Hong Kong Tourism Board chairman James Tien Pei-chun warned yesterday that this year's visitor arrival estimate of just over 29 million could prove to be too optimistic given the impact of the global economic downturn. The board will start reviewing its forecasts on a quarterly basis this year to account for economic developments. 'I do worry that the decline in visitor arrivals may be more than 1.6 per cent,' Mr Tien said. About two months ago, the board projected a 1.6 per cent drop in visitor numbers. The signs so far do not seem encouraging, with the number of mainland visitors shrinking about 15 per cent year on year during the first 15 days of this month. Over half of Hong Kong's visitor arrivals are from the mainland. Last year, visitors from Guangdong accounted for 78 per cent of the 16.8 million mainland arrivals.