Temporal evolution of trace gases in a sub-Himalayan region of India: WRF-Chem simulations versus observation

Abstract:

WRF-Chem has been used to simulate O₃ and its precursors, NO_x and CO and meteorological variables over the sub-Himalayn North-Eastern region of India including Bangladesh for the year 2012. The model simulation is compared with the observations carried at a location Dibrugarh (27.3ºN, 94.6ºE, 111 m amsl). WRF Chem is capable of simulating the diurnal variability in surface O₃ in all the seasons with a good correlation (R²>0.57) between the simulated and observed O₃. The model is found to capture the observed seasonal variations in O₃ concentrations with maximum during the spring and minimum in monsoon. However, the model overestimates both the minimum and maximum levels by >10ppb in monsoon season and > 20ppb in all other three seasons. Simulated NO is underestimated, mostly during the nighttime in all three seasons (5-20 ppb) except the monsoon (~5ppb) season, when it is overestimated. Simulated NO₂ and CO on the other hand, is underestimated in all the seasons. The observed difference may be contributed by the bias in the estimation of the O₃ precursors, NOx and CO in the emission inventories or the error in the simulation of the meteorological variables which influences O₃ concentration. For example, in the pre-monsoon and winter season, the model simulated shortwave flux overestimates the observation by ~500 Wm^-2. While in the monsoon and post monsoon season, simulated shortwave flux is equivalent to the observation. The model predicts high wind speed in all the seasons especially during night-time as against observation. In the post-monsoon and winter season, the simulated wind pattern is reverse to observation with daytime low and night-time high values. Rainfall is overestimated in all the seasons. The night-time overestimation of O₃ over this location may also be due to inadequacy of the model's chemistry to properly simulate the night-time level over this location.