1 Introduction

Aerosol number concentration and size distribution play a crucial role in Earth’s radiation budget by scattering and absorption of sunlight in atmosphere. Scattering and absorption coefficient is an important parameter which strongly depends on particle’s physical and chemical property. Particle number concentration and size distribution of atmospheric aerosols affect Earth’s radiation budget directly and indirectly through CCN activation (Stanier et al. 2004; Verma et al. 2006). The measurement of particle number concentration and size distribution is important to understand the effect of atmospheric aerosol on health, visibility and climate. A number of epidemiological studies showed correlation between fine particle concentration and respiratory and cardiovascular diseases (Samet et al. 2000; Pope 2000). The particles having diameter less than 10 μm are more responsible for health problem, because these particles can penetrate deep into the respiratory system (Dockery and Pope 1994). The particle number concentration of ultrafine particles is more important than mass concentration of fine particle. Thus, the measurement of particle number size distribution is important to understand the environmental effects of atmospheric particles (Peters et al. 1997; Penttinen et al. 2001).

The measurement of aerosol number size distribution and number concentration was conducted in urban environment mostly in developed countries (Hämeri et al. 1996; Williams et al. 1998; Woo et al. 2001; Wehner et al. 2002; Longley et al. 2003), while few measurements conducted in developing country (Dunn et al. 2004; Mönkkönen et al. 2005; Mohan and Payra 2006; Tiwari et al. 2011). The air pollution is a serious problem in mega cities of the South and the East Asia like Delhi, Kolkata (Calcutta), Mumbai (Bombay), Dhaka, Karachi, Bangkok, Beijing, Shanghai, Jakarta and Manila, and these cities are one of the most polluted cities of the world (Verma et al. 2012; Baldasano et al. 2003; Faiz and Sturm 2000). The air quality standards are based on the particle mass concentration. The mass concentrations of particle which have diameter <0.1 μm dominate the total particle number concentration are negligible in urban areas (Seinfeld and Pandis 1998). Most of the monitoring networks across the world measure PM10 and PM2.5 (mass of particulate matter smaller than 10 and 2.5 μm in aerodynamic diameter, respectively). However, less information is available on particle number concentration and size distribution. There have been several studies in rural areas (Horvath et al. 1996; Liu et al. 2008; Ketzel et al. 2004; Wiedensohler et al. 2004). However, in urban areas, very less measurements of aerosol number concentrations have been studied, e.g., Wehner et al. (2002), Väkevä et al. (1999), Shi et al. (2001), Buzorius et al. (1999). The measurement of particle number concentration and size distribution is more important than study of mass concentration because a single coarse particle has the equal mass of million ultrafine particles but coarse particles are less hazardous because it may not enter human respiratory system. Thus, it is important to measure the particle number size distribution to understand the aerosol behavior.

The purpose of this study is to determine the characteristic aerosol size distribution during two different events, i.e., dust storm (DS) and Diwali (DW) by examining the size-separated number concentrations measured over Jaipur, in India. Diwali (DW) is one of the famous festivals in India and is celebrated in all parts with display of huge fireworks. Firework is one of the most unusual anthropogenic activities that create short-term pollution and health hazards in all over the world. Due to burning of fire crackers, a high concentration of anthropogenic aerosol is mixed up in the atmosphere. Short-term variations in particulate matter are associated with decline in lung function and increased respiratory problems, and mortality from cardio respiratory causes (Dockery et al. 1993; Pope et al. 1991; Schwartz et al. 1993). The increase in concentration of anthropogenic aerosol affects day-to-day life, because these particles are small in size and enter human respiratory tract and pulmonary system. Fireworks have so many chemicals such as potassium nitrates, potassium chlorate, potassium perchlorate, sulfur, charcoal, manganese, and iron dust powder, strontium nitrate and barium nitrate (Mclain 1980). Various studies have been conducted in India, China, Germany, USA, etc. during fireworks (Zhang et al. 2010; Steinhauser et al. 2008; Wang et al. 2007; Moreno et al. 2007; Ravindra et al. 2003; Wehner et al. 2000; Liu et al. 1997). The recent and previous research studies to our knowledge are mostly focused on the metro cities of India and neither had they included measurements of particle number concentrations. Therefore, the purpose of this study was to investigate the number concentration of particles in urban atmosphere over Jaipur during DS and DW in northwestern India. We specifically focused on the following aspects: (1) the characteristics dispersion of aerosol particle behavior during a DS (May 22, 2011) and DW (October 26, 2011) and (2) the influence of DW and DS episodes on the characteristics of submicron particle number concentrations. This paper also explains the effect of meteorological parameter on particle number concentration with the additional information on the local weather conditions such as wind speed, wind direction, relative humidity and precipitation influence of the particle number size distribution.

2 Study area

The measurement site is located at Mansarovar (26.86°N 75.77°E), the largest housing colony in Asia. Mansarovar is a residential area in Jaipur, capital of Rajasthan. Jaipur is situated in the eastern boundary of the Thar Desert–a semi-arid land. The Thar Desert is the major source of dust storm located in western India and eastern Pakistan for Indian sub-continental (Pease et al. 1998; Washington et al. 2003) (Fig. 1).

Fig. 1
figure 1

The location of study region and observational site

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Jaipur is the first planned city of India, situated in the foot hills of Aravali mountain ranges at an altitude of 431 m a.m.s.l. Jaipur is surrounded by hills to the north and east and plains in western and southern sides (Tyagi et al. 2012). Total length of Jaipur from east to west is about 60 km, and total width from north to south is about 75 km. The population growth rate under the urbanization is quite high (53.09 %) in the last decade. There are small scale industries in Jaipur, and some of the major ones are located to the west and south of the city about 15–40 km from the site location. Farther to the south lie densely populated regions. New Delhi, the capital of India, is 230 km away in northeast direction.

3 Methodology

3.1 Sampling instrument

In this study, we use GRIMM Aerosol Spectrometer 1.108 (Grimm Aerosol Technik, GmbH, Germany), to measure particle size distribution in a residential area of Jaipur. GRIMM aerosol spectrometer is a portable optical particle counter which has been built for continuous measurement of airborne particles as well as for measuring the particle count distribution. This instrument provides four operational modes: environmental, occupational health, mass distribution and count distribution. The instrument measures particle concentrations in an optical size of 0.30–20 μm in 15 channels of differently sizes in concentration range of mass concentration range of 1–1,000,00 μg/m3 (for mass distribution, environmental and occupational health modes) or a 1–2,000,000 particles/l (for count distribution mode). The sensitivity of this instrument is 1 particles/l for count mode and 1 μg/m3 for mass mode, and instrument reproducibility is ±2 %. Ambient air is drawn into the unit via an internal volume-controlled pump at a rate of 1.2 L/min; for this, a stainless steel tube provided by the manufacturer was utilized as the spectrometer inlet. The instrument initiates a system self-test and zero calibration check for each measurement at the start. They possess an integrated gravimetric filter on which all particles are collected after the optical measurement and thus are available for further analysis. In this study, aerosol spectrometer was operated in count distribution mode to produce count distribution versus time with a temporal resolution of 15 min. The data have been collected from May to December in count distribution mode, and the data during rainy season (July, August and September) have been discarded.

4 Results

Several intense dust storms occurred over the north and northwestern India during pre-monsoon season, when dusts are transported by southwesterly wind from western Thar Desert (Sikka 1997). Dust storms are the major source for transportation of mineral dust and have considerable impacts on climate variability and ambient air quality (Dey et al. 2004; Payra et al. 2013; Verma et al. 2013). In this study, aerosol particle number concentration and size distribution have been presented during the two contrasting periods of DS on May 22, 2011 and DW during October 26, 2011. The presented aerosol data represent size distribution within the particle diameter size range 0.3–20.0 μm. This study shows that potential differences observed during DS and DW in aerosol particle number concentration and size distribution over Jaipur, and are discussed in subsequent sections.

4.1 Identification of study events

Figure 2 shows the GRIMM aerosols spectrometer measurements of total ANC from May to December, 2011. The figure shows a high variability in number concentration during the period of measurements. Two typical days in May, 2011 and October 2011, where a spike in the particle concentration is noticed, are marked by boxes in Fig. 2. The first one is identified as dust storm (DS) while second is identified as Diwali (DW), respectively. These episodes provide the opportunity to investigate the effects of severe high concentrations on the size distribution of particles.

Fig. 2
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Total aerosols number concentration over Jaipur from May to December 2011 as measured from GRIMM aerosols spectrometer

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The total particle concentrations (Fig. 2) are observed less during DS than DW. There is a considerable and rapid increase in concentration of aerosol particles during DS. The 24-h mean of total particle number concentration during DS and DW was 9.15 × 104 and 5.38 × 105 particles/l, respectively. During DS, the total particle number concentration increase about 4.81 times from the 24-h average value. However, during DW, the concentration of total aerosol particles was increased about 2.16 times from the average value.

4.2 Size-segregated aerosols number concentrations

The hourly variations in the particle number concentrations have been shown in Fig 3a and b for two different size ranges for two identified cases over a week long period. The concentrations have been size separated as fine (0.3 > Fine < 2.0 μm) and coarse particle (2.0 μm > Coarse < 20 μm).

Fig. 3
figure 3

Aerosol particle number concentration (particles/l) during a DS and b Diwali for a week long period in May and October 2011, respectively. The fine mode has been specified as size range from 0.3 to 2.0 μm and the coarse model from 2.0 to 20 μm

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Figure 3a shows the time series of hourly average aerosol particle number concentration for a week long ( May 18–24, 2011) period in two size ranges. The maximum change is found in coarse particle range (2.0–20 μm) and minimum for small particles (0.3–2.0 μm) during the period of DS. The particles in size range 0.3–2.0 μm and 2.0–20.0 μm increased from 1.20 × 105 to 2.08 × 105 and 4.32 × 103 to 4.63 × 104 particles/l during DS, respectively between 0030 and 0130 hours on 22 May.

Figure 3b represents the time series of fine and coarse particles from October 22 to 28, 2011. There has been markedly increase in the concentration of aerosols on the celebration day of DW (26 October) due to heavy burning of crackers. The concentration of particles with size range (0.3–2.0 μm) increased from 3.95 × 105 to 1.54 × 106 particles/l from 1830 to 2230 hours during Diwali. The hourly aerosol number concentration of particles with size range 0.3–2.0 μm increased 2.90 times during DW. The maximum change is found in small particles (0.3–2.0 μm) and minimum for large particle (2.0–20 μm) during DW.

The particle number concentration in coarse mode showed large fluctuations and was significantly higher than fine particles during DS. Conversely, the fine particle mode was markedly greater during DW. The shift in number concentrations of fine particles from DW to heavy dust period was opposite to that of coarse ones, which implies the presence of mineral dust as a component of aerosol over Jaipur during May period (Chun et al. 2001). Asian dust events are usually followed by the passage of strong frontal system, and it is highly possible that the movement of air was faster during the heavy dust incident (Sikka 1997; Chun et al. 2001). The high wind speed causes the transport of aerosol particles from one place to another. This fact is explained in details in the following sections by analyzing the 15-min averaged diurnal variations in ANC with prevalent meteorological conditions during the events.

4.3 Diurnal variation in ANC distribution and meteorology

The size-segregated number concentrations of aerosol observed in Jaipur for 24 h (1800–1745 hours) with 15-min interval during DS (May 22, 2011) and DW (October 26, 2011) are displayed in Figs. 4 and 5. The 15-min averaged values of ANC for 24 h are compared with the meteorological data over Jaipur.

Fig. 4
figure 4

Diurnal (24 h) variation in fine and coarse particle during (a) dust storm. The observed meteorological parameters (b) during dust storm

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Fig. 5
figure 5

Diurnal (24 h) variation in fine and coarse particle during (a) Diwali and respective (b) meteorological parameters

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4.3.1 ANC distribution during dust storm

Before DS, the aerosol number concentration was observed as 7.48 × 104 of fine and 3.0 × 103 particles/l of coarse particles (0015 hours) while during DS, the concentration of fine and coarse particle (Fig. 4a) increased to 4.20 × 105 and 1.12 × 105 particles/l (0030 hours), respectively. During the dust event, the total concentrations of aerosols were substantially increased by approximately 4.61 and 36.44 order of magnitude in the fine and coarse mode, respectively. After DS, the decline in particle number concentration in fine range was slower than coarse particle. The similar characteristic of fine and coarse particle was observed during second DS (1400 hours) but after DS, there was strong decline in particle number concentration of fine and coarse particle due to light rainfall which washes out atmospheric particles rapidly. Table 1 also shows that the maximum increase during DS is observed for 10.0–20.0 μm range. The maximum and minimum value of meteorological parameter during DS and Diwali are discussed in Table 2. The meteorological parameter gives an evidence of DS with a rapid increase in wind speed as well as humidity while a strong decline in temperature as shown in Fig. 4b and Table 2. The relative humidity and wind speed increased from 41 to 88 % and 13 to 33 km/h, respectively. While a strong decrease in temperature is observed (35 to 23 °C) during DS, the wind direction (Fig. 6a) was mostly western with occurrence probability of 45 %.

Table 1 The hourly increase during dust storm and Diwali for all size range
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Table 2 Comparative data of meteorological parameter during dust storm and Diwali
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Fig. 6
figure 6

Wind speed and wind direction during DS and DW

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4.3.2 ANC distribution during Diwali

Fine particle shows a distinct diurnal variation with minimum concentration observed during the day and maximum in the evening during DW (Fig. 5a). The fine particle concentration increased from 3.51 × 105 (1800 hours) to 1.70 × 106 (2200 hours) and coarse particle from 1.92 × 103 (1800 hours) to 3.78 × 103 particles/l (2200 hours) during DW. The hourly increase in different particle size range during DW is presented in Table 1 which also shows a maximum increase in particle number concentration for the size range 0.3–0.5 μm during DW. The fine and coarse particle increased 3.83 and 0.95 times, respectively, during Diwali. After 2200 hours, the particle concentration of fine and coarse particle decreases till 0330 hours and then start to increase. The meteorological parameter shows distinct behavior during DW. On October 26, 2011, the wind speed was around 2 m/s, indicating meteorological conditions to be very stable (Fig. 5b). After 1800 hours on 26 October, the particle concentration quickly increased owing to the strong primary emissions from Diwali firework celebrations. The diurnal variations of the particle number concentration are consistent with dynamics of the atmospheric boundary layer (Table 2). The particle number concentration shows a significant rise during evening due to the aerosols produced during emission from crackers. The stable atmospheric conditions allowed the particles to be accumulated in the urban atmosphere as evident from meteorological variables in Fig. 5b and Table 2. The emissions mostly get trapped in low mixing layer due to cold night as the nocturnal inversion layer also reaches near the Earth’s surface in winter. The wind was mostly (37.5 %) and prevailing (27 %) wind direction was northerly shown in Fig. 6b.

4.4 Lognormal distribution

Comparison of size-resolved number concentration for both events is shown in Fig. 7. The variation in size distribution of aerosols during two events is clearly reflected in the log-normal size distribution curves. It has already discussed that the total ANC is higher in DW compare to DS. The log-normal size distribution curves reveals that the particle size less than 0.8 μm has key contribution in DW for higher ANC. It shows that finer particles are less and coarser particles are more during DS. During DW, the aerosol size spectrum between 0.3 and 0.8 μm dominated while during DS, the aerosols number concentration in 0.8–10 μm modes is significantly high. The difference grew larger as we move to coarser particle size range. This is evident in Fig. 7 where size distribution curves indicated a progressive increase in particle number with decreasing size. The maximum, minimum and mean value of aerosol number concentration for different size range during dust storm and Diwali in Table 3 also depict that the concentration of small particle (0.3–0.8 μm) was greater during Diwali than dust storm. The remaining particle size range dominated during dust storm. High level of fine mode concentrations during DW containing primary particles from combustion sources and secondary particles from gas to particle conversion are produced in local emissions.

Fig. 7
figure 7

Log-normal distribution of aerosol particle number concentration

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Table 3 The maximum, minimum and mean value of aerosol number concentration for different size range during dust storm and Diwali
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5 Conclusions and perspective

We measured the submicron particle number concentrations with a GRIMM particle counter for two contradictory events during the May–December, 2011 at Jaipur, Rajasthan, India. The present study aimed at investigating the particle number concentrations and aerosol dispersion during two contradictory episodes and influence on size distribution characteristics of aerosols.

During dust episode, the maximum percentage change in hourly average of ANC (3.0 × 101 to 1.07 × 103 particles/l) was observed for the particle of size range (10–20 μm), and an extremely low number concentration of 0.3–0.5 μm particles (9.78 × 104 to 1.17 × 105 particles/l) was observed. The low number concentration during dust episodes suggests that dust particles act as a coagulation sink for the submicron particles where their concentrations are reduces significantly, and thus, the total number concentration is also reduced. In addition, the wind speed during the observed dust episode was high enough to dilute the locally produced urban particles and disperse them efficiently.

An intense increase in aerosols during the Diwali festival period over an urban area in Jaipur is observed due to use of various kinds of fireworks. The ANC of fine particles (0.3 < particle diameter < 2 μm) exhibits very high concentration during DW. The particles with size range 0.3–0.5 μm increased 2.94 times (3.65 × 105 to 1.44 × 106 particles/l). The variation in ANC during DW is clearly reflected in the log-normal size distribution curves. The maximum concentrations were attributed here for two reasons: locally emitted particles due to heavy burning of fire crackers and calm wind speed during DW. The low wind speeds and low mixing-layer heights lead to relatively high particle levels. Measurements indicate that both local emissions due to burning of fire crackers and meteorological conditions drive a very high ANC.

These results highlight the importance of routine measurements of aerosols to guide people and local administrators for usage of firecrackers during Diwali festival period and pollution episodes for understanding the changing climatology of the region.