Airborne measurements of the number concentration and size distribution of aerosols from 13 to 700 nm diameter have been made at four vertical levels across a coastline at Bhubaneswar (20° 25′N, 85° 83′E) during the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) programme conducted in March–April 2006. The measurements made during the constant-level flights at 0.5, 1, 2 and 3 km altitude levels extend ∼100 km over land and ∼150km over ocean. Aerosol number concentrations vary from 2200 to 4500 cm^-3 at 0.5 km level but are almost constant at ∼6000 cm^-3 and ∼800 cm^-3 at 2 and 3 km levels, respectively. At 1km level, aerosol number concentration shows a peak of 18,070 cm^-3 around the coastline. Most of the aerosol size distribution curves at 0.5 km and 1 km levels are monomodal with a maxima at 110nm diameter which shifts to 70 nm diameter at 2 and 3 km levels. However, at the peak at 1 km level, number concentration has a bimodal distribution with an additional maximum appearing in nucleation mode. It is proposed that this maxima in nucleation mode at 1 km level may be due to the formation and transport of new particles from coastal regions.

The effects of molecular diffusivity of H₂SO₄ and NH₃ vapours on nucleated particles of SO$^{2−}_{4}$ and NO$^{−}_{3}$ species are reported. Condensation sink and source rate of H₂SO₄ and NH₃ vapours, growth rates and ratios of real to apparent nucleation rates are calculated for SO$^{2−}_{4}$ and NO$^{−}_{3}$ aerosols using fractional contributions of them in total aerosol size-distribution during the measurement period at Pune, reported in Chate and Pranesha (2004). The percentage of nucleated SO$^{2−}_{4}$ and NO$^{−}_{3}$ aerosols of mid-point diameter 13 nm are 2% and 3% respectively of the total particles (13 nm ≤ $D_{p}$ ≤750nm) for both H₂SO₄ and NH₃ diffusion. In the diameter range $75 nm \leq D_{p} \leq 133 nm$, it is 48% and 45% of SO$^{2−}_{4}$ and NO$^{−}_{3}$ aerosols, respectively for NH₃ diffusion and 43% and 36% of SO$^{2−}_{4}$ and NO$^{−}_{3}$ for H₂SO₄ diffusion. Increase in percentage of nucleated particles of these species corresponding to mid-point diameter 133 nm around 0900 h IST is significantly higher than that of mid-point diameter 13 nm and it is due to photo-chemical nucleation, coagulation and coalescence among nucleated clusters. The ratios of real to apparent formation rates for SO$^{2−}_{4}$ and NO$^{−}_{3}$ aerosols are 12% and 11% respectively, corresponding to mid-point diameter 13 nm, 17% and 13%, for midpoint diameter 133 nm and 12% and 9.5%, for mid-point diameter 750 nm. The results indicate that nucleation involving H₂SO₄ and acidic NH₃ diffusion on SO$^{2−}_{4}$ and NO$^{−}_{3}$ particles is the most relevant mechanism in this region.

In spite of many experimental and theoretical studies the relationships between storm dynamics, severe weather,and lightning activity have been least understood.Measurements of electric field made under a severe thunderstorm at a northeastern Indian station,Guwahati,India are reported. Lightning flash rate increases drastically to about 84 flashes per minute (fpm)during the active stage which lasted for about 7 minutes,from about 15 flashes per minute during the initial phase of thunderstorm.Sudden increase in lightning flash rate (‘lightning jump ’)of about 65 fpm/min is also observed in the beginning of the active stage.The dissipating stage is marked by slow and steady decrease in lightning frequency.Despite very high flash rate during the active stage, no severe weather conditions are observed at the ground.It is proposed that the short duration of the active stage might be the reason for the non-observance of severe weather conditions at the ground.Analysis of Skew-t graph at Guwahati suggests that vertical distribution of Convective Available Potential Energy (CAPE)also may play some role in non-occurrence of severe weather at ground in spite of large lightning flash rate and lightning jump observed in this thunderstorm.Further,all electric field changes after a lightning discharge indicates the presence of strong Lower Positive Charge Centers (LPCC)in the active and dissipation stages. This suggests that LPCC plays an important role in initiation of lightning discharges in these stages.

Continuous measurements of aerosol size distributions in the mid-point diameter range 20.5–500 nm were made from October 2005 to March 2006 at Pune (18° 32′N, 73° 51′E), India using Scanning Mobility Particle Sizer (SMPS). Volatilities of atmospheric aerosols were also measured at 40°, 125°, 175°, 300° and 350°C temperatures with Thermodenuder–SMPS coupled system to determine aerosol volatile fractions. Aerosols in nucleated, CCN and accumulated modes are characterized from the measured percentage of particles volatized at 40°, 125°, 175°, 300° and 350°C temperatures. Averaged monthly aerosol concentration is at its maximum in November and gradually decreases to its minimum at the end of March. The diurnal variations of aerosol concentrations gradually decrease in the night and in early morning hours (0400–0800 hr). However, concentration attains minimum in its variations in the noon (1400–1600 hr) due to higher ventilation factor (product of mixing height and wind speed). The half an hour averaged diurnal variation of aerosol number concentration shows about 5 to 10-fold increase despite the ventilation factor at higher side before 1200 hr. This sudden increase in aerosol concentrations is linked with prevailing conditions for nucleation bursts. The measurement of volatile fraction of ambient aerosols reveals that there are large number of highly volatile particles in the Aitken mode in the morning hours and these volatile fractions of aerosols at temperatures > 150°C are of ammonium chloride and ammonium sulfate, acetic and formic acids.

A case of new particle formation observed during dissipation stage of a thunderstorm at a tropical station, Pune, India on 3 June 2008 is reported. The flash rate and rainfall intensity increased as high as 110 flashes per 5 minutes and 150 mm hour⁻¹ respectively during the active stage of thunderstorm, and then gradually decreased during the dissipation stage. The number concentration of particles in the size range of 10–100 nm sharply increased from 350 particles cm^-3 to ∼8000 particles cm^-3 during the dissipation stage of a thunderstorm and grew to larger diameter subsequently. Observations suggest that the atmospheric conditions such as (i) reduced background aerosol concentration after heavy rain, (ii) high humidity condition, and (iii) increased ion concentration during the dissipation stage by corona discharges, favoured generation of new particles by ion-induced nucleation (IIN). Observations also suggest that generation of unipolar ions by corona discharges may be more favourable for IIN and subsequent growth of the particles.