Air quality in Shanghai continued to improve in the first four months of the year. Average concentration of PM2.5 particles dropped to 33 micrograms per cubic meter between January and April, 8.3 percent lower than the same period last year, the Shanghai Ecology and Environment Bureau said on Tuesday. The city's air quality index was either "good" or "excellent" 97.5 percent of the time, an increase of 8.2 percentage points over the same period last year.