Wintertime optical properties of atmospheric particles and weather

Pergamon Atmospheric Environment Vol. 31, No. 24, pp. 4053-4062, 1997 @. 1997 Elsevier Science Ltd

All rights reserved. Printed in Great Britain P I I : S1352-2310(97)00289-6 1352-2310/97 $17.00 + 0.00

WINTERTIME OPTICAL PROPERTIES OF ATMOSPHERIC PARTICLES AND WEATHER

P E T E R H E R R M A N N and G O T T F R I E D H , ~ N E L Institut fiir Meteorologic und Geophysik, Johann-Wolfgang-Goethe-Universit~it, Georg-Voigt-StraBe 14,

D-60325 Frankfurt am Main, Germany

(First received 10 September 1996 and in final form 17 June 1997. Published September 1997)

Abstract--Atmospheric particles from the boundary layer were collected on Nuclepore filters at the summit of a mountain near Frankfurt am Main (Germany). The optical properties (absorption, scattering and extinction coefficients, the asymmetry parameter of the phase function of scattered light and the complex refractive index) and the soot content of the particles were determined using a photometric method and a subsequent mathematical inversion. The dependence of the optical properties and the soot content on meteorological parameters measured at the sampling site and on large and mesoscale motions of the atmosphere was investigated.

The major findings are: (1) The observed optical properties of the particles are correlated with the air temperature measured at the sampling site. (2) The optical properties change during the passage of a cold front. (3) The optical properties depend on the existence of a stable stratified layer in the lower troposphere. (4) The optical properties depend on the local weather situation and on the history of the air masses arriving at the sampling site. © 1997 Elsevier Science Ltd.

Key word index: Optical properties, atmospheric particles, weather situation.

1. INTRODUCTION

Atmospheric particles have a direct impact on climate and weather. They influence the radiation budget of the atmosphere, and thus the climate, because of their ability to absorb and scatter solar radiation. These particles may absorb up to 20-25% of the solar radiation (H/inel, 1987a). Heating of the particles due to absorption leads to local warming of the atmosphere in the boundary layer of up to 1.3 K during the daylight period (Busen and H~inel, 1987; H/inel et al., 1990). Absorption of solar radiation is almost exclusively caused by black carbon or soot (Horvath, 1993).

Scattering of solar radiation by atmospheric particles can also have the opposite effect. Model calculations have shown that backscattering of solar radiation by particles may lead to a cooling in the lower part of the atmosphere (Charlock and Sellers, 1980).

Another important aspect is the indirect influence of the particles on the climate. Particles can act as condensation or ice nuclei and thus influence the radiation budget of the clouds and the cloudy atmosphere.

Because of the impact of the atmospheric particles on both climate and weather through their interaction with radiation, a thorough knowledge of their optical properties is of great interest. Past investigations have

shown that each optical property can vary consider- ably at a specific place, and can have very different values at different locations at the same time (e.g. Fitzgerald, 1980; H/inel, 1994). The strong spatial and temporal fluctuations of the particles' optical properties are caused both by local anthropogenic particle sources and by vertical and horizontal air mass transport in the atmosphere. In this paper the fluctuation with time at a specific location will be studied.

The amount of scattering and absorption of solar radiation by particles in the atmospheric boundary layer above urban or industrial areas depends strongly on the time of the day. These diurnal variations are due to temporal emissions of particles by anthropogenic sources such as traffic, heating or industry (Habenreich and Horvath, 1984 Busen et al., 1987; Hitzenberger, 1990). The local weather situation at the measuring site also has a direct impact on the particles' extinction coefficient. Several authors have found that the variation in the particles' optical properties varies with different local wind speeds and directions (Peterson et al., 1981; Pirich and Horvath, 1983; Hitzenberger, 1986a, 1990). Convective processes can have a major effect on the extinction and absorption coefficient near the ground (H/inel, 1988; Horvath and Trier, 1993; Stettler and Hoynin- gen-Huene, 1994; Uhlig et al., 1994). During weather situations with a stable stratified layer in the lower

4053

4054 P. HERRMANN and G. H,~NEI.,

t roposphere, the ext inct ion coefficient is much larger than dur ing other weather s i tuat ions (Hoyningen- Huene, 1994). The optical propert ies of a tmospher ic particles have also been observed to change significantly dur ing the passage of a warm or cold front at the measur ing site (Flowers et ul.. 1969: Busen and Hfinel, 1987; Hitzenberger, 1986b, 1990).

Fur ther invest igations have shown that the optical thickness of particles' in the a tmosphere and the particles' extinction coefficient in the lower t roposphere at a fixed location both depend on the origin and history of the air mass arr iving at the measur ing site. The values of these particles' propert ies are significantly higher for cont inenta l air masses than for all o ther air masses. The extinction coefficient is always lowest in air of mar i t ime polar origin. These results were obta ined at several different measur ing sites in Europe as well as in the Uni ted States (Peterson et al.,

1981; HS, nel, 1994; Hoyningen-Huene and Wendisch, 1994; Nilsson, 1994). The temporal f luctuations in the particles' extinction coefficient caused by changes in the air mass are usually more significant than the var ia t ions caused by temporal f luctuations in local emissions (Uhlig et al., 1994). Longt ime measurements at background stat ions of the particles' scattering coefficient and the black ca rbon content in the a tmosphere (Bodhaine, 1995) show that these properties undergo annua l cycles of more than an order of magnitude, while at the same time the black ca rbon content shows shor t - term variat ions by more than two orders of magnitude. This order of magni tude of variabili ty has also been found by HS.nel (1994) for the extinction and absorp t ion coefficients of particles in Central Europe.

Dur ing all these earlier investigations the vari- a t ions in only one or two optical properties of a tmospher ic particles were measured simultaneously, and in most cases for only a few days or weeks.

Thus, no long-t ime observat ions of complete para- meier sets for the optical properties of a tmospher ic particles are currently available. Secondly, no quant i t - ative empirical models exist to explain the short- term variability of more than two orders of magni tude in the particles" extinction, scattering and absorpt ion coefficients. With this paper the authors want to take a first step towards filling that gap by examining how the optical properties and the soot content of the particles depend on meteorological parameters measured at the sampling site and on the weather situation.

2. E X P E R I M E N T A L SET t P ANt) INVERSION M E T H O D

In this section the sampling method and the sampling silc are described first. This is followed by a brief explanation of the mathematical inversion method used to determine the optical properties of the atmospheric particles from photometric data.

2.1. Samplin~l ol the particle.~

2.1 1 Samplino method. The atmospheric particles were sampled on Nuclepore filters with a pore diameter of 0.2 Inn. A special device allows sampling which is independent of wind direction. The air was sucked through the filter by a vacuum pump and its volume measured with a gas meter. Earlier investigators found that this sampling method en- sures that the sampled particles are representative with rc- gard to their size distribution in the atmosphere (Spurn 5 196566 and 1968. VDI 1974 and 1987). Due to the structure of the Nuclepore filters, most of the particles are deposited homogeneously on the surface of the filter (Spumy 1965,'66). For the purpose of deriving their optical properties, the deposited particles can be treated as a homogeneous layer that is independent of the filter (Hitzenberger 1993).

2.1.2. Samplinq site. Sampling was carried out on the summit of the mountain "Kleiner Feldberg", which is situ- ated northwest of the city of Frankfurt am Main (Germanyi at 826 m above MSL. The "'Kleiner Feldberg'" belongs to a cham of mountains called the "'Taunus" (see Fig. I ~. The

; 35° , -5

~ \ f - - J ~ ~ J ) " ~ . 50ON

~ ~ , Darmstadt

,0 m

/ 7 ! ' ' k Fig. 1. Map of the Taunus and the Rhein-Main area. The triangle gives the location of the sampling site. The Taunus is illustrated by an isoplete of 350 m and the Rhein and Main valley by 100 m [Wobrock er at.,

1994).

Atmospheric particles and weather 4055

sampling site is surrounded by an extended forest area. The nearest region with large anthropogenic particle emissions extends from east to southwest at a distance of 10-15km (the Rhein-Maln-Area). The sampling device was installed 7 m above ground level, i.e. at the same height as the tops of the surrounding trees. Due to the dense vegetation cover there is no disturbance of the samples by dust particles stirred up in the vicinity of the site. Some- times however, during weather situations with weak winds the assignment of the results to a specific air mass can become uncertain because local wind systems may carry particles from regional sources to the sampling site. This effect has been taken into account during the analysis of the results.

For the present investigation, atmospheric particles were sampled during January, February and March 1994. Samp- ling was carried out continuously with the exception of two breaks, which lasted about one day.

2.2. Determination of the optical properties of the particles The optical properties of the sampled atmospheric par-

ticles were obtained through photometric measurements and mathematical inversion of the measured data.

2.2.1. Photometric measurement. The filter and the particles deposited on the filter are illuminated with the light of a broadband radiation lamp. The divergence of the beam of radiation impinging on the filter is small enough that all the incident radiation can be treated as striking the filter perpendicularly. Incident radiation is transmitted and ab- sorbed as well as scattered into the forward and backward directions. A thermopile detector moving in a circle around the centre of the filter measures the scattered radiation, and in the forward direction the sum of the scattered plus transmitted radiation, at defined angles (see Fig. 2). Such measurements are made for both the unloaded and the loaded filter. In the case of the loaded filter the incident radiation strikes the particles first.

By integrating over the forward and the backward hemi- spheres the fraction of the incident radiation passing through the (unloaded and loaded) filter the transmitted radiation plus the radiation scattered into the forward hemisphere (p) and the fraction of the incident radiation scattered into the backward hemisphere (b) are obtained directly. The photometric data also allow the derivation of an upper limit for the transmitted fraction (t) of the incident radiation. The optical properties of the filter and of the particles deposited on the filter are derived from the measured values of the parameters p, b and the maximum value of t through a mathematical inversion method.

Hemisphere

forward :: backward

Detector ~ ~

/ . ; / "

I t ° "It

forward backward

scattered

'41 ...... : Radiation

Fig. 2. Schematic diagram of the experimental setup.

The measurements were made at a relative humidity of less than 30%. Under these conditions the particles on the filter are nearly dry, allowing the optical properties of dry particles to be obtained.

2.2.2. Mathematical inversion. The computation of the optical properties of the filter and of the particles on the filter from the results of the measurements is based on an exact radiation transfer model (H~inel, 1994). This model is based on the principles of invariance (Chandrasekhar, 1960, van de Hulst, 1980). Through the inversion of the measured parameters p and b, the optical properties optical depth (z), the single scattering albedo (co) and the asymmetry parameter (0) of the volume scattering function are obtained both for the filter alone and for the particles alone. The inversion is carried out numer- ically according to the procedure desc~bed in H~inel (1994), with an improved starting procedure by Kehr (1995).

The essential steps of the computation of the optical properties are:

1. Iterative determination of the optical properties ~, co and 0 of the unloaded filter

2. Calculation of the starting values for the iteration of the particles' optical properties

(a) Computation of the starting values for T and co based on a two-stream radiation transfer model. In this model it is assumed that the asymmetry parameter is a given constant (H~inel, 1987b).

(b) Correction of the starting values of z and co obtained in step 2(a).

(c) Computation of the starting value for g using the corrected starting values for ~ and co.

3. Calculation of z, co and 0 for the particles.

The values for the absorption, scattering and extinction coefficients, the complex refractive index and the soot content of the particles are obtained from the optical depth, the single scattering albedo of the particles and the volume of air sucked through the filter. In addition, the relative error due to short-term fluctuations of the detector signals is computed for each parameter.

After various corrections (H~inel et al., 1990) the results can be interpreted as spectral mean values for extraterrestrial solar radiation. The results can also be understood as spectral values for radiation at a wavelength of 0.7 #m (Blanchet, 1982).

The results from the method were compared with the results of independent methods, such as the determination of the extinction coefficient of particles in the troposphere through measurements of the transmission of a laser beam (Uhlig and H0yningen-Huene, 1993), calorimetric measurement of the absorption coefficient (Hgnel and Hillenbrand, 1989), visibility observations and other methods. It was found that the results of the new photometric method and the independent methods agreed well (H~inel, 1994).

3. UNCERTAINTIES

There are three categories of errors associated with the optical properties of particles: (1) Errors due to short-time fluctuations of the lamp and other inaccuracies in components used in the experimental setup. (2) Errors due to long-term changes in the lamp. (3) Errors due to inaccuracies in the inversion method.

In an earlier investigation (H/inel, 1994), it was found that the resulting relative errors in the scattering and extinction coefficients for particles are less than 13-20%. The error in the absorption coefficient

4056 P. HERRMANN and G. H,g, NEL

is smaller than 3.5 7.5% and in ,) less than 3 5%. For the real part of the refractive index the error is 1 - 2 % at max imum and the errors for the imaginary part of the refractive index and the soot content are about 14 30%. These errors are small compared to most of the effects described in this paper. Thus, they have no significant impact on the results of this investigation.

4. RESULTS

In this section the relat ion of the optical properties of dry particles to meteorological parameters measured at the sampling site or to mesoscale and large- scale mot ion processes in the a tmosphere are examined. Possible reasons for the ment ioned relations are discussed.

The optical propert ies of the particles to be discussed in detail are the absorpt ion, scattering and extinction coefficients. In the following discussion these parameters are also referred to as a t tenua t ion coefficients. The results for the particles ' single scattering albedo, their complex refractive index and their soot content are presented in the tables but are not discussed in detail. The air t empera ture and the wind direction at the sampling site are found to be the most impor tan t meteorological parameters with regard to the a t t enua t ion coefficients. They were recorded cont inuously dur ing the investigation period. The mean value for each parameter was calculated for each sampling period. The observed mean wind direc- t ion (dd) was classified as belonging to one of four s e c t o r s ( N E : 0 " < d d < 9 0 , S E : 9 0 < d d < 180<,SW: 180 -: < d d < 270:', NW: 270 < dd < 360 ). The stratification of the t roposphere was determined by analysis of vertical soundings taken by the Wet te ramt Frankfur t (Deutscher Wetterdienst). Weather charts ( D W D 1994) and backward trajectories were used to classify the weather s i tuat ion and the origin and t ranspor t route of the air masses arr iving at the sampling site. For the classification of the air masses, see Fig. 3.

4.1. Relation between the attenuation co¢~cients of the partieles and the air temperature

During the observat ion period the values of the a t t enua t ion coefficients were observed to increase with decreasing air temperature. At a tempera ture of - 1 0 C their mean values were about three to four

times larger than at 1 0 C (for the absorp t ion coefficient see Fig. 4). The individual da ta points show considerable scatter a round the regression line. Nevertheless, the l inear regression is significant at the 99.9% level.

As explanat ion for this observat ion, two effects have to be considered: (1) During winter t ime low temperatures are usually connected with easterly winds. This yields enhanced a t tenua t ion coefficients as described below. However, the effect of wind

r Z

m P

.- ,, "- /, -" ]-~\~::

/ / I

"' ! f;4 . :' c P

. ", cT

mT - / -, - - - :2

tqg 3 Classification of lhc mr mas,,, type according to ~t~ origin: mP: maritime polar, roT: maritime tropic, cP: comi-

nental polar, cT: continental tropic.

direction is not large enough to explain the temperature effect. (21 Consequently, the fact that there is increased heat ing and thus increased soot emission with decreasing temperatures also has to be taken into account.

This observed dependence on air temperature seems to contradict the results of an earlier case study (Hfinel el al., 1990). In that study the higher a t tenu- at ion coefficients were found at higher air temperatures. However, in that case the lower temperatures were associated with descending clean air, which led to a considerable lowering of the particle load of the a tmosphere at the sampling site. The higher temperatures were connected with ano ther extreme weather s i tuat ion in which air travelled at an almost cons tant height on a long path over industrialized regions of Central and Eastern Europe before reaching the sampling site. This led to considerable enr ichment of the air mass by particles. These cases show that specific weather si tuations and travelling paths can alter the a t tenua t ion coefficients of particles so much t h a t

the mean tempera ture effecl cannot be detected, even quanti ta t ively (see Fig. 4 for the scattering of the data round about the regression line).

4.2, ln/luence O/the passage q/ cohl.li'om,~ on the attenuation c(w~cients qf partieles

During the present investigation the passage of 12 cold fronts was observed. It was found that all the a t tenua t ion coefficients became smaller after the passage of a cold front. The coefficients decreased by a factor of 1.5 (see Fig. 5 and Table 1). O the r authors have fkmnd such decreases using nephelometer measurements (Hitzenberger, 1986b) as well as from observat ions of direct solar radiat ion (Flowers et al., 1969).


0.025

0.02

Eo < 0.015 ~ •

0.01 GO •

• • S S 0.0o5 - - . • .

o -12 -lb -~ -~ -i -i 6 i ~ ~ ; 1'0

T [*C]

Fig. 4. Correlation between the absorption coefficient of the particles (GA) and the air temperature (T).

[km"]

0.025

0.02

m prefrontal

[ ~ posffront=l

0.015

0.01

0.005

o 1 ~ 0 0 O

A S E

Fig. 5. Changes of the absorption (aA), scattering (as) and the extinction (as) coefficients of the particles during the

passage of a cold front.

The decrease of the attenuation coefficients after cold front passages is caused by the following processes: Behind cold fronts, air of maritime-polar origin with low attenuation coefficients is transported to the sampling site. Second, this kind of transport is usually accompanied by high wind velocities and thus by low particle loading of the atmosphere due to emissions at or near the ground. Third, the path over the continent is quite short for maritime-polar air masses, thus allowing minimum particle loading of the air. Fourth, a weak subsidence takes place behind cold fronts. Hence, air masses form higher levels in the troposphere can reach the boundary layer at the sampling site. This leads to a reduction of the particle concentration. Fifth, after the passage of a front the stratification in the lower troposphere is mostly unstable. This leads to strong vertical mixing of the atmosphere and causes a lowering of the particle concentration near the ground.

4.3. Relation between the attenuation coefficients of particles and the weather situation

4.3.1. The stratification of the lower troposphere. During this investigation, weather situations with a stable stratified boundary layer or an inversion between 850 and 1000m above MSL were occa- sionally observed. During the situations with stable stratification the partieles were sampled either at the top of alstable stratified layer or 50-150 m above the layer and in the latter case the particles were sampled below the inversion.

It was found that the attenuation coefficients depend strongly on the stratification of the lower troposphere. The attenuation coefficients are more than twice as high for stable conditions or for an inversion within the lower troposphere than for all the other situations (see Fig. 6 and Table 1).

This can be explained as follows: During weather situations with stable stratification the vertical mixing of the air in the boundary layer is weakened or even prevented. This yields an increasing particle concentration near the ground due to particle emissions. In addition, during these weather situations weak easterly winds were observed at the sampling site and these winds cause particles to be transported from regional sources to the sampling site.

Below an inversion the air can be mixed by turbulence or by convection and the particles origin- ating at or near the ground can be transported up towards the lower boundary of the inversion. No exchange can occur between air above and below the inversion layer and hence air from above the inversion which is less particle loaded cannot sink below the inversion, while the more strongly particle loaded air from below cannot rise above it. As a consequence, the particle loading of the air below an inversion increases.

4058 P. H E R R M A N N and G. H,~NEL

Table 1. The particles" optical properties and the soot content bel\)re and alter the passage of a cold front and the influence of the stratification of the lower troposphere on these properties

~r~ ,7 s ~r ~ ( l ¢'~J (1 krn 1) 1 km i} (I krn 11 ~t k :~s

Passage of a front Prefrontal

Mean I).{)179 0.0117 0.0062 0.347 1.60 0.140 0.338 S.D. 0.01 I)5 0.0083 0.~)025 0.084 0.09 0.049 0.213

Postlrontal Mean 0.0113 0.0077 l).OII37 0.323 1.59 0.131 0.316 S.D. 0.0047 0.0037 0.0015 0.09S 0.0S O.057 (/.248

Stratification of the lower troposphere Stahh,

Mean 0.0390 (/.(1268 0.c)121 0.371 1.59 0.125 0.308 S.D. 0.0049 0.0011 0.0046 o.081 {).0S {).(I49 (').212

Other Mean 0,0178 0.0118 {).(106ii 0.334 1.5S 0.1 It) 0.269 S.D. 0.0012 (/.0084 (}.0041 0.085 0,09 0.038 0.170

Note: aA, absorption coefficient: a~, scattermg coefficient: Gt., extinction coefficient: ,~, single scattering albedo: n, imaginary part: k, real part of the refractive index: :q, soot content.

[ km" ]

0.05

0.04

0.03

I stable conditions

' - - ' ] o the r conditions

0.02

0.01

O O O A S E

Fig. 6. Dependence of the absorption (C;Ah scattering (ors) and the extinction (~re) coefficients of the particles on the

stratification of the lower troposphere.

In this way the a t t e n u a t i o n coeff icients are in-

f luenced by reg iona l par t ic le sou rces d u r i n g s o m e

dis t rac t w e a t h e r s i t ua t ions . C o n s e q u e n t l y , this ha s to

be t aken in a c c o u n t d u r i n g fu r the r d a t a ana lys is .

4.3.2. Local wimt direction and air mass history. T h e w e a t h e r s i t ua t i on at the s a m p l i n g site ha s

been cha rac t e r i zed here by the local w ind d i rec t ion

and the ac tua l large a n d m e s o s c a l e t r a n s p o r t p a t t e r n s

of air m a s s e s in the a t m o s p h e r e . S i t ua t i ons with

a s tab le s t rat i f ied b o u n d a r y layer or a n inve r s ion were

ignored because in these cases the a t t e n u a t i o n coeffi-

c ients were in f luenced by local par t ic le sou rce s and

t hus were d is t inc t ly h ighe r t h a n in the r e m a i n i n g

cases.

As s h o w n in Tab le 2, the lowest va lues o f the

a t t e n u a t i o n coeff icients were found in m a r i t i m e - p o l a r

air. T h e re la ted w e a t h e r s i t u a t i o n s are cha r ac t e r i z ed

by qu ick ly m o v i n g l ow-p re s su re s y s t e m s o r i g ina t i ng

Table 2. Influence of the air mass history and of the local wind direction on the optical properties of the particles and the soot content (mean wdues)

Air mass Wind Number of cb ~-s era type Direction cases ( l k m l) t l k m ~t I l k m ~) II ,,J) . k xs

mp NE 1 0.0259 (/.0178 0.0081 0.313 1.59 0.125 0.306 mp SW 8 0.0109 0.0071 0.0038 0.328 1.55 0.080 0.196 mp NW 10 0.0133 0.0098 0.0035 0.264 1.53 0.045 (1.110

mT SE I 0.0t94 (I.0126 (I.0068 0.351 1.57 0.097 0.238 mT SW I 1 0.0169 0.(/111 0.0058 0.343 1.56 0.091 (I.223

cp NE 2 (/.0491 0.0301 0.0190 (/.368 1.58 0.162 0.386 cp SE 3 0.0694 0.0469 0.0225 0.324 1.55 0.077 0.190

cT SE 4 0.0152 0.0086 0.0067 0.437 1.63 (/.195 0.467 cT SW 2 0.0202 0.0126 (I.0077 0.379 1.59 0.122 0.299

Note: For symbols see Table 1.


over the Atlantic Ocean. The air was transported quickly from the northern Atlantic to central Europe. The transport routes over the continent were quite short. Consequently, the optical properties of particles in maritime-polar air masses were hardly influenced by particles from either anthropogenic or natural sources on the continent. In one case (see Table 2) significantly higher attenuation coefficients in polar- maritime air were observed. This was due to a lower wind velocity and longer route over the continent than in all the other cases.

In tropical air the attenuation coefficients are somewhat higher than in maritime-polar air. The arrival of tropical air masses at the sampling site was related to low pressure systems moving slowly from the Atlantic over northern Europe or to high- pressure areas over eastern Europe. On the one hand, in these cases the air masses often originated over a continent (especially the continental-tropical air masses) and the path over the continent was distinctly longer than in the maritime-polar case. This results in a stronger particle loading of the tropical air masses. On the other hand, only weak souther- ly winds were observed when tropical air masses ar- rived at the sampling site. This resulted in an increase in the particle concentration due to the nearby sources to the south of the sampling site (i.e. industry, heating).

The highest attenuation coefficients were found in continental-polar air. In these cases the weather situation is characterized by large high-pressure areas over eastern or northeastern Europe. In this way air that originated over Russia was transported to the sampling site. The transport routes lead only over the continent and over regions with strong particle sources like the Rhein-Main-Area. Consequently, the particle concentration is very high and the attenuation coefficients are large.

Other authors have investigated the dependence of the values of the extinction coefficient on the origin and transport route of air masses using different methods. The investigations were carried out both in the United States and in Europe and yielded similarly low values of the extinction coefficient in maritime- polar air. Larger attenuation coefficients in air that originated over the continent or with transport routes across regions with strong anthropogenic emissions were observed (Peterson et al., 1981; H~inel, 1994; Hoyningen-Huene, 1994; Nilsson, 1994; Uhlig et al., 1994).

4.4. Weather situation and the single scattering albedo, the refractive index and the soot content of the particles

The particles' refractive index and soot content were calculated from the ratio of the absorption to extinction coefficient (1 - to) using two different par- title models. This is caused by the different shapes of the size distribution for urban and nonurban particles. For urban parities the fraction of small particles is significantly larger than for nonurban particles. Ac-

cording to H~inel (1988) there are more small particles whose absorption cross sections are proportional to the imaginary part of the refractive index. In this case for a given (1 - to) the imaginary part becomes larger for nonurban than for urban particles.

For both models the relations between (1 - ~) and the refractive index and the soot content are highly nonlinear. As a consequence, in some cases the mean values of the refractive index and the soot content seem to be inconsistent with the mean values of ( 1 - to). However, these inconsistencies are merely caused by taking the mean over values obtained using the two different particle models.

4.4.1. Stratification of the lower troposphere. Dur- ing weather situations with stable stratification or an inversion in the lower troposphere the ratio of the absorption to extinction coefficient (1 - to) is about 0.037 (10%) larger than those observed during other weather situations (see Table 1). The values of the imaginary part of the refractive index and the soot content are about 12% larger for situations with stable conditions or an inversion than for the other situations. The higher values of the imaginary part of the refractive index and the soot content are caused by limited vertical mixing and the increasing loading of the lower troposphere by soot which results from this.

The values of the real part of the refractive index remain almost constant during all weather situations.

4.4.2. Passage of cold fronts. The values of (1 - to) measured just before the arrival of cold fronts at the sampling site were 0.022 (about 7%) larger than those obtained after their passage (see Table 1). The values of the imaginary part of the refractive index and the soot content were also higher before than after the passages. As mentioned above, this is caused by a lower particle loading of the air from soot behind cold fronts.

No significant changes could be found for the real part of the refractive index during the passage of cold fronts.

4.4.3. Local wind direction and air mass history. The largest values of (1 - to) were observed in continental-tropical air masses (because of the addition of soot from nearby sources) and the lowest values in maritime-polar air (see Table 2). The difference between the values for the air masses of maritime-tropic or continental-polar origin was quite small. The lowest values for the imaginary part of the refractive index and the soot content were found for maritime- polar air masses and the largest values in continental- tropical air. The real part of the refractive index was nearly the same for all cases. These results can be explained by the different origins and transport routes of each air mass type. As mentioned above, for air masses of maritime-polar origin the short transport routes over the continent combined with a high wind velocity prevent any significant loading of the air masses by strongly absorbing particles.

4060 P. HERRMANN and G. HANEL

5. D I S C U S S I O N

One goal of this paper is to show the variability in the optical properties of particles during wintertime. This variability was extremely large during the time interval covered by this investigation. For example, the minimum values of the particles' at tenuation coefficients are more than one order of magnitude smaller than the maximum values (see Table 1).

The variability in the optical properties of particles is related to the weather situation at the sampling site. It was found that the optical properties depend strongly on meteorological parameters measured at the sampling site as well as on the history of the air masses arriving at the sampling site. Beside this, the influence of regional anthropogenic particle sources (i.e. industry) could be detected.

However, it is neither possible to evaluate the influence of just one meteorological parameter on the optical properties, nor to estimate a quantitative empirical relation between the air mass history and the optical properties.

Another investigation showed that both the absorption and the scattering coefficients of the particles undergo large variabilities in the absence of local or regional particle sources (Bodhaine, 1995). This investigation was carried out over several years at three remote locations (Alaska, Hawaii and South Pole). When background air alone was considered, the daily values scattered by more than one order of magnitude around the monthly mean values. Taking all the measurement results into account, the difference between the minimum value of the absorption coefficient and the maximum value was larger than three orders of magnitude. It was suggested by Bodhaine that these variations are caused by mesoscale transport processes.

When all the facts mentioned above are considered, the present state of research can be summarized as follows:

1. It makes little sense to use monthly mean values of optical properties, e.g. for climate considerations. The differences between a daily value and the monthly mean can be of one order of magnitude or more/see Fig. 7: Bodhaine, 1995). In addition, during this investigation in many cases the day-to-day variations are also of factor two or larger (see Fig. 8).

2. The values of the optical properties depend strongly on the location of the measuring site. At present it is not possible to generalize the results from one measuring site so that they be considered representative as mean values for large areas.

As a consequence more measurements need to be carried out to investigate the large variability in the optical properties of particles and to obtain a better knowledge of the dependence of the optical properties of particles on the weather situation at the measuring site and on large and mesoscale transport processes in the atmosphere.

6. SL M M A R Y

During the winter of 1994 the temporal variations of the optical properties of atmospheric particles and their soot content were observed. The sampling was carried out at the summit of a mounta in near the city of Frankfurt am Main (Germany). The optical properties were determined by' polar photometry and a subsequent mathematical inversion. The relation of optical properties to the meteorological parameters measured at the sampling site and to large and meso

0.1

E O.01

February

t ~ March January c5

° t 0.001

I ~ Extinction coefficient I Absorption coefficient I

Fig. 7. Monthly means of the absorption and the extinction coefficients. The upper and lower bars show the standard deviation for each month.


0.07

0.06

0 . 0 5

0 . 0 4

0 . 0 3

0.02

0.01

n A l ' ~ n m t ~ n ~ a l f l ~ q N t

nt

3.01. 1.02. 1.03. 31.03.94 Date

Fig. 8. Time series of the absorption, the scattering and the extinction coefficients. The extinction coefficient is the sum of the absorption and the scattering coefficient. Each column

stands for one daily mean value.

scale transport processes in the atmosphere was examined. The essential results are:

1. The absorption, scattering and extinction coefficients of the particles were related to the air temperature measured at the sampling site. Large attenuation coefficients were associated with low temperatures and, similarly, small coefficients with high temperatures.

2. The optical properties of the particles change significantly during the passage of a cold front at the sampling site. Before the passage of the front the values of the attenuation coefficients were higher than afterwards.

3. During weather situations with a stable stratified boundary layer or with an inversion in the lower troposphere, the values of the particles' optical properties and the soot content were significantly higher than during other situations.

4. The lowest values for the attenuation coefficients, the ratio of the absorption to extinction coefficient (1 -co) , the imaginary part of the refraction index and the soot content were found in marit ime polar air, whereas the highest values of the attenuation coefficients were measured in continental polar air. The largest value of (1 - co), the largest real and imaginary parts of the refractive index and the largest soot content were observed in continental air moving from the southeast to the sampling site.

Acknowledgements--The authors cordially thank the Deut- scher Wetterdienst, especially the Wetteramt Frankfurt am Main, for putting the trajectories and the results of vertical soundings at their disposal.

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Wintertime optical properties of atmospheric particles and weather

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