summer variations of the atmospheric aerosol number concentration over roorkee, india

Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 1007–1019www.elsevier.com/locate/jastp

Summer variations of the atmospheric aerosol numberconcentration over Roorkee, India

D.K. Sharmaa ;∗, J. Raia, M. Israilb, Pratap SinghcaDepartment of Physics, Indian Institute of Technology, Roorkee 247 667, India

bDepartment of Earth Sciences, Indian Institute of Technology, Roorkee 247 667, IndiacNational Institute of Hydrology (NIH), Roorkee 247 667, India

Received 7 August 2001; received in revised form 9 April 2003; accepted 26 May 2003

Abstract

The atmospheric aerosol number concentration has been measured at Roorkee (29◦52′N, 77◦53′E, 275 m above mean sealevel) in northern India during November 1998–August 1999 at a height of 9 m above ground level. The aerosol numberconcentration in summer season (April–July 1999) for morning, noon and evening periods has been analyzed, and the dailyvariation of aerosol number concentration has been related to selected meteorological parameters like: relative humidity,temperature, rainfall and wind speed. The measurements were carried out with an optical particle counter. The counter monitorsthe number concentration of aerosols in the diameter size range from 0.3–5:0 �m. The aerosol number concentration for uppersize ranges (1.0–2.0 and 2.0–5:0 �m) is maximum in June and minimum in July. The aerosol number concentration in smallsize ranges (0.3–0.5 and 0.5–1:0 �m) decreases monotonically till the end of July.c© 2003 Elsevier Ltd. All rights reserved.

Keywords: Aerosols; Number concentration; Particle counter; Correlation coe:cient; Meteorological parameters

1. Introduction

Aerosols play an important role in the atmosphere. Theycontrol the atmospheric radiation budget and hence are im-portant in the variation of weather and climate. The atmo-spheric aerosols either independently or in combination withatmospheric ions act as cloud condensation nuclei and a;ectprecipitation (Singh, 1985). Devara and Raj in 1998 mea-sured the aerosols during two successive monsoon seasons.Hanel (1976) and Shaw (1988) have tried to study the sizedistribution of atmospheric aerosol in di;erent meteorolog-ical conditions. Singh et al. (1997) measured the concentra-tion of aerosols during monsoon period and found that theconcentration decreases with increasing precipitation. Thisphenomenon has been attributed to the scavenging of largeaerosol particles due to rain.

The study on aerosol distribution can be done by us-ing various techniques available such as Cascade impactor

∗ Corresponding author.E-mail address: dksphdes@redi;mail.com (D.K. Sharma).

(Pahwa et al., 1994; Zhang et al., 2001), Lidar (Devara andRaj, 1998; McCormick et al., 1978), Low-pressure impactor(Parameswarn and Vijaykumar, 1994), Laser scatterome-ter (Singh et al., 1997, 1999), Optical counter (Bansal andVerma, 1998; Sharma et al., 2002), etc.

In the present study, we have measured the aerosolnumber concentration with an optical counter. The summervariation of atmospheric aerosol number concentration indi;erent size bins for morning, noon and evening periodsand their behavior with some selected meteorological pa-rameters: relative humidity, temperature, rainfall and windspeed have been studied.

2. Experimental technique

The optical counter model KC-01A (Rion Co. Ltd. Tokyo,Japan) has been used in the present study, which is basedon the principle of Mie scattering of light. The instrumenthas been well calibrated and has a provision for inbuiltcalibration. A light beam intersects a Dow of ambient air

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1008 D.K. Sharma et al. / Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 1007–1019

containing aerosols, which is chopped by a light chopper.The light incident on aerosol particles is scattered and re-ceived by a photo-multiplier tube (PMT) at an angle of90◦. The output of the PMT is ampliFed and then fed toa pulse height analyzer, which divides the aerosol responseinto di;erent size ranges. The pulse height is directly pro-portional to the particle size. The instrument sucks the am-bient air at the rate of 1 l for 2 min. The sucked air ismonitored for 2 min only and the number concentration ofparticles is measured in the unit of particles/l. The wholeequipment is microprocessor-based and gives direct print-out of aerosol particle number concentration for di;erentsize ranges of 0.3–0.5, 0.5–1.0, 1.0–2.0 and 2.0–5:0 �m.The error in all size ranges measurement is less than 5%.The detailed description of the instrument can be found inthe work of Bansal and Verma (1998). The observationswere made at a height of 9 m above the ground surface onsecond Door of the physics department building inside theIIT campus. The IIT campus is located in the city of Roor-kee (29◦52′N; 77◦53′E) at a height of 275 m from the sealevel. The surrounding is free from industries; the man-madeaerosols are mainly due to household activities and automo-biles.

The observation times for the present study were chosenfor the period fromApril to July 1999 and data were recordedcontinuously from 9:00 AM to 6:00 PM. The number con-centration of aerosols in the above size ranges was recordedat every half an hour interval and the data of meteorolog-ical parameters were recorded at the National Institute ofHydrology, Roorkee, which is located at a distance about300 m from the aerosol observation site.

3. Results and discussion

The measurements have been grouped for morning, noonand evening variations. Analysis has been done in all thesize ranges and observation period from April to July hasbeen chosen for the present study because in this period boththe premonsoon and monsoon activities occur. The aerosolnumber concentration has a diurnal variation. In the eveningthe aerosol number concentration decreases as reported byVakeva et al. (2000). At noon it is increased due to prevail-ing sun and wind. The morning aerosols have di;erent char-acteristics due to continued condensation because of highhumidity and low temperature (Parameswarn and Vijay-kumar, 1994). Therefore, in the present study we havedivided the period of measurement in three groups, i.e.morning, noon and evening periods.

Aerosols data have been recorded for four size ranges(0.3–0.5, 0.5–1.0, 1.0–2.0 and 2.0–5:0 �m). However, wehave chosen to present two sizes (0.3–0.5 and 2.0–5:0 �m)only to reduce the number of Fgures and improve the pre-sentation quality. The average concentration of aerosols forthe evening period (16:00–18:00 h) for summer months isshown in Fig. 1. In the size range 0.3–0:5 �m [Fig. 1(a,b)],

the evening aerosol concentration varies from about 4×104

to above 2 × 105 particles=l in the month of April, mostof the particles remaining under the upper size range of105 particles=l. The concentration of the particles remainsnearly the same in the month of May 1999. However, in themonth of June the concentration of these particles was de-creased. Most of the particles remain in the range 1:5×104–9× 104 particles=l. In the month of July, the concentrationhad further decreased and most of the particles fall in therange 1× 104–8× 104 particles=l.

This trend continues in the size range 0.5–1:0 �m. Mostof the particles remain around 4×104 particles=l both in themonths of April and May. The concentration in this rangehad decreased in the months of June and July. The lowestconcentration goes up to about 103 particles=l in July.

In the size range 1.0–2:0 �m, the concentration is lowerthan the previous size ranges. In April, though the concen-tration ranges from above 1 × 103–4:75 × 103 particles=l,most of the particles remain within the upper limit of 2:5×103 particles=l. The concentration remains at the same levelin the month of May. In the month of June the average con-centration of particles (1 × 103–5:6 × 103 particles=l) hasslightly increased compared to the month of April and May,though there are high Ductuations in this month. In July theconcentration of particles had decreased drastically, most ofthe particles (leaving Frst few days) remaining in the rangeabout 200–1400 particles=l. In this size range the increaseof particles in the month of June can be attributed to thelarger particles being airborne due to heat, that causes des-iccation and wind. However, in the month of July the low-ering of aerosol number concentration can be attributed tothe scavenging of aerosols by monsoon rain.

A similar situation prevails in the size range 2.0–5:0 �m[Fig. 1(c,d)]. In the month of April and May, the level ofconcentration remains the same (300–1200 particles=l). InJune the concentration increases drastically ranging fromabout 400–5600 particles=l. However, in July the concen-tration had decreased drastically with most of the particleslying in the concentration range 50–300 particles=l.

During the noon period (from 10:30 AM to 3:00 PM),the daily average concentration [Fig. 2(a,b)] in the sizerange 0.3–0:5 �m is at the same level as for evening forthe whole summer from April to July. In the size range0.5–1:0 �m, the noon time average concentration is higherthan the evening concentration during the month of April(1 × 104–8 × 104 particles=l) but most of the particles re-maining within the upper limit of 4× 104 particles=l and inthe month of May it is slightly higher than the evening con-centration (1:5×104–5×104 particles=l). This higher trendfor noon concentration remains in the months of June andJuly. In the size range 1.0–2:0 �m, the noon concentrationis higher than the evening concentration in the months ofApril, May and July. However, in June they are almost sim-ilar. The noon concentration in the size range 2.0–5:0 �m is[Fig. 2(c,d)] maximum in June and minimum in July. Dur-ing all the 4 months the time variation of noon aerosols have

D.K. Sharma et al. / Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 1007–1019 1009

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similar pattern as for the evening aerosols. The nature oftime variation for evening and noon aerosols in the abovesize ranges are also similar.

The concentration of aerosols during morning period(9:00–10:00 AM) in the size range 0.3–0:5 �m is nearlyin the same range as for noon and evening period for thewhole summer of 1999 [Fig. 3(a,b)]. Same is the case forthe size range 0.5–1:0 �m. In the size range 1.0–2:0 �malso the level of concentration and time variation are thesame as for noon and evening periods. In the size range2.0–5:0 �m [Fig. 3(c,d)], the level of concentration andtime variation in all other months are the same as in theabove size ranges except in the month of May, where thelevel of concentration is higher and it varies from about800–5500 particles=l.

The time variation of aerosols in three size ranges showsthat for 0.3–0.5, 0.5–1.0 and 1.0–2:0 �m the morning con-centrations are larger than the evening concentration andshows a steady decrease from 09:00 to 14:00 h and thenbecame steady. Fig. 4 shows the daily behavior of 0.3–0.5and 2.0–5:0 �m size ranges. Because of the low tempera-ture and high humidity, the contribution to aerosol numberconcentration in the lower size ranges during morningperiod is due to water vapor condensation. With the riseof the Sun the temperature increases and the droplets getevaporated. However, the large size range of 2.0–5:0 �mdominates. With the rise of temperature and occurrence

of high-velocity winds the aerosol number concentrationincreases. The atmospheric temperature becomes higharound 14:00 h and after that both the temperature and windvelocity decrease. This causes a maximum around 14:00 hin large size range in the aerosol concentration. This trendis clearly visible in the months of April and July. In July theconcentration is lowest in all size ranges due to scaveng-ing. But the high velocity wind around 14:00 h might havecaused a peak at that time in the size range 2.0–5:0 �m inthe months of May and June. After about 10:00 h the con-centration (2.0–5:0 �m size) becomes almost constant tillevening. The absence of peaks in May and June and theirlower concentration compared to April has been attributedto the low-intensity rain in these months.

Devara and Raj (1998) studied the columnar content ofaerosols over Pune using lidar. Their results revealed con-trasting nature of variation in the years 1987 and 1988. In1987, the concentration was minimum in the month of Maywhile in 1988 it was maximum in this month, both in theheight range 50–1100 and 50–200 m from the ground sur-face. Their result showed that the aerosol concentration wasminimum during rainy season. This is in good agreementwith our observation that the aerosol number concentrationhad gone minimum in the month of July when the monsoonover Roorkee was in full swing.

Pahwa et al. (1994), studied the aerosols of di;erentcomposition and the total suspended particles (TSP) at


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Fig. 3. Variation of average number concentration of aerosol in morning period for summer season (April–July 1999) for lower and uppersize range.


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Fig. 4. The time variation of average number concentration of aerosols in summer season (April–July 1999) for lower and upper size range.

Delhi. Their study revealed that during May 1986, the TSPconcentration was 543:13 �g=m3. It had increased to777:17 �g=m3 in the month of June. In July and August,the concentration had gone to 231.88 and 173:36 �g=m3,respectively. As the onset of monsoon over Roorkee andDelhi are nearly at the same time, their results can be com-pared with our observation that in July the aerosol numberconcentration in all size ranges is decreased drastically. Intheir studies the minimum concentration was in August.The maximum rain occurs in August, which decreases theaerosol number concentration due to scavenging.

In Fig. 5, though day-to-day variation of aerosol numberconcentration occurs, it is seen that the variation of averageconcentration of aerosols for di;erent size ranges di;ers.For large size particles (2.0–5:0 �m), the concentration islowest and the concentration is maximum for small size par-ticles (0.3–0:5 �m) during the whole period of observation.The small size 0.3–0:5 �m, aerosol number concentrationis about 3 × 105 particles=l in the month of April. Most ofthe particles remain in the upper range of 105 particles=l[Fig. 5(a,b)]. The concentration of particles is nearly thesame in the month of May but is less in comparison to themonth of April. In the month of June, the concentration ofaerosol decreases and most of the particles remain in theconcentration range 2:3× 105–9× 104 particles=l. A furtherdecrease in concentration is observed in the month of July.

This trend continues in the size range 0.5–1:0 �m [Fig.5(c,d)]. Most of the particles remain in the range 7× 104–8 × 103 particles=l in the months of April and May. The

concentration in this range decreases in the month of Juneand July. The lowest concentration goes to 103 particles=l.In the size range 1.0–2:0 �m [Fig. 5(e,f)], the concentra-

tion is lower than that in the previous range. In the monthof April the concentration ranges from 2:5 × 102 to 6:4 ×103 particles=l. It is nearly the same in the month of Mayand is lower in June than in July. In the month of Julythe aerosol number concentration varies from 6:4 × 103–1× 102 particles=l.

A similar situation prevails in the size range 2.0–5:0 �m [Fig. 5(g,h)]. During the months of April and Maythe concentration of aerosol was nearly the same (50–100 particles=l), while in June, the concentration increases,ranging from 50–200 particles=l. There is a signiFcant de-crease in the concentration in July and most of the particleslie in the concentration range 15–50 particles=l.

The Ductuation of aerosol number concentration dependsupon meteorological parameters. The relative humidity(RH) was minimum in the month of April and increasesin May and became maximum in July [Fig. 6(a,b)]. In themonth of June the RH was less than the month of July. Thepresent study reveals that the aerosol number concentrationwas more a;ected by RH during South-East (SE) premon-soon season of the year 1999 over Roorkee. Parameswarnand Vijaykumar (1994) found that the RH signiFcantlya;ects the aerosol concentration and size distribution aboveabout 90%. Here at Roorkee in the months of June and July1999 average RH was almost close to this limit. Devara andRaj (1998) observed a higher humidity and lower temper-


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ature during South-West premonsoon in the year 1988 atPune. The e;ect of premonsoon scenario (Aher and Agashe,1997) on aerosols at Pune caused by the growth of clouddroplets results in higher rainfall. The same physical pro-cess appears to have taken place in 1999 at Roorkee duringSE premonsoon. The maximum temperature was lower in

April than the month of May (Fig. 6c) and is maximumin June and also decreases in the month of July (Fig. 6d).The minimum temperature was higher in July and nearlythe same in June and is lower in the month of April andincreases slightly in May [Fig. 6(e,f)]. The wind speed isnearly same in the months of April and May (Fig. 6i) and


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is higher in June and again decreases in the month of July(Fig. 6j). The rain plays an important role to modulate theaerosol size as larger particles take part in the scavengingprocess (Devara and Raj, 1998; Byrne and Jennings, 1993;Sparmacher et al., 1993). In the month of April and Maythe rainfall is nearly zero (Fig. 6g) so there is no signiFcant

e;ect on aerosol number concentration. The SE monsoonis e;ective (Fig. 6h) after mid-June, so the concentrationof aerosol decreases and it is in phase with the increasingmonsoon activity. This is attributed to the rainfall, which isa powerful factor to lower the aerosol number concentrationinvolving rain out process. The monthly means of aerosol


Table 1Monthly mean of aerosol number concentration and standard deviation (�)

Months Size range 0.3–0:5 �m Size range 0.5–1:0 �m Size range 1.0–2:0 �m Size range 2.0–5:0 �m

Conc. mean Standard Conc. mean Standard Conc. mean Standard Conc. mean Standard(l−1) deviation (l−1) deviation (l−1) deviation (l−1) deviation

April 16:46× 104 8:80× 104 35:03× 103 10:46× 103 29:57× 102 18:69× 102 7:78× 101 2:46× 101

May 11:51× 104 5:19× 104 25:95× 103 7:34× 103 16:32× 102 10:13× 102 9:87× 101 6:86× 101

June 8:73× 104 4:98× 104 28:07× 103 12:68× 103 16:86× 102 14:46× 102 8:92× 101 5:62× 101

July 5:61× 104 5:03× 104 13:96× 103 17:18× 103 8:50× 102 8:54× 102 2:56× 101 1:60× 101

number concentration along with standard deviations (�)have been shown in Table 1.

The average number concentration of aerosols is shownin Fig. 7(a,b) and the average of meteorological parameters(relative humidity, temperature, rainfall, and wind speed) inFig. 8(a–c) for summer season 1999 over Roorkee. In thesmall size ranges (0.3–0.5 and 0.5–1:0 �m) particle averageconcentration is maximum in the month of April and Mayand decreases continuously in June and July while the RHforms minimum in April and May and goes to maximumin June and July. The wind speed (Fig. 8c) remains almostconstant throughout the period of observation. In the largesize ranges (1.0–2.0 and 2.0–5:0 �m) the average particleconcentration increases, going to maximum in the middle ofJune, after which it starts decreasing.

The aerosol number concentration is expected to be de-pendent on meteorological parameters. Therefore, we havemade comparisons with meteorological parameters like rel-ative humidity, wind speed, temperatures and rainfall withaerosol number concentration. These have been shown inFigs. 9–13 along with the equation of linear regression Ftsand the correlation coe:cient.

Fig. 9 shows the correlation of average aerosol numberconcentration to the relative humidity. The number concen-tration in all size ranges decreases with humidity. The in-creasing humidity removes the aerosol particles in all sizeranges due to condensation on aerosol particles and sub-sequent falling down of the drops or droplets takes to theground. Pranesha and Kamra (1996) in their laboratory stud-ies have found that the particles are actually removed fromthe air due to condensation. The correlation coe:cient (R)is large for the lower size range particles (0.3–0.5 and 0.5–1:0 �m) and small for upper size range particles (1.0–2.0and 2.0–5:0 �m). This shows that the smaller particles actas condensation nuclei and the contribution of larger parti-cles to this process is very small.

In Fig. 10, there is slight increase in number concen-tration with increasing wind speed. This can be attributedto the dust particles from the soil becoming air-borne dueto wind and an increase in the number concentration isexpected. However, the correlation coe:cient in all thesize ranges is very small and a deFnite relation cannot beestablished.

The concentration decreases slightly with increasing min-imum temperature (Fig. 11). The correlation coe:cient (R)is about −0:50 for lower size ranges and it becomes verysmall for the upper size range particles (−0:22). This hasfurther been attributed to the process of condensation atsmall size particles. The increasing minimum temperaturedecreases the condensation and hence a negative trend in theregression line of Ft has been obtained. On the other hand,the average concentration slightly increases with increasingmaximum temperature (Fig. 12). With the rise of maximumtemperature, more and more drops/droplets are vaporizedleaving behind the aerosols.

In Fig. 13, the decrease of aerosol number concentrationwith rainfall is as expected which is attributed to the scav-enging of aerosol particles (Devara and Raj, 1998; Byrneand Jennings, 1993; Sparmacher et al., 1993 and others).One may doubt the conclusion drawn regarding the in-

Duence of meteorological parameters on aerosol numberconcentration. Pranesha and Kamra (1996) have performedlaboratory studies and have found that the suspended parti-cles are removed from air due to condensation.

Hanel (1976) found that the particle size decreases withdecrease in RH. Parameswarn and Vijaykumar (1994) stud-ied the aerosol size distribution using a low-pressure im-pactor and concluded that the aerosol size decreases withdecreasing RH. Vakeva et al. (2000) studied the nucleationphenomena in the humid urban atmosphere and found thatthe ultraFne particles (diameter 10 nm or less) are produceddue to nucleation. However, our interest lies in large par-ticles (0:3 �m and larger) and their results are not muchrelevant to our studies.

Vakeva et al. (2000) also studied the e;ect of wind onaerosol concentration. They found that during moderatewind the sub-micron size particles increase in concentra-tion in the urban environment. They also found that theaerosol concentration is a function of wind direction. In ourstudies we have not considered the wind direction but havefound that the aerosol number concentration increases withincreasing wind speed.

Shaw (1988) has found that the aerosol concentrationin the size range 0.36–0:40 �m increases with increasingtemperature. Summer months, in his studies were found tohave very high level of aerosol concentration (about 4 times


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°C

Max. Temp.

Min. Temp.

0

35

70

105

Ave

rag

e R

H, %

0

4

8

12

0 25 50 75 100 125

W-S

(km

/hr)

, RF

(m

m)

Wind speed

Rainfal

Time (days)

(a)

(b)

(c)

Fig. 8. The running mean average of meteorological parameters (relative humidity, temperature, rainfall and wind speed) in summer season(April–July 1999).

as a rough estimate) than the winter months. Further, thesun increased the aerosol concentration in the above sizerange. The aerosol number concentration dependence onboth minimum and maximum temperatures are shown in

Figs. 11 and 12. In all size ranges the aerosol number concen-tration decreases with increasing the minimum temperature.The correlation coe:cient ranges from −0:22 (for largestsize range) to−0:52 (for smallest size range). However, the


y = -0.1945x + 23.029R = -0.47

0

30

60

90A

vera

ge

con

c. (

x104 )

liter

-1

Size 0.3-0.5 y = -0.3841x + 50.333

R = -0.47

0

50

100

150

Ave

rag

e co

nc.

(x1

03 ) lit

er-1

Size 0.5-1.0

y = -0.3351x + 39.005R = -0.38

0

50

100

150

0 40 80 120

Relative humidity (%)

Ave

rag

e co

nc.

(10

2 ) lit

er-1

Size 1.0-2.0 y = -0.14x + 16.241

R = -0.44

0

30

60

90

0 40 80 120

Relative humidity (%)

Ave

rag

e co

nc.

(x1

0) li

ter-1

Size 2.0-5.0

µm

µm

µm

µm

Fig. 9. Variation of average aerosol number concentration versus relativity humidity during April–July 1999 for di;erent size ranges.

y = 1.3283x + 8.4961R = 0.13

0

30

60

90

Ave

rag

e co

nc.

(10

4 ) lit

er-1

y = 2.9205x + 21.17R = 0.14

0

50

100

150

Ave

rag

e co

nc.

(x1

03 ) lit

er-1

Size 0.5-1.0

y = 1.6845x + 14.922R = 0.08

0

50

100

150

0 1 2 3 4Wind speed (km/hr)

Ave

rag

e co

nc.

(x1

02 ) lit

er-1 Size 1.0-2.0

y = 1.526x + 4.8893R = 0.19

0

30

60

90

0 1 2 3 4Wind speed (km/hr)

Ave

rag

e co

nc.

(x1

0) li

ter-1 Size 2.0-5.0

Size 0.3-0.5 µm µm

µmµm

Fig. 10. Variation of average aerosol number concentration versus wind speed during April–July 1999 for di;erent size ranges.


y = -1.0802x + 35.516R = -0.52

0

30

60

90A

vera

ge

con

c. (

x104 )

liter

-1

Size 0.3-0.5

y = -1.9026x + 69.673R = -0.46

0

50

100

150

Ave

rag

e co

nc.

(x1

03 ) lit

er-1

Size 0.5-1.0

y = -2.193x + 68.189R = -0.50

0

50

100

150

10 20 30

Minimum Temp., °C

Ave

rag

e co

nc.

(x1

02 ) lit

er-1

Size 1.0-2.0

y = -0.3577x + 15.543R = -0.22

0

20

40

60

10 20 30

Minimum Temp., C

Ave

rag

e co

nc.

(x1

0) li

ter-1 Size 2.0-5.0

µm

µm µm

µm

°

Fig. 11. Variation of average aerosol number concentration versus minimum temperature during April–July 1999 for di;erent size ranges.

y = 1.9367x - 44.774

R = 0.50

0

50

100

150

Avera

ge c

on

c.

(x10

3)

lite

r-1

y = 1.4861x - 36.553

R = 0.36

0

50

100

150

20 30 40 50

Maximum Temp., ° °C

Avera

ge c

on

c.

(x10

2)

lite

r-1

y = 0.6468x - 16.269

R = 0.43

0

30

60

90

20 30 40 50

Maximum Temp., C

Avera

ge c

on

c.

(x10)

lite

r-1

y = 0.7651x - 17.281

R = 0.39

0

30

60

90

Avera

ge c

on

c.

(x10

4)

lite

r-1

Size 0.3-0.5 µm

Size 0.3-0.5 µm Size 0.3-0.5 µm

Size 0.3-0.5 µm

Fig. 12. Variation of average aerosol number concentration versus maximum temperature during April–July 1999 for di;erent size ranges.


y = -0.5345x + 20.045R = -0.30

0

30

60

90A

vera

ge

con

c. (

x104 )

liter

-1

y = -0.7766x + 28.482R = -0.40

0

30

60

90

Ave

rag

e co

nc.

(x1

03 ) lit

er-1

y = -0.6014x + 19.678R = -0.29

0

30

60

90

0 10 20 30 40Rainfall (mm)

Ave

rag

e co

nc.

(x1

02 ) lit

er-1

y = -0.2876x + 8.295R = -0.38

0

15

30

45

0 10 20 30 40Rainfall (mm)

Ave

rag

e co

nc.

(x1

0) li

ter-1

Fig. 13. Variation of average aerosol number concentration versus rainfall during April–July 1999 for di;erent size ranges.

variation with maximum temperature is quite contrary. In allsize ranges it increases with increasing maximum tempera-ture. The correlation coe:cients are 0.39 (0.3–0:5 �m), 0.50(0.5–1:0 �m), 0.36 (1.0–2:0 �m) and 0.43 (2.0–5:0 �m).In all the above discussions the correlation coe:cient (R)

between aerosol number concentration and meteorologicalparameters is not very high and the data points are scat-tered. The linear regression model here does not work well.The aerosol particles depend upon many parameters includ-ing natural and man-made sources and almost all the mete-orological conditions act the same time. Therefore, a linearrelationship between the aerosol number concentration (indi;erent size ranges) and meteorological parameters indi-vidually cannot be established.

A comprehensive model of the aerosol number concen-tration in di;erent size ranges as a function of meteorologi-cal parameters acting together becomes essential for properunderstanding of the subject matter.

4. Conclusion

In the present study, the behavior of aerosols numberconcentration during morning, noon and evening periods insummer season (April–July) for di;erent size ranges of theaerosols (0.3–0.5, 0.5–1.0, 1.0–2.0 and 2.0–5:0 �m) havebeen investigated. Aerosol number concentration for smallsize ranges (0.3–0.5 and 0.5–1:0 �m) was higher than that

for large size ranges (1.0–2.0 and 2.0–5:0 �m). The varia-tion of aerosol number concentration in morning was foundto be less than noon and it was observed maximum duringevening. The time variation of aerosols was highly depen-dent on size. For small size ranges the number concentrationdecreased monotonically till the end of July. For large sizeranges it increased becoming maximum in June and mini-mum in July. The study reveals the fact that the aerosol sizeand number concentration are greatly a;ected by the me-teorological parameters. The larger particles are especiallysensitive to variations in meteorological parameters. The rel-ative humidity, temperature, rainfall and wind speed playimportant roles to modulate the aerosol behavior but largeamount of precipitation (heavy rainfall) can alter the numberdensity and size distribution of atmospheric aerosols moree:ciently than RH and wind speed.

References

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summer variations of the atmospheric aerosol number concentration over roorkee, india

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