The dawn of pollution

By Priyanka Chandola
Published: Friday 31 July 2009

-- (Credit: AGNIMIRH BASU)A walk in pre-sunrise hours may increase chances of asthma

health enthusiasts living near freeways should minimize their outdoor exercise timings during the early morning hours; they should also keep their windows closed through the night. A team of environmental health scientists found the levels of air pollution to be really high during the pre-sunrise hours in the Interstate 10 freeway in Santa Monica, California.

Researchers from the University of California, Los Angeles and California Air Resources Board equipped an electric vehicle with no emissions of its own, with instruments to measure gaseous and particulate air pollutants, a global positioning system, video monitor and instruments to measure temperature and winds. For pre-sunrise measurement, the vehicle was driven on a fixed route over two days in the summer and three days in the winter of 2008 between 4 and 7 am.Covering a distance of 3,600 m this route passed through a dense residential neighbourhood below the freeway. Air pollutant concentrations were also measured on the freeway and streets transecting the route.

During pre-sunrise hours, the traffic volume was found to be 30 to 80 per cent lower compared to the morning or afternoon hours. However, the ultrafine particle concentrations were significantly higher. This, the key researcher Shishan Hu explained, was due to unique meteorological conditions that exist in dawn. "Pre-sunrise, although traffic is less on the freeway, the ambient air is relatively calm due to low wind speed and possible shallow temperature inversion, causing the pollutants to be trapped near the surface instead of being mixed and carried away. During the day traffic sources are more and strong mixing occurs," explained Hu. During temperature inversion, temperature increases with height--a reversal of the normal pattern.

Due to the specific wind direction, areas south of the freeway are downwind. Hence the impact of the air pollutants was found to be down south (as far as 2,000 m) during the pre-sunrise hours. Further north (upwind), it extended only 600 m.

Manju Mohan, professor at the centre for atmospheric sciences, iit, Delhi says, "These conditions can occur at sunrise causing a huge buildup of pollutants at the ground level. But this situation may not last for long because the heat from the sun is strong enough to cause complete mixing of pollutants."

Such wide pollutant-impact area downwind of a major roadway has much worse health effects. "Earlier researchers verified adverse conditions such as asthma and even morbidity associated with high pollutant concentrations in the vicinity of roadways. The impact zone was identified to be 300 m downwind side during daytime. Our study revealed much longer impact distance during pre-sunrise hours. This means more people living in the vicinity of roadways will be affected by freeway emissions," warned Hu.

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