Ozone and Precursor Gases at a Location in the Sub Himalayan Region of North East India: Measurement and Modelling
Bhuyan, Pradip Kumar; Pathak, Binita; Bharali, Chandrakala
Dibrugarh University, INDIA

The characteristics of the greenhouse gases O3, NOx, CO and SO2 and their interrelationship have been examined over a continental location in the sub Himalayan region of North East India. Ozone concentration starts to rise gradually after sunrise, attains its maximum level around 14 hrs IST due to photochemical production of O3, mainly from oxidation of natural and anthropogenic hydrocarbons, carbon monoxide (CO), and methane (CH4) by hydroxyl (OH) radical in the presence of a sufficient amount of NOx and then decreases rather fast in the afternoon hours to reach the near minimum level in the evening hours (Figure 1). Figure1: Diurnal variation of mean±1ó O3 for Pre-monsoon, Monsoon, Post-monsoon and Winter seasons for the period September 2010-January 2013 It remains low from evening till sunrise due to the absence of photolysis of NO2 and the continuous loss of O3 by NOx titration. The diurnal variation is smooth in monsoon and the difference between diurnal maximum and minimum is less compared to the other three seasons. Maximum and minimum concentration of O3 has been measured in the pre-monsoon (44.34± 10.2 ppb) and monsoon (24.8±7.44 ppb) season respectively. The low concentration in monsoon might be attributed to rainout of the precursor gases CO, NOx etc and obstruction of sunlight by clouds. The evening and night-time minimum ozone concentration remains around 3-4 ppb in the post-monsoon and winter seasons. On the other hand, the minimum ozone concentration is much higher at ~15 ppb and ~10 ppb in the pre-monsoon and monsoon season respectively. NO and NO2 (Figure 2) exhibits similar diurnal cycle with a daytime minimum and evening maximum which is due to the diurnal evolution of the planetary boundary layer. The lowest value for both NO2 and NO is ~0.5 ppb and is observed between 10-15 hrs and 09-16 hrs respectively in all the months. Peak NO2 is highest at 17.6±7.3 ppb in winter while highest NO concentration occurs in winter at 22 hr (65.8±55.2 ppb). Figure2: Diurnal variation of mean NO and NO2 for Pre-monsoon, Monsoon, Post-monsoon and Winter. The vertical bars represent 1ó deviation from the mean. CO is highest in the winter months ranging between 0.43±0.44 ppm and 1.59±0.55 ppm followed by post-monsoon season (0.43±0.19ppm-1.34±0.48 ppm) (Figure 3). However, SO2 does not show any significant variation (Figure 3) over the diurnal cycle as clear from Figure 3 and is highest in the post monsoon season with concentration in the range 2.2±0.51 ppb and 3.36±1.75 ppb and lowest in the pre-monsoon season with values in the range 1.02±0.83 ppb -1.63±0.77 ppb. Figure3: Diurnal variation of mean±1ó deviation of CO and SO2 measured during January 2012 to January 2013. Significant non linear correlation is found between O3 and NO, NO2 in all the seasons except in the monsoon indicating that ozone grows and decays exponentially with NO and NO2. Principal component analysis (PCA) on trace gases show that PC1 has high contribution from ozone (67.07%) which indicates that ozone formation is significant over Dibrugarh. PC2 is loaded with SO2 (17.03%) which could be attributed to the high S content in diesel fuel used in trucks resulting in high contribution of SO2. PC3 explains 10% variance but the contribution from any of the trace gases (O3, NOx, CO and SO2) is not significant. The measured ozone concentrations were simulated using the chemical Master Mechanism box model and utilizing simultaneous in situ measurements of NO, NO2 and CO. It is seen that the model, in general, captures the morning build-up and afternoon decay of ozone. However, the model simulations deviate from the measured ozone mixing ratio in most cases. The limitations and lack of complete information about background concentrations of other species (e.g. Methane and NMHCs, VOCs etc.) and dimensional limitation of the model result in the deviation of the observed concentration of ozone value. Figure 4 shows the MM simulated and observed ozone concentration for a representative day 14th October, 2012 and indicates that the precursor gases NOx and CO have major contribution to the observed variation of ozone concentration over the location. Figure 4: Diurnal cycles of ozone simulated using a Chemical Box model along with the observed data for a representative day (14th October 2012).