A TROPICAL depression began building up in the South China Sea yesterday and was expected to move closest to Hong Kong this afternoon. The Royal Observatory hoisted the number one standby storm warning signal at 4.45 pm yesterday when winds of 40 kilometres per hour and gusts of 60 km/h, were detected about 400 km east-southeast of the territory. It was moving in a west-northwesterly direction at 22 km/h and expected to hit the coast near Macau this afternoon. A spokesman for the Royal Observatory said today's weather was likely to be a combination of wind and rain. ''We expect the winds to strengthen by the time they reach Hong Kong. They should be moderate easterly winds and rain, but Thursday and Friday should be fine.'' He doubted whether it would turn into a repeat of last month's two severe storms, which caused chaos but helped clear the air. Air quality readings for last month showed Severe Tropical Storm Becky and Typhoon Dot brought in cleaner air and washed out many pollutants. But the rise in air quality was not so marked during Typhoon Dot because more vehicles braved the storm, leaving more nitrogen dioxide in the air. For the rest of the month, low wind speeds allowed a large build-up of pollutants. Nitrogen dioxide levels in Mongkok exceeded the air quality objective in two successive 24-hour periods on September 3 and 4. It was the fourth time this year the level had been exceeded. The Royal Observatory has raised the storm warning six times this year, with four being increased to the severe number eight signal.