Introduction
Biogenic secondary organic aerosol (BSOA), generated by gas-phase oxidation
of biogenic volatile organic compounds (BVOCs, such as isoprene,
monoterpenes, and sesquiterpenes) emitted from vegetation, is one of the
major contributors to the global SOA budget (Hallquist et al., 2009). As a
result of the atmospheric importance of BSOA, many laboratory studies have
focused on determining the mass yields and chemical characteristics of BSOA
from oxidation of different gas-phase precursors (e.g. Chhabra et al.,
2010; Lee et al., 2006a, b). However, chamber SOA is usually less oxidized
than ambient SOA (e.g. Aiken et al., 2008; Ng et al., 2011), indicating
that the current understanding of BSOA formation remains incomplete.
Furthermore, the SOA yield parameters for atmospheric models and the degree
of oxygenation of chamber SOA largely depends on the experimental conditions,
such as organic aerosol mass loading and seed particle surface area (Ehn
et al., 2012; Shilling et al., 2009; Zhang et al., 2014).
Forested environments, such as the boreal forest and Amazon rainforest can be
considered giant chemical reactors for BSOA production (e.g. Chen et
al., 2009; Ehn et al., 2012; Kanakidou et al., 2005; Slowik et al., 2010).
Elevated temperature and/or solar radiation over forests can enhance their
BVOC emissions from the forest (Rinne et al., 2002; Leaitch et al., 2011);
meanwhile, BSOA can be produced efficiently due to active photochemistry
during the day. Despite the fact that many field studies have been performed
near or in forested areas to investigate BSOA formation, anthropogenic
influence was frequently observed during those studies, further complicating
the SOA formation chemistry (e.g. Han et al., 2014; Setyan et al., 2012).
In particular, it has been hypothesized that the interactions of BVOCs with
anthropogenic pollutants contributed to high aerosol loadings in the
southeastern US (Goldstein et al., 2009). Shilling et al. (2013)
observed that the enhancement of SOA formation from isoprene was
strongly related to the NOx concentrations in Sacramento, California.
Xu et al. (2015) recently reported that isoprene-derived SOA was directly
mediated by sulfate, and NOx was shown to enhance nighttime SOA
formation via nitrate radical oxidation of monoterpenes in the southeastern US.
In addition to the uncertainties of anthropogenic–biogenic interactions on
BSOA formation, there is growing evidence of uncaptured BSOA formation
chemistry in smog chamber experiments. For example, Ehn et al. (2014)
recently showed that the wall effects of smog chambers can result in a
substantial loss of highly oxygenated organic compounds with vapour pressures
orders of magnitude lower than previously identified gas-phase oxidation
products of BVOCs (i.e. referred to as extremely low-volatility organic
compounds, ELVOCs). Furthermore, Zhang et al. (2014) illustrated that the
formation yields of toluene-derived SOA depend on the seed-to-chamber
surface area ratio predominantly due to the loss of SOA-forming vapour to
the chamber wall. The observed yield suppression likely extends to BSOA
formation systems. Müller et al. (2012) reported that cis-pinonic acid,
one of the first-generation products from α-pinene ozonolysis,
experienced a significant wall loss in smog chambers. Such wall losses have
not been adequately taken into consideration when aerosols yields were
reported from earlier chamber studies. As a result, conducting a field
measurement in a forested environment without significant influence of
anthropogenic emissions and without the constraints of smog chamber walls is
an attractive approach that can be used to re-evaluate our current knowledge
of BSOA formation.
In this study, we deployed an Aerodyne high-resolution time-of-flight
aerosol mass spectrometer (HR-ToF-AMS) to characterize ambient aerosol in a
coniferous forest mountain region in Whistler, British Columbia from 15 June
to 28 July 2010. Simultaneous measurements of gas-phase VOCs were made with
a high-resolution proton transfer reaction mass spectrometer (PTR-ToF-MS).
The measurement was part of the Whistler Aerosol and Cloud Study (WACS) 2010
campaign. The study duration covered a period when the forested region in
Whistler experienced persistently high levels of nearly pure BSOA up to
about 5 µg m3, providing a unique opportunity to investigate BSOA
formation in a coniferous forest. This amount of SOA formation is
considerably larger than that observed in pure isoprene-emission-dominated
forests, such as in the Amazon (Martin et al., 2010). Rather, it matches the
very high levels of BSOA observed in the summertime in another northern
location in central Canada (Slowik et al., 2010). Positive matrix
factorization (PMF) analysis was performed to understand the types of
organic aerosol that contributed to the total organic aerosol mass during
this time. The relative importance of different oxidation chemistry (i.e.
ozonolysis, OH radical and nitrate radical oxidation) on BSOA formation
between day and night are evaluated. Compared to previous laboratory
studies, the mass spectral characteristics of the BSOA factors identified in
the current study provide insight into the BSOA formation mechanisms.
Experiment
Sampling location and period
The Whistler Aerosol and Cloud Study (WACS) was a large-scale field campaign
conducted in Whistler, British Columbia from 15 June to 28 July 2010. One
of the two sampling sites on Whistler Mountain (Raven's Nest, see Fig. S1 in the Supplement)
sits within a coniferous forest mountain area at an elevation of 1320 m a.s.l.
In this paper, we focus on the observations at Raven's Nest from 1 to 19 July 2010,
covering a large BSOA event that lasted approximately 5 days (6–10 July)
and contrast them with those from a period with mixed biogenic and
regional background influence (13–18 July) (Figs. 1 and S2, and see
Sect. 3 for discussion). Key measurements used in this analysis are
briefly described in the following sections.
Aerosol measurements
An Aerodyne high-resolution time-of-flight aerosol mass spectrometer
(HR-ToF-AMS, Aerodyne Research Inc.) and scanning mobility particle sizer
(SMPS; TSI Inc., model 3936L75) were deployed to measure real-time,
nonrefractory particulate matter (NR-PM, i.e. ammonium, nitrate, sulfate, and
organic) and particle number size distributions (16–685 nm), respectively.
The ambient air was drawn through a stainless steel sampling line by the
HR-ToF-AMS, SMPS and other collocated particle instruments. The working
principle of the HR-ToF-AMS has been reported in detail previously (DeCarlo
et al., 2006). In brief, an aerodynamically focused particle beam impacts a
tungsten vaporizer that was maintained at ∼ 600 ∘C to vaporize
the NR-PM, and the resulting vapours are ionized with electron impact
ionization. The ions are then detected by a high-resolution time-of-flight
mass spectrometer, which was operated in V mode (5 min, mass resolving power
∼ 2000 with a higher sensitivity) and W mode (5 min, mass resolving
power ∼ 4000 with a lower sensitivity) alternatively. Ionization
efficiency (IE) calibrations were performed onsite using monodisperse
ammonium nitrate particles. The default relative IE (RIE) of nitrate,
sulfate, ammonium, chloride, and organic were used.
Time series (PST) profiles of (a) temperature and relative
humidity, (b) ozone, (c) wind speed and direction, (d) monoterpene and
isoprene measured by PTR-ToF-MS, (e) benzene toluene and acetonitrile
measured by PTR-ToF-MS, (f) organic, nitrate, sulfate, ammonium, and
chloride measured by HR-ToF-AMS, (g) background OA, BSOA-1, and BSOA-2
determined by PMF analysis, (h) particle number size distribution measured
SMPS. The gray and red dashed lines represent the boundaries of Period 1
(6–10 July) and Period 2 (13–18 July), respectively. Figure S2 enlarges the
time series during (a) 6–10 July and (b) 13–18 July.
The data were processed using the AMS data analysis software (Squirrel,
version 1.51H for unit mass resolution (UMR) data and Pika, version 1.10H for
high-resolution peak fitting,
http://cires.colorado.edu/jimenez-group/ToFAMSResources/ToFSoftware/index.html)
with the corrected air fragment column of the standard fragmentation table
(Allan et al., 2004; DeCarlo et al., 2006). The data reported here are
obtained from the high-resolution mass spectral fitting of W-mode data.
Elemental analysis (oxygen- and hydrogen-to-carbon ratios, O : C and
H : C) is based on the improved method proposed by Canagaratna et
al. (2015). Given that most HR-ToF-AMS studies reported O : C and H : C
values using Aiken's method (Aiken et al., 2008), comparison between the
Aiken's and improved method is shown in Fig. S5. Positive matrix
factorization (PMF) was performed to analyse the high-resolution AMS organic
measurement for identification of the forms of organic aerosol in this study.
The bilinear model was solved using the PMF2 algorithm in robust mode
(Paatero and Tapper, 1994) and the final solution was selected using the PMF
Evaluation Tool (PET) version 2.06 (Ulbrich et al., 2009). A three-factor
solution was selected as the optimum solution based on examination of the PMF
quality of fit parameter (Q/Qexpected) as a function of the number
of PMF factors up to seven factors (Fig. S9). Further increasing the number
of factors from three to four factors only splits a factor representing
background organic aerosol, and thus more than three factors were not
considered in this study. Time series and mass spectra of two-, three-, and
four-factor PMF solutions are shown in Figs. S10–S12, respectively.
VOC measurements
Measurements of volatile organic compounds (VOCs) were performed using a
high-resolution proton transfer time-of-flight mass spectrometer (PTR-ToF-MS,
Ionicon Analytik GmbH). The ambient air was drawn through a 5 m long
Perfluoroalkoxy (PFA) sampling line by a diaphragm pump at a rate of 5 slpm.
A 200 sccm sample was drawn into the instrument through a heated inlet from
the sampling line. Five-minute backgrounds were collected hourly through a
platinum wool catalyst heated to 350 ∘C. The time resolution of
measurement was 1 min. The E/N (where E is electric field strength and
N is buffer gas density) in the drift tube is kept at about
135 Townsend (Td). The raw PTR-ToF-MS data were acquired by the TofDaq
software (Tofwerk AG, Switzerland) and post processed by PTR-MS ToF Viewer
(Ionicon Analytik GmbH). Operations of proton transfer reaction mass
spectrometer (PTR-MS) with a quadrupole mass spectrometer have been presented
in great detail (Blake et al., 2009; de Gouw and Warneke, 2007). Briefly, a
PTR-MS is a soft ionization technique that allows for detection of VOCs that
have a greater proton affinity than water. While the ionization process is
the same in the PTR-ToF-MS as the PTR-MS, the high-resolution time-of-flight
mass spectrometer used in the PTR-ToF-MS captures the entire mass spectrum
from 12 to a high mass number (usually about 400) at high time resolutions
(e.g. 1 s). The high mass resolution of the mass detector
(∼ 4000–5000) allows isobars to be resolved. A list of
mass-to-charge (m/z) ratios of some selected VOCs is shown in
Table S1 in the Supplement.
Periodic calibrations of the PTR-ToF-MS were performed onsite for
α-pinene, isoprene, methanol, acetone, methyl vinyl ketone (MVK), and
2-methyl-3-buten-2-ol (MBO). Post-campaign calibration of formic acid was
obtained utilizing heated permeation tubes. The sensitivity of formic acid
was then ratioed to the acetone sensitivity measured both during and after
the campaign, and the ratio together with the acetone field calibration was
used to retrieve formic acid from the field measurements. The sensitivities
and limits of detection (LOD) of calibrated VOCs are shown in Table S1.
Three 1 h integrated VOCs samples were also collected each day in 3 L
stainless steel canisters. Analysis for both polar and nonpolar compounds
was carried out offsite by gas chromatography. It should be noted that the
PTR-ToF-MS isoprene signal at m/z 69.070 can be influenced significantly by
MBO fragments so that a correction factor based on the linear correlation
between the isoprene concentrations determined by PTR-ToF-MS and canister
samples (i.e. canister isoprene = 0.7 ⋅ PTR-ToF-MS isoprene,
R2 = 0.79) was used to correct the real-time isoprene concentration measured by
the PTR-ToF-MS for the MBO interference.
Ozone and OH radical measurements
Ozone was measured by a UV absorption monitor (Model TECO 49C, Thermo
Environmental Instruments Inc.). During the last 10 days of the campaign
(19–28 July), hydroxyl (OH) radicals were measured using the
well-established technique of chemical ionization mass spectrometry (OH-CIMS)
(Tanner et al., 1997). The measurement was used to estimate the
diurnal pattern of OH radical concentration from 13 to 18 July for
evaluating BSOA formation chemistry. The instrument has been previously
described (Berresheim et al., 2000; Sjostedt et al., 2007; Tanner et
al., 1997). Briefly, hydroxyl radicals are measured by titration with
isotopically labelled 34SO2 to produce H34SO4. The
isotopically labelled sulfuric acid molecules were then ionized by charge
transfer with nitrate ions (NO3-), produced by passing a flow of
nitrogen containing HNO3 through a 210Po ion source. In order to
minimize wall losses of OH radicals, a flow of 2400 slpm is drawn through an
inlet with 7.6 cm inner diameter and about ∼ 0.6 m in length. The instrument
background was determined periodically by adding hexafluoropropene (C3F6)
through the front injectors to scavenge the ambient OH
radicals. Calibrations were performed in situ by photolysing water vapour
with a mercury pen ray lamp in the sampling inlet. The detection limit for a
5 min integration of OH radical was 5 × 104 molecules cm-3.
Results and discussion
Figure 1 shows the time series of various measurements from 1 to 19 July,
which can be divided into three periods based on the meteorological
conditions previously reported (Ahlm et al., 2013; Pierce et al., 2012).
The first period (1–5 July) was humid and cloudy with low temperature. A
relatively high concentration of secondary sulfate indicates that this
period was strongly influenced by aged/transported air masses. The second
period (6–10 July) started with increasing ambient temperature and was
accompanied by rapidly increasing levels of biogenic volatile organic
compounds (BVOCs, e.g. monoterpenes and isoprene) which were followed by
increasing total organic aerosol mass (Fig. 1a, d, and f). This period is
referred to as the “biogenic period (or Period 1)” hereafter and will be the
focus of Sect. 3.1 and 3.2. The aerosol sulfate concentration was low
during the biogenic period. The third period (13–19 July) was cooler than
the biogenic period. Although the diurnal cycles of different measurements
indicate that the third period had both biogenic and regional background
influence (referred to as Period 2 hereafter), the observations provide
insights into the pathways of biogenic secondary organic aerosol (BSOA)
formation during the biogenic period (Sect. 3.3). The NOx (average ± SD (standard deviation) = 0.7 ± 0.9 ppbv), benzene
(0.02 ± 0.01 ppbv), toluene (0.11 ± 0.16 ppbv), and acetonitrile
(0.08 ± 0.04 ppbv) mixing ratios were generally low within 1–19 July, indicating
limited influence by local anthropogenic emissions and biomass burning. The
significance of nighttime nitrate radical chemistry will be discussed in
Sect. 3.4. Lastly, a comparison between previous laboratory studies and
these field observations will be discussed in Sect. 3.5.
Biogenic period
Enhancement of BVOC (monoterpenes and isoprene) emissions from the forest
due to high temperature and/or solar radiation was observed throughout the
campaign (Fig. 1a and d). Specifically, significant positive correlations
of daily average temperature and monoterpenes (R2 = 0.91) and
isoprene (R2 = 0.84) mixing ratios were obtained (Fig. S3). The
forested area in Whistler is dominated by conifers. Therefore, monoterpenes,
rather than isoprene, are the dominant BVOCs emitted into air. For example,
for the biogenic period, monoterpenes observed at the site were about a
factor of 3 higher than isoprene. Considering the low SOA formation
yield from gas-phase isoprene photo-oxidation chemistry determined in smog
chamber experiments, monoterpenes likely play a more critical role than
isoprene in BSOA formation in Whistler (Lee et al., 2006a, b). Note that
sesquiterpenes could not be determined by the PTR-ToF-MS; thus their
contributions to BSOA formation cannot be evaluated.
The uniform air mass within the biogenic period led to the accumulation of
organic aerosol (Fig. 1f) (Pierce et al., 2012). The organic aerosols
formed during the pristine event were almost entirely biogenic in origin as
determined by the FTIR analysis of filter samples during this period (Ahlm
et al., 2013). Inorganic constituents generally accounted for less than
5 % of the total NR-PM by mass. Even though the sulfate concentration was
slightly enhanced during the strong new particle formation events on 5 and 6 July
(i.e. particle number rapidly increased in the 20–30 nm size range),
the particle growth to diameters larger than 100 nm was primarily due to
condensation of BSOA materials on the nucleation mode particles (Fig. 1f
and h) (Pierce et al., 2012). There are strong correlations among
methanol, acetone, and total OA mass throughout the study (Figs. 2 and S4),
suggesting that they are likely from a similar origin. The
methanol-to-acetone ratio determined in this study is 3.61 (R2 = 0.94)
as shown in Fig. 2. The correlation between total organic aerosol
mass and formic acid (R2 = 0.77) is also presented in Fig. S4. Note
that methanol sources can be dominated by terrestrial vegetation in forest
areas. Overall, the biogenic episode provides a unique opportunity to
investigate the properties and formation mechanisms of BSOA in a forested
area dominated by terpene emissions.
Correlation of methanol and acetone measured by PTR-ToF-MS. The
colour scales represent the total organic mass measured by HR-ToF-AMS.
Chemical characteristics of BSOA
Positive matrix factorization (PMF) was performed to understand the chemical
characteristics of BSOA. The PMF analysis separates the total organic aerosol
into three factors: background organic aerosol (background OA), biogenic
SOA-1 (BSOA-1), and biogenic SOA-2 (BSOA-2). The organic speciated mass
spectra and time series of these factors are shown in Figs. 3a–c and 1g,
respectively. The background OA represents aged organic aerosol with an
intense signal of CO2+ fragments (i.e. a tracer of organic acid) and
a high degree of oxygenation (O : C = 0.87), and largely correlates
with secondary sulfate (see Sect. 3.3). The BSOA-1 and BSOA-2 factors
represent two different types of fresh BSOA based on their mass spectral
features and temporal profiles (see Sect. 3.3). Elemental analysis shows that
the degree of oxygenation of BSOA-2 (O : C = 0.58) and BSOA-1
(O : C = 0.56) is similar even though they have very different
m/z 44-to-m/z 43 ratios (a parameter for evaluating the
degree of aging of oxygenated OA (OOA) from unit mass resolution AMS spectra)
(Ng et al., 2010). This observation can be due to the fact that
CH2O+ and C2H3O+ fragments are the primary
contributors of organic m/z 30 and 43, respectively, for both
BSOAs. Note that the isoprene SOA signature (i.e. a high mass fraction of
m/z 82 (C5H6O+) to total organic mass, f82) was
not observed in the mass spectrum of BSOA-1 and BSOA-2 (Robinson et al.,
2011; Slowik et al., 2012; Hu et al., 2015). The f82 values of
background OA, BSOA-1, and BSOA-2 are 0.003, 0.007, and 0.006, respectively,
which match previous AMS measurements from studies strongly influenced by
monoterpene emissions (Hu et al., 2015).
Normalized unit mass resolution mass spectra of PMF factors
(a) background OA, (b) BSOA-1, and (c) BSOA-2. (d) Van Krevelen diagram: orange
and gray dots represent observations from the regional biogenic period (6–10 July)
and the whole study period, respectively. The cross symbols represent
the O : C and H : C ratios of α-pinene and its major oxidation products
(cis-pinonic acid, pinic acid, 3-methyl-1,2,3-butanetricarboxylic acid – MBTCA).
Comparison between the Aiken's and improved method for elemental composition analysis
is shown in Fig. S5.
The van Krevelen diagram in Fig. 3d shows that the ambient data (orange dots
for the biogenic period only) can be a linear combination of the three PMF
factors. Comparison between the Aiken's and improved method for elemental
composition characterization is shown in Fig. S5. The linear fit of ambient
data from the whole campaign (slope = -0.35 and
y intercept = 1.67) is shown in Fig. 3d. The y intercept is close to
the theoretical H : C of most BVOCs such as isoprene (C5H8),
α-pinene (C10H16), limonene (C10H16), and
β-caryophyllene (C15H24). Furthermore, some major products of
α-pinene ozonolysis including cis-pinonic acid, pinic acid,
3-methyl-1,2,3-butanetricarboxylic acid (MBTCA) fall along the linear fit of
the ambient data, consistent with the VOC measurements that terpenes are
likely one of the major BSOA precursors in the Whistler forest (see
Sect. 3.1). Cis-pinonic and pinic acids are α-pinene first-generation
oxidation products with the average saturation vapour
concentrations (C*) categorized as intermediate VOCs (IVOCs,
C* = 103–106 µg m3) and semi-VOCs (SVOCs,
C* = 102–100 µg m3), respectively
(Donahue et al., 2012). MBTCA is a low-volatility
(C* = 10-1–10-3 µg m3), later-generation
product of α-pinene SOA and a tracer of terpene SOA. It is important
to note that the observed growth for small particles (< 50 nm) at the
early stage of the biogenic period in Whistler must have an average C*
less than 10-2 µg m3 based on the modelling results from
Pierce et al. (2012). This implies that at least small amounts of organic
materials with volatility much lower than BSOA-1 and 2 (e.g. ELVOC observed
by Ehn et al., 2014) were required to permit the observed initial growth.
Understanding BSOA formation from its diurnal character
At the beginning of the biogenic period when there was significant
nucleation, BSOA-1 was more prevalent than BSOA-2. BSOA-2, however,
sustained the particle growth at the elevated temperature after the new
particle formation event on 7 July, likely because of its low volatility.
BSOA-1 started to decline from 8 July and the total organic mass was
dominated by BSOA-2 during the rest of the biogenic period. The temporal
profile of the BSOA-1-to-BSOA-2 ratio is shown in Fig. S6 to illustrate
the relative contribution of these BSOA factors within the biogenic period.
These observations imply different formation chemistry of the two BSOA
materials within the biogenic period (see later discussion).
The clear diurnal patterns of both gas- and particle-phase species within
the period with a mix of biogenic and regional background influence (Period 2)
permit evaluation of both the influence of photochemistry and
meteorological conditions on the formation of BSOA-1 and BSOA-2. Figure 4b
illustrates the morning increase in OH radical concentrations and ozone
mixing ratios due to increases in solar radiation (i.e. indicated by the
elevated ambient temperature), suggesting more active photo-oxidative
chemistry during the daytime and less deposition in the case of ozone. The
reaction rate constants of monoterpenes with ozone and OH radical are on the
order of 10-15–10-17 and 10-10–10-11 cm3 molecule-1 s-1,
respectively, and both oxidation processes
give comparable SOA formation yields for monoterpenes (Lee et al.,
2006a, b). Given that the concentrations of ozone and OH radical during the
daytime are on the order of 1011 and 105–106 molecules cm-3,
respectively (Fig. 4b), the overall contributions of
ozonolysis and OH oxidation of monoterpenes to daytime BSOA formation in
Whistler can be comparable to each other, even though ozonolysis may play a
larger role than OH oxidation in late afternoon and nighttime.
Diurnal cycles of (a) temperature, OH radical and ozone (b)
monoterpene (MT), isoprene, methyl vinyl ketone (MVK)/methacrolein (MACR),
and acetone, (c) background OA, BSOA-1, BSOA-2 and SO4, and
(d) elemental compositions of total organics (O : C and H : C) observed during
13–18 July (Period 2). Note that the OH radical concentration is the average
value measured after 19 July.
Monoterpene mixing ratios were highest during the night and then dropped
rapidly in the morning (Fig. 4c), which can be a combined effect of
chemistry and meteorological condition. Firstly, monoterpene emissions
generally increase with ambient temperature. This observation indicates that
monoterpenes were rapidly oxidized once the concentrations of ozone and OH
radicals built up during the daytime. Secondly, the increased mixing depth
in the morning and alternation of mountain flows from downslope (∼ 150∘)
to upslope (∼ 250∘) wind at
around 07:00–09:00 LT (local time, UTC - 8 h, Fig. 4a) might contribute
to this observation if the surface layer had relatively low monoterpene
mixing ratios. However, additional BVOC measurement in the valley is
required to confirm this argument. Furthermore, monoterpene mixing ratios
gradually increased in the afternoon likely in response to the decreasing
levels of atmospheric oxidants rather than the transition of upslope to
downslope wind in the late afternoon. It is worth noting that a
relatively strong downslope wind was observed when BSOA and BVOCs
accumulated in Whistler during the early period of the biogenic episode (Fig. 1c).
Other BVOCs and their oxidation products such as acetone, isoprene and
probably MVK/methacrolein (MACR; PTR-ToF-MS cannot separate MVK and MACR,
and other isoprene oxidation products may contribute to this mass-to-charge
ratio signal) increased in connection with ozone, providing further evidence
of active photochemistry during the day. The similar diurnal cycles of these
VOCs were also clearly observed during the later period of the biogenic
episode (i.e. 9–10 July, Fig. 1). Different diurnal cycles of isoprene and
monoterpene mixing ratios were observed previously in forests (Harrison et
al., 2001). Note that isoprene emission flux is sensitive to both solar
radiation and temperature (Rinne et al., 2002).
The diurnal variations of background OA and secondary sulfate that was
nearly neutralized by ammonium (Fig. S13) are almost identical, and their
concentrations peak at around 17:00 LT (Fig. 4d). Ozone level also peaks
at about the same time. These observations indicate increasing influences of
the regional background air mass in Whistler in the late afternoon. Upslope
wind likely carried the aged regional aerosol particles to the sampling site
and their mass loadings decreased when the transition of upslope to
downslope wind occurred. The background OA can be successfully separated
from the BSOA components using the PMF analysis. Comparing the total OA, the
BSOA components, and the background OA, it is clear that the total organic
mass within the biogenic period was not strongly influenced by anthropogenic
sources and/or transported air masses (Fig. 1g).
The BSOA-2 diurnal profile matches the variation of ozone plus OH radical
levels during the daytime, suggesting that oxidation of monoterpenes and its
oxidation products is likely one of the major BSOA-2 formation pathways.
BSOA-1 has a diurnal pattern clearly distinct from BSOA-2, with a nighttime
peak. BSOA-1 is likely more volatile than BSOA-2 based on their relative
fCO2+ values (Huffman et al., 2009). The low temperature at night
may favour partitioning of BSOA-1 materials to the particle phase, resulting
in a higher concentration of BSOA-1 at night. In addition to the
gas–particle partitioning of BSOA-1 materials, BSOA-1 formation via
ozonolysis of monoterpenes at night is possible because of the large
abundance of monoterpenes and presence of ozone. Nocturnal nitrate radical
chemistry can also contribute to BSOA-1 production significantly (see
Sect. 3.4). Lastly, BSOA-2 was composed of a higher fraction of organic
acids (i.e. fCO2+) compared to BSOA-1. Additional information is
required to evaluate the possibility of heterogeneous conversion of BSOA-1
to BSOA-2 during the daytime (see Sect. 3.5).
Nitrate radical chemistry at night
Nitrate radical (NO3⚫) is a product of ozone and NOx
and is a well-known nocturnal oxidant. Xu et al. (2015) recently reported
that NO3⚫ chemistry could play a key role in producing BSOA
at night in the southeastern US. The average ozone and NOx mixing
ratios at night during WACS 2010 were about 20–30 ppbv (Figs. 1b and 4)
and 0.34 ppbv (Fig. 5a) throughout the entire study, respectively, which
are comparable to the levels reported by Xu et al. (2015) (i.e.
O3 = 21 ppbv and NOx = 0.54 ppbv). Laboratory studies have shown that
organic nitrates can be produced from the reaction between
NO3⚫ and BVOCs (Ng et al., 2008; Fry et al., 2009).
Fragmentation of organic nitrates produces NO+ and NO2+
signals in AMS measurement with the NO+/NO2+ ratio much
higher than that of inorganic nitrate (e.g. Farmer et al., 2010). The
NO+ / NO2+ ratio of ammonium nitrate determined in our
calibration was approximately 3.38. In contrast, the average
NO+ / NO2+ ratio observed in the AMS measurement was 11.4 and
it was relatively constant throughout the whole sampling period (Fig. 5a).
The diurnal cycle of ambient NO+ / NO2+ ratio observed in
Period 2 is shown in Fig. 5c. The high values and small diurnal variations
of NO+ / NO2+ ratio indicate low levels of inorganic nitrate
and the presence of organic nitrates in the observed BSOA (e.g. Farmer et
al., 2010; Xu et al., 2015).
No diurnal cycles of NO+ and NO2+ AMS fragments were observed
(Fig. S7) to confirm significant NO3⚫ chemistry at night,
possibly due to the upslope mixing of anthropogenic/aged aerosol particles
mentioned in the previous sections. There was some covariance in the
temporal variations of sulfate and nitrate (Fig. S8). To evaluate the
possibility of nighttime NO3⚫ chemistry in this study, the
AMS nitrate mass is normalized by sulfate mass to eliminate the potential
effects of upslope mixing on nitrate aerosol concentrations. The
NO3- / SO42- ratio correlates extremely well with the
BSOA-1 (R2 = 0.71) for the whole sampling period (Fig. 5b).
Furthermore, the diurnal pattern of this ratio during Period 2 (13–18 July)
was almost identical to that of BSOA-1 (Fig. 5d), indicating the formation
of organic nitrates at night. The formation of organic nitrates during the
biogenic episode under relatively high temperature and low RH conditions
further suggests that the diurnal cycle of the
NO3- / SO42- ratio cannot be fully explained by
gas–particle partitioning of nitric acid which would occur preferentially at
lower temperatures.
Time series (PST) profiles of (a) NOx mixing ratio and
NO+ / NO2+ ratio measured by HR-ToF-AMS and
(b) NO3- / SO42- ratio and mass loading of BSOA-1. Diurnal
cycles of (c) NOx mixing ratio and NO+ / NO2+ ratio measured by
HR-ToF-AMS and (d) NO3- / SO42- ratio and mass loadings of
BSOA-1 and organic nitrates in Period 2.
Assuming the NO+ / SO42- and
NO2+ / SO42- mass ratios were constant in the air
mass carried by upslope winds and are approximately equal to 0.088 and 0.009,
respectively (i.e. the minimum values observed during the upslope wind
condition), the NO+ and NO2+ mass originating from the
upslope wind air masses can be subtracted from their total mass loadings.
After that, the mass loading of nitrate functional groups (-ONO2) in
organic compounds and thus the organic nitrate mass contributing to the
observed BSOA-1 mass can be estimated using the calculation approach used by
Xu et al. (2015) (see details in the Supplement) and assuming the
subtracted NO+ and NO2+ signal originated solely from organic
nitrates and the molecular weight of the organic nitrates to be 200–300 g mol-1.
The organic nitrate mass accounted for 22–33 % of BSOA-1 at night,
representing the upper limit, and its diurnal cycle (ranging from 0.05 to
0.2 µg m-3) is shown in Fig. 5d. Results of these calculations
suggest that organic nitrates can be a significant contributor to BSOA mass
produced via nighttime NO3⚫ chemistry. Similarly, Xu et al. (2015)
estimated that the organic nitrates accounted for 20–30 % of less-oxidized
OOA (LO-OOA) mass observed during the nighttime.
Comparison to previous laboratory studies
Potential formation pathways of BSOA can be evaluated through examination of
specific mass fragments. Organic fragments at m/z 91 have been observed in
laboratory-generated and ambient BSOA, and the BSOA-1 and BSOA-2 factors
have a distinct peak at m/z 91 mostly due to the presence of the
C7H7+ fragment (Fig. 3). Although major contributors of
C7H7+ observed in this study are not well characterized, it
is well known that formation of tropylium ion (C7H7+) by
electron impact ionization of benzyl compounds can attribute to this organic
fragment (McLafferty and Turecek, 1993). While combustion processes are
major sources of aromatic compounds, Gratien et al. (2011) observed
formation of p-cymene via oxidation of α-pinene in laboratory
experiments, suggesting possible biogenic origins of aromatic compounds.
Other terpenes perhaps undergo similar chemistry but their atmospheric
significance remains unclear.
Even though the mass fraction of m/z 91 to total organic (f91)
is not a unique tracer for BSOA (Ng et al., 2011), f91 can be
potentially used to evaluate formation pathways of BSOA in this unpolluted,
biogenic-rich environment (e.g. evaluate relative importance of BSOA
formation pathways and precursors). The f91 values of both raw data and
PMF results are shown in Fig. 6 along with f91 values from previously
published laboratory and field studies. Recent inter-comparison of Aerodyne
aerosol chemical speciation monitors (ACMS) highlighted a significant
variability of mass fraction of specific m/z to total organic
between instruments (Fröhlich et al., 2015), and hence the comparisons of
f91 values between studies are only semiquantitative. Nevertheless, Chen
et al. (2015) reported that the f91 value of α-pinene SOA
generated in smog chamber experiments at low NOx conditions is much
lower than β-caryophyllene SOA generated in the same study. Previous
plant chamber experiments (i.e. oxidation of a mixture of BVOCs) further
suggest that the value of f91 likely depends on the relative
contribution of sesquiterpenes to total BVOCs; the average value of f91
of SOA generated by the oxidation of different plant emissions is reported
here (Kiendler-Scharr et al., 2009). The above observations indicate that
sesquiterpenes may play an important role to the production of
m/z 91 (or C7H7+ fragments) observed in Whistler and
other forest region such as a boreal forest in Finland (Finessi et al.,
2012).
The mass fraction of m/z 91 (f91) observed in this study (overall
OA, regional biogenic period, background OA, BSOA-1, and BSOA-2),
α-pinene and β-caryophyllene SOA generated by chamber experiments
(Chen et al., 2015), pinonaldehyde uptake on sulfuric acid seeds (Liggio
and Li, 2006), plant chamber SOA (Kiendler-Scharr et al., 2009), cis-pinonic
acid (Lee et al., 2012), and BSOA observed in boreal forest in Finland
(Finessi et al., 2012).
Laboratory studies have also shown that some of the first-generation
products of α-pinene ozonolysis can have a relatively high level
of f91 in their AMS spectra, including cis-pinonic acid droplets generated by
atomization (Lee et al., 2012) and pinonaldehyde uptake to sulfuric acid
particles (Liggio and Li, 2006). Although cis-pinonic acid and
pinonaldehyde are rather volatile, they may experience significant wall loss
in smog chambers during the SOA formation experiments previously reported
(Müller et al., 2012; Zhang et al., 2015). The wall effect can suppress
subsequent gas-phase reactions of intermediate products that may produce
less volatile, SOA-forming organic acids with the same f91 signature. Since
the surface characteristics of the land and vegetation in the remote
forested region can be very different to the wall surface of smog chambers,
α-pinene is possibly an important precursor of C7H7+
fragments in the real atmosphere.
The difference in the f91 values between BSOA-1 and BSOA-2 can be further
used to evaluate the relationship between these two PMF factors. In the same
campaign in Whistler, Slowik et al. (2012) performed a set of in situ
heterogeneous OH oxidation experiments using a flow tube reactor to age BSOA
sampled during the biogenic period. They clearly observed that the f91 value
declined with heterogeneous oxidation and the mass spectra of reaction
products determined by PMF analysis did not contain a high value of f91.
Because BSOA-2 has a higher level of f91 than BSOA-1 (Fig. 6), we conclude
that there is little heterogeneous oxidative conversion of BSOA-1 to BSOA-2.
Conclusions and atmospheric implications
The strong biogenic episode observed in this study provides a unique
opportunity to improve current understanding of BSOA formation chemistry in
a coniferous forest. The high levels of SOA formation are comparable to
those previously observed in the summertime in another Canadian forested
location also dominated by terpene emissions (Slowik et al., 2010). Given
the considerable emphasis placed on isoprene SOA in the past few years and
given the high SOA levels observed in this study, there is merit to
addressing the formation of SOA in these coniferous forested regions.
Indeed, with pronounced high-latitude warming it is very important to better
understand SOA formation in such regions to know how they will respond to
changing climate conditions (Leaitch et al., 2011).
The BSOA observed during the biogenic episode was primarily due to
ozonolysis and OH oxidation of BVOCs (i.e. monoterpenes and perhaps
sesquiterpenes) during the day. We also provide evidence that nitrate
radical chemistry with BVOCs at night can be significant. PMF analysis
identified two types of BSOA, namely BSOA-1 and BSOA-2, and separated them
from the background organic aerosol. BSOA-1 represents gas-phase ozonolysis
and nitrate radical oxidation products, and is likely semivolatile in
nature, resulting in higher concentrations at low ambient temperature during
the night. BSOA-2 has a much stronger CO2+ signal than
BSOA-1, and consists of products from gas-phase oxidation by OH radical and
ozone during the day. Hence, the temporal variations of BSOA-1 and BSOA-2
observed here are due to their gas–particle partitioning in response to
ambient temperature, the relative importance of different oxidation
chemistry between day and night, and the gradual oxidation of early-generation
gas-phase oxidation products. The calculation in Sect. 3.4
suggests that BSOA-1 can be largely contributed by organic nitrates at night
(22–33 % by mass) due to nitrate radical chemistry.
This study evaluates the values of f91 obtained from the AMS measurements as
a tracer to investigate the BSOA-2 potential formation pathways. We
demonstrated that heterogeneous oxidation of BSOA-1 is expected to be a
minor production pathway of BSOA-2. Nevertheless, we cannot rule out the
possibility that some BSOA-1 material repartitions to the gas phase due to
elevated temperature during the daytime and then undergoes gas-phase
oxidation to produce BSOA-2 materials. This may partly explain the decay of
BSOA-1 and a higher level of BSOA-2 observed in the later period of the biogenic
episode. Although sesquiterpenes were not measured in this study, they can
be potentially important to generate BSOA with distinct peak at m/z 91 (or
C7H7+ fragments) based on previous laboratory observations.
Moreover, recently chamber experiments suggested that high signals at
C7H7+ in ambient aerosol mass spectrum can be potentially
used to indicate the presence of SOA products generated from β-pinene
and nitrate radical reactions (Boyd et al., 2015).