Characterization of atmospheric trace gas andaerosol concentrations at forest sites insouthern and northern Finland using backtrajectories
Markku Kulmala1), Üllar Rannik1), Liisa Pirjola1), Miikka Dal Maso1),Janne Karimäki1), Ari Asmi1), Arto Jäppinen1), Veli Karhu1),Hannele Korhonen1), Suvi-Päivi Malvikko1), Arto Puustinen2),Jukka Raittila1), Sami Romakkaniemi2), Tanja Suni1),Sari Yli-Koivisto1), Jussi Paatero3), Pertti Hari4) and Timo Vesala1)
1) University of Helsinki, Department of Physics, P.O. Box 9, FIN-00014 Universityof Helsinki, Finland
2) University of Kuopio, Department of Applied Physics, P.O.Box 1627, FIN-70211 Kuopio, Finland
3) Finnish Meteorological Institute, Sahaajankatu 20 E, FIN-00810 Helsinki,Finland
4) University of Helsinki, Department of Forest Ecology, P.O. Box 24, FIN-00014University of Helsinki, Finland
Kulmala, M., Rannik, Ü., Pirjola, L., Dal Maso, M., Karimäki, J., Asmi, A.,Jäppinen, A., Karhu, V., Korhonen, H., Malvikko, S.-P., Puustinen, A., Raittila,J., Romakkaniemi, S., Suni, T., Yli-Koivisto, S., Paatero, J., Hari, P. & Vesala,T. 2000. Characterization of atmospheric trace gas and aerosol concentrationsat forest sites in southern and northern Finland using back trajectories. BorealEnv. Res. 5: 315–336. ISSN 1239-6095
The trace gas and aerosol concentrations as well as meteorological data (radiation,temperature, humidity) measured in Hyytiälä and Värriö, southern and northern Fin-land, respectively, were investigated with air mass analyses. The back trajectories of airmasses arriving to the sites on the 925-hPa pressure level were calculated 96 hoursbackwards in time. Two trajectories per day, arriving at 00 UTC and 12 UTC, werecomputed. The studied time period covered December 1997 to August 1998, Novem-ber 1998 to July 1999, and September 1999. The arriving air masses were divided intofive sectors according to their origin: I = North–West (Arctic Ocean), II = North–East
316 Kulmala et al. • BOREAL ENV. RES. Vol. 5
(Northern Russia, Kola Peninsula), III = South–East (Southern Russia, St. Petersburg),IV = South–West (Central Europe, Great Britain), and V Local, circulating air masses.The climatology of various properties of air masses originating from different sectorswas studied. The analysis showed differences between typically clean (sectors I and II)and polluted (sectors III and IV) air masses. In air masses from sectors III and IV, theNOx concentrations were high during all seasons, but the O3 concentrations were highduring the spring and summer seasons and low in winter. The highest SO2 concentra-tions arrived from sector III. In Värriö also the air from sector II was accompanied withhigher SO2 concentrations and some very high peaks were observed in winter when airmasses passed through the industrial area of Kola Peninsula. Nucleation events typi-cally occurred in clean air masses and accumulation mode concentrations were higherin polluted air masses. Although there were some differences between Hyytiälä andVärriö, the overall behaviour was similar at both sites.
The formation of new aerosol particles, calleda nucleation event, and the subsequent growth ofthese particles was observed at SMEAR stationsin the 1990s (Mäkelä et al. 1997, Kulmala et al.1998, Pirjola et al. 1998, Mäkelä et al. 2000).Although meteorological, chemical and biologi-cal factors are known to affect the particle forma-tion (Kulmala et al. 2000), the role of each of thesefactors is yet to be fully understood. The experi-mental capacity of both SMEAR stations enablesus also to analyse trace gas (O3, NOx, SO2) con-centrations, the atmosphere–forest gas exchange,and meteorological factors affecting environmen-tal issues (Ahonen et al. 1997, Vesala et al. 1998).In this paper, we present trace gas and aerosolproperties as well as canopy and shoot scale fluxesmeasured at SMEAR stations as a function of airmasses. The air mass analysis was performed us-ing back trajectories. Although back trajectoriestypically include some uncertainties (see Stohl1998), they have been used in the source analysis(e.g. Virkkula et al. 1995, Virkkula et al. 1997,Stohl 1996, Wotawa and Kröger 1999) and alsoin analysing air pollution according to air masshistories (e.g. Beine et al. 1996, Solberg et al.1997, Simmonds et al. 1997, Avila and Alarcon1999). Our aim was to find out the characteristicsof aerosol concentrations, trace gas concentra-tions, canopy scale fluxes, and shoot scale fluxesfor different air masses in order to reveal linksbetween air masses, particle formation, canopygas exchange, and chemistry.
Introduction
Aerosol particles and trace gases are ubiquitousin the Earth’s atmosphere and contribute signifi-cantly to both biogeochemical cycling and to theEarth’s radiative flux budget. IntergovernmentalPanel on Climate Change gave in their 1995 re-port (Houghton et al. 1996) an estimation of theglobally and annually averaged radiative forcingfor the direct and indirect contributions of green-house gases, for the direct and indirect aerosoleffect, and for natural changes in solar output.Since each of these contributions reflects the in-tegrated effects of various anthropogenic and bio-genic pathways, a critical task is to describe thecontent of each contribution and its sources, andto reduce the uncertainties.
A major problem in the analysis of the envi-ronmental issues is a lack of combined physico-chemical and biological knowledge. Practicalexamples of combination of physico-chemical andbiological knowledge and possibility to utilizeversatile up-to-date instrumentation for continu-ous long-term field measurements are illustratedat the SMEAR stations (Station for Measuringforest Ecosystem–Atmosphere Relations). Thefacility comprises two installations: SMEAR I inVärriö (northern Finland) and SMEAR II inHyytiälä (southern Finland). The stations are simi-larly equipped and form a combined facility op-erating within the boreal Scots pine forest underdifferent climatic conditions.
317BOREAL ENV. RES. Vol. 5 • Gas and aerosol concentrations at two forest sites
Experiments
SMEAR stations
SMEAR I station (67°46´N, 29°35´E, 390 m a.s.l)is situated in a 40-year-old and 7-m-tall Scots pine(Pinus sylvestris L.) stand on the top of a hill atthe Arctic timberline. It complements the researchdone at SMEAR II station by operating under dif-ferent climatic conditions but representing thesame boreal vegetation type. The severe arcticconditions cause an additional challenge to the ar-rangement of measurements. The annual meantemperature in Värriö is around –1.5 °C and south–west is the prevailing wind direction.
SMEAR II station (61°51´N, 24°17´E, 181 ma.s.l) is located in a homogenous Scots pine standat Hyytiälä Forestry Field Station of the Univer-sity of Helsinki, 220 km NW from Helsinki. Thenatural managed stand was established in 1962by sowing after the area had been treated withprescribed burning and light soil preparation. Thestand consists only of 1% of other species besidesScots pine close to the station. The mean heightof the trees on the site is 12 m, their mean diam-eter at the breast height is 13 cm, and the pro-jected leaf area is 3 m2 (8 m2 in the case of totalarea) per unit area of soil. The air quality at thesite represents typical background conditions. Theannual mean temperature in Hyytiälä is 3 °C.
Practically no local sources of pollutants existclose to the SMEAR I station. However, regu-larly occurring episodes of heavy sulphur diox-ide and aerosol pollution are observed duringnortheasterly winds, transported from the KolaPeninsula industrial areas (Ahonen et al. 1997,Virkkula et al. 1997), which are located less than200 km away from Värriö. The St. Petersburg areaand Russia have been determined as being sourceareas for SO2, O3 and fine aerosol particles(Virkkula et al. 1995, Beine et al. 1996). Also thecontinental Europe and British Isles are responsi-ble for pollution in Finnish Lapland (Virkkula etal. 1995, 1997). The continental Europe is a sourceof NOx all over the year, such that the productionand depletion of O3 occurs in summer and winter,respectively (Laurila 1999, Simmonds 1997).These are also the main known source areas forpollution at SMEAR II, with the exception thatKola industrial areas influence less and St. Peters-
burg and Central and Western Europe more theair quality in Hyytiälä. The local pollution fromthe station buildings (0.5 km) and the city ofTampere (60 km), both located west-south-westfrom the station, affect occasionally the air qual-ity at SMEAR II.
More information on the sites as well as de-tails of measurements can be found in Ahonen etal. (1997), Hari et al. (1994) and Vesala et al.(1998), see also the SMEAR homepage at http://honeybee.helsinki.fi/HYYTIALA/smear), and theEU funded project BIOFOR (Biogenic aerosolformation in the boreal forest) homepage (http://mist.helsinki.fi/Biofor/index.html).
Back trajectories
The air mass back trajectories needed in this workwere calculated with the long-range transportmodel TRADOS (Pöllänen et al. 1997, Valkamaand Pöllänen 1996). The model is an air parceltrajectory model of the Lagrangian type. Thethree-dimensional trajectories and the dispersionparameters are computed by using numerical me-teorological forecasts obtained from the NordicHIRLAM (High Resolution Limited-AreaModel) weather prediction model which contains31 vertical levels. The horizontal grid resolutionof the HIRLAM version used in this study was 44× 44 km. Parameters obtained from the HIRLAMdatabase are the surface data (the surface pres-sure and ground surface temperature, air tempera-ture, relative humidity, surface wind speed, andprecipitation) and the pressure level data (ambi-ent air temperature, relative humidity, vector wind,and geopotential height) at seven different con-stant pressure levels: 1 000, 925, 850, 700, 500,300, and 100 hPa.
In the present study, air parcel back trajecto-ries arriving at Hyytiälä and Värriö on the 925-hPa pressure level were calculated typically 96hours (66% of trajectories belonging to classes Ito IV, see next section for trajectory classes) back-wards in time. The trajectories were shorter in timewhen they left the geographical region of the cur-rent version of HIRLAM, approximately the areabetween (47°N, 103°W), (88°N, 180°E), (62°N,100°E), (50°N, 55°E), (27°N, 33°E), (35°N, 0°E),(22°N, 43°W), and (37°N, 70°W), i.e. from cen-
318 Kulmala et al. • BOREAL ENV. RES. Vol. 5
tral Canada to North Pole, central Siberia, Cas-pian Sea, Egypt, Gibraltar Strait, central AtlanticOcean, and Eastern USA. The 925-hPa level waschosen because the TRADOS output for the 1000-hPa level is generally less accurate due to topog-raphy effects. Two trajectories per day, arrivingat 00 UTC and 12 UTC, were computed. The timeperiod covered from 3 Dec. 1997 to 22 Aug. 1998,from 18 Nov. 1998 to 28 Jul. 1999 and from 9Sep. 1999 to 22 Sep. 1999.
Trajectory classification
The trajectories were plotted on maps and whichtheir classification established (see Fig. 1). The
area was divided into squares 30° times 10° inlongitudional and latitudional directions. All thegrid squares were numbered and all the squaresthat the trajectory passed through were tracked. Ifthe trajectory passed the areas of a special inter-est (Kola Peninsula), this was also marked. Ac-cording to this analysis, trajectories were dividedinto five main classes corresponding to four maindirections (Table 1). If the trajectory did not be-long clearly to classes I to IV or it was circulatingaround Finland, it was not considered in the fol-lowing analysis (class V). The back trajectoriescoming from sector IV to Hyytiälä and from sec-tor I to Värriö are presented in Fig 1a and b, re-spectively. The definition of these four areas wasbased on the large-scale concentration data of
Arrival: Värriö 67.77 N, 29.58 E, 25.1.1998 00 UTC, 925 hPa, Length 52 hours
TRADOS Back Trajectory, FMI
I
Fig. 1. — a: 49 hours long TRADOS back trajectory arriving at Hyytiälä 25 May.1999 12 UTC on the 925 hPapressure level; — b: 52 hours long back trajectory arriving at Värriö 25 Jan.1998 00 UTC on the 925 hPapressure level.
Arrival: Hyytiälä 61.84 N, 24.3 E, 25.5.1999 12 UTC, 925 hPa, Length 49 hours
TRADOS Back Trajectory, FMI
12
22
32
13
23
33
14
24
34
a b
Table 1. Sectors used in trajectory classification.—————————————————————————————————————————————————Sector Code Comments1)
—————————————————————————————————————————————————The Arctic I Trajectories originating from squares 12, 13, 14, or 24, and not passing overOcean, N–W pollution sources or square 22
Russia northeast II Trajectories that passed square 22 before arriving to the measurement siteof Finland, N–E
Kola Peninsula IIb Trajectories passing Kola Peninsula(Värriö only)
Russia southeast III Trajectories for which the last or second to last square passed through was 32of Finland, S–E
Central Europe IV Trajectories for which the last square was 33 or 34and Great Britain,S–W
Not classified V Not belonging to I to IV—————————————————————————————————————————————————1) Routes for different classes, i.e. the combinations of squares corresponding to a certain class, were deter-mined by plotting all the trajectories and choosing the appropriate class for all the combinations of squares thetrajectories passed.
319BOREAL ENV. RES. Vol. 5 • Gas and aerosol concentrations at two forest sites
various air pollutants (Tarrasón et al. 1998, Hjell-brekke 1999). According to Kahl et al. (1989),only large geographical domains can be identifiedwith trajectory statistics.
The number of trajectories coming from eachsector during different seasons to Hyytiälä andVärriö are presented in Fig. 2. The seasons weredefined according to months as follows: winter(December, January, February), spring (March,April, May) and summer (June, July, August).Since the trajectories were available only for ashort period in autumn, this season was not ana-lysed separately. The air masses arriving fromsector IV dominated in Hyytiälä (more than 350during the whole period). However, in spring thesector I was dominating. In case of Värriö, thearriving air masses originated equally frequentlyfrom sectors I and II + IIb. The extra sector IIbdirectly from Kola Peninsula industrial areas is
important for the air quality in Värriö and wasanalysed separately for gas and particle concen-trations. Corresponding observations were notthen included in class II results. The trajectorysectors I and II (+IIb for Värriö) were much moredominating for Värriö than for Hyytiälä. This isin accordance with long-term wind direction sta-tistics (Tammelin 1991) as well as with trajectoryflow climatology in northern Finland reported byRummukainen et al. (1996).
Results and discussion
In the following the meteorological parameters(radiation, temperature, humidity), gas (CO2, H2O,O3, NOx, SO2) and aerosol concentrations, themicrometeorological fluxes of momentum, heatCO2, H2O and aerosol particles, and shoot scale
I II IIb III IV V 0
20
40
60
80
100
120
140
Sector
Winter
I II IIb III IV V 0
20
40
60
80
100
120
140
Sector
Spring
I II IIb III IV V 0
20
40
60
80
100
120
140
Sector
Summer
Fig. 2. The frequency of air mass origin calculatedusing back trajectories, divided according to seasons,arriving at Hyytiälä (black column) and Värriö (whitecolumn). The number of trajectories in winter is 341/342 (Hyytiälä/Värriö), in spring 355/344 and in sum-mer 282/282.
320 Kulmala et al. • BOREAL ENV. RES. Vol. 5
gas exchange were analysed according to the ori-gin of the arriving air masses and seasons. Table2 summarises the analysed parameters and obser-vation levels. The observed range of variation wasdescribed by percentile statistics, which are notvery sensitive to possible erroneous results inobservations. In Tables 3 and 4, a summary ofstatistics is presented. Before statistical analysis,to suppress short turbulence-time-scale variationsin observations, the averages of the measuredquantities were calculated around the arrival timeof trajectories over forty minutes (aerosol parti-cle concentrations) to hourly (all other quantities)period.
Meteorology
Wind direction
Measurements of wind direction in Hyytiälä andVärriö were compared with the directions of ar-riving trajectories. The wind direction was meas-ured at 50 meters height in Hyytiälä and at 10meters height in Värriö. The trajectories arrivingat Hyytiälä and Värriö on the 925-hPa pressurelevel were used. The corresponding height de-pends on surface pressure and is about 600 me-
ters when surface pressure is 1000 hPa.In the boundary layer the wind direction is not
generally constant with height. Frictional forces,temperature advection, and topography affect thewind direction. Because boundary layer frictiondecreases with height, wind direction tends to turnclockwise with increasing height in the northernhemisphere (Holton, 1992). Cold and warm airadvection cause counterclockwise and clockwiseturning of wind direction with height, respectively.At Hyytiälä the wind direction was generally by20–30 degrees more at 925-hPa pressure level thanat 50 meters height. This difference was quite in-dependent of the wind direction. At Värriö, how-ever, the respective difference was dependentstrongly on the wind direction. This implies theeffect of local topography on the wind directionclose to the surface.
Temperature
In winter and in spring, the lower temperatures inHyytiälä corresponded to northeast and southeastair masses. This did not apply to the spring sea-son in Värriö (temperatures for winter were notavailable). In summer, the air coming from south-east and southwest was generally warmer, and the
Table 2. List of parameters and observation levels at the SMEAR stations, relevant to current study.—————————————————————————————————————————————————Parameter Measurement level, SMEAR I Measurement level, SMEAR II—————————————————————————————————————————————————Wind direction 10 m 50 mTemperature 15 m 67 mGlobal radiation 15 m 15 mUVA and UVB 15 m 15 mRH 67 mO3 15 m 67 mNOx 15 m 67 mSO2 15 m 67 mH2O 67 mCO2 67 mParticle concentration 15 m 2 mSize distribution (3–500 nm) 2 mExchange CO2, H2O, NOx
and O3 by shoots Canopy top Canopy topTurbulent fluxes of momentum, heat,
CO2, H2O and aerosol particles(bigger than 10 nm in diameter)by eddy covariance 23.3 m and 46.0 m
—————————————————————————————————————————————————
321BOREAL ENV. RES. Vol. 5 • Gas and aerosol concentrations at two forest sites
Table 3. Percentile statistics for parameters measured at Hyytiälä corresponding to trajectory arrival classesduring different seasons.—————————————————————————————————————————————————
Percentile Winter Spring Summer—————————————— ————————————— —————————————
I II III IV I II III IV I II III IV—————————————————————————————————————————————————Temperature (°C) at 12 UTC
323BOREAL ENV. RES. Vol. 5 • Gas and aerosol concentrations at two forest sites
Table 4. Percentile statistics for parameters measured at Värriö corresponding to trajectory arrival classesduring different seasons.—————————————————————————————————————————————————
Percentile Winter Spring Summer————————————— —————————————— —————————————
I II/IIb III IV I II/IIb III IV I II/IIb III IV—————————————————————————————————————————————————Temperature (°C) at 12 UTC
northern air masses brought with them lower tem-peratures.
In summer, the median temperature in Hyytiälävaried between 15 and 20 °C at daytime. At night,the temperature was approximately 5 °C lower.In winter, there was more variations in the data,the median was between 0 and –10 °C at day andat night. In Värriö, the median varied between 12and 17 °C in summer at day and was again lowerat night.
Radiation
The global radiation statistics were evaluated for12 UTC (Fig. 3). The lowest radiation fluxes dur-ing all seasons corresponded to trajectories arriv-ing from southwest (III) and southeast (IV) sec-tors. This reflects the higher cloudiness associ-ated with these air masses. An exception was thesummer season in Hyytiälä, when the lowest ra-diation values were measured during the airmasses of northeast origin (sector II). However,in winter this was the direction which gave thehighest global radiation values, in Hyytiälä andVärriö 90th percentiles were about 250 and 160W m–2, as compared with about 100 W m–2 corre-sponding to other sectors. The contrast betweenthese winter and summer radiation values was ac-companied by a similar contrast in surface pres-sure values, which indicates that prevailingly high
and low pressure weather systems correspondedto air masses arriving from sector II in winter andsummer, respectively. The seasonal migration ofweather systems has to be responsible for this. Inspring, which is the season of frequent nucleationevents as will be seen later, the highest radiationcorresponded to northern air masses (sectors I andII).
The UV-A and UV-B radiation statistics be-haved similarly to global radiation (not shown).The median of UV-A radiation in summer inHyytiälä was 25–35 W m–2 and in Värriö 15–25W m–2. In both cases, UV-A radiation intensity inwinter was only few watts per square meter. Themedian of UV-B radiation data in Hyytiälä in sum-mer was 1–1.5 W m–2 and in Värriö 0.6–1.1 Wm–2. In winter it was less than 0.05 W m–2.
Relative humidity
Relative humidity was measured only in Hyytiälä.In spring and in summer, the median of RH was50%–70% at daytime, and higher at night. In win-ter, it was nearly 100%, and the variation of therelative humidity was much smaller than duringother seasons. This was due to the low saturationvapour pressure. Back trajectory direction did nothave a clear connection to relative humidity, ex-cept at daytime in spring and in summer the humid-ity was higher during southwest winds (sector IV).
325BOREAL ENV. RES. Vol. 5 • Gas and aerosol concentrations at two forest sites
Gases
The O3, NOx and SO2 concentrations discussed inthis section were measured at both the sites. Thewater and carbon dioxide concentrations wereanalysed only for Hyytiälä measurements.
Ozone
A seasonal cycle in ozone concentrations wasobserved. At both the sites the highest ozone con-centrations were measured in spring and the low-est in winter (Fig. 4). For example, the seasonalmedians in Hyytiälä were 27.2 ppb (winter), 44.1ppb (spring) and 34.6 ppb (summer). Scheel et al.(1997) reported similar behaviour in seasonal dif-ferences of O3: the average concentration in UtoIsland (59.8°N, 21.4°E, 7 m a.s.l) was 27.0 ppb in
winter and 40.8 ppb in summer over the period1989 to 1993.
In winter, the higher O3 concentrations bothin Hyytiälä and Värriö corresponded to trajecto-ries from sectors I (the Arctic Ocean) and II (north-west Russia). On the contrary, in spring there wereobservations of higher O3 concentrations fromsectors III (south-west Russia, including St.Petersburg’s area) and IV (Central Europe), eventhough the distributions were rather wide. AlsoBeine et al. (1996) observed higher ozone con-centrations at Ny-Ålesund Zeppelin mountain sta-tion (78°55´N, 11°53´E, 474 m a.s.l) in air massesfrom Russia and Western Europe during the springseason in 1994. This is consistent with the resultsof Laurila (1999) and Simmonds (1997) on thebehaviour of continental Europe as a source andsink of O3 in summer and winter, respectively. Inwinter the polluted air masses (sectors III and IV)
Fig. 3. Global radiation in Hyytiälä (left) and Värriö(right) at 12 UTC, classified according to trajectorysectors (see Table 1) and seasons. The presentedstatistics are the 10th, 25th, 50th, 75th and 90th per-centiles.
I II III IV 0
50
100
150
200
250Winter
Glo
bal r
adia
tion
[W m
–2]
0
200
400
600
800Spring
0
200
400
600
800Summer
I II III IV
Glo
bal r
adia
tion
[W m
–2]
I II III IV
Glo
bal r
adia
tion
[W m
–2]
Sector Sector
Sector
326 Kulmala et al. • BOREAL ENV. RES. Vol. 5
act as a sink for O3, and in spring when the photo-chemical reactions start to play an important role,the polluted air masses behave also as a sourcefor O3 (for chemistry see Seinfeld and Pandis 1998,Finlayson-Pitts and Pitts 2000). Thus the biggestseasonal variation between the winter and springO3 concentrations was from sectors III and IV.
In summer, ozone concentrations were lowerthan in spring, being highest from sector IV (Cen-tral Europe) in Värriö and also from sector III(south-west Russia) in Hyytiälä. In summer, whenthe photochemical radical production is not a lim-iting factor as in winter, ozone production is moresensitive to NOx concentrations (Kleinman 1991).The NOx concentrations were lowest in summer(Fig. 5). Also the dry deposition velocity onto thevegetation is higher in summer due to stomatalactivity.
Only a small difference between the nighttimeand daytime values of ozone were observed dur-
ing all seasons in Värriö, but in Hyytiälä the diur-nal cycle was clearly seen, especially in summer.At night the concentration decreases because ofdeposition and limited transport from above dueto stable stratification, but at day O3 is transportedfrom above due to intensive mixing and there isadditional photochemical production of O3 (seealso e.g., Lopez et al. 1993). The difference be-tween Hyytiälä and Värriö can be explained bythe same mechanisms: larger deposition due tovegetation characteristics and more photochemi-cal production due to higher NOx concentrationsin Hyytiälä.
Nitrogen oxides
Maximum NOx concentrations were observed inwinter and spring at both the sites (Fig. 5). Thelowest seasonal NOx concentrations were meas-
I II IIb III IV 10
20
30
40
50
60Winter
O3
conc
entr
atio
n [p
pb]
I II IIb III IV 10
20
30
40
50
60Spring
O3
conc
entr
atio
n [p
pb]
I II IIb III IV 10
20
30
40
50
60Summer
O3
conc
entr
atio
n [p
pb]
Sector Sector
Sector
Fig. 4. O3 concentrations in Hyytiälä (left) and Värriö(right), classified according to trajectory sectors andseasons. The presented statistics are the 10th, 25th,50th, 75th and 90th percentiles for the observationsboth at 00 UTC and 12 UTC.
327BOREAL ENV. RES. Vol. 5 • Gas and aerosol concentrations at two forest sites
ured in summer, when in Värriö the concentrationswere close to the detection limit (0.1 ppb). Theseasonal cycle can partly be explained by differ-ences in atmospheric mixing during the long-rangetransportation and partly by an increase in emis-sions from closely located sources during wintermonths (e.g. an increase in energy consumptiondue to heating). A clear southward NOx gradientwas observed when comparing the concentrationsat two sites. This is in agreement with the observa-tions in Norwegian Arctic (Solberg et al. 1997).
The highest NOx concentrations were meas-ured in air masses arriving at Hyytiälä and Värriöfrom more polluted continental Europe, i.e. fromsouth-west Russia (III) and from Central Europe(IV). Then the measured concentrations were ap-proximately twice as high as those from sectors Iand II. This was a trend observed regardless ofthe season. Similar air mass dependence was ob-served in Norwegian Arctic in spring 1994
I II IIb III IV 0.1
1
10Winter
NO
x co
ncen
trat
ion
[ppb
]
I II IIb III IV 0.1
1
10Spring
NO
x co
ncen
trat
ion
[ppb
]
I II III IV 0.1
1
10Summer
NO
x co
ncen
trat
ion
[ppb
]
IIb
Sector Sector
Sector
Fig. 5. NOx concentrations in Hyytiälä (left) and Värriö(right), classified according to trajectory sectors andseasons. The presented statistics are the 10th, 25th,50th, 75th and 90th percentiles for the observationsboth at 00 UTC and 12 UTC.
(Solberg et al. 1997). The concentrations inHyytiälä were two to four times higher than theconcentrations in Värriö because of the smallerdistance to the industrial sources, especially in St.Petersburg’s area southeast of Finland. The airfrom the Arctic Ocean brought especially low NOx
concentrations to Värriö.No significant diurnal variation in NOx con-
centration was found in Värriö. However, inHyytiälä the daytime concentrations were slightlyhigher than the nighttime ones in winter, espe-cially in cases when the air masses came fromsectors I and IV.
Sulphur dioxide
The highest SO2 concentrations were observed inwinter at both the stations, and the lowest con-centrations in summer (Fig. 6). This can be ex-
328 Kulmala et al. • BOREAL ENV. RES. Vol. 5
plained by human activities (burning of fossil fu-els) and an increased transport range due to thelack of photochemical transformation of SO2 andless mixing with the higher atmosphere in winter.
The concentrations arriving from the ArcticOcean (sector I) were very low at both measure-ment sites all over the year (Fig. 6). Also the con-centrations from Central Europe (IV) were quitelow. The highest concentrations in Hyytiälä weremeasured throughout the year in air masses com-ing from south-west Russia and the industrial dis-trict in St. Petersburg region (III). In Värriö, thehighest concentrations were detected in air massescoming from Russia: from Kola Peninsula (IIb)in winter and summer, and from Russia (sectorsII and III) in spring.
In Värriö, the concentrations from area II weremuch higher than in Hyytiälä. Some very high
peaks were seen when air masses passed throughthe industrial area of Kola Peninsula (IIb) espe-cially in winter (90th percentile value near 12 ppb).High peaks were also observed in spring and sum-mer. No significant diurnal variation in SO2 con-centrations was found.
Water
Seasonal variation in water concentration showsthat high values (median over all trajectory classes11.8 g m–3) were measured in summer and lowvalues (median 4.2 g m–3) in winter. In winter,and in spring air masses containing more waterarrived mainly from sectors IV and I (CentralEurope and the Arctic Ocean, Fig. 7), as expected.In summer, the highest water content of air came
I II IIb III IV 0.1
1
10
Winter
SO
2 co
ncen
trat
ion
[ppb
]
I II IIb III IV 0.1
1
10
Spring
SO
2 co
ncen
trat
ion
[ppb
]
I II IIb III IV 0.1
1
10Summer
SO
2 co
ncen
trat
ion
[ppb
]
Sector Sector
Sector
Fig. 6. SO2 concentrations in Hyytiälä (left) and Värriö(right), classified according to trajectory sectors andseasons. The presented statistics are the 10th, 25th,50th, 75th and 90th percentiles for the observationsboth at 00 UTC and 12 UTC.
329BOREAL ENV. RES. Vol. 5 • Gas and aerosol concentrations at two forest sites
from sector III (south-west Russia), with waterconcentrations more than four times as high aswintertime concentrations from the same area.Furthermore, the lowest values were measuredfrom sector I (the Arctic Ocean). No significantdiurnal variation in air water content was observed.
Carbon dioxide
In winter and in spring, high CO2 concentrationscome to Hyytiälä from sectors III and IV (south-west Russia and Europe) (Fig. 8). This is a verycommon feature for pollution-related gases. CO2
emissions to the atmosphere are due to burning offossil fuels and biomass. In winter and spring, themedian CO2 concentration was about 369 ppmfrom sectors III and IV. The air masses from the
other sectors brought approximately 364 ppm. Insummer, the values were lower than in winter andin spring.
Also the forest-atmosphere CO2 exchange isimportant for the concentrations in Hyytiälä. Gen-erally the respiration of the ecosystem is higherin autumn because of the higher temperature, anddepressed in winter when the temperature is lower.In autumn, the forest becomes a source of CO2 tothe atmosphere, and starts to act as a sink in spring,when daily photosynthesis exceeds respiration.Therefore, in summer the median of carbon diox-ide concentration over all trajectories and arrivaltimes was about 359 ppm and at other seasons itwas about 366–367 ppm. For the same reason,the concentrations at night were higher than atday, except in winter when the values were al-most the same. In summer, the median CO2 con-
I II III IV 0
2
4
6
8Winter
H2O
con
cent
ratio
n [g
m–3
]
I II III IV 0
2
4
6
8
10
12Spring
I II III IV 5
10
15
20Summer
H2O
con
cent
ratio
n [g
m–3
]
H2O
con
cent
ratio
n [g
m–3
]
Sector Sector
Sector
Fig. 7. Water vapour concentrations in Hyytiäläclassified according to trajectory sectors and seasons.The presented statistics are the 10th, 25th, 50th, 75thand 90th percentiles for the observations both at 00UTC and 12 UTC.
330 Kulmala et al. • BOREAL ENV. RES. Vol. 5
centration was about 364 ppm at night and 355ppm at day.
Aerosols
The aerosol concentrations in Värriö were typi-cally smaller than in Hyytiälä (Fig. 9). Thenighttime concentrations were somewhat smallerthan daytime concentrations at both the sites. Thewintertime concentrations were significantlysmaller than the spring and summer concentra-tions. In spring, the highest concentrations oc-curred in air masses from sector I and were re-lated to nucleation events. Beine et al. (1996)observed lowest aerosol particle concentrationsin Arctic air masses during the spring of 1994 atNy-Ålesund Zeppelin mountain station (78°55´N,11°53´E, 474 m a.s.l). This might indicate the
importance of biological factor in particle forma-tion events.
Nucleation events are observed quite often inHyytiälä and also in Värriö. Fig. 10 presents thefraction of trajectories accompanied with nuclea-tion bursts for different sectors. Note that the lownumber of observed events can lead to unreliableresults (particularly true in the winter plot). Thehighest fractions were observed in springtime forair masses coming from sectors I and II to Hyytiälä(almost 0.25) and to Värriö (around 0.1). In sum-mer, the fractions were significantly smaller thanin spring. In winter, no nucleation events wereobserved in Värriö.
The modal aerosol concentrations, obtainedfrom the size spectrum measurements in Hyytiälä,were investigated. Modal structure of the aero-sols was analysed by fitting two or three (depend-ing on the concentration of smallest particles) log-
I II III IV 350
360
370
380Winter
CO
2 co
ncen
trat
ion
[ppm
]
I II III IV 350
360
370
380Spring
I II III IV 350
360
370
380Summer
CO
2 co
ncen
trat
ion
[ppm
]
CO
2 co
ncen
trat
ion
[ppm
]
Sector Sector
Sector
Fig. 8. Carbon dioxide concentrations in Hyytiäläclassified according to trajectory sectors and seasons.The presented statistics are the 10th, 25th, 50th, 75thand 90th percentiles for the observations both at 00UTC and 12 UTC.
331BOREAL ENV. RES. Vol. 5 • Gas and aerosol concentrations at two forest sites
normal modes to the measured particle size spec-trum. From this data the concentration in eachmode (nucleation, Aitken and accumulationmodes) was calculated. The concentrations werecalculated for each trajectory arrival time as anaverage over 40 minutes period.
The nucleation mode is defined as the smallestmode with a modal diameter less than 20 nm. Thedaytime nucleation mode had the highest concen-tration in spring (Fig. 11), particularly when theair was coming from the Arctic Ocean sector. Thenighttime concentrations were always low.
The Aitken mode concentrations did not seemto vary much according to the sector of air origin(Fig. 11). The air masses from south-west Russia(III) and Central Europe (IV) sectors clearly domi-nated the accumulation mode concentrations, ex-cept in summer, when also sector II had quite highconcentration values.
Chamber measurements
Gas exchange of pine shoots was measured dur-ing summers 1998 and 1999 in Hyytiälä andVärriö. The gas exchange measurements allowedus to study the net CO2 exchange, the transpira-tion, the NOx exchange, and the O3 deposition onthe shoot.
No significant variation of photosynthesis ac-cording to trajectory directions could be seen (Fig.12). The main controlling environmental factorfor photosynthesis is radiation. However, over twosummers factors other than radiation affected theCO2 exchange and the correspondence with ra-diation (see Fig. 3) could not be observed. Thephotosynthesis and transpiration in Hyytiälä wereapproximately twice as large as in Värriö (notshown). In Hyytiälä, the nighttime transpirationvalues were practically zero and the net CO2-ex-
I II IIb III IV 10
2
103
104
Sector
Aer
osol
num
ber
conc
entr
atio
n [c
m–3
] Winter (1 Dec–28 Feb.)
I II IIb III IV 10
2
103
104
Sector
Aer
osol
num
ber
conc
entr
atio
n [c
m–3
] Spring (1 Mar.–31 May)
I II IIb III IV 10
2
103
104
Sector
Aer
osol
num
ber
conc
entr
atio
n [c
m–3
] Summer (1 Jun.–31 Aug.)
Fig. 9. Total aerosol number concentration measuredin Hyytiälä and Värriö at 12 UTC, classified accordingto trajectory sectors and seasons. The presented sta-tistics are the 10th, 25th, 50th, 75th and 90th percen-tiles.
332 Kulmala et al. • BOREAL ENV. RES. Vol. 5
change was slightly negative, indicating respira-tion instead of photosynthesis. In Värriö, thenighttime values of photosynthesis and transpira-tion were often small but positive because of brightnights.
The ozone deposition was stronger when airarrived from sectors III and IV, when also the high-est O3 concentrations were observed. No connec-tion between the O3 deposition and radiation wasobserved. However, the NOx emissions were larg-est for the Central Europe direction (sector IV),and the pattern of the NOx emission statistics ac-cording to trajectory origin was similar to radia-tion. There was virtually no NOx emission at night.
Fluxes
The turbulent fluxes of momentum, sensible heat,water vapour, carbon dioxide and aerosol parti-
I II III IV 0
0.01
0.02
0.03
0.04
Sector
Frac
tion
of tr
ajec
torie
s le
d to
nuc
leat
ion Winter (1 Dec.–28 Feb.)
I II III IV 0
0.05
0.1
0.15
0.2
0.25
Sector
Frac
tion
of tr
ajec
torie
s le
d to
nuc
leat
ion Spring (1 Mar.–31 May)
I II III IV 0
0.02
0.04
0.06
0.08
0.1
Sector
Frac
tion
of tr
ajec
torie
s le
d to
nuc
leat
ion Summer (1Jun.–31 Aug.)
Fig. 10. Fraction of analysed trajectories arriving at12 UTC that accompanied with nucleation events inHyytiälä (left) and Värriö (right) according to trajectorysectors during different seasons. The numbers of tra-jectories are the same as in Fig. 2.
cles, measured in Hyytiälä by the eddy covariancetechnique, were classified according to the trajec-tory origin.
The sensible heat fluxes behaved similarly toglobal radiation values shown in Fig. 3, as ex-pected. As a feature in winter, the sensible heatfluxes within sector III air masses were predomi-nantly upwards, while sensible heat was frequentlytransported also downwards during the other airmasses. This is explained with the colder (rela-tive to the surface) continental air arriving fromsector III. The sensible heat fluxes at night weretowards the surface and small compared to day-time values.
The absorption of momentum by the surfaceis determined by local characteristic, the aerody-namic roughness of the surface, and by the driv-ing force, the large-scale air flow. The differencesin momentum flux reflected the differences in pre-vailing synoptic situations, accompanying with
333BOREAL ENV. RES. Vol. 5 • Gas and aerosol concentrations at two forest sites
I II III IV 10
0
101
102
103
Winter
Mod
al n
umbe
r co
ncen
trat
ion
[cm
–3]
Sector I II III IV
100
101
102
103
Spring
Mod
al n
umbe
r co
ncen
trat
ion
[cm
–3]
Sector
I II III IV 10
0
101
102
103
Summer
Mod
al n
umbe
r co
ncen
trat
ion
[cm
–3]
Sector
Fig. 11. Classification of number concentrations ofnucleation (left), Aitken (middle) and accumulation(right) mode particles in Hyytiälä at 12 UTC accordingto trajectory sectors and seasons. The presented sta-tistics are the 10th, 25th, 50th, 75th and 90th percen-tiles.
trajectory sectors. The momentum fluxes weresignificantly lower at night compared to daytimevalues during spring and summer seasons.
The water vapour flux was almost always up-wards, and the rare negative values were close tozero. During the growing season, the main con-tribution to water flux comes from transpirationby trees, which is linked to their photosyntheticactivity and ambient relative humidity. In winter,the water fluxes were small and rather well corre-lated with global radiation. The pattern was simi-lar also for the spring and summer seasons.
In winter, the forest is a small source of CO2
to the atmosphere. In spring, the highest CO2 up-take occurred during northeast air origin. The highradiation corresponding to the Arctic Ocean tra-jectories (see Fig. 3) accompanied probably withlow temperatures, and was not so favourable forphotosynthesis. In summer, the CO2 uptake wasrelatively even for all sectors, although there were
significant differences in global radiation. Thiswas similar to CO2 exchange of shoots (Fig. 12).
The particle fluxes were available only forspring and summer seasons, from March to Au-gust. In the analysis the cases corresponding tolocal wind direction between 220 and 260 degreeswere neglected as possibly affected by local pol-lution sources (the station buildings). Negativeflux values correspond to the deposition of parti-cles. The statistics have been obtained from lim-ited number of cases for each sector: 20, 26, 20and 38 together for spring and summer seasonsfor the sectors from I to IV, respectively. Sector Idiffers clearly from others: the trajectories arriv-ing from the Arctic Ocean often brought nuclea-tion events with them, and large downward parti-cle fluxes were associated with the events (Fig.13). For the spring period, only one observationof a large particle flux shifted the 10th percentile,corresponding to sector II trajectory, to a large
334 Kulmala et al. • BOREAL ENV. RES. Vol. 5
negative value. In summer, the statistics corre-sponding to southeast trajectories (sector III)reflect presence of large negative fluxes, charac-teristic to nucleation events. These correspondhowever only to 7 cases, three of which weresmaller than –5 × 106 m–2 s–1.
Conclusions
In winter, the air masses from over the northeastand southeast Russia (sectors II and III; see Table1 for sectors) brought lower temperatures, whichwas not the case in summer. In summer, the south-ern air masses were the warmest. The highest ra-diation fluxes in winter were observed duringairflow from northeast (sector II), which corre-sponded in summer to lowest short wave radia-
tion input in Hyytiälä, but not in Värriö. The high-est radiation in summer was observed during theair masses from the Arctic Ocean. In spring thehighest radiation corresponded to northern airorigins (sectors I and II). The high radiation wasrelated typically to low cloudiness arriving fromthese sectors. In these airflows also nucleationevents occurred frequently. In spring and sum-mer the relative humidity was higher in south-west airflow and lower in the air from the ArcticOcean.
According to concentrations of tracers of airpollution in air masses, the sectors can be corre-spondingly divided into typically clean sectors (Iand II) and polluted sectors (III and IV). South-west Russia together with the industrial districtof St. Petersburg is located in sector III and thepolluted areas of Central Europe in sector IV.
I II III IV 0.05
0.1
0.15
0.2
0.25Summer
Pho
tosy
nthe
sis
[g m
–2 s
–1]
I II III IV
0
2
4
6
8
10
12Summer
Tran
spira
tion
[g m
–2 s
–1]
I II III IV 20
40
60
80
100
120Summer
O3
depo
sitio
n [n
g m
–2 s
–1]
I II III IV –5
0
5
10
15
20Summer
NO
x em
issi
on [n
g m
–2 s
–1]
Sector Sector
Sector Sector
Fig. 12. Photosynthesis, transpiration, O3 deposition and NOx emission from/to pine shoot per unit needle areain Hyytiälä at 12 UTC during the summer season, classified according to trajectory origin. The presented statis-tics are the 10th, 25th, 50th, 75th and 90th percentiles.
335BOREAL ENV. RES. Vol. 5 • Gas and aerosol concentrations at two forest sites
I II III IV –80
–60
–40
0
20
Spring
Par
ticle
flux
[x 1
06 m
–2 s
–1]
I II III IV –30
–20
–10
0
10
20
Summer
Par
ticle
flux
[x 1
06 m
–2 s
–1]
Sector Sector
Fig. 13. Particle number fluxes measured in Hyytiälä at 12 UTC as classified according to air mass sectors andseasons. The presented statistics are the 10th, 25th, 50th, 75th and 90th percentiles.
What it comes to ozone concentrations, there wasa clear difference between clean and polluted airmasses. In wintertime the polluted air acts as asink for ozone and in spring and summertimemainly as a source. Therefore, the air masses fromsectors III and IV brought to both Hyytiälä andVärriö high O3 concentrations in spring and sum-mer, and low concentrations in winter. Consist-ently, also higher deposition of O3 on pine shootswas observed in summer during the flow fromsectors III and IV. Regardless of the season, theseair masses contained the highest NOx levels. TheNOx concentrations in flows from these sectorswere almost twice as high as in flows from sec-tors I and II. The SO2 concentrations in sector IVair masses were quite low. The highest SO2 con-centrations were measured in Hyytiälä in airmasses from south-west Russia (III). In Värriöthe highest SO2 concentrations were detected inair masses arriving from Russia: from south-westRussia (III) in winter and in spring, but also fromnorthern Russia and Kola Peninsula (II). In Värriö,the SO2 concentrations from area II were muchhigher than in Hyytiälä, and some very high peakswere observed in winter when air masses passedthrough the industrial area of Kola Peninsula.
Some clear differences also in aerosol proper-ties between clean and polluted air were observed.In polluted air masses accumulation mode con-centrations were higher. The analysis of nuclea-tion events showed that nucleation typically oc-curred in clean air masses. In spring, the nuclea-tion occurred in about 25% of cases in Hyytiälä
and about 10% of cases in Värriö when air massoriginated from clean sectors. The frequency ofnucleation events corresponding to polluted airsectors was much lower. During the nucleationevents large downward aerosol particle numberfluxes were measured.
Although there were some differences in airmass characteristics in Hyytiälä and Värriö, theoverall behaviour was similar at both the sites.
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