Over the Pune region thunderstorms are frequently observed during April to May. Table 1 shows the detail statistical information about total number of events studied during the premonsoon and monsoon in the period of 2005–2008. We analyzed a total of 60 events of thunderstorms of which, 45 cases belong to premonsoon season and 15 to monsoon season. In this study we analyzed electrical parameters (lightning flash rate, electric field) with the chemical species (NOx, O3) during the thunderstorm activity using the thunderstorm and nonthunderstorm day data. Figures 3a and 3b show the lightning flash rate per five minutes and electric field during thunderstorm observed on 3 May 2005, respectively. Lightning flash rate has been derived from electric field data. Figures 4a and 4b show diurnal variations of NOx and ozone observed on 3 May 2005 as well as 5 days before and after the thunderstorm day (nonthunderstorm days). The thunderstorm developed southeast of the observatory at about 16:30 h Local Time (LT). As shown in Figure 3b the electric field became positive and increased rapidly around 17:30 and lightning activity began. Intense lightning activity was observed for 2 h from 17:10 to 19:10 with decreases in frequency thereafter. The lightning activity stopped completely by 19:15 h LT. It is well known that the lightning is a major source of NOx in the middle and upper troposphere and contributes significantly to abundance of NO2 [MacGorman and Rust, 1998; Choi et al., 2005; Beirle et al., 2006; Martin et al., 2006; Schumann and Huntrieser, 2007]. However, there are only a few studies about surface variation of NOx and ozone associated with thunderstorms [Betts al., 2002; Ott et al., 2010; Minschwaner et al., 2008]. The lightning frequency and electric field records shown in Figures 3a and 3b clearly suggest that the dissipation stage of this thunderstorm had started at about at 19:00 h. The concentrations of NOx and ozone observed on nonthunderstorm days are considered as representative of the background levels. As shown in Figure 4a NOx concentration (14 ppb) after 2–3 h (21.00) of peak lightning activity is more than the twofold compared to the nonthunderstorm days. This increase in NOx suggests the transport of NOx from the upper or middle troposphere by downdraft of the thunderstorm during its dissipation stage. Figure 4b shows the variation of ozone on 3 May 2005 thunderstorm day with 5 days before and after the thunderstorm day (nonthunderstorm days). A study by Minschwaner et al.  shows enhancements in ozone between about 3 and 10 km altitude within an electrically active storm in central New Mexico. However, our analysis shows a decrease in ozone following the dissipation stage of thunderstorm (Figure 4b). Several studies have shown that the impacts of NOx on ozone are complex and nonlinear [Lin et al., 1988]. The decrease of ozone following the thunderstorm on 3 May 2005 can be attributed due to the “titration reaction” which became dominant only after the NOx titration threshold is reached. Ozone production is typically associated with hydrocarbon oxidation that produces HO2 or peroxy radicals (RO2) such that
Combined with reactions (R3) and (R4), these reactions convert NO to NO2 without the consumption of ozone, resulting in net ozone production.
O3 production increases linearly with hydrocarbon concentrations but varies inversely with NOx concentrations (hydrocarbon-limited regime) because the O3 production rate is limited by the supply of hydrocarbons. The dependence of O3 production on NOx and hydrocarbons is very different between the two regimes. Ozone concentrations (ppbv) simulated by a regional photochemical model is a function of NOx and hydrocarbon emissions [Sillman et al., 1990]. In the NOx-limited regime, hydrocarbon emission controls are of no benefit for decreasing O3. In the hydrocarbon-limited regime, NOx emission controls cause an increase in O3. O3 production varies linearly with the NO concentration but is independent of hydrocarbons (NOx-limited regime) because the O3 production rate is limited by the supply of NOx. On a thunderstorm and nonthunderstorm day peak of O3 occurs during afternoon hours (12:00–15.00) because of strong solar radiation (Figure 4b). After 2–3 h of thunderstorm activity (21.00 h) NOx slowly increases and reaches concentration (14 ppb) above the titration threshold level and starts to destroys the O3 (∼15 ppb) via titration reaction. Peak of NOx and dip of O3 at 21:00 h on thunderstorm day is clearly seen in Figures 4a and 4b because of thunderstorm and lightning activity. However, the threshold limit is not fixed and varies from region to region. Based on the long-term data and their analysis as reported earlier from the same station [Beig and Brasseur, 2006; Beig et al., 2007], the threshold limit is estimated to be between 12 and 20 ppb [Beig et al., 2010].
 We discuss one more case of premonsoon thunderstorm observed on 3 June 2008. This thunderstorm developed at about 16:00 h LT local time and lasted for about 2 h; the flash rate along with 5 min average of electric field is shown in Figures 5a and 5b. Lightning flash rate increased very rapidly and reached 120 flashes per 5 min during the active stage of the thunderstorm and then decreased gradually to less than one flash per minute at about 18:00 h. As shown in Figure 5b with the onset of the thunderstorm, the electric field increased to 2 kV/m and lightning activity started at about 16:20 h. In Figures 6a and 6b we have plotted diurnal variations of NOx and ozone observed on 3 June 2008 as well as 5 days before and after the day of the thunderstorm (nonthunderstorm days). As shown in Figure 6a, NOx shows considerable increase at 21:00 h. This considerable increase in NOx observed on 3 June 2008 at 21:00 h is clearly due to transport of NOx from the upper or middle troposphere with the downdraft of the thunderstorm during its dissipation stage. Figure 6b shows the variation of ozone on 3 June 2008 and 5 days variations of ozone before and after thunderstorm day. During the dissipation stage (20:00–21:00 h) of the thunderstorm, the surface NOx (17 ppb) increases sharply and at the peak level becomes four times more than the normal fair weather days (Figure 6a). The increase at peak hours is found to be from 4 ppb for nonthunderstorm days (fair weather days) to 17 ppb during thunderstorm hours. The rate of increase is high in the magnitude of NOx concentration which is close to titration threshold limit. As a result the ozone level drops down due to the titration reaction. We did not find any enhancement in ozone during the active period of the thunderstorm (Figures 4b and 6b) as also reported earlier by Minschwaner et al. . In fact it is observed that in most of the cases, ozone showed low concentrations during the active period of thunderstorms. This decrease in concentration may be due to reduced solar radiation because of the presence of thunderclouds. However, it is observed that there is a reduction in ozone during the dissipation stage of the thunderstorm as seen in Figures 4b and 6b. During premonsoon season 10 out of 45 cases shows that there is large enhancement in NOx concentration above the titration threshold during or after the dissipation of the thunderstorm and reduction in surface ozone concentration.