New particle formation (NPF) occurs frequently in the global atmosphere. During recent years, detailed laboratory experiments combined with intensive field observations in different locations have provided insights into the vapours responsible for the initial formation of particles and their subsequent growth. In this regard, the importance of sulfuric acid, stabilizing bases such as ammonia and amines as well as extremely low volatile organics, have been proposed. The instrumentation to observe freshly formed aerosol particles has developed to a stage where the instruments can be implemented as part of airborne platforms, such as aircrafts or a Zeppelin-type airship. Flight measurements are technically more demanding and require a greater detail of planning than field studies at the ground level. The high cost of flight hours, limited time available during a single research flight for the measurements, and different instrument payloads in Zeppelin airship for various flight missions demanded an analysis tool that would forecast whether or not there is a good chance for an NPF event. Here we present a methodology to forecast NPF event probability at the SMEAR II site in Hyytiälä, Finland. This methodology was used to optimize flight hours during the PEGASOS (Pan-European Gas Aerosol Climate Interaction Study)–Zeppelin Northern mission in May–June 2013. Based on the existing knowledge, we derived a method for estimating the nucleation probability that utilizes forecast air mass trajectories, weather forecasts, and air quality model predictions. With the forecast tool we were able to predict the occurrence of NPF events for the next day with more than 90 % success rate (10 out of 11 NPF event days correctly predicted). To our knowledge, no similar forecasts of NPF occurrence have been developed for other sites. This method of forecasting NPF occurrence could be applied also at other locations, provided that long-term observations of conditions favouring particle formation are available.
Formation and growth of secondary aerosol particles has been observed in numerous locations and in different environments in the planetary boundary layer (for an overview see, e.g. Kulmala et al., 2004; Kulmala and Kerminen, 2008). Numerous investigations have attempted to connect new particle formation (NPF) to atmospheric trace gas concentrations, atmospheric chemistry, and meteorological processes (e.g. Weber et al., 1995; Riipinen et al., 2007; Paasonen et al., 2010). Most of the NPF observations are based on stationary ground-level measurements during which the sampled air masses and prevailing meteorological conditions are continuously changing. Typically the growth of the newly formed particles can be followed for several hours from these fixed point measurements, indicating that NPF usually occurs over large areas (Dal Maso et al., 2007; Hussein et al., 2009). In order to obtain more information on the spatial extent of NPF events both in the vertical and horizontal directions, measurements using aircrafts are needed. As part of the 4-year-long EU funded PEGASOS (Pan-European Gas Aerosol Climate Interaction Study) project, a Zeppelin NT (Neue Technologie) airship was performing atmospheric aerosol, trace gas, and photochemistry measurement flights in central Finland during May–June 2013. In order to most efficiently utilize the flight hours of the airship, it was necessary to prepare forecasts on the probability of NPF events in the coming days.
Most of the Zeppelin measurement flights during the campaign were directed to the vicinity of the University of Helsinki SMEAR II measurement station in Hyytiälä (Hari and Kulmala, 2005). Measurements of aerosol number–size distributions, trace gas concentrations and basic meteorological quantities were started at the SMEAR II station in January 1996. These long time-series records have been used extensively to characterize the conditions in which NPF occurs (or does not occur) in this boreal forest environment, based on both the local atmospheric conditions as well as the synoptic situation and air mass origins and transport route to the station (Boy and Kulmala, 2002; Boy et al., 2003; Lyubovtseva et al., 2005; Dal Maso et al., 2007; Sogacheva et al., 2008; Nieminen et al., 2014).
Field observations, laboratory experiments, and theoretical considerations have shown that sulfuric acid is one of the key components in atmospheric NPF events, but in addition also trace amounts of other vapours such as ammonia, amines, or oxidized organics are needed (e.g. Kulmala et al., 2013). Particularly the contribution of extreme low volatile organics seems to be crucial in the boreal forest environment (Kulmala et al., 1998; Yli-Juuti et al., 2011; Ehn et al., 2014). Proxies for the concentrations of these trace gases or their precursors have been developed based on campaign-wise measurements (Petäjä et al., 2009; Lappalainen et al., 2009). Based on the concentrations and emissions of these trace gases, several parameterizations have been developed to describe the occurrence and intensity of NPF (e.g. Buzorius et al., 2003; Bonn et al., 2008; Paasonen et al., 2010; Häkkinen et al., 2013).
In this work, we describe forecasts for the occurrence of NPF at the SMEAR II station. The forecasts are based on the above-mentioned long-term time series observations of the typical conditions during NPF days and non-NPF days, the air mass origins as well as weather and air-quality forecasts.
The main objective of the NPF forecasts was to predict whether during the next 3 days NPF events were likely to occur at the SMEAR II station area. A time period of 3 days was chosen in order to have long enough time for preparing the measurement instruments needed on different flights while still maintaining reliability of the input data used in making the NPF forecasts. The final NPF forecast was always provided for the next day, as the Zeppelin measurement flights were typically planned 1 day in advance. All the NPF forecast results presented in this work refer to the final NPF forecasts, i.e. forecasts for the next day.
Forecasts for concentrations of trace gases SO
As supporting data, we also used several “traditional” weather forecasts available on the internet (including forecasts by the Finnish Meteorological Institute, Foreca, and Norwegian Meteorological Institute), mainly to evaluate the probabilities of cloudiness and rain. During the campaign time, the weather was rather variable and the forecasts were changing rapidly (even several times a day) from clear skies to partly cloudy and possibly rainy. All these conditions are known to affect directly the probability of NPF.
Air mass arrival directions and source areas were forecast for 96 h prior to
the arrival of air at Hyytiälä using the HYSPLIT single particle
Lagrangian transport model developed by NOAA and freely available on the
internet (
Typical conditions on NPF and non-NPF days in Hyytiälä are shown in
Table 1 for May and June during years 1996–2012. The conditions are shown
for the time window 08:00-11:00, which is the time when NPF typically starts in
Hyytiälä. In a data-mining study of the SMEAR II station long time-series records of aerosol size distributions and meteorological parameters,
Hyvönen et al. (2005) found that the condensation sink (describing the
pre-existing aerosol surface area) and relative humidity were the two
parameters most effectively separating NPF days from non-NPF days. Particle
formation was occurring only on days with a low CS and low RH. On the other
hand, photochemical production of vapours participating in nucleation and
growth, namely sulfuric acid and oxidation products of organics, is more
efficient in clear-sky conditions with high UV radiation intensity compared
to cloudy conditions. Thus, our main criteria in forecasting NPF to occur
were clear sky conditions, low condensation sink (in practice low PM
Conditions observed at Hyytiälä during NPF and non-NPF days between 08:00 and 11:00 (local time) in months May–June 1996–2012. For each variable the median value is given and the interquartile range (25th and 75th percentiles) is shown in brackets. The median and interquartile values are calculated from all data at 30 min time resolution in the time window 08:00–11:00.
The air mass source area and transport route to Hyytiälä were
considered when making the NPF forecasts. In the long time-series analysis by
Dal Maso et al. (2007), the occurrence of NPF in Hyytiälä was
observed to be highly favourable in air masses originating from the Arctic and North Atlantic oceans, and on the other hand suppressed in southern air
masses. This is typically connected to clean air arriving from the west and
more polluted air originating from central and eastern Europe, directly
influencing the sink for newly formed particles. However, in air masses
originating from the south and south-east to Hyytiälä, SO
Flowchart of the decision making process for the NPF forecasts.
Overview of the meteorological parameters, trace gas concentrations and particle size distributions during the campaign 3 May–11 June 2013.
Criteria for the NPF forecasts (the source for each data is shown in parentheses). All the criteria within the category must be fulfilled; i.e. the individual criteria are combined with logical operator AND.
We also developed several “nucleation parameters” to forecast the intensity of NPF. The parameters that worked best were either related to only the proxy concentration of sulfuric acid, or were related to proxies for both sulfuric acid and oxidation products of volatile organic compounds (such as monoterpenes). Paasonen et al. (2010) studied several different parameterizations for the formation rate of 2 nm particles, and found that at the Hyytiälä site nucleation rate could be mainly explained by the sulfuric acid concentration to the power of 1 or 2.
The simplest nucleation parameter is described by the following equation:
A nucleation parameter taking into account the oxidation products of
monoterpenes, in addition to sulfuric acid, is described by the following
equation:
The PEGASOS–Zeppelin Northern mission was a 40-day-long measurement campaign
between 3 May and 11 June 2013. An overview of the meteorological conditions
as well as trace gas and particle concentrations observed at the SMEAR II
station during the campaign is shown in Fig. 2. Most of the days were sunny
with either clear or partly clear skies. Rain occurred on 13 days during the
campaign. The air was rather clean from anthropogenic pollution, especially
in the first and last week of the campaign. Occasionally, there were
pollution episodes seen e.g from a 10-fold rise of the SO
Air mass arrival trajectories to Hyytiälä 3 May–11 June 2013 calculated using HYSPLIT model. The colour indicates the arrival date and each trajectory represents air mass route during 96 h before arrival. Air mass trajectories arriving on NPF days between 10:00–14:00 local time are marked with black lines.
Particle number concentration size distributions (top panel), and
nucleation parameters NP
Figure 3 shows the arrival routes of air masses to Hyytiälä during
the period of our measurement campaign. These trajectories were calculated
for the 250 m arrival height above ground, and 96 h backwards in time. From
the beginning of the campaign until middle of May, approximately 17 May,
the air masses originated mainly from over the Atlantic, and arrived at
Hyytiälä either directly from the west over Scandinavia or from
the south-west, making a turn over the Baltic Sea. Air in Hyytiälä was
relatively clean during this time, characterized by low particulate mass and
trace gas concentrations. Especially SO
Figure 4 shows the particle number size-distributions along with the
forecasted NPF occurrence and the time series of the nucleation parameters
NP
NPF event forecasts (second column), and NPF event classification based on measured particle size distributions (third column) for each day of the campaign. Class I and II NPF events refer to the classification by Dal Maso et al. (2005). Remarks on the fourth column show the basis for the NPF event forecast.
Continued.
Continued.
Comparison of the NPF classification based on DMPS data (rows), and the NPF forecasts (columns). On days marked in bold the forecasts were successful in predicting whether NPF occurred in Hyytiälä or not, and on days marked in italic the forecast was wrong according to observations. The days classified as undefined according to observations are left out of the comparison with forecasts.
After mid-May until early June, the air masses arrived at Hyytiälä
mainly from the east, either spending several days over continental Russia
or, in some cases, coming more directly from over the Arctic Ocean via
north-west Russia. The air mass circulation was driven by a persistent
high-pressure system residing over central Finland. This resulted in a
rather unusual air mass transport pattern to Hyytiälä, and also made
the NPF forecasting more challenging. During this time, there were
situations when the polluted air masses resulted in a high condensation
sink, preventing the occurrence of NPF. Also the SILAM forecasts for the
SO
The nucleation parameter NP
The nucleation parameters NP
The particle number size distributions measured by the differential mobility
particle sizer (DMPS) during the whole campaign are shown in the upper panel
of Fig. 4. Using the criteria developed by Dal Maso et al. (2005), each day
was classified as either an NPF event, non-event, or undefined day. On NPF
event days a new mode of particles smaller than 25 nm is observed and these
particles can be observed growing to larger sizes during several hours. NPF
event days are further classified according to the possibility to reliably
derive particle formation and growth rates (Class I) or not (Class II). The
days when no new sub-25 nm particles appeared were classified as
non-NPF days. Undefined days are those days for which it was not possible to
unambiguously determine whether NPF occurred or not. Table 3 shows the
forecast and the corresponding event classification for each day. During the
40-day campaign, clear regional NPF events lasting for several hours were
observed on 11 days in Hyytiälä. Six of these days were also forecast
to be NPF days, and four to have a possibility of NPF to occur. The NPF day
which we forecast to be a non-NPF day (9 June) was cloudy and had a
possibility of rain according to weather forecasts, and the air masses were
forecast to originate from the west, which is not the direction from where
air masses typically arrive to Hyytiälä on NPF event days (Dal Maso et
al., 2007). On 10 days of the campaign there was no particle formation
occurring in Hyytiälä, and these were also forecast to be non-NPF
days, except for 2 days (17 and 28 May) for which a possible NPF event
was forecast. This was most probably caused by the very low SO
Comparison of the event classification and the event forecasts is shown in
Table 4. We follow the method of Hyvönen et al. (2005) for calculating
the score indices for the performance of the event forecasts on the 21 days
classified as either NPF or non-NPF days (undefined days are removed from
this comparison). Out of these 21 days our forecasts had two false NPF event
days (non-event day forecast to be either event or to have a possibility for
event) giving a 10 % false-event fraction, and one NPF event day forecast
to be a non-event day giving a 5 % missed-event fraction. The total error
of the NPF forecasts (false and missed events) during the 21 classified days
of the 40-day campaign was (
Here we present a way to forecast new particle formation events. Being able to make such forecasts accurately is very important, for example, when airborne measurements are performed. As a summary, we made an NPF forecast for 40 days. The forecasts were found to work reasonably well. Only 1 day when nucleation was forecast to occur was a non-nucleation event day. In total, 24 days were predicted to be either NPF event days or probable NPF event days; 10 days were NPF event days, 11 were undefined (when it could not be reliably determined whether NPF occurred or not), and 2 were non-event days.
The main challenges in making the NPF forecasts were to obtain as reliable input data as possible from SILAM, HYSPLIT, and weather forecasts. The methods utilized here are most likely also applicable to other locations where there is sufficiently long data sets available to characterize the conditions favourable for the occurrence of regional-scale particle formation. In urban areas, and within cities our methods are less likely to be applicable due to the day-to-day variation of emissions of vapours and particles from local anthropogenic sources.
This research is supported by the Academy of Finland Centre of Excellence
programme (project numbers 1118615 and 272041). The EU FP7 project PEGASOS
(project number 265148) is acknowledged for the Zeppelin NT measurements.
T. Yli-Juuti acknowledges financial support from Max Planck Society.
H. E. Manninen acknowledges support by the Finnish Cultural Foundation. The
authors acknowledge the NOAA Air Resources Laboratory (ARL) for the provision
of the HYSPLIT transport and dispersion model and READY website
(