Atmospheric Environment Vol. 32, No. 14/15, pp. 2601—2608, 1998( 1998 Elsevier Science Ltd. All rights reserved

Printed in Great Britain1352—2310/98 $19.00#0.00PII: S1352–2310(97)00483–4

PARTICULATE ORGANIC NITRATES: SAMPLING ANDNIGHT/DAY VARIATION

TORBEN NIELSEN,*,- JESPER PLATZ,- KIT GRANBY,‡ASGER B. HANSEN,‡ HENRIK SKOV‡ and AXEL H. EGEL"V‡

-Chemical Reactivity, Department of Environmental Science and Technology, Building 313, Ris+l NationalLaboratory, P.O. Box 49, DK-4000 Roskilde, Denmark; and ‡National Environmental Research Institute,

P.O. Box 358, DK-4000 Roskilde, Denmark

(First received 3 March 1997 and in final form 15 October 1997. Published June 1998)

Abstract—Atmospheric day and night concentrations of particulate organic nitrates (PON) and severalother air pollutants were measured in the summer 1995 over an open-land area in Denmark. The samplingof PON was evaluated comparing 24 h samples with two sets of 12 h samples. These results indicate that theobserved low contribution of PON to NO

yis real and not the result of an extensive loss during the

sampling. Empirical relationships between the vapour pressure and chemical formula of organic com-pounds were established in order to evaluate the gas/particle distribution of organic nitrates. A positivecorrelation between the PON to NO

;ratio and the inverse temperature may indicate that the organic

nitrates associated with particles also may be present in the gas phase. The observed day and nighttimelevels of PON were of the same magnitude, although the day to night ratio varied from 0.4 to 4.4. ( 1998Elsevier Science Ltd. All rights reserved

Key word index: Organic nitrates, atmospheric chemistry, daytime, nighttime, sampling, particles, vapourpressure.

1. INTRODUCTION

The atmospheric formation of ozone over the openland is controlled by the presence of NO

xat least

during the summertime (Finlayson-Pitts and Pitts,1996; Nielsen et al., 1996). Bifunctional organic ni-trates have been found in the gas phase as well asunidentified bi- and/or multifunctional organic ni-trates associated with particles in smog-chamber ex-periments and are formed by reactions of alkenes withOH radicals at day-time (Palen et al., 1993; Tuazonand Atkinson, 1990) and with NO

3radicals at night-

time (Atkinson, 1991; Hoffmann et al., 1997; Skov etal., 1992). These compounds are considered to bea possible source of NO

2in remote areas (Wangberg

et al., 1996) and they may thus affect the formation ofozone from the photooxidation of CO and CH

4(Ayers et al., 1992). Recently, some gas phase C

2—C

4hydroxynitrates and dinitrates have been identified.Daytime processes were estimated to be the majorsource of these compounds (O’Brien et al., 1995, 1997).Organic nitrates are suspected of making up a signifi-cant part of the so-called missing NO

y. Concurrent

*Author to whom correspondence should be addressed.

measurements of NOy

and the individual nitrogenspecies (NO, NO

2, HNO

3, PAN, PPN and inorganic

nitrate) over the open land and background areasindicates that the unidentified fraction is 6—44% of thetotal NO

y(Crosley, 1996; Fahey et al., 1986; Nielsen et

al., 1995; Parrish et al., 1993). The atmospheric levelsof the particulate organic nitrates (PON), i.e. organicnitrates associated with particles, have earlier beendetermined to be 14$5 ng (N) m~3 (0.25% of totalNO

y) in over an open area in Denmark (Nielsen et al.,

1995) and 45 ng(N) m~3 in Los Angeles air (Mylonaset al., 1991). Both investigations applied the IRabsorption features of the nitrate groups for thedetermination.

In order to determine whether the low contribu-tion of PON to NO

yis real or caused by extensive

loss during the sampling, the possible influenceof sampling artefacts has been studied. The day andnight levels are compared. The measurements wereperformed in the summer 1995. The sampling sitewas over open land at a location relatively close toNO

xsources. The results are also compared with

the previous results from the spring and summer1993 (Nielsen et al., 1995). Also, the relation be-tween vapour pressure and structure and the tem-perature effect on the gas/particle distribution isdiscussed.

2601

2. EXPERIMENTAL

2.1. Site description and meteorological conditions

The measuring station is located in a horizontally homo-geneous agricultural field in a typical Danish farming areaon Zealand. The geographical co-ordinates are 12°07@07AE,55°41@42AN and the altitude above sea level is 15 m. Thenearest town is Roskilde (40,000 inhabitants), located 6 kmSSW of the site. The Danish capital, Copenhagen (850,000inhabitants), is located 35 km to the east. Located 2 km westof the site is Roskilde inlet. A highway with traffic load ofappr. 6000 cars each day runs in the north-south directionalong the coast. The terrain altitude variation is less than 5 min a radius of 1 km from the site. Maps of the area have beenpresented by Nielsen et al. (1981 and 1994). The vegetation ofthe surroundings includes grass, barley, rye, wheat, rape,potato plants, bushes and different deciduous and coniferoustrees, although the number of coniferous trees is small.

The particulate organic nitrates (PON) samples were col-lected mainly in warm and sunny periods in May—July 1995.The winds originated from the west sector half of the timeand otherwise from east and southeast in the other cases.The wind speed was typical for Danish conditions at about4 m s~1.

2.2. Particulate organic nitrates

Sampling: 12- and 24 h samples were collected on glassfiber filters with Hi-Vol samplers and stored in a freezer(!18°C) until analysis. The sampling volumes were typi-cally 750 m3 for 12-h samples and 1500 m3 for 24 h samples.The 24 h samples were collected in the period 0700—0700, andthe 12 h samples in the periods 0700—1900 and 1900—0700.The 0700—1900 samples are called the day-samples andthe 1900—0700 samples are called the night-samples in Sec-tion 3, although the period from sunset to sunrise was notmore than 61

2—71

2h during this investigation. However, the

appellations appear reasonable, as the global radiation (seeTable 2) during the 12 h night period was only 7% of thatduring the 12 h day period. 33 samples were collected in theperiod 23 May—21 July 1995. Two of the samples werecollected in May, 11 were collected in June and 20 werecollected in July. 23 of the 33 samples were 12 h and 10 were24 h. Results from nine sets of two 12h samples and one 24 hwere collected.

Extraction and analysis: The filters were extracted threetimes, 30 min each time, with 100 ml chloroform (Merck p.a.)in an ultrasonic bath. 7 ml tetrachlorethylene (Merck,Uvasol) were added to the concentrated extracts, and themixture was concentrated further to 5 ml. The C

2Cl

4solu-

tions were analysed using Fourier transformation infraredspectroscopy (FTIR, Perkin-Elmer Spectrometer 1760X).Each sample was analysed with 1000 scans. The peak areabetween 1617 and 1663 cm~1 was determined representingthe total amount of organic nitrates in the sample. A typicalspectrum of a sample has been published elsewhere (Nielsenet al., 1995). C

2Cl

4solutions of different organic ni-

trates, i-propyl nitrate (Fluka, 95%), 3-nitrooxy-2-butanoneand 3-nitrooxy-2-butanol, were used for calibration.By comparing the measured sample areas to the standardareas the concentrations of PON in the samples werecalculated.

2.3. Other components

Nitric acid and particulate inorganic nitrate were collectedon NaCl coated denuders (0.4 mm i.d.* 50 cm, 12 h samplingtime, flow 1 l min~1) followed by NaF coated filters, anddetermined by ion chromatography with UV detection(Nielsen et al., 1995; Skov et al., 1997). PAN was measuredby gas chromatography with electron capture detection(Nielsen et al., 1982). O

3was measured by a UV-absorption

monitor, NO and gas NOyby a two chamber chemilumines-

cence monitor, SO2by UV-fluorescence (Nielsen et al., 1994).

twelve hour samples of formic acid, acetic acid and methanesulphonic acid were collected by filterpacks. The particlesincluding MSA were collected on a Teflon filter in the frontand the gas phase fraction on two back-up cellulose filtersimpregnated with Na

2CO

3. The analyses were performed by

means of ion-chromatography (Granby et al., 1997). Thesampling of VOC (C

2—C

8) was performed with three 4 h

(07—11, 11—15 and 15—19) and one 12 h samples (19—07) in thecampaigns. The samples were analysed by trapping on ac-tivated carbon, thermal desorption, cryofocussing and GC-FID analysis (Hansen and Palmgren, 1996).

The meteorological station determined wind speed, winddirection, temperature, temperature difference (1.5 and 9 m),humidity, global radiation, pressure and precipitation every30 min with conventional micro-meteorological instruments(Nielsen et al., 1994).

3. RESULTS AND DISCUSSIONS

3.1. Possible identity, vapour pressure and gas/particledistribution

Organic nitrates may be formed by three pathwaysin the atmosphere (Flocke et al., 1991; Tuazon andAtkinson, 1990; Wayne et al., 1991):

(1) Reactions of peroxy radicals with NO. The re-action is a source of monoalkyl nitrates.

(2) Reactions of alkenes with HO radicals, forma-tion of a hydroxyperoxy radical and subsequentreaction with NO. The reaction is a source of hy-droxynitrates and smog-chamber experiments withisoprene, b-pinene and 1-octene have shown forma-tion of PON (Palen et al., 1993).

(3) Reactions of alkenes with NO3, formation of

a nitrooxyperoxy radical and subsequent oxidation toan organic hydroxy nitrate or oxonitrate compound.The gas-phase derivatives of, e.g. isoprene have beenidentified in smog-chamber experiments (Skov et al.,1992).

In ambient air a number of different C1—C

17alkyl

nitrates have been identified in gas phase (Flocke etal., 1991; Luxenhofer et al., 1996). Furthermore,O’Brien et al. (1995, 1997) have identified 6 C

2—C

4hydroxynitrates and 1,2-dinitrooxybutane in the gasphase. Luxenhofer et al. (1996) were unable to identifyalkyl nitrates associated with particles.

The gas/particle distribution of organic compoundsdepends on the vapour pressure of the compound. Ingeneral, there is a strong need for physical/chemicalparameters of organic pollutants in order to estimatetheir fate in the environment and a number of papershave shown that it may be possible to provide rela-tionships between these parameters and chemicalstructure of the compound. These parameters include,e.g. water solubility, octanol/water partition coeffi-cients and sorption coefficients (Dickhut et al., 1994;Helweg et al., 1997; Nielsen et al., 1997a; Schwarzen-bach et al., 1993). The important recent work ofHoffmann et al. (1997) illustrates, that there also isa strong need for relations between the vapour pres-sure and the chemical structure.

2602 T. NIELSEN et al.

Table 1. Estimated vapour pressure (P, mm Hg) of different organic nitrates at 20°Ca

No. of carbon atoms Dihydroxynitrates Hydroxynitrates Carbonylnitrates Alkylnitrates

5 2]10~4 0.02 0.2 310 1]10~6 2]10~4 0.002 0.0315 1]10~8 2]10~6 2]10~5 3]10~420 1]10~10 2]10~8 2]10~7 2]10~625 1]10~12 2]10~10 2]10~9 2]10~8

! The expression for the calculation of P is shown in the legend to Fig. 1.

The following relationship describes the influenceof the number of carbon atoms, nitrate, hydroxy andcarbonyl group on the vapour pressure of the com-pound in liquid phase at ambient temperature: logP"!(0.4069$0.0057)]no. of C-atoms!(2.144$0.070)]no. of nitrate groups!(1.961$0.057)]no.of hydroxy groups!(1.130$0.071)]no. of carbonylgroups #(4.466$0.077), n"183, r"0.98, p(0.001(see Fig. 1). The applied data set for the correlationincludes C

7—C

29hydrocarbons, C

1—C

18alcohols,

C2—C

10diols, C

5—C

18carbonyls, C

1—C

20alkyl ni-

trates and C2—C

3hydroxynitrates and dinitrates

(CRC, 1982; Hallquist et al., 1996; Kames andSchurath, 1992; Luxenhofer et al., 1996; Saxena andHildemann, 1996). Most of the experimental datawere achieved by extrapolating to 20°C. The datafrom Luxenhofer et al. (1996) and Saxena andHildemann (1996) were at 25°C. The calculatedP values of pinone- and caronealdehyde and C

8-C

20alkyl nitrates agreed with 56$20% with those re-cently determined experimentally by Hallquist et al.(1996) and Luxenhofer et al. (1996). The relation canalso be applied to compounds being in solid phase intheir pure state at ambient temperature, because thegas-particle partitioning in ambient air is determinedby the vapour pressure of the sub-cooled liquid andnot by the sublimation pressure of the solid com-pound (Pankow and Bidleman, 1992). The correlationincludes 183 compounds having P varying 10 ordersof magnitude. Most of the P values have been attained

Fig. 1. The figure shows the relation between the vapourpressure of organic compounds (x-axis) and the vapour pres-sure calculated from their chemical structure (y-axis) ap-plying the equation: log P"!0.4069]no. of C-atoms!2.144]no. of nitrate groups !1.961]no. of hydroxy

groups !1.130]no. of carbonyl groups #4.466.

by applying the Clausius—Clapeyron equation(Schwarzenbach et al., 1993) predicting an inverserelation between ln P and ¹:

ln P"!*H](R¹)~1#A

where *H is the heat of vapourisation and A is a con-stant. *H appeared to be independent of the temper-ature, as the mean r$2p was 0.9996$0.0003 forthe ln P!¹~1 correlations. The relation betweenthe vapour pressure at ambient temperature and thechemical structure has been applied to calculate thevapour pressure of different organic nitrates (seeTable 1). Organic nitrates are a group of compoundswhich may have great variations in their vapour pres-sure varying 12 orders of magnitude from C

5alkyl

nitrates to C25

dihydroxynitrates. Thus, the class oforganic nitrates will contain compounds, which areonly present in ambient air in the gas phase, somewhich is associated with particles and some will bepartly gas phase and particle associated. At equilib-rium, the gas-to-particle distribution is dependent onthe vapour pressure, temperature and particle concen-tration (Pankow, 1987, 1994; Pankow and Bidleman,1992), but the nature of the particles probably alsoaffects the distribution. As a matter of fact, the distri-bution of polar organic compounds between waterand suspended solid matter is affected by polar inter-actions between functional groups in the substrateand the compound itself (Nielsen et al., 1997a).A strong inverse relationship between vapour pres-sure and particle proportion values exists for non-polar compounds in typical ambient conditions usingfield data including alkanes, PAH and organochlorinecompounds (mainly PCB). The following relationhave been derived (Finizio et al., 1997; Pankow andBidleman, 1992):

F]A~1"10~b3]TSP]P~1

where F is amount of the compound associated withparticles, A the amount in gas phase, TSP the concen-tration of particles (kg m~3) and b

3a constant depen-

dent on the temperature. At 20°C an appropriatevalue of b

3is !8 (Finizio et al., 1997; Pankow and

Bidleman, 1992). Thus, the F/A ratio is smaller than0.01 if P'2]10~5 mm Hg and TSP"20 kg m~3,i.e. for P'2]10~5 mm Hg one can consider the com-pound to be exclusive in the gas phase. Correspondingly,

Particulate organic nitrates: variations 2603

one can consider the compound to be exclusivelyassociated with particles, if P(2]10~9 mm Hg. Al-though the extrapolation of this data set to organicnitrates should be done cautiously, the set andTable 1 confirm, that C

17alkyl nitrates (P"3]

10~5 mm Hg), C4

hydroxynitrates (0.05 mm Hg) andC

4dinitrates (0.04 mm Hg) should be present in the

gas phase in ambient air. From the data set above andTable 1, it can be estimated that higher alkyl nitrates('C

18) may contribute to the presence of PON com-

pounds. However, if they are present in ambient air, itshould be possible also to identify these compounds inthe gas phase, as P"1]10~8 mm Hg for C

25alkyl

nitrates. The PON compounds may also be bi- andmultifunctional organic nitrates. For bifunctional or-ganic nitrates the number of carbon has to be at least12. For nitrates with three functional groups the num-ber of carbon atoms has to be at least 7. Therefore, itwill be very difficult, if not impossible, to determinethe exact chemical structure of bi- and multifunctionalorganic nitrates present as PON in ambient air con-sidering the low amounts of PON, the large numberof possible isomers and difficulties in preparation andhandling of bi- and multifunctional organic nitrates.A major VOC source for PON formation may bebiogenic emissions, terpenes (C

10) and sesquiterpenes

(C15

) (Calogirou et al., 1996; Konig et al., 1995). Dieselexhaust emissions of alkenes may also be a sourcefor VOC precursors. The content of alkenes in dieselfuel is in the range 0.2—2.2% (Siegl and Wallington,1996).

Table 2 shows the night and day mean concentra-tions of C

2—C

5alkenes, C

2—C

8alkanes and the traffic

VOC, toluene. Nielsen et al. (1997b) have shown thatmost of the alkenes had originated from biogenicsources. This also indicates that the emissions of ter-penes and sesquiterpenes are important (Konig et al.,1995). The measured levels of ethylene and the otheralkenes were 6—20 times higher than the modelledvalues, while the modelling of the traffic VOC, tol-uene, agreed with the measured values. Also, theC

2—C

8alkanes had a much lower ratio of the meas-

ured to the modelled values (2.8) than the alkenes. Theconclusion of Nielsen et al. (1997b) is that the emissioninventories underestimated the biogenic emissions ofalkenes and this implied that biogenic emissions werethe major source of alkenes.

3.2. Sampling validation

As discussed in the previous section some organicnitrates may be present both in the gas phase andassociated with particles (Nielsen et al., 1995). Phys-ical as well as chemical processes may interact in thesampling of these compounds. Physical losses byevaporation of the organic nitrates from sampled par-ticles may affect the results (Finlayson-Pitts and Pitts,1986). Another possible error is chemical decomposi-tion of PON during sampling. In order to validatethe sampling technique the results from 12 h and24 h samples were compared. However, first the two

Hi-Vol samplers were intercompared by collectingseven sets of identical 24 h samples and comparing theamounts of collected sulphate. This test was satisfac-tory as the amounts of sulphate agreed to within1—2%. The comparison of PON in 12 h and 24 hsamples is shown in Fig. 2. On average the content inthe 24 h samples was 85$18% ($2p) of the contentof PON in the set of two sequential 12 h samplescovering the same time interval. The variation in thedata set was investigated most thoroughly with re-spect to variations in the meteorological parametersand the air pollution parameters, but no systematiccorrelation was found. Thus, it seems to be accidentalthat the early sample set is much closer to equivalencebetween 12#12 and 24 h than the last four. Theresults indicate that the low PON to NO

yratio found

previously (Nielsen et al., 1995) and in this investiga-tion (see Table 2) is real and not caused by an exten-sive loss during sampling. Otherwise, the ratio shouldbe much lower than 85%. Also, the samples werecollected in connection with warm and sunnyweather. It is believed that the risk for blow-off in-creases with the temperature and the risk for chemicaldegradation increases with the presence of photo-chemical oxidants. Therefore, the comparison shouldrepresent a worst case situation for Danish condi-tions.

3.3. PON occurrence and day/night variation

The measured PON made up, as mentioned earlier,a minor proportion of total NO

y(0.17—0.28%, see

Table 2). The mean PON levels in the day (11$3 ng(N) m~3) and night samples (10$4 ng (N) m~3) in1995 and in the 24 h samples from 1993 (14$5 ng(N) m~3) were not significantly different (t-test,p"0.16—0.26). As discussed in Section 3.1 the organicnitrates contributing to PON is believed to consist ofsemi-volatile species. The particle to gas ratios ofthese species are expected to increase with decreasingtemperature. The correlation between the ratioPON (kg (N) m~3) to NO

;(kg (N) m~3) (NO

z"

NOy—NO

x) and the inverse temperature applying

both the 1993 and the 1995 data set support this:

PON/NOz"(60$26)](1/¹(K))!(0.19$0.10),

r"0.33, p(0.05.

In the 1993 data set a remarkable correlation betweenthe 24 h levels of PON and the night NO

3dose was

observed (Nielsen et al., 1995). However, it was notevident, if this implied that NO

3chemistry had been

the dominant PON source. From Fig. 3, it can bededuced that the ratio between the day and sub-sequent night PON concentrations varies strongly.Thus, the PON concentration of the 9 July daysample was 4.4 times higher than the following nightsample, while the PON concentration of the 11—12July night sample was 2.8 times higher than that of the11 July day sample. In 6 of the 10 sets of samples the dayconcentration was higher than the night concentration,

2604 T. NIELSEN et al.

Tab

le2.

Ave

rage

conc

entr

atio

n($

2p)o

fpar

ticu

late

orga

nic

nitra

tes(P

ON

)and

other

pollut

ants

and

aver

age

met

eoro

logi

calc

onditio

nsin

the

nigh

tan

dda

ysa

mple

sain

the

per

iod

23/0

5—21

/07

1995

,in

24h

sam

ples

!in

the

period

23/0

3—25

/06

1993

and

the

nig

ht

of11

—12

July

and

the

day

of13

July

1995

Com

pound

Conce

ntra

tion

24h

1993

Conce

ntra

tion

nig

ht19

95C

once

ntra

tion

Day

1995

Day

toni

ght

ratio

Nig

htco

nc.11

July

1995

Day

conc

.13

July

1995

PO

N(n

g(N

)m~3)

14$

510

$4

11$

31.

220

.721

.5N

O(k

g(N

)m~

3)0.

9$0.

31.

0$1.

10.

7$0.

40.

71.

8#H

NO

3(k

g(N

)m~3)

0.19

$0.

060.

036$

0.01

20.

13$

0.05

3.5

0.02

10.

26G

asN

Oy(k

g(N

)m~

3)4.

9$1.

35$

43.

2$0.

90.

75.

25.

5#In

orga

nic

nitra

te(k

g(N

)m~3)

0.6$

0.3

0.9$

0.7

0.9$

0.5

1.0

0.43

1.60

PA

N(k

g(N

)m~

3)0.

32$

0.04

"0.

12$

0.07

0.25

$0.

072.

10.

060.

45C

2-C8

alka

nes

(ppb

C)

45$

1328

$6

0.6

6440

C2-C

5al

kene

s(p

pbv)

3.9$

0.9

7.6$

1.6

1.9

5.3

9.2

Tolu

ene

(ppb

v)0.

17$

0.05

0.26

$0.

091.

50.

150.

34O

zone

(ppb)

38$

226

$7

40$

51.

56

38#

Form

icac

id(k

gm

~3)

1.7$

0.6

1.3$

0.3

2.3$

0.6

1.8

1.2

3.9

Ace

tic

acid

(kg

m~

3)2.

3$0.

81.

7$0.

53.

0$0.

91.

71.

74.

4M

ethan

esu

lphon

icac

id(k

gm

~3)

0.09

$0.

030.

13$

0.05

0.12

$0.

040.

90.

050.

07SO

2(p

pb)

2.0$

0.8

0.49

$0.

130.

9$0.

41.

90.

441.

1W

ind

spee

d(m

s~1)

5.1$

0.7

3.4$

0.7

4.5$

0.7

1.3

2.5

2.1

Tem

pera

ture

(°C

)10

$2

16$

221

$2

1324

Rad

iation

(Wm

~2)

203$

2538

$8

522$

4614

4837

7PO

Npro

por

tion

(%)ofto

talN

Oy

0.25

0.23

0.34

1.5

0.37

0.30

No.of

sam

ple

s21

1013

11

!D

aydefi

ned

as07

00—1

900

and

nigh

tdefi

ned

as19

00—0

700.

"O

nly

7d.

#M

ean

from

700—

1300

.

Particulate organic nitrates: variations 2605

Fig. 2. Comparison of nine sets of one 24 h sample and two12 h samples. The figure shows the ratio between the amountof particulate organic nitrates (PON) in the 24 h sample andthe sum of the PON amounts in the two sequential 12 h

samples.

Fig. 3. The figure shows the day and night concentrations ofparticulate organic nitrates (PON). The day concentrationscover the period 0700—1900. The night concentrations coverthe period 1900—0700. The x-axis shows the start date for thesampling, i.e. the night-sample for the 23 May covers the

period 23 May 1900—24 May 0700.

in 2 sets the night concentrations were highest, and inthe remaining 2 sets the day and night concentrationswere similar. The 10 sets were carefully investigatedby means of air mass trajectory plots and weathermaps from each day at midnight and noon from theDeutscher Wetterdienst (1995). This analysis showedthat in 4 sets (23 May, 26 June, 10 and 11 July) the dayand night samples represent air pollution having thesame origin of the air masses. The trajectories in thesefour sets had not been affected by the passage offrontal systems. Despite the similarity in source, theday/night PON ratios for these sets were also highly

variable (0.4—2). A closer look at the meteorology andair chemistry, however, can provide an explanationfor this variability.

On 9 July a high-pressure system moved from theNorth Sea to Scandinavia forcing a front aboveDenmark towards the south. The high-pressure sys-tem gave very stable weather in Denmark during thefollowing days with winds from the east. The air masstrajectories passed over Southern Sweden. On the 12July, the high--pressure system moved further to-wards southeast. At noon on the 12 July the winddirection changed from east to southeast. The frontpassed Denmark from the south in the early morninghours of the 13 July. Just after the front passage theSO

2concentration increased from (0.25 to 5.5 ppb

indicating long-range transport of air pollutants fromCentral Europe. The air mass trajectory arriving atLille Valby on 13 July 1200 had passed Poland 24 hearlier. Thus, the air mass trajectories also confirmedthat the air masses giving the night PON on 11—12July and those giving the day episode of 13 July hada different origin. The night of 11—12 July was charac-terised by clear sky and very low wind speeds (seeTable 2). The concentrations of most of the VOC wereelevated. C

2—C

8alkanes and C

2—C

5alkenes were 1.4

times higher than their average night concentration,and acetylene (11—12 July: 0.73 ppb, night average:0.61$0.20 ppb) and benzene (11—12 July 0.73 ppb,night average: 0.59$0.17 ppb) were also slightly elev-ated. The poor mixing of air pollutants (low windspeeds during the night) may also imply that signifi-cant concentrations of terpenes may have been pres-ent, although the emissions are much lower at night-time than at daytime. The so-called photochemicalage of NO

yappeared to be young, as the ratio of

inorganic nitrate, PAN and nitric acid to NOy

wasa factor of two lower than that on the other nights.NO was not present besides in the period 500—700,which was more than 1 h after sunrise, and thereforenot had caused depletion of NO

3during the night.

Considering the young age of NOy

it is difficult toimagine that most of the PON had been formed atday—time and transported to the measuring site dur-ing the night. The 13 July day sample was character-ised by a long-range transport episode as indicated bythe morning SO

2peak in addition to photochemical

activity. PAN was 0.55 kg N m~3 at 1300 and had itsmaximum (0.75 kg N m~3) at 1700. Correspondingly,the H

2O

2maximum (1.1 ppb) also was late in the

afternoon (1800—1830). Ozone was 59 ppb at1230—1300, but unfortunately a technical failurecaused the ozone measurements to stop at 1300. Thelevels of formic acid, acetic acid, nitric acid and partic-ulate inorganic nitrate was also elevated (see Table 2).From the discussion above, PON production inducedboth by OH at daytime and by NO

3at nighttime

appears possible. The 1993 correlation between the24-h levels of PON and the night NO

3dose (Nielsen

et al., 1995) apparently due to a combination of rea-sons, i.e. that the NO

3chemistry is an important

2606 T. NIELSEN et al.

PON source and episodes with night NO3doses usual

are associated with elevated photochemical activity inthe daytime hours.

4. CONCLUSIONS

The low contribution of particulate organic nitrates(PON) to NO

yappears to be real and not a sampling

artefact. The major VOC sources for PON formationare estimated to be biogenic alkenes, terpenes andsesquiterpenes. The PON compounds are probablybi- and multifunctional organic nitrates. The organiccompounds associated with particles may also bepresent in the gas phase. Empirical relationships be-tween the vapour pressure and the chemical formulawere established for organic compounds containingnitrate, hydroxy and/or oxo groups. PON may beformed both during the daytime and at night. A con-dition for the latter is that the night concentrations ofhigher alkenes is sufficient. The observed day andnight concentrations of PON were in the same magni-tude, although the day to night ratio varied from 0.4to 4.4.

Acknowledgements—Financial support of the Danish Centrefor Air Pollution Processes and Models under the DanishEnvironmental Research Programme and the DanishScience Research Council is grateful acknowledged.Dr S+ren Hvilsted, Ris+, is acknowledged for his permissionto apply the Perkin-Elmer FTIR spectrophotometer for thisinvestigation. Ole Hertel, National Environmental ResearchInstitute, is acknowledged for providing air mass trajectoriesfor this investigation.

REFERENCES

Atkinson, R. (1991) Kinetics and mechanisms of the gas-phase reactions of the NO

3radical with organic com-

pounds. Journal of Physical Chemical Reference Data 20,459—507.

Ayers, G. P., Penkett, S. A., Gillett, R. W., Bandy, B., Gal-bally, I. E., Meyer, C. P., Elsworth, C. M., Bentley, S. T.and Forgan, B. W. (1992) Evidence for photochemicalcontrol of ozone concentrations in unpolluted marine air.Nature 360, 446—449.

Calogirou, A., Larsen, B. R., Brussol, C., Duane, M. andKotzias, D. (1996) Decomposition of terpenes by ozoneduring sampling on Tenax. Analytical Chemistry 68,1499—1506.

CRC (1982) Handbook of Chemistry and Physics, 62nd edn.CRC Press, Boca Raton, FL.

Crosley, D. R. (1996) NOy

blue ribbon panel. Journal ofGeophysical Research 101D, 2049—2052.

Delmas, R., Lacaux, J. P., Menaut, J. C., Abbadie, L., Roux,X. le, Helas, G. and Lobert, J. (1995) Nitrogen compoundemission from biomass burning in tropical African savan-nah FOS/DECAFE 1991 experiment (Lamto, IvoryCoast). Journal of Atmospheric Chemistry 22, 175—193.

Deutscher, Wetterdienst (1995) European MeteorologicalBulletin, Amtsblatt des Deutschen ¼etterdienst, Offenbach,Germany.

Dickhut, R. M., Miller, K. E. and Ardren, A. W. (1994)Evaluation of total molecular surface area for predictingair—water partitioning properties of hydrophobic aro-matic chemicals. Chemosphere 29, 283—297.

Fahey, D. W., Hubler, G., Parrish, D. D., Williams, E. J.,Norton, R. B., Ridley, B. A., Singh, H. B., Liu, S. C. andFehsenfeld, F. C. (1986) Reactive nitrogen species in thetroposphere: Measurements of NO, NO

2, HNO

3, partic-

ulate nitrate, peroxyacetyl nitrate (PAN), O3, and total

reactive nitrogen (NOy) at Niwot Ridge, Colorado. Jour-

nal of Geophysical Research 91D, 9781—9793.Finizio, A., Mackay, D., Bidleman, T. and Harne, T. (1997)

Octanol-air partition coefficients as a predictor of par-titioning of semi-volatile organic chemicals to aerosols.Atmospheric Environment 31, 2289—2296.

Finlayson-Pitts, B. J. and Pitts, J. N. Jr. (1986) AtmosphericChemistry: Fundamentals and Experimental techniques.Wiley, New York.

Flocke F., Volz-Thomas, A. and Kley, D. (1991) Measure-ments of alkyl nitrates in rural and polluted air masses.Atmospheric Environment 25, 1951—1960.

Granby, K., Egel+v, A. H., Nielsen, T. and Lohse, C. (1997)Carboxylic acids: Seasonal variation and relation to chem-ical and meteorological parameters. Journal of Atmo-spheric Chemistry (in press).

Hallquist, M., Wangberg, I. and Ljungstrom, E. (1997) At-mospheric behaviour of dicarbonyl products from theatmospheric oxidation of selected terpenes. In ¹he Oxidiz-ing Capacity of the ¹roposphere. ed. B. Larsen, B. Versino,and G. Angeletti. EUR 17482, pp. 345—349. EuropeanCommission, Brussels, Belgium.

Hansen, A. B. and Palmgren, F. (1996) VOC air pollutants inCopenhagen. Science of ¹otal Environment 189/190, 451—457.

Helweg, C., Nielsen, T. and Hansen, P. E. (1997) QSPRscompared with HPLC experiments for the estimation ofK

08for N-PAC’s. In Procedings of the seventh Interna-

tional ¼orkshop on QSARs in Environmental Sciences. ed.G. Schuurmann, Soc. Environmental Toxicology andChemistry (accepted for publication).

Hoffmann, T., Odum, J. R., Bowman, F., Collins, D.,Klockow, D., Flagan, R. C. and Seinfeld, J. H. (1997)Formation of organic aerosols from the oxidation of bi-ogenic hydrocarbons. Journal of Atmospheric Chemistry26, 189—222.

Kames, J. and Schurath, U. (1992) Alkyl nitrates and bifunc-tional nitrates of atmospheric interest: Henry’s law con-stants and their temperature dependencies. Journal of At-mosheric Chemistry 15, 79—95.

Konig, G., Brunda, M., Puxbaum, H., Hewitt, C. N.,Duckham, C. S. and Rudolph, J. (1995) Relative contribu-tion of oxygenated hydrocarbons to the total biogenicVOC emissions of selected mid-European agricultural andnatural plant species. Atmospheric Environment 29, 861—874.

Luxenhofer, O., Schneider, M., Dambach, M. and Bal-lschmiter, K. (1996) Semivolatile long chain C

6-C

17alkyl

nitrates as trace compounds in air. Chemosphere 33, 393—404.Mylonas, D. T., Allen, D. T., Ehrmann, S. H. and Pratsinis,

S. E. (1991) The sources and size distribution of organoni-trates in Los Angeles aerosol. Atmospheric Environment25, 2855—2861.

Nielsen, T., Samuelsson, U., Grennfelt, P. and Thomsen,E. L. (1981) Peroxyacetyl nitrate in long-range transportedpolluted air. Nature 293, 553—555.

Nielsen, T., Hansen, A. M. and Thomsen, E. L. (1982) A con-venient method for preparation of pure standards of per-oxyacetyl nitrate for atmospheric analyses. AtmosphericEnvironment 16, 2447—2450.

Nielsen, T., Granby, K., Egel+v, A. H., Hummelsh+j, P.,Jensen, F. P. and Skov, H. (1994) Lille Valby, Denmark. In¹he ¹OR Network. A Description of ¹OR MeasurementsStations, eds T. Cvitas and D. Kley, pp. 36—45. EURO-TRAC, International Scientific Secretariat, Garmisch-Partenkirchen, Germany.

Nielsen, T., Egel+v, A. H., Granby, K. and Skov, H. (1995)Observations on particulate organic nitrates and unidenti-fied components of NO

y, Atmospheric Environment 29,

1757—1769.

Particulate organic nitrates: variations 2607

Nielsen, T., Pilegaard, K., Egel+v, A. H., Granby, K., Hum-melsh+j, P., Jensen, N. O. and Skov, H. (1996) Atmo-spheric nitrogen compounds: Occurrence, compositionand deposition. Science of ¹otal Environment 189/190,459—465.

Nielsen, T., Siigur, K., Helweg, C., J+rgensen, O., Hansen,P. E. and Kirso, U. (1997a) Sorption of polycyclic aro-matic compounds to humic acid as studied by high perfor-mance liquid chromatography. Environmental Science and¹echnology 31, 1102—1108.

Nielsen, T., Hertel, O., Christensen, C. S., Egel+v, A. H.,Granby, K., Hansen, A. B., Platz, J. and Skov, H. (1997b)Evaluation of a trajectory model to simulate formation oftropospheric ozone, other photochemical oxidants andatmospheric reaction products. In ¹he Oxidizing Capacityof the ¹roposphere. eds B. Larsen, B. Versino, and G.Angeletti, EUR 17,482, pp. 426—430. European Commis-sion, Brussels, Belgium.

O’Brien, J. M., Shepson, P. B., Muthuramu, K., Hao, C.,Niki, H., Hastie, D. R., Taylor, R. and Roussel, P. B. (1995)Measurements of alkyl and multifunctional organic ni-trates at a rural site in Ontario, Journal of GeophysicalResearch 100D, (22) 795—22,804.

O’Brien, J. M., Shepson, P. B., Wu, Q., Biesenthal, T.,Bottenheim, J. W., Wiebe, H. A., Anlauf, K. G. andBrickell, P. (1997) Production and distribution of organicnitrates, and their relationship to carbonyl compounds inan urban environment. Atmospheric Environment 31,2059—2069.

Palen E. J., Allen D. T., Pandis S. N., Paulson S., Seinfeld,J. H. and Flagon R. C. (1993) Fourier transform infraredanalysis of aerosol formed in the photooxidation of 1-octene. Atmospheric Environment 27, 1471—1477.

Pankow, J. F. (1987) Review and comparative analysis of thetheories on partitioning between the gas and aerosol par-ticulate phases in the atmosphere. Atmospheric Environ-ment. 21, 2275—2283.

Pankow, J.F. (1994) An absorption model of gas/particlepartitioning of organic compounds in the atmosphere.Atmospheric Environment 28, 185—188.

Pankow, J. F. and Bidleman, T. F. (1992) Interdependence ofthe slopes and intercepts from log-log correlations of mea-sured gas-particle partitioning and vapour pressure—I.

Theory and analysis of available data. Atmospheric Envi-ronment 26, 1071—1080.

Parrish, D. D., Buhr, M. P., Trainer, M., Norton, R. B.,Shimshock, J. P., Fehsenfeld, F. C., Anlauf, K. G., Botten-heim, J. W., Tang, Y. Z., Wiebe, H. A., Roberts, J. M.,Tanner, R. L., Newmann, L., Bowersox, V. C., Olszyna,K. J., Bailey, E. M., Rodgers, M. O., Wang, T., Berresheim,H., Roychowdhury, U. K. and Demerjian, K. L. (1993) Thetotal reactive oxidized nitrogen levels and the partitioningbetween the individual species at six rural sites in EasternNorth America. Journal of Geophysical Research 98D,2927—2939.

Saxena, P. and Hildemann, L. M. (1996) Water-soluble or-ganics in atmospheric particles: a critical review of theliterature and application of thermodynamics to identifycandidate compounds. Journal of Atmospheric Chemistry24, 57—109.

Schwarzenbach, R. P., Gschwend, P. M. and Imboden, D. M.(1993) Environmental Organic Chemistry. Wiley, NewYork.

Siegl, W. O. and Wallington, T., Ford Motor Company,1996, private communication.

Skov, H., Hjorth, J., Jensen, N. R. and Restelli, G. (1992)Products and mechanisms of the reaction of the nitrateradical (NO

3) with isoprene, 1,3-butadiene and 2,3-

dimethyl-1,3-butadiene in air. Atmospheric Environment26, 2771—2783.

Skov, H., Egel+v, A. H., Granby, K. and Nielsen, T. (1997)Relationships between ozone and other photochemicalproducts at Ll. Valby, Denmark. Atmospheric Environment31, 685—691.

Tuazon, E. C. and Atkinson, R. (1990) A product study of thegas-phase reaction of isoprene with the OH radical in thepresence of NO

x. International Chemistry Kinetics 22,

1221—1236.Wangberg, I., Barnes, I. and Becker, K. H. (1996) Atmo-

spheric chemistry of bifunctional cycloalkyl nitrates.Chemical Physical ¸etters 261, 138—144.

Wayne, R. P., Barnes, I., Biggs, P., Burrows, J. P., Canosa-Mas, C. E., Hjorth, J., Bras, G. le, Moortgat, G. K., Perner,D., Poulet, G., Restelli, G. and Sidebottom, H., 1991, Thenitrate radical: Physics, chemistry, and the atmosphere,Atmospheric Environment 25, 1—203.

2608 T. NIELSEN et al.