Budapest platform for
Aerosol Research and Training (BpART) was created for advancing long-term
on-line atmospheric measurements and intensive aerosol sample collection
campaigns in Budapest. A joint study including atmospheric chemistry or
physics, meteorology, and fluid dynamics on several-year-long data sets
obtained at the platform confirmed that the location represents a well-mixed,
average atmospheric environment for the city centre. The air streamlines
indicated that the host and neighbouring buildings together with the natural
orography play an important role in the near-field dispersion processes.
Details and features of the airflow structure were derived, and they can be
readily utilised for further interpretations. An experimental method to
determine particle diffusion losses in the differential mobility particle
sizer (DMPS) system of the BpART facility was proposed. It is based on
CPC–CPC (condensation particle counter) and
DMPS–CPC comparisons. Growth types of nucleated
particles observed in 4 years of measurements were presented and discussed
specifically for cities. Arch-shaped size distribution surface plots
consisting of a growth phase followed by a shrinkage phase were characterised
separately since they supply information on nucleated particles. They were
observed in 4.5 % of quantifiable nucleation events. The shrinkage phase
took 1 h 34 min in general, and the mean shrinkage rate with standard
deviation was
It is increasingly recognised that atmospheric new particle formation (NPF)
and consecutive particle growth events are also relevant in urban
environments (e.g. Alam et al., 2003; Wehner et al., 2004; Borsós et al.,
2012; Dall'Osto et al., 2013; Xiao et al., 2015). The events observed in
cities can be interconnected to regional NPF (Salma et al., 2016), and,
therefore, their occurrence frequency is also considerable (up to
25–30 % on an annual scale), and the process often shows a remarkable
seasonal dependency, with a monthly maximum of up to 60–75 % in spring.
New particle formation can increase the pre-existing particle number
concentrations by a factor of 2–3 (Salma et al., 2011a). As an overall
effect, daily mean relative contributions of NPF events to ultrafine (UF)
particles (with diameter
New particle formation and the growth process is superimposed on
high-temperature emission sources and atmospheric transformation processes,
which can – in particular in cities – complicate the identification and
classification of NPF events. Higher particle number concentration levels
and their fluctuation, a larger number of relevant emission sources of UF
particles, spatial distribution and temporal variability of the sources, and
the fact that the Aitken-mode particles usually have larger abundance and
smaller median diameters in cities than at remote sites are the main
disturbance or complicating factors. Studies on NPF processes require long
and continuous atmospheric measurements of aerosol, air pollutant gases and
meteorological variables. Such experimental data sets have been becoming
gradually available for some cities in the world. It is increasingly
necessary that a systematic overview of the peculiarities and specific features of urban NPF considering several aspects is given. The major objectives of this paper are (1) to demonstrate and evaluate various types of NPF and growth processes
specifically for cities, (2) to discuss the reasons for their occurrence, (3) to investigate and quantify the shrinkage of newly formed particles, (4) to
interpret the effect of surrounding airflow patterns and spatial
representativity of a new urban research facility, the Budapest platform for
Aerosol Research and Training (BpART, URL:
The superstructure
The experimental work was realised in Budapest, Hungary. Most measurements
were performed at the BpART facility (Fig. 1). The platform is to serve and
advance the research of atmospheric aerosols through complex surface-based
and satellite-borne measurements, as well as to promote the education and
training of PhD students and young researchers. It is based on an insulated
metal container with internal dimensions of 2.00 m (width), 2.80 m (length)
and 2.10 m (height) dedicated to on-line aerosol measurements and aerosol
sample collections. It is located on the second-floor balcony of the northern block of the Lágymányos Campus, Eötvös University.
The WGS84 coordinates (altitude, longitude and height) of the NW upper
corner of the container are 47
Sampling inlets and sensors are set up at heights between 80 and 150 cm above the rooftop level (Fig. 1a). The platform has several waterproof vertical inlets through the ceiling and some horizontal inlets with diameters of 64, 10, 8 and 6 mm. The aerosol instruments available include a filter dynamics measurement system tapered element oscillating microbalance (FDMS-TEOM 1400a, Rupprecht and Patashnick, USA), RT-OC/EC analyser (Sunset Laboratory, USA), differential mobility particle sizer (DMPS), stacked filter unit (SFU) sampler, micro-orifice uniform-deposit impactor (MOUDI), nano-MOUDI sampler (both MSP, USA) and high-volume dichotomous sampler (HiVol virtual impactor). The air pumped inside and processed by the instruments is forwarded to the outside via closed internal and external tubing to a distance from the receptor site of at least 3 m in the prevailing downwind direction. Meteorological data are available from both a regular urban climatological station (station number: 44505; name: Budapest Lagymanyos) of the Hungarian Meteorological Service operated at a height of 10 m above the roof level of the building (at a height of 39 m above the ground) at a distance of about 70 m from the BpART facility, and from a simpler on-site meteorological station. Aerosol optical thickness data can be retrieved via satellite receivers, which are located next to the regular meteorological station, from the Moderate Resolution Imaging Spectroradiometer (MODIS) on the Terra satellite.
Some additional experimental data utilised in the present paper were measured
either in a near-city
background on the western border of the city in a wooded area (latitude
47
The principal measuring system for the present study was a flow-switching
type DMPS. Its main components are a radioactive bipolar charger, a Nafion
semipermeable membrane dryer, a 28 cm long Hauke-type differential mobility
analyser and a butanol-based condensation particle counter (TSI, model 3775,
labelled as CPC1). Particles with an electrical mobility diameter from 6 to
1000 nm are recorded in their dry state (with a typical relative humidity
< 30 %) in 30 channels. Sample flow is 2.0 L min
Particle transport losses in the DMPS system were determined in an
intercomparison exercise by using an identical comparative CPC (labelled as
CPC2, TSI, model 3775) for ambient air in two steps. First, the CPC
ordinarily built in the DMPS system (CPC1) was compared to CPC2 to check
their coherent functioning by operating them with an inlet sample flow of
0.3 L min
Standardised meteorological measurements of air temperature (
The evaluation of the measured DMPS data was performed according to the
procedure protocol recommended by Kulmala et al. (2012). Mathematically
inverted size distributions were utilised for calculating particle number
concentrations in the particle diameter ranges from 6 to 25 nm
(
Airflow and dispersion of aerosol particles in the surroundings of the BpART
research facility were investigated by using a three-dimensional
computational fluid dynamics (CFD) model of the adjacent area in order to
estimate the representativity of the results and conclusions obtained at the
single measurement point. The model includes a detailed geometrical
description of the host university building (where the platform is located)
as well as simplified representations of the neighbouring buildings and
vegetation within a 600 m radius area (Fig. 1c). In order to specify the
boundary conditions, a 300 m wide relaxation zone was added to the periphery
of the orographic model artificially, which equalises the terrain elevation
height. The vertical profiles of velocity and turbulent quantities were
specified at a cylindrically shaped lateral boundary. The domain height was
set at 200 m, and symmetrical boundary condition was used at the top of the
domain. The numerical solution was developed in the ANSYS-FLUENT solver
(version 16.2) by using a realisable
Ranges and medians of the daily median particle number concentrations and UF particle contribution for central Budapest are summarised in Table 1 for general orientation.
Evaluation of the concentration data measured by CPC1 (ordinarily built into
the DMPS system) and CPC2 (comparative instrument) is shown in Fig. 2. The
agreement between the two CPCs near and above the limiting range between the
single-particle counting mode and photometric mode is not sufficient. Both
the scatter plot and concentration ratios show systematic deviations. This is most likely caused
by differences in the live time correction at high counting rates and in the
actual transition ranges between the operating modes for the two instruments,
which are based on the manufacturer's calibration (Hermann et al., 2007).
Fortunately, such high concentrations do not occur when the CPC measures
size-separated particle fractions in the channels of the DMPS system (see
later). Considering the data below
Ranges and medians of daily total particle number concentrations
(
Scatter plot and CPC2
Temporal response of two identical CPCs to the concentration change
caused by adjusting the same HEPA filter to them in a stepwise manner. The
dashed lines show the fitted linear curves. Relaxation time
Scatter plot and CPC
Evaluation of the data obtained from the DMPS system and CPC2 operated in
parallel is shown in Fig. 4. It is worth noting that no individually
measured DMPS concentration data (in a channel) was larger than
Ranges, medians and means with standard deviations for air
temperature (
Averages of some basic meteorological data inside and outside the BpART
facility (based on ca.
Streamlines (trajectories of massless particles) were traced back by using a reversed flow field from the observation volume element to a distance of 600 m in radius. The histogram of the starting point height for the eight major wind sectors is displayed in Fig. 6. It reveals that the effective sampling height changes with WD. The most probable height of origin was 3–8 m for most WDs (of N, NE, E, SE and NW). In the case of S, SW and W wind directions, however, when the observation volume element falls onto the lee side of the host building, most air parcels arrived from a height of 25–40 m. This suggests that the airflow around the host building has a major impact on the sampling height. No substantial dependency on the canopy drag was found. Flow structure around the BpART facility for the eight major wind sectors in summer are shown in Fig. 7. The path lines indicate that the host and neighbouring buildings together with the natural orography play an important role in the near-field dispersion processes. Velocity distributions revealed that the vegetation causes a minor upward displacement of the streamlines, as expected, and that the near-ground velocity decreases. The airflows arriving from the directions N, NE, E and SE bring in air masses directly from further distances to the BpART facility. When the wind blows from S, SW, W or NW, the effects of eddies and turbulence at the platform cannot be neglected, and the on-site WSs are usually decreased and on-site WDs usually substantially modified. The details and features of the airflow structure should be taken into consideration when interpreting the horizontal extension of atmospheric processes and phenomena.
Many NPF and consecutive particle growth processes were observed as banana-shaped plots (Fig. 8a), which indicate that the measurement site was often exposed to spatially homogenous air masses (Kivekäs et al., 2016). This shape was usually associated with regional nucleation phenomenon in the Carpathian Basin (Salma et al., 2016) and occurred under relatively clean atmospheric conditions. Some plots appeared with a narrow onset as an NPF burst. A few similar banana plots were also observed in rather polluted air. The basic preconditions of NPF events are realised by competing source and sink terms, and, therefore, NPF can occur even in polluted environments (with large condensation and scavenging sinks), provided that the source of condensable chemical species is large enough and other conditions are also favourable (Fig. 8b). Changes in the intensity of local emission sources can cause considerable fluctuations of the nucleation-mode NMD in the overlapping diameter range. A flow-mediated banana plot (Fig. 8c) was observed in the street canyon, which has one end at the bank of the River Danube, and the other end in the city. There are two essential horizontal airflows possible inside the canyon: either from or toward the river, which lead to rather diverse atmospheric conditions. In some cases, NPF events were clearly recognisable as banana plots for several hours when the wind blew from the river (bringing in cleaner and regionally more representative air masses), while the curve representing NPF disappeared when the airflow changed to the opposite direction (introducing more polluted air masses, in which there was no NPF and growth process going on or which inhibited the ongoing process).
Wind roses based on the modern Beaufort scale for the data sets
obtained above the rooftop level of the building
Figure 9 shows NPF and particle growth events with multiple consecutive
onsets. During the relevant time intervals, the properties which are related
to NPF events (e.g.
Histogram of the starting point height at a distance of 600 m from the BpART facility for the eight major wind sectors (N, NE, E, SE, S, SW, W and NW) for summer (vegetation with leaves) and winter (no vegetation). The sampling height of the platform is indicated by dashed lines.
Figure 10a shows an NPF with an uninterrupted particle growth which was
limited in time. The duration of the growth phase can be variable. In some
cases, the particle growth was followed by a reverse process, i.e. by a
continuous decrease in nucleation-mode NMD as displayed in Fig. 10b. This
combination results in an arch-shaped surface plot. In some cases, the nucleated
particles shrank back to the smallest measurable diameter limit of 6 nm for the DMPS. The actual plot (Fig. 10b) was created in a blizzard when
SO
Size distribution surface plots generated by NPF and growth events are
usually superimposed on the effects of other urban emission sources and
transformation processes. Some of them, such as vehicular road traffic,
happen almost regularly since they often follow a diurnal activity pattern of
inhabitants on workdays (Morawska et al., 2008; Salma et al., 2011b). Some
other ordinary substantial emission sources, such as boat traffic on rivers
in cities or diesel-driven single heavy-duty trucks or buses, cause sudden
and considerable changes (red stripes) on surface plots in an irregular or
occasional manner. Furthermore, number concentrations can be also modified
very much or sometimes even in a determinative way by local meteorology and
atmospheric mixing. A typical surface plot reflecting some of these effects
is shown in Fig. 11a. Impacts of residential heating and combustion
activities, mainly on winter evenings, can also add to the situation.
Figure 11b shows a surface plot for a very specific activity, namely
extensive grass cutting in the park next to the BpART facility. It was likely
generated by organic emissions from the lysed vegetation tissue combined with
the exhaust emissions from the internal combustion engines, and it is
expected to affect a very local area only and should not be classified as a
NPF on a larger spatial scale. Even the particle growth observable around
21:00 UTC
Path lines and speeds around the BpART facility for the eight major wind sectors (N, NE, E, SE, S, SW, W and NW) in summer (vegetation with leaves) in a perspective view. The wind directions are marked by an arrow and letters, and the location of the platform is indicated by a red dot in the panels.
There were 178 quantifiable NPF and consecutive particle growth events
identified in Budapest in 4 years. Of these, 4.5 % (8 days) yielded
well-developed uninterrupted arch-shaped surface plots. Most of them occurred
in spring (50 %) and summer (25 %), although the number of cases was
rather too limited to arrive at reliable conclusions. Arc-shaped surface
plots were observed in NPF classes other than quantifiable events as well, but
their rigorous evaluation was hindered by their ambiguous or interrupted
growth or shrinkage phases and fluctuating data. The decreasing process of
the Aitken-mode NMD was also detected on some non-nucleation days. The
quantifiable events were only evaluated because they
supply representative and reliable information on nucleated
particles. The SR varied from
Means and their mean relative change rates (in an hour expressed as
percentage of the mean) separately for the growth (
N.r.: not relevant (below 1 % in absolute value).
The shrinkage of newly formed particles was earlier attributed to the
evaporation of reversible condensed chemical species (for urban environments,
see Yao et al., 2010, Backman et al., 2012, Young et al., 2013, Skrabalova et
al., 2015; for a regional background, see Cusack et al., 2013). Its primary
causes were explained either by changes in meteorological conditions or in
atmospheric vertical mixing. They both lead to decreased atmospheric
concentration of vapours below their supersaturation. Decrease in the
nucleation-mode NMD due to coagulation and to the collapse
of structured particles with void volume was not considered,
especially since the latter possibility does not seem very realistic for
nucleated particles. Recent studies reported that the particles which are
grown after NPF contain organics (including carboxylic and hydroxyl acids and
N-containing organic compounds; Smith et al., 2008), nitrate, sulfate and
ammonium ions as major chemical compounds (Bzdek et al., 2012). Some of them
can be indeed semi-volatile. Another study (Kivekäs et al., 2016)
concluded, however, that this explanation needs further elaboration since
newly formed particles in their first stage are expected to consist of
compounds having a low or extremely low vapour pressure (Ehn et al., 2014).
Moreover, freshly nucleated particles with a median mobility diameter of ca.
20 nm collected on transmission electron microscopy (TEM) grids survived
high-vacuum conditions in the TEM column, at least for a limited time
(several tens of minutes; Németh et al., 2015). They evaporated when
exposed to the electron beam. In addition, a simplified atmospheric transport
model which considered temporal or spatial variations or both in the
formation rate (
Banana-shaped size distribution surface plots for regional NPF and
consecutive particle growth with narrow onset
Size distribution surface plots for NPF and consecutive particle
growth as banana-shaped plot with double onset
In order to investigate the effect of the local meteorology and some air
pollutants, two time intervals corresponding to the growth phase (increase of
the nucleation mode NMD) and shrinkage phase (decrease in the nucleation mode
NMD) of the arc-shaped plots were selected. The aerosol-related and
meteorological properties characteristic of the growth and shrinkage
conditions were averaged separately for these time intervals. The median
values and change rates derived in this way were finally averaged for the
identified 8 days together with the mean growth / shrinkage ratios
(Table 3). It can be seen that evident particle growth and shrinkage happened
for similar time intervals of ca. 1.5–2 h. Concentrations
Size distribution surface plots for NPF and consecutive particle
growth as banana-shaped plot limited in time
Size distribution surface plots showing the typical effects of
vehicular road traffic emission sources and local meteorology
Chemical and physical conditions in urban atmospheric environments can be
rather complex and variable both in space and time. They are also affected by
the close surroundings, constructions and orogeny. Long-term, continuous and
representative measurements, which are the basis of contemporary aerosol
studies, require a dedicated research infrastructure and devices. This also
means that supplementary and interdisciplinary studies need to be performed
at these measurement sites. Here we demonstrated a joint detailed study in
atmospheric chemistry or physics, meteorology, and fluid dynamics for this
purpose. The results and conclusion achieved represent a reliable background
for later interpretations and conclusions. Various shapes of particle growth
processes observed in 4-year-long measurements were presented and their
classifications were discussed in detail. Misleading conclusions could be
drawn if the actual activities around the measurement sites in cities, in
particular in combination with transformation and other disturbance effects
and fluctuating data were not considered. The particle diameter range
< 10 nm of the measuring (DMPS) systems seems invaluable for correct
and reliable NPF identification in urban environments. Arch-shaped surface
plots provide information on freshly nucleated particles. Their shrinkage
could be associated with the changes in local meteorology or atmospheric
conditions, in particular decreased GRad and the gas-phase H
The relevant observational data used in this paper are available at
Financial support by the Hungarian Scientific Research Fund (contracts K116788 and K108936) is appreciated. We are grateful to Regina Hitzenberger and Anna Wonaschütz, both of the University of Vienna, for providing the CPC2 for the intercomparison exercise and would like to thank Veronika Varga (Eötvös University) for her help in the data treatment of the intercomparison exercise and Mihály Vince (Budapest University of Technology and Economics) for his assistance in preparing the CFD modelling. Edited by: V.-M. Kerminen