Particle nucleation is one of the main sources of atmospheric particulate
matter by number, with new particles having great relevance for human health
and climate. Highly oxidized multifunctional organic molecules (HOMs) have
been recently identified as key constituents in the growth and, sometimes,
in initial formation of new particles. While there have been many studies of
HOMs in atmospheric chambers, flow tubes, and clean environments, analyses of
data from polluted environments are scarce. Here, measurements of HOMs and
particle size distributions down to small molecular clusters are presented
alongside volatile organic compounds (VOCs) and trace-gas data from a
campaign in June 2017, in Beijing. Many gas-phase HOMs have been
characterized and their temporal trends and behaviours analysed in the
context of new particle formation. The HOMs identified have a
degree of oxidation comparable to that seen in other, cleaner, environments, likely
due to an interplay between the higher temperatures facilitating rapid
hydrogen abstractions and the higher concentrations of
Atmospheric particle nucleation, or the formation of solid or liquid particles from vapour-phase precursors, is one of the dominant sources of global aerosol by number, with primary emissions typically dominating the mass loadings (Tomasi et al., 2017). New particle formation (NPF) or the secondary formation of fresh particles is a two-step process comprising initial homogeneous nucleation of thermodynamically stable clusters and their subsequent growth. The rate of growth needs be fast enough to outcompete the loss of these particles by coagulation and condensation processes in order for the new particles to grow, and hence NPF is a function of the competition between source and sink (Gong et al., 2010). New particle formation has been shown to occur across a wide range of environments (Kulmala et al., 2005). The high particle load in urban environments was thought to suppress new particle formation until measurements in the early 2000s (McMurry et al., 2000; Shi et al., 2001; Alam et al., 2003), with frequent occurrences observed even in the most polluted urban centres. NPF events in Beijing occur on about 40 % of days annually, with the highest rates in the spring (Wu et al., 2007, 2008; Wang et al., 2016). Chu et al. (2019) review many studies of NPF which have taken place in China and highlight the need for long-term observations and mechanistic studies.
NPF can lead to the production of cloud condensation nuclei (CCN) (Wiedensohler et al., 2009; Yu and Luo, 2009; Yue et al., 2011; Kerminen et al., 2012), which influences the radiative atmospheric forcing (Penner et al., 2011). A high particle count, such as that caused by nucleation events, has been shown to precede haze events in environments such as Beijing (Guo et al., 2014). These events are detrimental to health and quality of life. The sub-100 nm fraction of particles to which new particle formation contributes is often referred to as the ultrafine fraction. Ultrafine particles (UFPs) pose risks to human health due to their high number concentration. UFPs exhibit gas-like behaviour and enter all parts of the lung before penetrating the bloodstream (Miller et al., 2017). They can initiate inflammation via oxidative stress responses, progressing conditions such as atherosclerosis and initiating cardiovascular responses such as hypertension and myocardial infarction (Delfino et al., 2005; Brook et al., 2010).
Highly oxidized multifunctional molecules (HOMs), organic molecules with O : C
ratios
Recent technological advances have facilitated insights into the very first steps of nucleation, which were previously unseen, with mass spectrometric techniques such as the atmospheric-pressure-interface time-of-flight mass spectrometer (APi-ToF-MS) and its chemical ionization counterpart (CI-APi-ToF-MS) allowing for high-mass and high-time-resolution measurements of low-volatility compounds and molecular clusters. Diethylene glycol-based particle counters, such as the particle size magnifier (PSM), allow for measurements of particle size distributions down to the smallest molecular clusters nearing 1 nm. Recent chamber studies have elucidated the contribution of individual species to particle nucleation, ammonia, and amines, greatly enhancing the rate of sulfuric acid nucleation (Kirkby et al., 2011; Almeida et al., 2013). In these studies, HOMs have been identified, formed through autoxidation mechanisms (Schobesberger et al., 2013; Riccobono et al., 2014; Ehn et al., 2014). These are key to early particle growth (Tröstl et al., 2016) and can nucleate even in the absence of sulfuric acid in chambers (Kirkby et al., 2016) and in the free troposphere (Rose et al., 2018). In this paper, we report the results of HOM and particle size measurements during a summer campaign in Beijing, China.
Sampling was performed as part of the Air Pollution and Human Health in a
Developing Megacity (APHH-Beijing) campaign, a large international
collaborative project examining emissions, processes, and health effects of
air pollution. For a comprehensive overview of the programme, see Shi et al. (2019). All sampling was conducted across a 1-month period at the
Institute for Atmospheric Physics (IAP), Chinese Academy of Sciences,
Beijing (39
The Aerodyne nitrate chemical ionization atmospheric-pressure-interface time-of-flight mass spectrometer (CI-APi-ToF-MS) was used to make measurements of
neutral oxidized organic compounds, sulfuric acid, and their molecular
clusters at high time resolution with high resolving power. The ionization
system charges molecules by adduct formation, such as in the case of organic
compounds with two or more hydrogen bond donor groups (Hyttinen et al.,
2015), or proton transfer in the case of strong acids like sulfuric acid.
Hydroxyl or hydroperoxyl functionalities are both common hydrogen-bond-donating groups, with hydroperoxyl being the more efficient hydrogen bond
donor (Møller et al., 2017). This instrument has been explained in great
detail elsewhere (Junninen et al., 2010; Jokinen et al., 2012), but briefly
the front end consists of a chemical ionization system where a 10 L min
Two scanning mobility particle sizer (SMPS) instruments measured particle
size distributions at 15 min time resolution, with one long SMPS (TSI 3080 EC,
3082 long DMA, 3775 CPC, TSI, USA) and one nano SMPS (3082 EC, 3082 nano DMA,
3776 CPC, TSI, USA) measuring the ranges 14–615 and 4–65 nm respectively.
A particle size magnifier (A10, Airmodus, FN) linked to a CPC (3775, TSI,
USA) measured the sub-3 nm size fraction. The PSM was run in stepping mode,
operating at four different saturator flows to vary the lowest size cut-off
of particles that it will grow (this cut-off is technically a point of
50 % detection efficiency) of
The condensation sink (CS) was calculated from the size distribution data as
follows:
Measurements of the classical air pollutants were measured at the same site
and have been reported in the campaign overview paper (Shi et al., 2019).
A total of 5 d of CI-API-ToF-MS data were collected successfully, from
21 June 2017 midday through 26 June 2017 midday. New particle formation events
were observed on 24 June in the late afternoon and 25 June at
midday. Some nighttime formation of molecular clusters was seen earlier in
the campaign, as were several peaks in the 1.5–100 nm size range, likely
from pollutant plumes containing freshly nucleating condensable materials.
The trace gases,
For the peaks that have had chemical formulae assigned, oxidation state of
carbon, or OS
Oxidation state of carbon calculated as 2 times the oxygen-to-carbon ratio minus the hydrogen-to-carbon ratio against carbon number for (coloured) individual ions and (blue circles) signal-weighted average for each carbon number. Area and colour are both proportional to the peak area for each ion.
The degrees of OSc observed here are similar to those seen in other
environments such as during the SOAS campaign in 2013 in the southern United
States, characterized by low
Mass defect plot of fitted mass spectral peaks between
100 and 600 mass units at
A mass defect plot is shown in Fig. 2, which shows nominal mass plotted
against mass defect for all peaks in this dataset. Mass defect is defined as
the ion mass minus integer mass. This is shown for two separate daytime
periods, one where nucleation was not occurring and HOM concentrations are
lower (10:30–12:00 CST 23 June 2017) and one where nucleation was occurring
under high HOM concentrations (10:30–12:00 CST 25 June 2017). The band of lower
mass defect is characterized by a number of large peaks with high signal,
for example, at
Temporal trends of HOMs in the urban atmosphere can reveal their sources and
behaviour in the atmosphere. Most of the HOM species peak in the daytime.
These species all follow a similar diurnal trend, as shown in Fig. 3.
The concentrations of both
Summed time series of the normalized signals of
The identified compounds have been roughly separated into several
categories, each of these plotted in Fig. 3. Figure 3a shows the separation
of components into non-nitrogen-containing HOMs and nitrogen-containing
HOMs, or organonitrates (ONs). The ON signal is much higher than that of the
HOM, attributable in part to a few ions of high signal, such as the isoprene
organonitrate
Despite the very large fluxes of anthropogenic organic pollutants in
Beijing, biogenic emissions are still an important source of reactive VOCs
in the city, with abundant isoprene oxidation products observed (see above),
as well as monoterpene monomers (
Other identified peaks are plotted in Fig. 3c. The
Time series for the whole sampling campaign for the concentrations of (left axis) VOCs as measured by PTR-ToF-MS and (right axis) a selected HOM product associated with that precursor.
Nearly all ions with the exception of the larger compounds attributed to the
cross reaction of
The first half of campaign measurements are marked by an episode of low HOM
signals. A diurnal cycle still exists but it is weak. The radiation
intensity was significantly lower on these prior days than it was on
24 June. No data are available for the final period of measurement. Ozone is
higher on the prior measurement days with lower HOM signals (see Fig. S1).
Little agreement is seen between VOC concentration and HOM signals on these
days. The condensational sinks are roughly similar to those on days of
higher HOM concentrations, but temperature and solar radiation are much
lower. HOM formation is largely dependent upon VOC concentration, oxidant
concentration (which will be lower if solar radiation is lower, especially
in the case of
The
Nearly all the signal intensity in the CI-APi-ToF-MS instrument arises from
molecules charged by
Normalized unit mass
A burst in the signal seen by the CI-APi-ToF-MS occurs first in the late
morning in Fig. 5a, and this is at the same time as peaks
begin to rise in the identified HOMs (see Fig. 3). Here, the PSM is not
available due to an instrumental fault until 16:00 CST; however, at that point,
an elevation to particle count and a large elevation to cluster count can be
seen. Moving into the evening period, the mass contour shows peaks in larger
masses
SMPS
The second day plotted in Fig. 5b (25 June 2017) shows a
strong afternoon peak to the HOMs (for most HOMs, stronger than that on the
day prior). Particle formation is shown in the PSM data. A strong midday
peak to particle number is seen with two distinct peaks in cluster count.
These two peaks are not coincidental with the two peaks in HOM signal (i.e.
nitrogen-containing HOMs in Fig. 3a peaking at 11:00 and 16:00 CST). Sulfuric
acid, however, does peak synchronously with the particle number count.
Sulfuric acid is plotted across the contour plot in Fig. 6, where PSM data
are also shown in the bottom panel. The peak in CI-APi-ToF-MS mass signal,
visible in Fig. 5, occurs at around 12:00–13:00 CST; peaks in the PSM cluster
count occur at 10:00 and 13:00 CST. Peaks in mass up to 550
There is recent strong evidence to suggest that the driving force of the
earliest stages of particle formation in urban Shanghai is sulfuric
acid and
The average degree of HOM oxidation in Beijing is comparable with that seen
in other environments. Rapid intramolecular hydrogen shifts during
autoxidation due to the higher temperatures are probably offset by the
frequent termination reactions due to high
The temporal trend of nearly every HOM shows afternoon or evening maxima.
Both
Initial particle formation coincides with peak sulfuric acid signals, while
the growth of the particles correlates more closely with the signals of
HOMs. This is very similar to behaviour observed in a study of NPF in
Shanghai which was attributed to sulfuric acid–dimethylamine–water
nucleation with condensing organic species contributing to particle growth
(Yao et al., 2018), and this is further backed up by numerous SA–DMA
clusters present in this dataset. The freshly formed particles grow and
contribute significantly to total particle loading. This is visible when the
unit mass CI-APi-ToF-MS data are plotted as a contour plot, and further this
is visible in the PSM data, with bursts in both total number count
Data supporting this publication are openly available from the UBIRA eData
repository at
The supplement related to this article is available online at:
The study was conceived and planned by RMH and ZS. DCSB and JB set up and operated the main instrumental measurements, and JB prepared the first draft of the paper and responded to comments from RMH and ZS. CNH and WJFA contributed the hydrocarbon data and provided comments on the draft paper, and FAS and JL contributed the gas-phase pollutant data.
The authors declare that they have no conflict of interest.
This article is part of the special issue “In-depth study of air pollution sources and processes within Beijing and its surrounding region (APHH-Beijing) (ACP/AMT inter-journal SI)”. It is not associated with a conference.
This was part of the APHH-Beijing programme funded by the UK Natural Environmental Research Council, the National Centre for Atmospheric Science, and the Natural Sciences Funding Council of China. We thank Xinming Wang from the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences; Brian Davison from Lancaster University; and Ben Langford, Eiko Nemitz, Neil Mullinger, and other staff from the Centre for Ecology and Hydrology, Edinburgh for assistance with the VOC measurements and associated infrastructure.
This research has been supported by the Natural Environmental Research Council (grant no. NE/N007190/1) and the Natural Sciences Funding Council of China. It was additionally facilitated by the National Centre for Atmospheric Science ODA national capability programme ACREW (NE/R000034/1), which is supported by NERC and the GCRF.
This paper was edited by Kimitaka Kawamura and reviewed by three anonymous referees.