Variations in O<sub>3</sub>, CO, and CH<sub>4</sub> over the Bay of Bengal during the summer monsoon season: shipborne measurements and model simulations

Abstract

Abstract. We present shipborne measurements of surface ozone (O 3 ), carbon monoxide (CO), and methane (CH 4 ) over the Bay of Bengal (BoB), the first time such measurements have been performed during the summer monsoon season, as a part of the Continental Tropical Convergence Zone (CTCZ) experiment during 2009. O 3 , CO, and CH 4 mixing ratios exhibited significant spatial and temporal variability in the ranges of 8-54 nmol mol −1 , 50-200 nmol mol −1 , and 1.57-2.15 µmol mol −1 , with means of 29.7 ± 6.8 nmol mol −1 , 96 ± 25 nmol mol −1 , and 1.83 ± 0.14 µmol mol −1 , respectively. The average mixing ratios of trace gases over BoB in air masses from central/northern India (O 3 : 30 ± 7 nmol mol −1 ; CO: 95 ± 25 nmol mol −1 ; CH 4 : 1.86 ± 0.12 µmol mol −1 ) were not statistically different from those in air masses from southern India (O 3 : 27 ± 5 nmol mol −1 ; CO: 101 ± 27 nmol mol −1 ; CH 4 : 1.72 ± 0.14 µmol mol −1 ). Spatial variability is observed to be most significant for CH 4 with higher mixing ratios in the air masses from central/northern India, where higher CH 4 levels are seen in the SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CartograpHY) data. O 3 mixing ratios over the BoB showed large reductions (by ∼ 20 nmol mol −1 ) during four rainfall events. Temporal changes in the meteorological parameters, in conjunction with O 3 vertical profile, indicate that these low-O 3 events are associated with downdrafts of free-tropospheric O 3 -poor air masses. While the observed variations of O 3 and CO are successfully reproduced using the Weather Research and Forecasting model with Chemistry (WRF-Chem), this model overestimates mean concentrations by about 6 and 16 % for O 3 and CO, respectively, generally overestimating O 3 mixing ratios during the rainfall events. An analysis of modelled O 3 along air mass trajectories show mean en route O 3 production rate of about 4.6 nmol mol −1 day −1 in the outflow towards the BoB. Analysis of the various tendencies from model simulations during an event on 10 August 2009, reproduced by the model, shows horizontal advection rapidly transporting O 3 -rich air masses from near the coast across the BoB. This study fills a gap in the availability of trace gas measurements over the BoB and, when combined with data from previous campaigns, reveals large seasonal amplitude (∼ 39 and ∼ 207 nmol mol −1 for O 3 and CO, respectively) over the northern BoB.

WRF-Chem simulated surface ozone over south Asia during the pre-monsoon: effects of emission inventories and chemical mechanisms

“…Further, nudging towards reanalysis data limits deviations in simulated meteorology (e.g. Kumar et al, 2012a;Ojha et al, 2016;Girach et al, 2017). Nevertheless, we include an evaluation of model-simulated water vapour, temperature and wind speed against radiosonde observations (Fig.…”

“…Therefore, our conclusions are not affected. The net photochemical O 3 production rate (ppbv h −1 ) from sunrise to noontime (06:30-12:30 IST), when most of the photochemical buildup of ozone takes place leading to its peak noontime mixing ratio, has been calculated utilizing the chemical tendencies in WRF-Chem (Barth et al, 2012;Girach et al, 2017). A comparison of monthly average O 3 production rates among the three inventories is shown in Fig.…”

“…Following up on this, here we compare modelled ozone mixing ratios obtained with these two extensively used chemical mechanisms over south Asia: RADM2 (e.g. Michael et al, 2013;Ojha et al, 2016;Girach et al, 2017) and MOZART (e.g. Ghude et al, 2014Ghude et al, , 2016, by keeping the same input emission inventory (HTAP).…”

“…Kumar et al, 2012a, b;Michael et al, 2013;Gupta et al, 2015;Jena et al, 2015;Ansari et al, 2016;Ojha et al, 2016;Girach et al, 2017). WRF-Chem simulations at higher spatial resolution employing regional emission inventories have been shown to better reproduce the observed spatial and temporal heterogeneities in ozone over this region as compared to the global models (e.g.…”

Response to referee 1 (RC1)

“…The Weather Research and Forecasting model with Chemistry (WRF-Chem) (Grell et al, 2005;Fast et al, 2006), a regional simulation system, has been popular for use over the south Asian region in numerous recent studies to simulate the meteorology and spatiotemporal distribution of ozone and related trace gases (e.g. Kumar et al, 2012a, b;Michael et al, 2013;Gupta et al, 2015;Jena et al, 2015;Ansari et al, 2016;Ojha et al, 2016;Girach et al, 2017). WRF-Chem simulations at higher spatial resolution employing regional emission inventories have been shown to better reproduce the observed spatial and temporal heterogeneities in ozone over this region as compared to the global models (e.g.…”