Tropospheric ozone maxima observed over the Arabian Sea during the pre-monsoon

Abstract. An enhancement of the tropospheric ozone column (TOC) over Arabian Sea (AS) during the pre-monsoon season is reported in this study. The potential sources of the AS spring ozone pool are investigated by use of multiple data sets (e.g., SCIAMACHY Limb-Nadir-Matching TOC, OMI/MLS TOC, TES TOC, MACC reanalysis data, MOZART-4 model and HYSPLIT model). Three-quarters of the enhanced ozone concentrations are attributed to the 0–8 km height range. The main source of the ozone enhancement is considered to be caused by long-range transport of ozone pollutants from India (∼  50 % contributions to the lowest 4 km,  ∼  20 % contributions to the 4–8 km height range), the Middle East, Africa and Europe (∼  30 % in total). In addition, the vertical pollution accumulation in the lower troposphere, especially at 4–8 km, was found to be important for the AS spring ozone pool formation. Local photochemistry, on the other hand, plays a negligible role in producing ozone at the 4–8 km height range. In the 0–4 km height range, ozone is quickly removed by wet deposition. The AS spring TOC maxima are influenced by the dynamical variations caused by the sea surface temperature (SST) anomaly during the El Niño period in 2005 and 2010 with a  ∼  5 DU decrease.