Analysis of Ambient Air Quality Based On Exceedance Factor and Air Quality Index for Siliguri City, West Bengal

Bad air is one of the key concerns for most of the urban centres today, and Siliguri is no exceptions to this. In order to assess the air quality of Siliguri, Exceedance factor (EF) method was applied based on the average annual concentration of the pollutants named as; NO2, SO2, PM2.5 and PM10 and it is found that PM2.5 and PM10 are the major pollutants that pose a severe threat for the city. After applying the EF method, it is found that the values of PM2.5 was between moderate to high pollution level and for PM10 it falls under high to critical pollution level. On the other hand, the concentration of NO2 and SO2 falls under moderate to low pollution level. Through trend analysis of the various pollutants, it is found that their concentration was varying in nature. In case of PM10, the trend shows high concentration which exceeds national standard; whereas PM2.5 shows its concentration near towards violating the national standard soon if not checked. In contrast, trends of NO2 and SO2 were recorded lower than the national standard. The present situation of ambient air of Siliguri was analyzed based on Air Quality Index which reveals that air quality of the city can be classified into two seasons, i.e. clean air period (from April to October) and polluted period (from November to March). Lastly, the annual trends of PM2.5 and PM10 were constructed as they are the major pollutants, and it shows their skewed nature during winter months which results in smog episodes. It unveils how critical the situation of air quality of Siliguri became especially during winter months which seek immediate attention. Thus the study tries to present a vivid scenario about the present air quality of Siliguri, which concludes with some of the suggestions to restrain the air quality.

The Limit Exceedance Factor Importance Measure: An Application to the Prism Reactor Design

Probabilistic Risk Assessment has several different standard importance measures for structures, systems, and components (SSCs). These importance measures are Risk Achievement Worth (RAW), Risk Reduction Worth (RRW), and Fussell-Vesely (FV). In particular, RAW and FV have been used to identify SSCs of low risk significance. However, when designing a conceptual reactor with a safety goal in mind such as those proposed in various technology neutral frameworks, these importance measures are quite crude. RAW is defined such that a component is always in the failed state, and a designer may not be looking to remove a system but to simplify it to improve economics. The Limit Exceedance Factor has been developed as a more informative importance measure when there is a goal in mind. It is defined as the factor by which the failure probability of a component may be multiplied by such that the end state (e.g. core damage) frequency exceeds a limit. With a living PRA available throughout the design process, it could allow a designer to know how much room there is for possible simplification in redundant systems. Alternatively, in the case where a system does not meet the frequency limit it can show which systems might be ideal targets for improvement to reach the limit. For a sodium-cooled fast reactor design, using the goals and limits outlined in NUREG-1860 (Technology Neutral Framework), one end state that must stay below a threshold is a very large release due to an energetic scenario. Several SSCs can be identified as risk insignificant with RAW values below two. The other SSCs tend to have very high RAW values on the order of one thousand. When the LEF is applied to the SSCs that seem to be very important using traditional importance measures, we see that some may actually have a fairly wide margin from their cited failure probabilities and still maintain the frequency goal.