Elsevier

Atmospheric Environment

Volume 190, October 2018, Pages 10-22
Atmospheric Environment

Classification of the new particle formation events observed at a tropical site, Pune, India

https://doi.org/10.1016/j.atmosenv.2018.07.025Get rights and content

Highlights

  • NPF events observed at a tropical site have been classified.

  • A new type of event, of truncated banana shape is reported.

  • Monthly distribution of events shows that they occur in the hottest season.

  • Backward air-mass trajectories and ion characteristics on events days were studied.

  • Positive ion concentration on event days are more than of the negative ions.

Abstract

A total number of 109 new particle formation events identified in the ion-mobility spectra measured with a Neutral Cluster and Air Ion Spectrometer in the mobility range of 3.16–0.00133 cm2 V−1s−1 (Diameter* range 0.36–47.1 nm) at a tropical site at Pune, (18.53 °N, 73.85 °E, 573 m amsl) India from March 08, 2010–December 31, 2012 are classified based on their shape characteristics under four categories. Most of these events occurred in the morning hours of the pre-monsoon season during the hottest months (April and May) of the year. The meteorological conditions and the changes in ion characteristics associated with some typical events are examined. Average ion-mobility spectrum for the event days shows a minimum in the negative big cluster ion (diameter, 0.85–1.6 nm) concentration and two maxima in the positive intermediate (diameter, 1.6–7.4 nm) and large ion (diameter, 7.4–47.1 nm) concentrations as compared to the average spectrum for all days. Analysis of 7-days airmass back trajectories shows that since the only source of big cluster ions is through the growth of small cluster ions (diameter, 0.36–0.85 nm) the growth of small to big cluster ions is faster when the airmass approaches our site from the land. Further, the concentrations of positive intermediate and light large (diameter, 7.4–22 nm) ions is more when the airmass approaches from the Arabian Sea.

Introduction

2Atmospheric aerosols can be either injected into the atmosphere directly from land or sea surface - primary aerosols, or can be generated in the atmosphere by the gas-to-particle conversion processes - secondary aerosols. Atmospheric ions are actively involved in the formation of secondary aerosol particles (Hoppel, 1985: Yu and Turco, 2000; Kulmala et al., 2004; Gopalakrishnan et al., 2005; Yu, 2010; Tammet et al., 2014). Knowledge of the changes in size and mobility of air ions in the initial stages of their growth provides valuable information for understanding the formation and growth processes of aerosols. Knowledge of the process responsible for the growth of the newly formed particles to the size of cloud condensation nuclei where they can influence the Earth's radiation budget and cloud processes is much needed under different environmental conditions for estimating the atmospheric radiative forcing on the regional and global scales. Observations to investigate the formation and growth of ultrafine particles have been made under a variety of environmental conditions (Kulmala et al., 2004, 2013; Dunn et al., 2004; Laakso et al., 2004; Hirsikko et al., 2005; Siingh et al., 2005, 2011a; Tammet et al., 2006; Qian et al., 2007; Kamra et al., 2009, 2015a; Asmi et al., 2010; Manninen et al., 2010; Betha et al., 2013; Kanawade et al., 2014; Young et al., 2013; Bianchi et al., 2016; Huang et al., 2016). Measurement sites for such observations extend over large ranges of altitude, latitude and degree of air pollution. For example, such measurements have been made at different altitudes in the atmosphere (Lee et al., 2003; Bianchi et al., 2016), in the tropical, mid- and high-latitude and polar regions (Dhanorkar and Kamra, 1993a, Dhanorkar and Kamra, 1993b; Horrak et al., 1998; Siingh et al., 2007, 2011a,b; Kamra et al., 2009; Manninen et al., 2010; Jung et al., 2013), and in the remote marine, polluted marine, remote continental, rural and urban areas (Hoppel et al., 1994; Zhang et al., 2004; McMurry et al., 2005; Qian et al., 2007; Hirsikko et al., 2011; Gagné et al., 2011; Herrmann et al., 2014; Kanawade et al., 2014; Santos et al., 2015). Urban areas are an important source for the global aerosol CCN load and most of CCN come from the primary emission and only 10% from nucleation in the boundary layer (Merikanto et al., 2009). Nevertheless, the number of studies focusing on the behaviour of air ions and particularly their association with the NPF events in urban areas around the world is still somewhat limited (Manninen et al., 2010). The main emphasis in the past studies, some of which were based on the observations made over a period of several years, was to study the nucleation events for the formation of new particles.

A variety of different mechanisms such as the binary (Kulmala and Laaksonen, 1990; Viitanen et al., 2008), ternary (Kulmala et al., 2000; Viitanen et al., 2008), ion-induced (Yu, 2001) and ion-mediated nucleation (Yu and Turco, 2000; Svensmark et al., 2007; Kirkby et al., 2011; Nagato and Nakauchi, 2014; Riccobono et al., 2014), and the nucleation mechanisms involving organic vapors (O'Dowd et al., 2002a) or iodine (O'Dowd et al., 2002b) have been proposed to be responsible for the nucleation of the newly formed particles. Moreover, vapours that participate in different steps of nucleation are most likely different from those that participate in the growth of particles (DallÓsto et al., 2018). A satisfactory dominant mechanism for the nucleation and growth of the particles at most of the locations has remained elusive. Moreover, such measurements and knowledge of the mechanisms responsible for the formation and growth of the particles in tropics are scarce and are much needed for understanding the aerosol effects on the climate and climatic change.

Formation of new particles in nucleation events mostly occurs in “bursts” (Horrak et al., 1998) which have been classified in different categories based on their shapes. For example, Hirsikko et al. (2007) and Vana et al. (2008) have classified the New Particle Formation (NPF) events in different categories based on their observations made at mid-latitude stations at Hyytiala (boreal forest) and Mace Head (west coast of Ireland), respectively. To our knowledge, such a classification has never been reported for the NPF events observed at a tropical site. In the present work, we classify the NPF events observed over a period of ∼3 years under four categories and report on the seasonal distribution of each category of the events. The airmass back trajectories and the diurnal variations and changes in the ion size and mobility characteristics associated with each category are also examined and reported. We also compare the characteristics of the average mobility spectra (i) on the event days and on all days, and (ii) when the airmass is reaching our measurement site from the ocean or land are also compared. Towards this objective, we have measured the ion mobility spectra during March 2010 to December 2012 at Pune (18° 31′ N, 73° 55′ E, 560 m above mean sea level) with a Neutral Air Ion Spectrometer (NAIS) in the mobility range of 3.16–0.00133 cm2 V−1 s−1 which corresponds to the particle size range of 0.36–47.1 nm diameter.

Section snippets

Instrumentation and observation site

Measurements of both positive and negative ions in the mobility range of 3.16–0.00133 cm2 V−1 s−1 (diameter range 0.36–47.1 nm) have been made with a Neutral Air Ion Spectrometer (NAIS) of Airel Ltd, Estonia (Mirme et al., 2007). The NAIS records mobility spectra for positive and negative ions in a 5-min period, based on 200-s of sample air and 100-s offset-level measurements. The entire mobility distribution of the atmospheric ions can be divided in two main classes, (i) cluster ions

Criteria for the classification of the new particle formation events

Based on the visual inspection of the shape characteristics of the new particle formation (NPF) events (henceforth called events) during our observation period at Pune, we have classified them in different categories viz. Type Ia (Truncated banana), Type Ib, Type II (Patch type), Type III (Inverted cup type), type IV (Undefined type), Type V (Non-event). To complete the discussion and avoid the need of readers to often refer back to our earlier works, a short description and an example of each

Periods of enhanced concentration of ions and aerosols (PECIA)

The NPF events occurring in the morning hours were generally preceded by a period of 2–3 h of enhanced concentration of ions and aerosols (PECIA). The PECIA generally started between 0600 and 0800 LT. During our observation period we have observed 112 PECIA events. Fig. 7 shows an example of this on April 7, 2010. It is yet to be further investigated whether it is an NPF event. Ions did not show much growth during these periods. However, the concentrations of ions and aerosol particles

Rain - induced event

In this type, ion concentration of all categories showed almost simultaneous and instantaneous increase as soon as the rain started to fall on the ground (Fig. 8). However, none of the ion-category showed any growth in the ion size and the ions disappeared soon after the rain stopped. Horrak et al. (2006) investigated such events. During our observation period we have observed 42 rain-induced events. Detailed analysis of these events associated with rain is given by Kamra et al. (2015b). Kamra

Event statistics in different seasons

A total of 109 events were observed during our observation period (excluding PECIA and rain-induced events). Out of these 109 events, 68 were of Type Ia (Truncated banana type), 3 of type Ib, 10 of Type II, 14 of Type III and 14 of Type IV. Fig. 9 shows the distribution of these events in different months during winter (January, February), pre-monsoon (March, April and May), monsoon (June, July, August, September) and post-monsoon (October, November, December) seasons of each year. Fig. 9 also

Airmass back trajectories

Fig. 10(a–d) shows the 7-days back-trajectories of airmass arriving at our site at 0300 UT (0830 LT) at 500 m level on the event days, drawn from HYSPLIT transport and dispersion model (Draxler and Rolph, 2010). On the most number of days with a Type 1a event in the pre-monsoon/summer season, the air masses originated and approached our site from the Arabian Sea side after travelling ∼100 km over the land and crossing the Western Ghats. However, on a few days, although the airmass originated

On all days vs event days

The daily average mean mobility spectra of the atmospheric ions of both polarity for all days and only for the event days during the observation period (March 2010–December 2012) are shown in Fig. 11. Particle sizes corresponding to the electrical mobility are calculated by assuming the particles to be singly charged (Tammet, 1995). Fig. 11 clearly shows three distinct groups for small, intermediate and large ions and is similar to that observed by Horrak et al. (2000) at Tahkuse observatory,

Discussion

The data collected during our two earlier campaigns which extended over periods of only 47 and 64 days (Siingh et al., 2013; Kamra et al., 2015a) was not sufficient to classify the events in different categories and to look into their seasonal distribution. To our knowledge, this is for the first time in this study that such a classification is being reported for a tropical site. The data collected for the present study extends over a period of ∼3 years and shows that the NPF events were most

Conclusions

We have classified 109 events of NPF that were observed during the period of March 8, 2010 to December 2012 at Pune in four categories. The statistical seasonal distribution of these events shows that most of them occurred in the morning hours of the hottest months of the pre-monsoon season. One event of Type Ia and two of Type II also occurred under cloudy sky conditions. The NPF events of “Type Ia” have been termed as “Truncated banana type” and are different from the traditional “banana

Acknowledgments

The Indian Institute of Tropical Meteorology, Pune is supported by the Ministry of Earth Sciences, Government of India. Authors thank the India Meteorological Department Observatory at Pune for providing the meteorological data. We acknowledge NOAA, ARL for making the HYSPLIT air mass back trajectories available at (http://www.arl.noaa.gov/ready.html). AKK acknowledges the support under Honorary Scientist Programme of the Indian National Science Academy. The authors wish to thank the anonymous

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    Present Affiliation: Department of Physics, HNB Garhwal University, Srinagar, India.

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