New particle formation in the fresh flue gas plume increase the effective particle number emissions of a coal-fired power plant

Atmospheric emissions, including particle number and size distribution, of a 726 MWth coal-fired power plant were studied experimentally from power plant stack and from flue gas plume dispersing in the atmosphere. Experiments were conducted under two different flue gas cleaning conditions. The results were utilised in a plume dispersion and dilution modelling taking into account nucleation particle precursor (H2SO4 resulted from the oxidation of emitted SO2) formation 5 and assessment related to nucleation rates. The experiments showed that the primary emissions of particles and SO2 were effectively reduced by flue gas desulphurization and fabric filters, especially the emissions of particles smaller than 200 nm in diameter. Primary pollutant concentrations reached background levels in 200–300 seconds. However, the atmospheric measurements indicated that new particles are formed in the flue gas plume, even in the very early phases of atmospheric ageing. 10 The effective number emission of nucleated particles were several orders of magnitude higher than the primary particle emission. Modelling studies indicate that regardless of continuing dilution of the flue gas, nucleation precursor (H2SO4 from SO2 oxidation) concentrations remain relatively constant. In addition, flue gas nucleation is more efficient than natural atmospheric nucleation. Especially, the observation of the new particle formation with rather low flue gas SO2 concentrations 15 changes the current understanding on the air quality effects of coal-combustion. The results can be used to evaluate the optimal ways to achieve better air quality particularly in polluted areas like India and China. 1 Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2015-990, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: 5 February 2016 c © Author(s) 2016. CC-BY 3.0 License.


Introduction
In global scale, nearly 40 % of annual production of electricity is covered by coal combustion (EU, 2014).In addition to CO 2 emissions, known to have climatic effects, coal combustion causes emissions of other harmful pollutants like NO x , SO 2 , and particles, all decreasing the air quality and increasing health related risks but also affecting climate.Coal combustion related air quality problems exist especially in developing countries like China (Huang et al., 2014) where the power production is not always equipped with efficient flue gas cleaning systems.However, with proper combustion and flue gas cleaning technologies the fine particle emissions of coal combustion can be decreased to very low level and also the emissions of gaseous pollutants other than CO 2 can be decreased.
Particle mass and number emission factors for the 300 MW coal-fired power plant with electrostatic precipitator (ESP) and flue gas desulphurization unit (FGD) have been reported by Frey et al. (2014): the emission for particle mass (PM1) was 0.18 ± 0.06 mg MJ −1 and for fine particle number 2.3•10 9 ± 4.0•10 9 MJ −1 .However, it can be expected that particle emissions and also the characteristics, such as particle size, are highly dependent on technologies used in the power production.Only few studies have reported particle number size distributions and mean particle diameter for the coal combustion emissions.The mean particle diameters have been reported to be between 100 nm (Frey et al., 2014;Yi et al., 2008) and 1 µm (Yi et al., 2008;Lee et al., 2013).According to Saarnio et al. (2014), chemical composition of particles in the efficiently cleaned flue gas after the FGD is shifted towards desulphurization chemicals.Interestingly, sulphate particle emissions from coal combustion with proper cleaning technologies can restrain the global warming due to cooling effect of the particles (Frey et al., 2014;Charlson et al., 1992;Lelieveld et al., 1992).
Due to the emission limits of power plants, driven by needs for healthier environment, emissions should be kept at minimum.This can be achieved by different technologies.Flue gas NO x emissions can be reduced in the power plant boiler by applying low-NO x burners, whereas SO 2 emissions can be reduced by flue gas desulphurization (FGD) (Srivastava et al., 2001).Particle emissions can be reduced by electrostatic precipitators (ESP) and fabric filters (FF).Very low emission levels can be achieved by these techniques.For example from particle emission point of view, ESP typically removes 99% (Helble, 2000) of the fine particles.Further, Saarnio et al. (2014) showed that desulphurization plant with fabric filters remove up to 97 % of the fine particles.Combination of these techniques would then remove 99.97 % of the fine particle emissions of the particles formed in combustion.However, particle emission as well as the effects of technologies can differ from these if the emissions are measured from the diluted flue gas in the atmosphere.In principle, particle number and even particle mass can increase in the atmosphere for example due to the nucleation and condensation processes (Marris et al., 2012;Buonanno et al., 2012).However, there are very few observations of the processes in the diluting flue gas during the first few minutes after the stack.
Power plant plumes have been studied with aircrafts by measuring long distance cross-wind profiles of gases and particles (Stevens et al., 2012;Brock et al., 2002;Lonsdale et al., 2012;Junkermann The flying altitude of the helicopter was 150 meters above ground level or higher which corresponds to the LIDAR (Halo Photonics Streamline Doppler lidar with full-hemispheric scanning capability) (SI2) results for plume altitude.It should be noted that only the flue gases from the boiler under investigation were steered to bypass FGD and FF.Thus, in the "FGD+FF off" situation flue gas plume consisted of both the cleaned flue gas and the flue gas cleaned by ESP.This has to be kept in 95 mind in the analysis of atmospheric measurements.
Weather conditions were quite stable during the study.The wind direction and speed were 210 • and 6.5 m s −1 in "FGD+FF off" case and 260 • and 4 m s −1 in "FGD+FF on" case, respectively.The marine boundary layer height was 246-258 meters and the planetary boundary layer heights were 360-530 meters.However the calculations were made within the marine boundary layer because 100 the flue gas plume did not arise above it.The background aerosol concentrations for each measured gaseous component were: CO 2 403 ppm, SO 2 less than 2-8 ppb.The ambient temperature was 6.6-6.9 • C, the global radiation was 346-466 W m −2 and the visibility was over 30 000 meters.

Model description: Gaussian plume model 125
The Gaussian plume model is a solution to an advection-diffusion equation that describes the changes in the pollutant concentrations due to advection of wind and turbulent mixing with the surrounding air (Stockie, 2011).Accordingly, the concentration of a pollutant i, C i , emitted from a point-like source, can be expressed as follows: Here x, y and z are the spatial coordinates, aligned so that x axis corresponds to the wind direction, and H is the height at which i is emitted (stack height).Also, Q i is the emission rate of i at the source, U is the mean wind speed, and σ z as well as σ y are the so called dispersion coefficients which reflect the spatial extent of the plume as a function of the downwind distance x.The dispersion coefficients were calculated using the parameterization of Klug (1969) and the atmospheric stability class, which 140 It is worth noting that the background concentration of i is zero according to eq. 1: C i → 0 when z → ∞ or y → ±∞ However, the flue gas emitted from the stack was actually cleaner, in terms of particle number concentration, than the background air when the flue gas was cleaned properly.In order to take into account for such cases, the following equation was used instead of eq.1: where C ∞ is the background concentration of i, and C 0 is its concentration at the source.It can be readily shown that eq. 2 is a solution the advection-diffusion equation underlying eq. 1.Also, it is easily verified that Ĉ → C ∞ when z → ∞ or y → ±∞.Finally, the value of Q i in eq. 1 was chosen so that Ĉ → C 0 when z → H and x,y → 0.
An important output of the model is the dilution ratio of the flue gas plume, DR, which is calcu- are the modelled CO 2 concentration at time t and the CO 2 concentration measured in the stack, respectively.

Model description: Nucleation rate and particle formation calculations
The particle appearance (driven by nucleation and growth) rates at the lowest diameter detected by 155 the CPC (2.5 nm) were calculated using the parameterization developed by Lehtinen et al. (2007).
The key input parameters for the model are the nucleation rate (J nuc ), the particle growth rate (GR), and the coagulation sink which describes the rate at which clusters are removed via coagulational scavenging (CoagS).The parameter J nuc is calculated based on the estimated sulphuric acid concentrations as function of plume age as detailed below, and the particle growth rates are calculated 160 by assuming growth only via irreversible condensation of sulphuric acid.Also, CoagS is calculated from the condensation sink CS (which is calculated in a fashion described below) using the eq.8 in Lehtinen et al. (2007).Also, the initial size of the freshly nucleated clusters was varied, and the value of the shape factor (m in Eq. 6 in Lehtinen et al. (2007)) was set equal to -1.6.
The nucleation rates J nuc in the studied plume were calculated using the parameterization devel-165 oped by Kulmala et al. (2006) which has also been applied previously to model nucleation in plumes (Stevens et al., 2012(Stevens et al., , 2013)).Accordingly, ) is the sulphuric acid concentration.The value of A=1•10 −7 s −1 was chosen according to the study of Stevens et al. (2012Stevens et al. ( , 2013)).
Formation of [H 2 SO 4 ] was calculated assuming that it is produced only via the OH + SO 2 reaction It should be noted here that in the calculations the background concentration of NO x is assumed to be of minor importance when compared to NO x emitted by power plant.To support this, 185 the study of Pirjola et al. (2014) indicates that in the harbour area close to the power plant studied here the NO x concentration level is typically clearly lower than 100 ppb.

Primary emissions of the coal-fired power plant
The SO 2 and particle emissions of the power plant were strongly dependent on flue gas cleaning 190 system.This can be seen in Table 1 which shows flue gas concentrations for CO 2 , SO 2 , NO x , O 2 , particle number (N tot ), dust as well as flow rate in the duct in both flue gas cleaning conditions.In the shift from "FGD+FF off" to "FGD+FF on" situation the SO 2 concentration decreased nearly to fifth part, the concentration of dust decreased by a factor of 50 and the N tot decreased by a factor of four thousand.For other parameters the effect of FGD+FF was insignificant. 195 Figure 2 shows the particle number size distributions of flue gas in the stack in both cleaning conditions.These were measured using an electrical low pressure impactor (ELPI) and a scanning mobility particle sizer (SMPS) in both "FGD+FF on/off" cases.In the "FGD+FF on" case the SMPS measurement is a median value over few hours of operation due to low particle number concentrations in the stack.Based on the SMPS measurement the particle mean mobility diameter was 80 200 nm and the width of particle number size distribution (geometric standard deviation, GSD) was 1.45 for "FGD+FF off" case.In comparison, the mean mobility diameter was 31 nm for "FGD+FF on" and the width of particle number size distribution was 2.15.Based on the measurements using the ELPI mean aerodynamic diameter was 141 nm and GSD was 1.41.The difference in mean diameters measured using the ELPI and the SMPS comes from the size classification principle of the ELPI, 205 which is sensitive to particle density.In fact, the difference indicates effective density larger than unit density for emitted particles (approximately 3.1 g cm −3 ).In comparison, Saarnio et al. (2014) used effective density of 2.5 g cm −3 to convert the electrical mobility diameter measured using SMPS to aerodynamic diameter.When studying "FGD+FF on" case there was no difference between aerodynamic particle diameter and mean mobility diameter and, thus no difference in the density of the 210 particles.
Flue gas sample from the stack was diluted with hot dilution air before the particle instruments and thus the particle number concentrations (Table 1) and particle size distributions (Figure 2) are for non-volatile particles.In combustion studies the hot dilution air is typically used to prevent the formation of liquid nucleation particles and to minimize the effects of condensation of semi-volatile 215 compounds on particles.However, to ensure the measured particles were non-volatile and not affected by the dilution method itself, a thermodenuder (Rönkkö et al., 2011) was used periodically after the sampling and dilution.The thermodenuder did not affect the particle number size distribution, which confirms the non-volatile nature of the measured particles.Due to this non-volatility of the particles, the life time of the primarily emitted particles in the atmosphere can be longer than that 220 of volatile particles, e.g.nucleation mode particles observed in vehicle exhaust (Lähde et al., 2009).

Atmospheric measurements
The measurement results are shown in Figure 3 describing the flue gas plume concentrations as a function of plume age.The data was recorded based on gps-coordinates which were used to calculate distances from the stack, and the distances were changed to correspond plume age using wind speeds 225 6.5 m s −1 and 4.0 m s −1 (LIDAR, S3).The vertical lines denote the 2 km distance from the stack.
Figure 3 shows the dilution time scale of the flue gas in terms of CO 2 and SO 2 in both operation conditions.Same trend in SO 2 and N tot concentrations as observed in Table 1, was measured by instruments installed in the helicopter; in "FGD+FF off" situation the particle and SO 2 concentrations were higher than the "FGD+FF on" situation.It should be kept in mind that in "FGD+FF off"  in Figure 3a and 3b when approaching plume age zero.Thus, the dilution process is discussed below, mainly, from the maximum concentrations forward.

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An increase in total particle concentration can be seen in Figure 3 after 400 seconds aged the flue gas plume.This tendency can be seen in both flue gas cleaning situations.Based on Figure 3a, for "FGD+FF off" case the background particle concentration was 1430 cm −3 , after 200 seconds the concentration was at the background level and after 400 seconds it increased to 10 000 cm −3 .Based on CO 2 measurements, the dilution of flue gas was practically complete at 200 seconds.Similarly, 250 in "FGD+FF on" case after 500 seconds the particle concentration was slightly above background, after which increasing even up to 5 000 cm −3 after 700 seconds.Thus, the concentrations in the diluted and aged flue gas plume were higher than the background and significantly higher than could be expected based on the primary particle concentrations and observed dilution profiles.There is a moderate increase in CO 2 , and SO 2 concentrations at 350 sec in "FGD+FF on" case and at 250 255 seconds (SO 2 , CO 2 and N tot ) in "FGD+FF off" case (Figure 3).The moderate increase in "FGD+FF on" case can be seen in EEPS data (Figure 4) but not in "FGD+FF off" case.To authors knowledge the moderate peaks cannot be explained by additional external emission sources because there should not be any sources at the same altitude in the flight directions (Figure 1) or upwind of the plume direction.Thus, the increased concentrations seem to be caused by occasionally different plume 260 mixing.In general, taking into account the fact that there is no comprehensive measurement of the primary precursor matrix (only [SO 2 ] is measured), the primary precursor matrix might include low- volatile organics and SO 3 which can increase the probability of new particle formation.Due to the increasing trend in particle concentration, some estimation about nucleation rates can be calculated.
Depending on the plume age the mean nucleation rates calculated from the data shown in Figure 3 265 depended on the plume age being for "FGD+FF off" case 0-81 cm −3 s −1 and for "FGD+FF on" case 0 cm −3 s −1 to 18 cm −3 s −1 (average change in total particle number concentration at 400-482 s and 500-692 s).
Figure 4 shows the flue gas plume particle number size distribution as a function of plume age.
Distributions were calculated from the EEPS data measured from the helicopter in both "FGD+FF 270 on/off" situations as 10 second moving median method.In Figure 4  and SO 2 in Figure 3.The comparison between CPC (Figure 3) and EEPS particle size distribution (Figure 4) shows that the flue gas is diluting in 0-300 seconds in "FGD+FF off " and 0-550 seconds in "FGD+FF on " and after that more small particles (and some larger particles) are detected in both cases.Although, EEPS total particle number concentration cannot be compared to total concentration of CPC because Levin et al. (2015) showed that EEPS total particle number concentration is not 280 comparable with a CPC.Further, the Figure 4 the EEPS particle size distribution data is noisy and based on Awasthi et al. (2013) can show maximum of 67 % wrong compared to SMPS.

Model Calculations: Modelled vs measured CO 2 concentrations
The validity of the Gaussian plume model was tested against CO 2 measurements from the plume.
Median CO 2 concentrations were calculated using the measurement data at a five seconds interval 285 separately for the "FGD+FF on/off" cases, and the locations of the peak CO 2 concentration (tmax, [CO 2,max ]) were identified from the resulting time series.The value C 0 was chosen to eq. 2 so that the modelled CO 2 concentration, ĈCO2 , was around [CO 2,max ] when t = t max .The choice of C 0 was made in this manner rather than initializing the model to use the stack concentrations due to the following two reasons.First, Gaussian plume model does not yield reliable results close, i.e.

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within a few tens of meters, to the source (Arya, 1995).Second, the comparison of the results near (first 10-50 seconds) the source is problematic because the helicopter was not located at the plume centerline during the initial stages of the measurements.
Comparison of the measured and modelled CO 2 concentrations is shown in Figure 5.The chosen stability classes were 'b' and 'c' as well as 'c' and 'd' for the "FGD+FF on" and"FGD+FF 295 off" cases, respectively, corresponding to the stability conditions ranging from unstable to neutral (Pasquill, 1961).As can be seen, the model reproduces the observed trends rather well, in particular for the "FGD+FF off" case, while the model tends to slightly overestimate the observed concentrations for the "FGD+FF on" case.The modelled and measured concentrations were within one standard deviation in general.Mean relative error (MRE) and correlation coefficients (R 2 ) were 300 calculated between the measured and modelled values.For the "FGD+FF off" case, MREs were between 5 and 25%, depending on the stability class, and R 2 around 0.97, respectively.Corresponding values were between 29 and 40% (MRE) and around 0.86 (R 2 ) for the "FGD+FF on" case.In  into account by the model.For SO 2 measurement and model comparison, the values of MRE varied between 271-578% ("FGD+FF off" case) and between 291-413% ("FGD+FF on" case), depending on the stability class.Also, R 2 was 0.93 and 0.90 for "FGD+FF off" and"FGD+FF on" cases, re-310 spectively.However, this discrepancy does not affect the model performance as the measured SO 2 concentrations, instead of modeled, were used in the plume model simulations.The modelled nucleation rate J nuc is directly proportional to the sulphuric acid concentration and 330 hence the trends in [H 2 SO 4 ] are directly reflected in J nuc (Figure 6).The mean values of J nuc were around 0.4 or 0.7 cm −3 s −1 , in the "FGD+FF off" case and 0.1 or 0.17 cm −3 s −1 in the "FGD+FF on" case, both nucleation rates dependent on the stability class.Furthermore, apparent particle formation rate were calculated at the lowest CPC detection limit which was 2.5 nm, J25.According to the scheme applied here, fraction of freshly nucleated particles that survive into detectable sizes depends 335 mainly on their growth rate (GR) and on the condensation sink (CS).The average GRs were 0.34 or 0.19 nm/h in the "FGD+FF off" case, and 0.07 or 0.04 nm/h in the "FGD+FF on" case; both cases are depending on the stability class.These values are clearly smaller than estimations for atmospheric observations (e.g.Kulmala et al., 2001) and thus, the modelling results do not explain the observed particle formation in the flue gas plume.

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A series of additional calculations were performed in order to investigate the sensitivity of the results to the values of the key input parameters.First, J nuc is proportional to the constant A whose exact value is not accurately known, and this uncertainly translates directly into the calculated nucleation rates.A sensitivity analysis was made for the nucleation model in order to evaluate the sensitivity of nucleation rates to the value of A (shown in Table 2).In these calculations, a value of 345 1•10 −6 was chosen for A which is an order of magnitude higher than in base case simulations.The choice of the value was based on the study of Sihto et al. (2006) who investigated NPF events occurring on boreal forest.As can be seen, increased value of A is not sufficient alone to explain observed new particle formation.A second source of uncertainty is terms of the sulfuric acid concentration which was calculated using a rather simple scheme (see section 2.1.1).Increases in [H 2 SO 4 ] leads 350 to both increased Jnuc and GR and ultimately to larger J25.Results displayed in Table 2 show that J25 is more consistent with observations when [H 2 SO 4 ] is increased five or ten-fold and when A is In comparison, concentrations of around 1•10 6 cm −3 have been reported during the daytime around noon in various atmospheric environments (Hofzumahaus et al., 2009;Petäjä et al., 2009)  pounds may increase also the formation rate of nucleation particles (Pirjola et al., 2015) which may also explain the discrepancy between measurements and model calculations.

Discussion
Each power plant (over 50 MW) in EU has emission limits for SO 2 , NO 2 , and particle mass concentrations, for this studied power plant the limits are 600 mg Nm −3 (210 ppm), 600 mg Nm −3 370 (290 ppm), and 50 mg m −3 n, respectively.Comparison between Table 1 results with the emission limits above shows that the emissions were clearly below these limits when the power plant operation was normal e.g."FGD+FF on".It was observed that these low emissions can be achieved by properly working flue gas cleaning systems.In addition to primary emissions, flue gas cleaning systems seemingly affect also the amount of [H 2 SO 4 ] of new aerosol particles, such as SO 2 which tends to oxidate in the atmosphere to SO 3 and, further, to form H 2 SO 4 .This study shows clearly the importance of flue gas cleaning technologies, and underlines the proper usage of the technologies when the atmospheric pollution is discussed in terms of coal combustion.E.g. according to Huang et al. (2014) in Xi'an and Beijing 37% of sulphate in the atmospheric particles is emitted from coal burning.

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The power plant plume dilutes to background levels in 200 seconds which is faster than indicated in other in-flight measurements (Stevens et al., 2012;Junkermann et al., 2011).According to mod- ), but this study shows that nucleation can take place in lower SO 2 concentrations.In general, the particle number concentrations in the urban atmosphere may be underestimated due to particle formation in power plant plumes.
In the light of the new results authors would like to distinguish the primary particle emission from the newly formed particle emission because those particles have different effects on the atmosphere 395 and different formation mechanisms.Comparing primary particle emission with newly formed particle emission, the effects of different particles in the atmosphere could be taken into account more precisely in aerosol models or air quality assessment.For instance, in the plume for "FGD+FF off" case rough estimates can be calculated for the particle number emission (from CPC, Figure 3) per grams of CO 2 for particles existing in the plume ages of 25-55 seconds was 2.0•10 10 (g CO 2 ) −1 400 and in ages over 400 seconds 8•10 10 (g CO 2 ) −1 and, in the "FGD+FF on" case between 55-85 seconds 4•10 9 (g CO 2 ) −1 and after 500 seconds 3.74•10 10 (g CO 2 ) −1 .In comparison, the primary emissions were 1.75•10 10 (g CO 2 ) −1 for "FGD+FF off" case and 8.0•10 6 (g CO 2 ) −1 for "FGD+FF on" case.Thus, new particle formation can increase the real atmospheric particle number emissions even several orders of magnitude.It should be noted that the particle formation depends strongly 405 on the plume age, [SO 2 ] and primary particle concentrations, and it is possible that there are some low-volatile organics or SO 3 present at the plume affecting the nucleation.
Coal combustion is harmful for climate due to the CO 2 emission.However, also atmospheric particles affect the climate having either cooling or warming effect depending on their chemical and physical characteristics.It is known, that soot or black carbon containing particles have a warming total particle number concentrations in the primary emission.SO 2 concentration was reduced to fifth of "FGD+FF off" situation compared to "FGD+FF on" situation and the total non-volatile particle 425 number concentration was reduced by orders of magnitude.Similar trend in primary emission reduction was detected in the atmospheric measurements.In addition, the reduction in primary emissions affects directly the concentrations of gaseous precursors (SO 2 ) for secondary particle formation in the atmosphere.
It was observed that the flue gas dilutes to background concentrations in 200-300 seconds.This 430 dilution time scale is faster than reported in previous studies.However, the concentration profiles also showed an increase in particle number concentration in an aged flue gas, dilution and dispersion processes.
To validate the dilution time scale, a Gaussian model was used to calculate the dilution in the atmosphere taking into account the primary emission and weather conditions.The Gaussian model formation rate.These were calculated because the measurement results showed an increase in particle number concentrations in the flue gas plume during the dilution process.The modelling results

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for [H 2 SO 4 ] formation rate support the hypothesis of sulphuric acid formation, but the sulphuric acid formation itself does not totally explain the increase in the total particle number concentration, therefore, e.g.low-volatile organics may excist on the flue gas plume.The sensitivity analysis of the [H 2 SO 4 ] formation showed that the atmospheric parametrization is not enough to explain the processes in the flue gas plume.

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Comparison between the primary particles and newly formed particles, calculated based on the atmospheric results, show in the flue gas plume of coal-fired power plant the concentration of newly formed atmospheric particles can be several orders of magnitude higher than the primary particles from the flue gas duct.This is the reason why the newly formed particles should be taken into account when discussing power plant emissions in the future.The formation of these particles in the power 450 plant plumes should be properly parametrized to implement power plants more efficiently e.g. in air quality and climate models.
It is widely known that CO 2 emissions are harmful for the climate.However, the CO 2 emission is Fig.1shows the helicopter measurement routes for "FGD+FF on" and "FGD+FF off" situations.The objective of flight routes was to follow the centre line of the flue gas plume.Helicopter flew both up and down of the plume; the gps-data was used to separate these two flight situations to calculate the distance and the age of the plume separately.

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is needed to calculate the dispersion coefficients.Atmospheric stability classes were estimated based 5 Atmos.Chem.Phys.Discuss., doi:10.5194/acp-2015-990,2016 Manuscript under review for journal Atmos.Chem.Phys.Published: 5 February 2016 c Author(s) 2016.CC-BY 3.0 License.on the measurements of the wind speed and solar radiative flux at the surface.Moreover, the pollutant concentrations were calculated along the centerline of the plume, the value of U was set to constant and equal to the average wind speed during the flights.Finally the value of z was set equal to the stack height (150 meters).

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and the only loss pathway for H 2 SO 4 is condensation onto the particle surfaces.When steady-state 6 Atmos.Chem.Phys.Discuss., doi:10.5194/acp-2015-990,2016   Manuscript under review for journal Atmos.Chem.Phys.Published: 5 February 2016 c Author(s) 2016.CC-BY 3.0 License. is assumed, the following equation is thus obtained [H 2 SO 4 ] = k 1 ×[SO 2 ]×[OH]/CS where k 1 is the reaction constant between OHand SO 2 (Table B.2 in Seinfeld and Pandis, 2008).The SO 2 concentrations were taken from the helicopter measurements, and the time development of CS and [OH] in the plume were modelled as follows.First, CS was calculated using the relation CS= CS stack /DR 175 + CS ∞ ×(1 -1/DR) where CS stack is the condensation sink of aerosols measured in the stack, and CS ∞ is the condensation sink of the background aerosols.The value of the latter parameter was calculated from the size distributions measured at the SMEAR III station(Junninen et al., 2009) which is located around two kilometers away from the power plant.Second, [OH] was calculated using the parameterization ofStevens et al. (2012) which has downward shortwave radiative flux at the surface 180 and [NO x ] as main inputs.Value for the former parameter was taken from the measurements (using the value averaged over the measurement periods), and the NO x concentrations were calculated as follows: [NO x (t)]= [NO x,stack ]/DR(t) where [NO x,stack ] is the NO x concentration measured in the stack.

Figure 2 .
Figure 2. Particle size distributions measured with ELPI and SMPS from the flue gas in the stack.ELPI and SMPS data is shown in operation conditions, "FGD+FF on" and "FGD+FF off".The x-axis is aerodynamic diameter for ELPI data and electrical mobility diameter for SMPS data.

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situation only one of the two flue gas cleaning systems was bypassed.Plume dilution can be evaluated by the CO 2 concentrations (in Figure3 a and b), which show that the "FGD+FF off" case dilutes to approximately background level in 200 seconds (0.74 km) and the "FGD+FF on" case in 300 seconds (1.5 km).The peak values for CO 2 , SO 2 and N tot were 3195 ppm, 2193 ppb, 3.•10 4 cm −3 in the "FGD+FF off" situation and 3254 ppm, 585 ppb, 0.4•10 4 cm −3 , 235 respectively, for the "FGD+FF on".However, the dilution decreases the CO 2 , SO 2 and N tot concentrations in the atmosphere to 422 ppm, 52 ppb in "FGD+FF off" situation, and 473 ppm, 89 ppb in"FGD+FF on" situation.Respectively, the N tot reached nearly to background concentrations after 200 seconds and 300 seconds.The background concentrations for each measured gaseous component were 403 ppm and < 25 ppb, for CO 2 and SO 2 respectively.The boundary layer mixing started 240 during the "FGD+FF on" measurements and thus the closest background values were subtracted from both "FGD+FF on/off cases".It can be noted that very near (first 10-50 seconds) the stack the helicopter was not in the plume.This can be seen from CO 2 and SO 2 concentration values presented

Figure 3 .
Figure 3. Concentrations of power plant flue gas components measured by instrument installed in to the helicopter as a function of plume age; "FGD+FF off" on the left and "FGD+FF on" on the right.SO2 (ppb, black line) and CO2 (ppm, blue line) concentrations on the left axes and total particle number concentration ∆Ntot (1 cm −3 , red line) on the right axes.The ∆Ntot is calculated using the closest background value.The grey vertical lines denote 2 km distance from the stack in "FGD+FF on/off" cases.The presented results are 5 second median values.

Figure 4 .
Figure 4.The particle number size distribution calculated from EEPS data as a functions of plume age.Measurement was made with the EEPS installed to helicopter.The results are calculated 10 second moving median values.
Figure 3.The particle size distribution had a mode around 80 nm, which refers to the solid particle median diameter measured with the SMPS from the flue gas in the stack.Figure 4 indicates similar dilution profile for particles, if N tot is calculated from EEPS data, than the dilution was for CO 2 275 order to further investigate the performance of the model, comparison was made between measured and modelled SO 2 concentrations.The results showed that the model consistently overestimates the Atmos.Chem.Phys.Discuss., doi:10.5194/acp-2015-990,2016 Manuscript under review for journal Atmos.Chem.Phys.Published: 5 February 2016 c Author(s) 2016.CC-BY 3.0 License.

Figure 5 .
Figure 5.Comparison of measured and modelled CO2 concentrations.Median of measured values are shown with black (circle) symbols along with the standard deviations.Dashed and dotted red lines correspond to model results for stability classes 'b' and 'c' (above) and 'c' and 'd' (below), respectively.
Modelled and measured CO 2 concentrations showed that the model reproduced the observed dispersion of the plume relatively accurately.Thus the model was applied to calculate [NO x ], [OH], and 315 [H 2 SO 4 ] which were needed to investigate possibility of new particle formation in the plume.These results are summarised in Figure 6.It is seen that sulphuric acid concentrations exponentially increase during the initial stages of the simulation and then reach constant concentration around 1•10 6 Atmos.Chem.Phys.Discuss., doi:10.5194/acp-2015-990,2016 Manuscript under review for journal Atmos.Chem.Phys.Published: 5 February 2016 c Author(s) 2016.CC-BY 3.0 License.
set equal to 1•10 −6 like inSihto et al. (2006).Therefore, underestimation of [H 2 SO 4 ] may explain the discrepancy between the obeservations and base case model results.This might caused by underestimation of[OH]  or overestimation of CS.Regarding the modeled OH concentrations, it can 355 be noted that they are relative low, reaching values of around 1•10 5 cm −3 by the end of the flights.
elling results ofStevens et al. (2012), atmospheric new particle formation via coal combustion orig-Atmos.Chem.Phys.Discuss., doi:10.5194/acp-2015-990,2016   Manuscript under review for journal Atmos.Chem.Phys.Published: 5 February 2016 c Author(s) 2016.CC-BY 3.0 License.inated sulphuric acid nucleation begins at 5 km distance from the source whereas the sulphuric acid formation begins right after emission.Experiments of this study indicates that the nucleation may 385 take place in the aged plume and being the most effective after 400 seconds, corresponding approximately 2 km distance from the emission source in atmosphere.Also this distance is significantly less than 5 km distance indicated byStevens et al. (2012).Thus, this study indicates that atmospheric nucleation in power plant plumes takes place faster than the models and measurements have suggested before.Also, it has been known that new particles form in sulphur-rich plumes(Junkermann et al.,   390

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effect on climate.This study indicated that most of the particles originated from coal combustion are formed in the atmosphere.Based on that knowledge, it can be assumed that the formed particles are more scattering than absorbing.Also,Frey et al. (2014) have shown that primary emission of the coal-fired power plant has a scattering effect, at least when compared to other combustion originated primary particles.This study gives new insights when the effects of coal combustion are studied in a 415 global scale.Finally, the results help to understand the formation of atmospheric particles in polluted areas, such as India and China.Atmos.Chem.Phys.Discuss., doi:10.5194/acp-2015-990,2016 Manuscript under review for journal Atmos.Chem.Phys.Published: 5 February 2016 c Author(s) 2016.CC-BY 3.0 License.stack, and modelling studies for atmospheric processes of flue gas plume.The stack measurements were made to estimate the effectiveness of flue gas cleaning technologies, such as filtering and desulphurization.It was shown that the flue gas cleaning technologies had a great effect on the SO 2 and

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confirms the dilution time scale, and the dilution ratio could be used to calculate the theoretical maximum values for different components in the flue gas plume.Weather conditions and theoretical maximum value for [NO x ] was used to calculate the [OH] formation rate and further [H 2 SO 4 ] not the only one to have effects on the climate.Other anthropogenic gases can increase the climatic Atmos.Chem.Phys.Discuss., doi:10.5194/acp-2015-990,2016 Manuscript under review for journal Atmos.Chem.Phys.Published: 5 February 2016 c Author(s) 2016.CC-BY 3.0 License.

Table 1 .
Flue gas concentrations of CO2, SO2, NOx, O2, total particle number (Ntot), dust, and flue gas flow rate in the stack.Mean values (+ standard deviation) are presented for both flue gas cleaning conditions ("FGD+FF on" and "FGD+FF off").
. Relative low modeled OH concentrations can be explained by high NO x concentrations which were calculated to decrease consistely from several tens of ppm down to around 200 ppb during the flights (not 360 illustrated here).Such high concentrations of NO x are consistent with low [OH] (see Figure 1 in Lonsdale et al., 2014).It could be thus speculated that model underestimates [H 2 SO 4 ], and consequently rate of new particle formation, due to overestimation of [NO x ].Moreover, it should be noted that neither SO 3 nor low-volatile organic vapours that might have been present in the measured flue

Table 2 .
Sensitivity analysis made for number of particles formed with diameters above 2.5 nm during the flight (1 cm −3 (600 s) −1 ) in the atmosphere with different values of A and [H2SO4].The [H2SO4] is calculated based on the measurement results and scaled up to test faster nucleation rate for both "FGD+FF on" and "FGD+FF off" cases and stability classes (sc).