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Atmospheric Environment 95 (2014) 296e304

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Atmospheric Environment

journal homepage: www.elsevier .com/locate/atmosenv

Air ion mobility spectra and concentrations upwind and downwind ofoverhead AC high voltage power lines

Matthew D. Wright a, b, *, Alison J. Buckley a, James C. Matthews a, b, Dudley E. Shallcross b,Denis L. Henshaw a

a H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UKb Atmospheric Chemistry Research Group (ACRG), School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK

h i g h l i g h t s

* Corresponding author. Current address: AtmoGroup (ACRG), School of Chemistry, University of Brist1TS, UK.

E-mail address: matthew.wright@bristol.ac.uk (M.

http://dx.doi.org/10.1016/j.atmosenv.2014.06.0471352-2310/© 2014 Elsevier Ltd. All rights reserved.

g r a p h i c a l a b s t r a c t

� Air ion concentration and mobilitymeasurements near AC high voltagepower lines.

� Corona observed during 33 of 46measurements, at 9 of 11 sites.

� Bipolar (both polarities) and positiveion production observed mostfrequently.

� Polarity imbalance observed for pos-itive (15 of 24 days) but not negativeions.

� Particular line configurations shownto be ‘heavy producers’ of coronaions.

a r t i c l e i n f o

Article history:Received 14 March 2014Received in revised form19 June 2014Accepted 21 June 2014Available online 21 June 2014

Keywords:Small ionsCorona dischargePower linesElectrical mobilityCharged particlesIon-aerosol attachment

a b s t r a c t

Corona ions produced by high-voltage power lines (HVPLs) can alter the nearby electrical environment,potentially increasing aerosol charge levels downwind. However, there is a lack of knowledge concerningthe concentration and mobility of ions from AC HVPLs and their dispersion away from the line. Wepresent ion concentration and mobility measurements made near AC HVPLs in South-West England.Examples of typical mobility spectra are shown highlighting features commonly observed. Corona wasobserved during 33 of 46 measurements, at 9 of 11 sites, with positive or ‘bipolar’ (both polarities) ionproduction commonly seen. Ion production usually increases atmospheric concentrations by only amodest amount, but extreme cases can enhance concentration by an order of magnitude or more. Apolarity imbalance is required to increase aerosol charge via ion attachment; this was observed on 15 of24 days when positive corona was observed, but was not seen for negative ions. Ion mobility was higherdownwind compared with upwind for both ion polarities, but the increase was not statistically signifi-cant. Future work should focus on identifying and characterising ‘heavy-producing’ HVPLs, and obtainingresults in conditions which may favour negative ion production e.g. high humidity, inclement weather orduring nighttime.

© 2014 Elsevier Ltd. All rights reserved.

spheric Chemistry Researchol, Cantock's Close, Bristol BS8

D. Wright).

1. Introduction

Atmospheric small ions (clusters of molecules around a centralion) are produced by ionisation of air molecules mainly by cosmicrays or radioactive decay (Harrison and Carslaw, 2003). However,

M.D. Wright et al. / Atmospheric Environment 95 (2014) 296e304 297

overhead direct current (DC) or alternating current (AC) high-voltage power lines (HVPLs) can produce ‘corona’ ions when theelectric field near the conducting cable exceeds a critical voltage,leading to localised electrical breakdown and ionisation (Maruvada,2000). Various factors can affect corona ion production (Maruvada,2000), including meteorological conditions, line voltage and thenumber, dimensions and surface roughness of the cables (includingprotrusions due to dirt and water droplets). Some corona ions maybe transported into the local environment by a crosswind, and havebeen detected at distances of several kilometres downwind fromHVPLs (Chalmers, 1952).

Corona ions can collide with aerosol particles, transferring theircharge to them. If sufficient corona production occurs, the aerosolcharge state may be altered (Buckley et al., 2008). It is hypothesisedthat corona ion production by HVPLs may lead to increased risk ofill health through charge-mediated enhanced deposition of inhaledpollutant aerosols in the lung (Fews et al., 1999). Such increaseddeposition has previously been observed in human volunteers(Melandri et al., 1983) and hollow cast models (Cohen et al., 1990).This hypothesis may in part explain the increased risk of leukaemiain childhood (Draper et al., 2005) and adults (Lowenthal et al.,2007) observed near HVPLs at distances where direct HVPL elec-tric and magnetic fields are negligible. Aerosol charge, if modifiedby corona ions, will only return to background levels once neu-tralised by background, bipolar ions (on timescales of tens of mi-nutes or longer; Fews et al., 2002), so there may be a regiondownwind of HVPLs in which aerosol charge is elevated andenhanced lung deposition may occur. The electrical mobility ofions, defined as the drift velocity per unit electric field, is importantin determining the rates of recombination of bipolar ions and theattachment to aerosol particles (Hoppel and Frick, 1986). Deter-mining ion concentrations from measurement of atmosphericconductivity also requires knowledge of ion mobility. Ion andaerosol concentrations, their mobilities and the aerosol chargedistribution are linked by the ion- and aerosol population balanceequations (Fuchs,1963; Hoppel and Frick,1986; H~orrak et al., 2008).

There are several studies reporting atmospheric ion mobility(e.g. Nagato and Ogawa, 1988; H~orrak et al., 1994; Komppula et al.,2007; Wright et al., 2014), but very few on AC corona ions. Thiswork presents air ion mobility spectra and concentrations obtainedusing a Gerdien-type Aspiration Condenser Ion Mobility Spec-trometer (ACIMS; Fews et al., 2005) on 29 measurement days be-tween 2006 and 2008, at 11 AC HVPL sites in South-West Englandduring mostly dry weather conditions; to our knowledge the mostextensive survey to date of AC HVPL corona incorporating mea-surement of ion mobility.

2. Background

2.1. Atmospheric potential gradient perturbations, corona ionproperties and aerosol charging

Much research on HVPL corona ion production utilises mea-surements of perturbations in the Earth's natural potential gradient(PG) downwind of HVPLs (Chalmers, 1952; Fews et al., 2002;Bracken et al., 2005; J-Fatokun et al., 2010). Both the mean andthe variability in PG can be altered. Previous observations at HVPLsin England (Fews et al., 2002), showed both positive and, lesscommonly, negative perturbations, with evidence for corona pro-duction in 21 of 22 measurements. Long term measurementsshowed PG downwind was more negative overnight and duringrainfall or high humidity, while increasing wind speed increased PGstandard deviation and interquartile range computed over 10 minperiods (Matthews, 2012a,b). Although these data may be used toinfer an airborne (i.e. at some height above ground level) ion

concentration (e.g. Fews et al., 1999, 2002), accurately describingthe location and quantity of the ions causing the perturbations isdifficult (J-Fatokun et al., 2010; Matthews et al., 2012). Downwindof HVPLs, aerosol charge may be modified by the attachment ofcorona ions to the particles (Buckley et al., 2008). The ion-aerosolattachment rate and the ultimate charge distribution on aerosolparticles are sensitive to the positive and negative ion mobilitiesand concentrations (Hoppel and Frick, 1986; L�opez-Yglesias andFlagan, 2013). Therefore, a model to predict additional aerosolcharging due to an influx of ions from HVPLs requires knowledge ofthe ion mobilities and concentrations from the instant of produc-tion through to the point at which re-neutralisation of particles bybackground ions is complete. However, due to difficulty inmeasuring ion properties at height, or in proximity to HVPL cables,measurements at ground level are to date the primary means ofassessing ion properties near HVPLs.

2.2. Corona ion concentrations

Several studies have reported elevated ion concentrations atground level downwind of HVPLs compared with upwind orbackground control measurements, with much work undertakenon DC lines. Hendrickson (1986) observed nearly unipolar plumes,with dominant polarity that of the conductor furthest downwind.Ion concentrations were enhanced by a median factor of 4e8 at adistance of 300 m and a charge imbalance was observed out to1600 m. Suda and Sunaga (1990) observed downwind concentra-tions ranging from 2000 to 37,000 cm�3 (mean 19,000 cm�3),compared with 200e1400 cm�3 (mean ~ 650 cm�3) in backgroundconditions.

A limited number of studies have investigated AC lines. Moststudies report a bias towards positive ion production, althoughthere are instances where both polarities are observed or negativeion production is favoured. Bracken et al. (2005) concluded that ACHVPLs have only a minor impact on space charge; but, this wasbased on comparison of downwind electric field and ion concen-tration with the maximum (99th percentile) values of upwindmeasurements. Median values for ‘no-wind’ (<0.5 m s�1) anddownwind were consistently higher than upwind, for all sites andboth polarities. Fews et al. (2005) presented one example showingan excess of negative (1200 cm�3) over positive (690 cm�3) ions,both of which exceeded upwind levels (<200 cm�3). A series ofstudies in Australia also report elevated ion concentrations due toHVPLs, with one survey showing downwind net ion concentrationexceeded the background mean at 31 of 41 sites (Jayaratne et al.,2008; J-Fatokun et al., 2010).

2.3. Corona ion electrical mobility

In general, studies on corona ion electrical mobility outside thelaboratory are scarce, in particular for AC HVPLs. Suda and Sunaga(1990) observed mean mobility of 1.34 ± 0.31 and1.56 ± 0.21 cm2 V�1 s�1 for positive and negative ions respectivelydownwind of a 750 kV DC line, higher than those observed inbackground conditions at the same site, ~0.8 to 1.3 cm2 V�1 s�1.Previously, Mohnen (1977) collated ion mobilities from the litera-ture, showing that ‘artificial’ ions (e.g. from corona discharge orradioactive decay in charge neutralisers) had higher mobilities(1.33 and 1.84 cm2 V�1 s�1 for positive and negative ions respec-tively) than ‘natural’ atmospheric ions (1.15 and 1.25 cm2 V�1 s�1).Recently, Alonso et al. (2009) measured the mobility spectrum ofnewly-produced corona ions with a high-flow Differential MobilityAnalyser (DMA). Negative ions showed two distinct peaks at 1.7 and2.0 cm2 V�1 s�1, while positive ions had a broader spectrum be-tween 0.8 and 1.6 cm2 V�1 s�1. However, in this work, clean, dry air

M.D. Wright et al. / Atmospheric Environment 95 (2014) 296e304298

was used and ions were fresh (age < 0.1 s), whereas it has beenshown that absolute humidity can affect ion mobility in the first 3 sfollowing production (Fujioka et al., 1983; Harrison and Aplin,2007).

3. Experimental method

3.1. Instrumentation and measurement sites

Two Aspiration Condenser Ion Mobility Spectrometers (ACIMS)were used to determine ion mobility and concentration, details ofthe operational and data inversion procedures and instrumentresolution are given by Fews et al. (2005) and Wright et al. (2014).Themobility limits of the device as operated here are an upper limit~3 cm2 V�1 s�1, and a lower limit ~0.03 cm2 V�1 s�1, correspondingto sizes ~0.4 to ~7.5 nm according to the diameteremassemobilityrelationship of Tammet (1995). ‘Small ions’ are defined as thosewith mobility >0.5 cm2 V�1 s�1 (~0.4e1.8 nm size). One ACIMS scan(incorporating 54 voltage steps for each polarity, from 0 to±1000 V) takes ~15 min, with typically at least 6 scans made duringeach measurement and averaged data used in the subsequentinversion. For some measurements, a Sequential Mobility ParticleSizer and Counter (SMPSþC, GrimmAerosol Technik) was available,with which aerosol size distributions in the diameter range10e1000 nm were obtained.

11 HVPL sites were visited, with measurements made at oneupwind (UW) and at least one downwind (DW) location at eachsite, details of which are given in Table 1 and are designated by two-letter codes. We also supply a KMZ file (Supplementary Material)detailing measurements at each site, colour coded by line type e.g.132 kV, single conductor (Sites BH, LS, NS, RW, TM, YT); 275 kV, 2sub-conductor (Sites EL, WF); 400 kV, 2 sub-conductor (Sites HH,LG) and 400 kV, 4 sub-conductor (Site DC).Where possible, UWandDW data were collected contemporaneously, using two ACIMS, toallow direct comparison. Where this was not possible, non-contemporaneous (sequential) measurements were made, whereone set of measurements (at least 6 cycles) were made UW, fol-lowed by a similar number DW; or vice versa. We did not alternatebetween UW and DW locations multiple times, in order to mini-mise instrument down-time, typically ~15 min between the end ofthe UW measurement and restarting DW. In total, 46 measure-ments (over 29 days between April 2006 and October 2008) weremade, of which 29 had contemporaneous data collection and 17had sequential data collection. The actual distance travelled by ionsfrom the line to the measurement location depends on the winddirection. Meteorological data including wind speed and direction,

Table 1Details of the 11 locations, HVPL types and perpendicular distance from measurement s

Site Latitude Longitude Line voltage (kV) No. of conduc

(Site 1)

HH 51� 530 1500 N 2� 00 4100 W 400 2LG 51� 100 5500 N 2� 450 2400 W 400 2DC 51� 370 900 N 1� 140 1700 W 400 4YT 51� 230 1600 N 2� 500 3900 W 132 1NSb 51� 250 5400 N 2� 460 5100 W 132 1TM 51� 300 5400 N 2� 180 5500 W 132 1RW 51� 340 1900 N 2� 260 2600 W 132 1BH 51� 200 1200 N 2� 390 2400 W 132 1LS 51� 320 5000 N 2� 200 2800 W 132 1EL 51� 350 5200 N 2� 340 3300 W 275 2WF 51� 340 1000 N 2� 370 4700 W 275 2

a With wind from the regional prevailing wind direction (SW), at all locations site 1 wastated.

b There are two HVPLs (both 132 kV) at this site; Site 2 is 70 m downwind of one line

temperature and relative humidity (RH) were collected at the UWlocations using a weather station (Davis Instruments). The transittime from the line to each DW location was calculated using theaverage wind speed and direction throughout each individualmeasurement. Instrumentation was located in the back of a vehicleopen to the air, with inlets at ~1 m above ground level.

3.2. Determination of HVPLs ‘in corona’

An intercomparison based on 200 h of data collected in thelaboratory was made to ensure the responses of the two ACIMSwere similar. The mean difference between concentrations(±standard deviation) measured using the two ACIMS was3.8 ± 6.4% and 4.1 ± 8.0% for positive and negative ions respectively,while for mobility the mean difference (±standard deviation) be-tween the two ACIMS was 4.9 ± 5.0% and 2.5 ± 6.6% for positive andnegative ions respectively. Additional information is given in theSupplementary Material. For field measurements, the HVPL wasdefined as being ‘in corona’ when DW measurements of ion con-centration exceeded UW measurements by a threshold value, foreither or both ion polarities. For ion properties to be consideredsignificantly different between UW and DW sites (at the p ¼ 0.05confidence level) this was taken to be the mean value plus 2standard deviations of the difference in response of the two ACIMSfor each polarities, which was 16.6% and 20.1% respectively forpositive and negative ion concentration, and 14.8% and 15.7%respectively for positive and negative ion mobility. If the DW con-centration of both ion polarities exceeded this threshold, the HVPLwas designated as in ‘bipolar’ corona; if only one polarity exceededthe threshold, the line was designated as in either ‘negative’ or‘positive’ corona; and if neither polarity exceeded the threshold,the line was designated ‘no corona’.

4. Results and discussion

4.1. Ion concentrations and designation of HVPLs ‘in corona’

Fig. 1 shows the ratio of DW/UW ion concentration for eachmeasurement, along with the threshold for determining the pres-ence of corona. Corona ion concentrations were elevated during 33/46 measurements, with ‘positive only’ (17/46) and ‘bipolar’ (15/46)corona most evident, while ‘negative only’ corona occurred onlyonce. In fair weather, EPRI (2003) suggest that, for equal corona‘loss’ current in each AC half-cycle (and hence nþmþ ¼ n�m�, wheren± and m± refer respectively to the concentration and electricalmobility of each ion polarity), the higher negative ion mobility

ites to the HVPL at each location.

tors Perpendicular distance from line to measurement site (m)

1a 2 3 4 5 6

190 75 220350 65 125 245 320 640250 115 260105 20 75 155 27080/145b 70/25b 135/60b 220/100b

145 30060 90 200120 45 95 145110 300190 170150 40 260

s upwind of the HVPL and sites 2e6 were downwind of the HVPL, unless otherwise

, 25 m upwind of the second line. Sites 3 and 4 are downwind of both lines.

Fig. 1. Ratio of corona ion concentration downwind of HVPLs to upwind measurements. Asterisks indicate non-contemporaneous measurements. Dashed line shows the (negativeion) threshold above which lines were considered ‘in-corona’ (ratio of 1.2).

Fig. 2. All 46 individual downwind positive and negative ion concentration mea-surements (in order of increasing magnitude) and the mean value for each polarityfrom corresponding upwind measurements.

M.D. Wright et al. / Atmospheric Environment 95 (2014) 296e304 299

leads to a net escape of positive ions from the vicinity of theconductor cables. An alternative approach, considering the regionaround the cables in which ions of each polarity will be attractedback to the cable surface and be removed from the air (Maruvada,2000), would suggest that more mobile negative ions might beremoved from a larger region, leading to positive ions being locatedfurther from the cable surface than negative ions, potentiallyfacilitating their increased escape from the vicinity of the cable.Whilst these considerations are quite simplistic (e.g. there is noexplicit account for a crosswind, and the expressions shown areonly valid very close to conducting cables and in cylindrical sym-metry) nevertheless our experimental data does show that, ingeneral, positive corona is detected downwind more often thannegative corona. Corona was observed at 9 of 11 sites visited, asimilar proportion to that observed by Jayaratne et al. (2008) for netion concentrations and Fews et al. (2002) for PG. Considering onlycontemporaneous measurements we find qualitatively similar ob-servations of positive (10/29), negative (1/29), bipolar (11/29) andno corona (7/29), suggesting that both contemporaneous and non-contemporaneous results are valid in determining corona polarity.Subdivision of the ‘bipolar’ classification in terms of the dominantpolarity shows that 8/15 were biased towards positive, and 7/15towards negative, ion production. Those biased towards positivecorona tended to have greater difference between positive andnegative DW/UW concentration ratios (average ratios 5.4 and 1.9for positive and negative ions respectively), whereas for negativebias the DW/UW positive and negative ratios were in general closer(average ratios 1.7 and 2.1 for positive and negative ionsrespectively).

Sites that were visited several times often showed consistencyin terms of the nature of corona observed e.g. HH consistentlyshowed very high positive ion production, with negative ion sup-pression; LG was characteristically similar to HH but at lower ionconcentration enhancement factors; WF showed the greatestnegative bias; while TM showed no evidence of ion production.However, in the latter case, the same HVPL at a different site (RW)did show elevated ion concentrations, highlighting the potentialimportance of local conditions on corona production. In addition,we observed corona production at several 132 kV sites. Only a smallproportion of 132 kV lines (those managed by National Grid) wereincluded in the study of Draper et al. (2005). However, in the designof future studies of this nature, our results suggest that possibleeffects due to all 132 kV, as well as higher voltage, HVPLs should beconsidered.

Site HH, a 400 kV, 2-conductor bundle HVPL, consistentlyshowed the highest DW/UW ion concentration ratios, with amaximum DW/UW enhancement factor of 20. The electric field

near the surface of the conductor determines whether coronaionisation occurs, and for a given line voltage this is higher forconductors with a smaller diameter. Also, for conductors of a givendiameter, the surface electric field is higher in bundles with fewerconductors. On each occasion where multiple DW locations werevisited on the same day at HH, the positive ion DW/UW concen-tration ratio was lower further away from the line, while thenegative ion concentration ratio was higher. Using data from 17/08/07 as an example, the location nearest the HVPL (75mDW) showeddepletion of negative ions (DW/UW ratio 0.22) alongside a largeincrease in positive ions (ratio 20.6). This suggests that the biastowards positive ion production is great enough to neutralise asignificant fraction of negative ions via recombination, while stillleaving a large excess of positive ions. The more distant location(220 m DW) shows a more moderate positive DW/UWexcess (ratio5.9) and a less depleted negative ion concentration ratio (0.89) dueto entrainment of background air containing a more even ion bal-ance into the plume. We note that LG also has the same cable ge-ometry but did not display such high corona production, showingthat, whilst possibly important, this particular cable geometry isnot the only factor responsible for high corona production bycertain HVPLs.

Fig. 2 shows mean DW positive and negative ion concentrationsduring the 46 measurements, along with the overall mean of thecorresponding UW data. The mean (±standard deviation) of theDW concentrations were 387 (±530) cm�3 and 129 (±117) cm�3 forpositive and negative ions respectively, compared with UW con-centrations 137 (±83) cm�3 and 117 (±69) cm�3 for positive and

Fig. 3. Ion mobility spectra obtained from HH on 17/08/07.

M.D. Wright et al. / Atmospheric Environment 95 (2014) 296e304300

negative ions respectively. The maximum ion concentrationobserved (over a single ACIMS scan) was 3015 cm�3 for positiveions and 1478 cm�3 for negative ions, both at HH on 31/08/07. Forthe most part, corona ion production by HVPLs increases overallconcentration by a modest amount, often within the rangeobserved in ambient fluctuations, but the most extreme cases canenhance (especially positive) ion concentrations by an order ofmagnitude or more. 29 DWmeasurements exceeded the UWmeanfor positive ions, compared with only 19 for negative ions; similarto results obtained by Jayaratne et al. (2008).

In Fig. 1, some DW/UW concentration ratios are <1, while inFig. 2 several individual DW concentrations are below the meanUW value. This is more evident for negative ions than positive ions,and in most cases (especially at HH, RWand LG) is accounted for byexcess positive ion production leading to neutralisation of negativeions, reducing the concentration relative to that UW. Some in-stances of ‘no corona’ measurements where both polarities appearreduced DWmay be a result of differences in aerosol concentrationbetween UW and DW sites leading to greater ion loss via attach-ment, and spatial or, in the case of non-contemporaneous mea-surements, temporal variability in ion concentrations.

Meteorological parameters can influence the amount and po-larity of corona produced by HVPLs. Measurements in this studywere taken throughout the year, with corresponding variation intemperature, RH, wind speed and direction. Summary statistics formeteorological parameters are given in the SupplementaryMaterial. High RH can result in increased negative ion production(Chalmers, 1952; Matthews, 2012a). The only site at which the DW/UW ratio was higher for negative ions was WF. The most extremeratio during the 2 days at this site corresponded with the highestRH recorded in this study (81e84%). However, across all sites, norelationship between RH and either ion concentration or polaritybias was observed. Similarly, while increased wind speed increasescorona production from single points at HV (Large and Pierce, 1957)

and HVPLs (Matthews, 2012a), it also significantly influencesdispersion and ioneion and ioneaerosol interactions, and a simplerelationship between wind speed and DW ion concentration wasnot observed.

The ion mobility spectra (and ion properties) obtained from anindividual ACIMS scan (or number of scans) reflect the averageproperties throughout themeasurement period. However, previousmeasurement of PG fluctuations (Fews et al., 2002) and ion con-centration (Jayaratne et al., 2008; Matthews, 2012b; Matthewset al., 2012) at 1 s time resolution, showing considerable vari-ability on this timescale, suggests the likelihood of shorter periodswith higher ion concentration than those reported here (usually>1 h) (Fig. 2).

4.2. Corona ion mobility spectra

Figs. 3e5 show example ion mobility spectra from three sites,along with approximate ion masses and diameters (calculated at20 �C and 1013.25 kPa using the algorithm of Tammet, 1995).Spectra shown in Fig. 4 (NS, 12/09/08) and Fig. 5 (BH, 17/09/08)were obtained from data collected at UW and DW locationscontemporaneously. In Fig. 3 (HH, 17/08/07), the UW spectra is anaverage of the whole measurement period, encompassing both DWlocations. At HH (Fig. 3) a greatly increased number of positive ionswere observed at both DW locations, compared with UW. There is arelatively broad spread in ionmobility UW, with a single peak at 1.4and 1.55 cm2 V�1 s�1 for positive and negative ions respectively,similar to that observed in previous studies (e.g. H~orrak et al.,2000). At both DW sites, two clear peaks are observed for posi-tive ions, at around 1.3 and 2.0 cm2 V�1 s�1 at HH2 and 1.1 and2.0 cm2 V�1 s�1 at HH3, while for negative ions a very small peak isobserved at 2.4 cm2 V�1 s�1 at HH2 and 1.7 cm2 V�1 s�1 at HH3. Therapid neutralisation of negative ions by a large excess of positiveions would shorten the lifetime of negative ions, and may be

Fig. 4. Ion mobility spectra obtained from NS on 12/09/08.

M.D. Wright et al. / Atmospheric Environment 95 (2014) 296e304 301

responsible for the increasedmobility observed DW comparedwithUW. The relative importance of the two positive ion mobility peaksdiffers between measurements at HH2 and HH3. During thismeasurement, average transit times (calculated from meteorolog-ical data) from the HVPL centre to locations HH2 and HH3 were 28and 68 s respectively. The reduction in prominence of the lowermobility peak may be due to changes in trace gas chemistry be-tween these measurement times, known to influence ion compo-sition and hence mass and mobility (Mohnen, 1977; Harrison andCarslaw, 2003; Aplin, 2008; Luts et al., 2011), favouring produc-tion of higher mobility species; however, no ion composition ormass measurements were made here. There is recent evidence thatevolution of (negative) ions can persist for up to 20 s, and may beinhibited by high RH (Luts et al., 2011). Aerosol size distributionmeasurements were made at both HH2 and HH3, with observedmean (±standard deviation) concentration, NT, 4910 ± 1240 cm�3

and diameter 28 ± 5 nm. The effective attachment coefficient, beff,(Hoppel, 1985) was 8.8 � 10�7 cm3 s�1 and the ion sink to aerosols,

Fig. 5. Ion mobility spectra obta

beffNT, was 4.3 � 10�3 s�1 (neglecting effects due to changes inaerosol charge), implying a characteristic time for attachment~230 s. This suggests that, between HH2 and HH3, some attach-ment should occur, but may not account for the decreased positiveion number in the lower mobility peak completely, and other fac-tors (dilution with background air, chemical reaction) may play amore important role.

In Fig. 4, there is a substantial change in the positive ionmobilityspectrum between the UW and DW measurements. UW, a singlepeak at 1.5 cm2 V�1 s�1 was observed, while DW the spectrum ismuch broader, with several peaks at 1.15, 1.9 and 2.3 cm2 V�1 s�1.The negative ion spectrum is less affected. Ion concentrations forboth polarities were enhanced (~170 cm�3 DW compared with~90 cm�3 UW for both positive and negative ions). The effect onpositive, but not negative, ion mobility may be due to humidityeffects. During the afternoon, intermittent rainfall occurred and RHincreased (from ~50% at the beginning of the measurement to >70% towards the end of the measurement). Harrison and Aplin

ined from BH on 17/09/08.

M.D. Wright et al. / Atmospheric Environment 95 (2014) 296e304302

(2007) noted that positive ions showed decreased mobility above athreshold value for water vapour pressure (4.1 hPa), attributed tochanges in the hydration level of the ion clusters, while negativeion mobility was unaffected. Although the range of water vapourpressure here, 12.5e15 hPa, was significantly higher than that inHarrison and Aplin (2007), it is possible that, as humidity increased,positive ion mobility decreased, which may manifest itself in themobility spectrum in Fig. 4 as a series of peaks corresponding totimes during the measurement at which those particular positiveion mobilities were favoured due to prevailing conditions. Otherstudies (Luts et al., 2011) have shown a humidity influence onnegative ion mobility and composition, not observed in this work.

Fig. 5 shows an example of a measurement at BH4 (transit time220 s), where ion concentrations DW remain similar to those UW,but the mobility of both positive and negative ions is higher DW(positive: 1.8 cm2 V�1 s�1; negative: 2.4 cm2 V�1 s�1) comparedwith UW (positive: 1.5 cm2 V�1 s�1; negative: 2.0 cm2 V�1 s�1).Closer to the line at BH2 (transit time 100 s) ion concentrations areelevated DW compared with UW for both polarities, indicating thatthe HVPL was in corona on this measurement day, with increasedmobility DW compared with UW and peaks in the DW mobilityspectrum (d) at the same mobilities as in the earlier measurement(b). The ions produced directly from the HVPL are likely to havebeen lost by the time of measurement (b), since the transit timeexceeds typical ion lifetimes of ~2min (Tammet et al., 2006), yet theHVPL appears to retain an influence over the ion mobility at thisdistance.

The frequency histogram of mean ion mobility (grouped in0.1 cm2 V�1 s�1 mobility bins) for each measurement UW and DW(Fig. 6), suggests that this increased mobility may be a commonfeature of the ion environment DW of HVPL. The mean (±standarddeviation) mobility over all measurements was1.93 ± 0.36 cm2 V�1 s�1 UW and 1.99 ± 0.27 cm2 V�1 s�1 DW fornegative ions, and 1.65 ± 0.37 cm2 V�1 s�1 UW and1.79 ± 0.21 cm2 V�1 s�1 DW for positive ions. Mobility was 3%higher for negative ions and 9% higher for positive ions DWcompared with UW, amoremodest increase than observed by Suda

Fig. 6. Frequency histogram for mean ion mobility for each measurement upwind anddownwind for a) positive ions, b) negative ions.

and Sunaga (1990) DW of a DC HVPL (28% and 47% increase forpositive and negative ions respectively) and was not statisticallysignificant. Due to the nature of the corona production anddispersion processes, DC HVPLs typically produce unipolar, high ionconcentrations. This may increase the rate of dispersion and drydeposition to the ground, leading to shorter lifetimes and moremobile ions, relative to those detected DW of AC HVPLs. It is alsopossible that production of ozone, nitrogen oxides and radicals inthe region near HV cables modifies air chemistry, altering thespecies potentially able to form cluster ions in the DW plume.Measurement of trace gas composition UW and DW of HVPLsalongside cluster ion mass spectrometry (Junninen et al., 2010)would assist in determining the extent of such an effect on coronaion composition, complementing existing laboratory studies ofnewly-produced corona ions (Nagato et al., 2006; Alonso et al.,2009; Manninen et al., 2011).

4.3. Ion polarity imbalance, aerosol charging and implications forhealth effects

Fig. 7 shows the DW polarity imbalance in ion concentrations.For a positive imbalance, nþ/n� is shown, while for a negativeimbalance the inverse, n�/nþ, is shown. Also shown is themaximum imbalance (either positive, 1.52, or negative, 1.31)observed in UW measurements over the entire study. When mul-tiple measurements were made during one day, only the highestimbalance is shown. Again, positive ion imbalances were observedmore frequently, and had much higher magnitude, than negativeion imbalances. No negative ion imbalances exceeded themaximum UW value, while for positive ions this was observed on15/24 days.

For an atmosphere containing diffusing ions and particles, ionsof both polarities will ‘attach’ to aerosol particles, leading to a‘steady-state’ aerosol charge state, in which particles of a given sizehave a certain probability of holding a given number of electroniccharges (Fuchs, 1963). A detailed description of the process ofaerosol charging by ion-aerosol attachment can be found in theliterature (Fuchs, 1963; Hoppel and Frick, 1986; H~orrak et al., 2008;L�opez-Yglesias and Flagan, 2013). A full treatment of aerosolcharging by AC HVPLs requires knowledge of the ion concentrationand mobility for both polarities and the aerosol size distribution atall points between corona ion production, close to the conductorcables, through to the measurement location at ground level; wellbeyond the scope of this work. Nevertheless, the extreme bias to-wards positive ions observed in particular at HH (maximumimbalance 112.5), but also several other sites showing imbalances>3 (YT, RW, LG), implies that, given sufficient time for ion-aerosolattachment, the charge state of aerosols could be significantlyaltered by corona ions from some AC HVPLs. Subsequent dis-charging is likely to take longer, since concentrations of bipolarbackground ions (required to re-neutralise particle charge) arelower. Thus, theremay be an elevated particle charge state for somedistance DWof HVPLs in corona, extending beyond the distance theions themselves persist for. The distance over which charge statesremain higher than background levels is central to determining anypotential increase in dose of particulates and associated healthimpact.

An ion imbalance (of either polarity) is required to enhanceaerosol charge state, so in the case of equally-enhanced productionand dispersion of both ion polarities, aerosol charge state alterationwould not occur. For positive corona production, the ions producedmust first neutralise the observed slight natural negative bias inaerosol charging (Wiedensohler, 1988), reducing the overall netmagnitude of particle charge, before increasing this again in thepositive direction (if sufficient imbalance is present). HVPLs biased

Fig. 7. Ratio of downwind positive-to-negative, or negative-to-positive ion concentrations. Grey shaded area represents the highest positive (1.52) or negative (1.31) imbalanceobserved during upwind measurements.

M.D. Wright et al. / Atmospheric Environment 95 (2014) 296e304 303

towards negative corona would immediately begin to increaseaerosol charge state. Only 5/46 measurements showed a negativebias in DW ion concentration, none of which exceeded themaximum observed UW negative bias, and only 1 (WF) wasdesignated as being in ‘negative’ corona. Further study in inclementweather conditions and during nighttime would help determinewhether this was more frequent under these environments, asprevious works suggest (Chalmers, 1952; Matthews, 2012a,b).

5. Conclusions

We report a series of measurements of corona ion mobilityspectra and concentrations upwind (UW) and downwind (DW) ofhigh-voltage power line (HVPL) sites in South-West England.Corona of either (or both) polarity was observed during 33 of 46measurements, at 9 of 11 sites. Variability both in the level andpolarity of corona between sites and, to a lesser extent, at each siteon different days, was observed. Positive coronawas detected morefrequently, and in higher concentration, than negative corona. Themost extreme case was a 400 kV, 2-conductor bundle HVPL, wherethe DW/UW ratio of concentrations for positive ions exceeded 20during one measurement. Ion production by HVPLs usually in-creases overall ion concentration by only a modest amount, oftenwithin the range observed in ambient fluctuations, but the highestproducers can enhance concentrations by at least an order ofmagnitude. We observed corona from 132 kV as well as 275 kV and400 kV HVPLs, suggesting that in future epidemiological studiesthis line voltage should be considered.

Overall, ion mobility was higher DW compared with UW (by 3%and 9% for positive and negative ions respectively) but this was notstatistically significant. In individual mobility spectra, increasedDW mobility was sometimes observed when the transit time fromHVPL to measurement location exceeded the expected ion lifetimei.e. the ions were likely to have been produced within the DWplume, rather than directly near the HVPL conductors. This may bebecause shorter ion lifetimes, due to increased losses in the elec-trical environment DW of HVPLs, allow less time for ion growth,resulting in a smaller, more mobile ion population. Ion mobilityalteration by HVPLs is most apparent during significant increases inconcentration for one polarity, leading to depletion of the otherpolarity. Limited evidence in individual measurements for a hu-midity effect on positive, but not negative, ion mobility wasobserved.

Aerosol charge enhancement due to corona ions from HVPL canonly occur when there is a polarity imbalance, otherwise the net

charge state of the aerosol remains unaltered. A positive imbalance,exceeding themaximumvalue observed in UWmeasurements overthe entire study, was observed on 15 of 24 measurement days; nonegative ion imbalance exceeded the maximum UW value. Thelevel of imbalance required to significantly enhance aerosolcharging could be determined via charge measurement with twoSequential Mobility Particle Sizers (Buckley et al., 2008) in additionto ACIMS, or with other instruments capable of measuring ambientcharging of ultrafine particles (e.g. Ion-DMPS, Gagn�e et al., 2008),along with full consideration of the production, ion-aerosol andioneion interaction and dispersion of ions from HVPL. Anotherfocus for future work should be measurements near HVPLs in highhumidity, inclement weather and during nighttime, where nega-tive ion production is favoured and where current results arescarce.

Acknowledgements

This work was supported by CHILDREN with CANCER UK,Registered Charity (UK) No. 298405. HVPL conductor geometry waskindly provided by Dr. John Swanson, National Grid (UK). We thankJonathan Ward and Andrea Lazenby for assistance during experi-mental measurements.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.atmosenv.2014.06.047.

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