S5), which was different from that in NYC, where 12


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MO-OOA showed a weak correlation with CxHyNp+.37 In addition, LO-OOA showed

263

lower N/C than MO-OOA during all seasons (Figure S6), consistent with those

264

observed in Sichuan 69 Beijing 70 and Kaiping, 71 but different from those in Changdao

265

72

266

reaction ways. The N/C ratios of COA varied from 0.010 – 0.015 except the winter of

267

2013 (0.004), which is overall consistent with those (0.008 – 0.018) from different

268

cooking emissions. 74 Comparatively, the N/C ratios of BBOA showed large variations

269

(0.004 – 0.082), likely suggesting different biomass burning in different seasons. Such

270

large variations were also observed in previous studies, for example, < 0.018 for fresh

271

biomass burning 74 and ~0.06 in BBOA from ambient measurements. 75

and HongKong,73 implying that ON can be generated in different secondary

272

Figure 1 also presents the average contribution of each OA factor to the total ON

273

in each season. It is clear that ON was dominantly contributed by MO-OOA during

274

three seasons (spring, summer and autumn), ranging from 39-44%, while 15-22% of

275

ON was contributed by LO-OOA. These results indicate that ON in these three

276

seasons are dominantly from secondary aerosol formation. Note that OA from

277

cooking emissions also showed a considerable contribution to ON (17-21%) although

278

the N/C ratios were generally low (< 0.015, Figure S6). In contrast, BBOA showed a

279

large contribution to ON accounting for 23% and 44% in winters of 2013 and 2014,

280

respectively, while the contributions of MO-OOA to ON in winter were decreased to

281

23 – 24%. These results indicate that biomass burning is a significant source of ON in

282

winter season in Beijing. As shown in Figure S5, BBOA was highly correlated with

283

major N-containing ions in winter of 2014 (r2=~0.9), and still moderately correlated 13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

284

with those in winter of 2013 (r2=~0.5). This is also consistent with higher N/C ratios

285

of BBOA (0.042 and 0.082 in winters of 2013 and 2014, respectively) than other OA

286

factors. Similarly, high N/C ratio in BBOA was observed in previous studies, e.g., 0.06

287

at urban and rural sites in PRD.71, 75 Note that BBOA was also resolved in spring and

288

autumn, however, the N/C ratio (< 0.015) was much lower than those in winter, and

289

the contribution to ON was correspondingly low (3% and 6%). These results might

290

indicate that biomass burning in winter has different composition and properties

291

from those in other seasons.

292

Diurnal cycles

293

Figure 2 shows the diurnal profiles of ON, ON/TN and correlation coefficients

294

between ON and POA and SOA during four seasons. The diurnal profiles of ON were

295

relatively flat in spring and summer except lower concentrations during the late

296

afternoon. However, ON/TN presented a pronounced diurnal cycle which was

297

characterized by two peaks during mealtimes. Such a diurnal pattern was consistent

298

with those of C4H9+ and COA observed previously,70, 74 suggesting the influences of

299

cooking emissions. Although the N/C ratio showed an opposite diurnal pattern to

300

ON/TN, high concentrations of COA during mealtimes increased the total ON, leading

301

to correspondingly high ON/TN. ON was tightly correlated with SOA throughout the

302

day (r2 > 0.60) in spring and summer, but weakly correlated with POA for most of the

303

time (e.g., r2 0.03 as O/C increased from

310

~0.2 to > 0.6, suggesting the formation of more nitrogen-containing compounds

311

during the aging of OA. Similar increases in ON and OC in the afternoon due to the

312

enhanced photochemistry were also observed in California67 and the Southeastern

313

U.S. 11

314

The diurnal cycles of ON in autumn and winter were relatively similar, which

315

were both characterized by nearly twice higher concentrations at nighttime than

316

daytime. Similarly, higher ON/TN ratios were observed at nighttime, but the

317

influences of cooking OA on ON/TN were much less than those in spring and summer.

318

ON shows tighter correlations with SOA and CO2+ than POA and C4H9+ in autumn,

319

indicating a dominant source of secondary ON (56%). In contrast, the correlations of

320

ON with SOA and POA were comparable during the winter of 2013 (r2 = ~0.8) and

321

2014 (r2 = ~0.9), suggesting the ON sources from both primary emissions and

322

secondary formation. This result is consistent with the observations in Xi’an that

323

water-soluble ON was correlated with both secondary inorganic species and K+.10 In

324

fact, as shown in Figures 3d and 3e, N/C was positively correlated with O/C in winter,

325

and ON concentration increased from 0.08 to 1.25 μg m-3 as photochemical age

326

increased from 10 h to 22h (Figure 3f). Also, N-ion families (CnH2n+2N+ and CHyON+)

327

show increasing trends as a function of photochemical age (Figure S8), indicating that 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

328

photochemical production still played a role in the formation of ON in winter.

329

RH effects on ON

330

The variations of ON and N/C as a function of RH during four seasons are

331

presented in Figure 4. At low RH levels (RH < 60%), ON showed an increase as the

332

increase in RH during all seasons, yet the increasing rates were different with the

333

highest values in winter (0.23 and 0.26 μg m-3/10% RH in 2013 and 2014, respectively)

334

and the lowest one in summer (0.05 μg m-3/10% RH). Because aerosol liquid water

335

predicted with ISORROPIA-II was generally low (< 20 μg m-3),61 the increases in ON at

336

low RH level were mainly due to the decreases in wind speed (Figure S9) that

337

facilitated the accumulation of air pollutants. However, the N/C ratios also showed

338

increases as a function of RH at low RH levels during all seasons suggesting the

339

changes in nitrogen-containing compounds. By contrast, the variations in mass

340

concentrations of ON at RH>60% showed much differences between four seasons.

341

While the ON concentration showed almost a linear increase with RH in autumn, it

342

showed overall decreases at high RH levels during the other three seasons. The

343

decreases were mainly caused by several precipitation events that scavenged OA

344

substantially in summer (Figures S10) 61 and other factors, e.g., wind speed in spring

345

and winter of 2013 (Figures S11-S12). For example, by excluding the precipitation

346

events in summer, ON remained at relatively high concentrations at high RH levels.

347

Similarly, the normalized ON using HOA as a reference76 also showed high

348

concentrations at high RH levels in spring and winter. Although ON did not show a

349

large increase at high RH levels, the N/C showed an increasing trend as a function of 16


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350

RH during all seasons, suggesting that aqueous-phase processing likely have played

351

an important role in the formation of nitrogen-containing compounds. For example,

352

previous studies showed that aqueous-phase reactions of glyoxal and methylglyoxal

353

with amines, amino acids and ammonium sulfate can form abundant

354

nitrogen-containing oligomers.4 This is also consistent with our previous results that

355

MO-OOA with higher N/C ratio showed much higher contribution to OA at higher RH

356

levels due to aqueous-phase processing.61 In fact, we found that the aqueous-phase

357

related MO-OOA and ON in this factor showed large increases at high RH levels

358

(Figure S10-S12), while ON in other OA factors either showed decreases or remained

359

relatively constant. 61 The different trends of ON and N/C as a function of RH can be

360

explained by the fact that the variations in N/C ratios are mainly driven by MO-OOA

361

that showed much higher values than other factors (Figure S6), whereas those of ON

362

can be affected by both primary and secondary OA factors. Consistent with our

363

conclusion, higher concentrations of ON were also observed at higher RH levels in

364

summer and winter in Beijing47 and higher WSON in wet days than dry days in

365

southern Scotland.77

366

Figure 4 compared the average mass spectra of N-ion families (CxHyNp+ and

367

CxHyOzNp+) between low and high RH levels. While the average N/C ratios at RH>60%

368

(0.027-0.035) were ubiquitously higher than those at RH < 60% (0.020-0.025), the

369

mass spectra also have some differences. Generally, CxHyNp+ families showed 9%-30%

370

higher fraction at high RH levels than those at low RH values, and ~8%-44% for

371

CxHyOzNp+, consistent with the increases in N/C and ON at higher RH levels. The 17



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Page 18 of 33

372

different spectra between high and low RH (Figure S13) were characterized by

373

prominent peaks of CH4N+ (m/z 30), C2H6N+(m/z 44) and C3H8N+(m/z 58) during four

374

seasons, and CnH2n+2N+ was correspondingly higher by 47%-60% at high RH levels.

375

This result indicates that aqueous-phase processing appears to produce a

376

considerable amount of amine-related nitrogen compounds. The ratios in Figure

377

S13a also showed pronounced CH5N+ (m/z 31) and C2H7N+ (m/z 45), further

378

supporting the formation of amines at high RH levels. Also note that the ratios of

379

C1HxN+ between high and low RH levels were higher than those of C2HxN+ and C3HxN+,

380

suggesting

381

nitrogen-containing compounds with different molecular weights. We also observed

382

enrichments of CHON+(m/z 43) and CH2ON+ (m/z 44) at high RH levels except

383

summer, likely indicating the formation of amide-related compounds (R-C(O)-NH2) in

384

aqueous-phase.

385

4. Limitations

that

aqueous-phase

processing

have

different

impacts

on

386

In this study, we have demonstrated the capability of HR-AMS in quantification

387

and characterization of ON in atmospheric aerosols. The highly time-resolved ON

388

combined with PMF analysis provides more insights into the sources and processes

389

of ON. However, limitations in quantification of ON with HR-AMS are still remained.

390

Firstly, HR-AMS only measures ON in submicron aerosols due to the transmission

391

efficiency of aerodynamic lens while a considerable fraction of ON often exists in

392

particles with diameter larger than 1 µm. For example, the average concentration of

393

WSON was 5.5 N mol m-3 in PM1.3-10, which is nearly 50% of that in PM1.3 at a remote 18


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394

marine location in the Eastern Mediterranean.65 Therefore, the HR-AMS equipped

395

with the newly developed aerodynamic lens that can transmit ~90% of PM2.5

396

should to be deployed further to characterize ON in PM1-2.5. Secondly, HR-AMS with

397

thermal vaporization (~600°C) only detect non-refractory ON, but may underestimate

398

ON by missing the fraction in refractory materials, e.g., dust.

78

399

Thirdly, the N/C ratio is a key factor affecting the accuracy in ON quantification,

400

which depends on the separation and quantification of N-containing ions (CxHyNp+,

401

CxHyOzNp+, NHx+, NOx+, and minor CxHyOzNpSq+). The current N/C ratio is overall

402

underestimated because: 1) the contribution of ON in NOx+ and NHx+ families (e.g.,

403

from fragmentation of organic nitrates and some amino compounds) cannot be

404

separated. For example, the NO+/NO2+ ratio in this study is close to that of

405

ammonium nitrate (=2.28), and quantification of ON in NOx+ and NHx+ is impossible;

406

2) AMS elemental analysis excludes the most important nitrogen-containing ion,

407

CH2N+ (m/z 28) due to the overwhelming interference of adjacent N2+ ion. Recent

408

HR-AMS offline analysis of filter samples with argon as carrier gas illustrated the

409

most prominent CH2N+ peak in CxHyNp+ families.79 It is estimated that the current N/C

410

ratio is underestimated by ~20% on average by ignoring the ON in CH2N+ at m/z 28,

411

and NHx+ and NOx+ families, and the underestimation can be up to ~42% if organic

412

nitrates are dominant in OA. 30

413

Fourthly, accurate determination of N/C ratio strongly depends on the mass

414

resolution (m/Δm) because of much lower signals of CxHyNp+ and CxHyOzNp+ than

415

CxHy+ and CxHyOz+. The V-mode of HR-AMS that has a typical mass resolution of 19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

416

~2000, which is difficult to separate and quantify most of nitrogen-containing ions

417

above m/z 50 (Figure S1). While the W-mode with nearly twice higher mass

418

resolution (~4000-5000) is better for separating nitrogen ion fragments, it also has

419

larger uncertainties in high resolution peak fitting due to much lower signals. Thus,

420

an optimized peak shape tuning before sample analysis is necessary and of great

421

importance for a better quantification of ON. The recently developed long

422

time-of-flight AMS with a much improved mass resolution (~8000) yet similar

423

sensitivity as that of V-mode of the HR-AMS will greatly help the future ON

424

characterization. Fifthly, uncertainties in elemental analysis of OA can also influence

425

the ON quantification. For example, the recently proposed I-A method produces ~9%

426

higher OM/OC ratio than that of A-A method,59 which would lead to a decrease in OC,

427

and then ON concentration according to equations (2) and (3).

428

ACKNOWLEDGEMENTS

429

This work was supported by the National Key Project of Basic Research

430

(2014CB447900), and the National Natural Science Foundation of China (41575120,

431

41571130034).

432

Competing financial interests: The authors declare no competing financial interests.

433

Supporting Information

434

1 Table (Table S1) and 13 Figures (Figures S1 – S13).

435 436

References

437 438 439

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27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

731

Table 1. A summary of average mass concentrations of ON (μg m-3) in different OA

732

factors and the total OA during four seasons.

733

LO-OOA MO-OOA BBOA CCOA HOA COA FOA Total OA

Spring Summer Autumn 2013 Winter 2014 Winter 0.066 0.057 0.100 0.074 0.056 0.190 0.110 0.230 0.100 0.130 0.013 0.035 0.098 0.250 0.030 0.053 0.040 0.040 0.120 0.077 0.092 0.047 0.100 0.023 0.063 0.067 0.43 0.26 0.59 0.43 0.57

734

28


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735

Figure Captions:

736

Figure 1. Mass spectra of ion categories of CxHyNp+ and CxHyOzNp+ in SOA, POA and

737

OA in (a) spring, (b) summer, (c) autumn, and winters of (d) 2013 and (e) 2014. The

738

pie charts show the average contributions of OA factors to ON during four seasons.

739

Figure 2. Diurnal profiles of N/C, mass percentages of ON to TN, mass concentration

740

of ON, and correlation coefficients between ON and POA and SOA in (a) spring, (b)

741

summer, (c) autumn, and winters of (d) 2013 and (e) 2014.

742

Figure 3. Variations of N/C as a function of oxygen-to-carbon ratios (O/C) in (a)spring,

743

(b)summer, (c) autumn, winters of (d)2013 and (e)2014. The variations of ON as a

744

function of photochemical age in the winter of 2014 are shown in (f). The data were

745

binned according to the O/C (photochemical age), and mean (cross), median

746

(horizontal line), 25th and 75th percentiles (lower and upper box), and 10th and 90th

747

percentiles (lower and upper whiskers) are shown for each bin.

748

Figure 4. Variations of ON and N/C as a function of RH, and the average mass spectra

749

of N-ion categories (CxHyNp+ and CxHyOzNp+) for submicron aerosols at high RH

750

(RH>60%) and low RH levels (RH60% CxHyNp CxHyOzNp N/C: 0.028

0.8

0 20 40 60 80 100 RH (%)

0 20 40 60 80 100 RH (%)

0.00 0 20 40 60 80 100 RH (%)

+

N/C: 0.027

N/C: 0.035

N/C: 0.032

N/C: 0.033

N/C: 0.025

N/C: 0.025

N/C: 0.024

0.6 0.4 0.2 0.0 1.0

% of Total Signal

0.04

1.2

N/C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.8

+

RH60%) and low RH levels (RH

Seasonal Characterization of Organic Nitrogen in Atmospheric

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