AIR quality improved in January, as the concentrations of all pollutants fell below the hourly and 24-hour air quality objective values throughout the month. But a spokesman for the Environmental Protection Department (EPD) said concentrations varied at all sites, indicating the effects of regional weather conditions rather than the influence of local sources. The findings came from regular measurements at EPD monitoring stations in Mongkok, Central/Western and Kwai Chung. Air pollutants measured include sulphur dioxide (SO2), nitrogen dioxide (NO2), respirable suspended particles (RSP), and total suspended particles (TSP). The four pollutants have different origins. SO2 originates mostly from the combustion of sulphur containing fossil fuels. NO2 is formed either directly during combustion or by atmospheric oxidation of nitric oxide (NO) which is also produced during combustion. Vehicle exhaust is a major source of NO and NO2. It is also a major source of airborne particles. Other sources include industry, furnaces and boilers, construction work, the seas and soil dust. The EPD monitors air quality continuously at a total of 11 stations. Short-term measurement campaigns are undertaken as and when the need arises.