Monitoring air pollutants is important to understand the impact of policy changes. Vivek Chattopadhyaya explains the existing technologies for it and some of their limitations
Why air quality monitoring is essential
The starting point of air quality monitoring is to first study if an area has an air pollution problem. Monitoring helps in assessing the level of pollution in relation to the ambient air quality standards. Standards are a regulatory measure to set the target for pollution reduction and achieve clean air. Robust monitoring helps to guard against extreme events by alerting people and initiate action. We regulate a total of 12 pollutants, including SO2, NO2, PM10, PM2.5 (particulate matter of up to 10 micron and up to 2.5 micron size), ozone, lead, arsenic, nickel, CO, NH3, benzene, and BaP (particulate phase). Across cities, only SO2, NO2 and RSPM / PM10 are monitored regularly. Other pollutants, such as PM2.5, O3, CO, BTX, heavy metals are monitored in select cities as capacity is still being built. India has set a target for states to meet National Ambient Air Quality Standards (NAAQS) in urban areas by 2017.
How accurate and absolute is the air quality data reported
The present national air quality monitoring network is limited in scope as the recorded values are indicative and there is immense time lag in reporting the data. So real time action is not possible. Also, involvement of various monitoring agencies, personnel and equipments in sampling, chemical analyses and data reporting brings uncertainty and biases. But even with the existing system the non-compliance with standards in cities is found to be enormous. As many as 131 cities are exceeding the permissible limit for PM 10 and 18 cities are exceeding the permissible limit for NO2. Therefore, it is the action that matters even as we upgrade our monitoring systems.
Who carries out monitoring in India
The ambient air quality in India is monitored collectively by CPCB, state pollution control boards (SPCBs), pollution control committees (PCCs), and National Environmental Engineering Research Institute (NEERI) in cities, and covers 215 cities and towns. A total of 523 manual monitoring stations are being operated across states. Some states have set up additional monitoring stations in cities. However, there is a shortfall in operation as about 1,000 stations with additional continuous ambient air quality monitoring (CAAQM) stations, that report data real time, are required. As per 2011 census, 46 cities have million-plus population and in 16 cities CAAQMS have already been installed and commissioned. The CAAQM stations collect the data of 8 pollutants, except metals and BaP.
Real time monitoring results will help in calculating air quality index to issue health advisories as well as for formulation of action plan to meet standards.
What plagues the current manual method
To ensure the quality of data, analytical quality control and for following guidelines for monitoring and calibration, repair of instruments and evaluation of ambient air quality monitoring stations are must. But there are certain limitations to manual monitoring. Different measuring techniques and instruments give varying results on ambient concentration in the same location. Studies have found that average error ranges from 10 to 26 per cent for PM monitoring, primarily because of incorrect flow measurement and calibration. Moreover, inconsistent power supply and voltage fluctuation affect monitoring. In case of gaseous pollutants, duration of sampling, sample dilution and temperature controls are essential to ensure that tests are done properly. Monitoring of benzene and O3 by manual method has been found to be very difficult.
CAAQM’s sophistication deals with these issues, as it is a compact set up of different pollutant analysers and even calibration units. CAAQMS are equipped with UPS along with backup capacity. But it costs quite a lot. The set of analysers, calibrations systems, meteorological instrumentation, sensors, sampling lines and display systems, among others, cost about Rs 90 lakh. Overhead and maintenance costs go up to Rs 10 lakh a year.
How many stations are sufficient?
Opinions differ on an appropriate grid density for capturing air quality profile. Most experts agree that the current grid density is inadequate. The criteria followed in India requires that class I cities (cities with 100,000 population or more) should have a minimum of three stations each; mega cities nine each; industrial areas should have about six and capital cities, six each. An analysis of the National Ambient Air Quality Monitoring Programme (NAMP) data shows that 82 cities have only one station and 66 have two stations and about 80 class I cities do not meet the criteria for monitoring stations. Big cities fare better. But do we increase the numbers with high investments? Given the resource crunch and limited technical skills in this field, many experts advise against setting up elaborate monitoring systems. They suggest acting on the information currently available from the present network, instead. Setting up more stations should be considered based on where most people live, the pollutants they are exposed to and the need for a daily alert system.
Weakest links in air quality management
Most SPCBs keep their special monitoring and surveillance efforts confined to industrial areas. Very few turn their attention towards the urban air quality of cities. As a principle, what matters most is the ability of the authorities to understand the profile of a city’s air pollution sources and their emissions rates and trends; cities also must go beyond routine monitoring to generate specialised data. Unfortunately, Indian cities do not score on these counts.
How effective is satellite usage?
Satellite data can fill the coverage gaps in the existing network to support routine monitoring. They could also be used to identify potential air quality hot spots. It has been found that using satellite data is more economical than setting up and operating a number of fixed stations. For instance, most of the NO2 in the atmosphere is contained within 1 km from the surface of the earth. That makes satellite measurements a useful representation of surface NO2 concentrations. For PM assessment, too, satellite based assessment of aerosol optical depth is used. In summary, both magnitude and spatial extent of sources can be determined and exposure can be assessed with the help of satellites.
Satellite observations can change what otherwise is known about emissions and concentrations observed through ground-based measurements. In a study in South Africa, large difference between the estimated emissions values and actual values from the satellites was found, especially with respect to location and strength of emissions of sources.
In India, a recent satellite-based study indicated that contrary to the decreasing trend of SO2 concentrations reported by ground based monitors across cities, the SO2 concentrations in coal-fired power plant regions actually increased by >60 per cent during 2005–2012, implying the air quality monitoring network needs to be optimised to reflect the true SO2 situation in India.
But there are limitations, too. For instance, to assess the PM levels, satellite-based data retrieval gets obscured due to cloud cover. Therefore, data of ground-monitors are also essential to validate the results. But the future is promising if satellite information and ground monitoring data are used as complementary systems. Use of atmospheric transport models can even enable fusing of surface PM2.5 measurements and satellite-estimated PM2.5 concentrations to provide additional air quality information in areas which do not have ground-based monitors.
Ascertaining air quality
How good or bad is the air we breathe is known through monitoring and interpretation of data vis a vis the standards. In most stations in India, ambient air quality monitoring is carried out manually using high volume samplers and respirable dust samplers with gaseous attachments, primarily to monitor PM, SO2 and NO2. The monitoring of pollutants is needed to be carried out for 24 hours (4-hourly sampling for gaseous pollutants and 8-hourly sampling for particulate matter). The sampler uses a blower to suck in air. Particle size classifier separates particles greater than 10µm size from the air stream which is then passed through a filter paper to collect particles lesser than 10µm size to get PM10. The filter paper can be used to determine metals, sulphate and nitrate levels. The sampler is also equipped with bubblers to assess gaseous pollutants. The site operators have to manually record the flow rates, change the filter paper, reagents and preserve the samples and take them to a lab for analysis. Automatic monitors are used to monitor PM2.5. Take a look at two filter papers depicting the unexposed and exposed PM2.5 filter papers.
The automatic or Continuous Ambient Air Quality Monitoring Stations (CAAQMS) can monitor pollutants using different analysers, thereby, reducing the chances of manual error, generate data at time-intervals of minutes and transmit the data. The data generated is disseminated online through a digital display board to public. These systems are now used in cities. Some countries use mathematical interpolation of the readings to estimate concentrations in surrounding areas. Indian cities are gradually improving the monitoring capacity and several metro cities including state capitals have begun to set up CAAQMS.
For manual monitoring, the criteria of minimum 104 observations in a year is not met in many stations and now even 50 days of monitoring is considered adequate for annual average estimation. Power failure, instrumental failure, paucity of trained manpower affects the frequency.
Several cities fulfill the minimum days requirement and some cities such as Guwahati, Agra, Chandigarh and Parwanoo are performing well in the manual monitoring with respect to number of days monitored which goes up 280 days in some stations. But significant missing data impedes accurate exposure-response relationships to assess the health impacts.
Bengaluru stands out for its proactive move to generate data on the impact of policy action or special events on air quality. The most significant step has been to scan the effects of the “bus day”, organised every month in the city by the Bangalore Metropolitan Transport Corporation. Specialised monitoring indicates these measures exert a positive impact. It has also shown how on days when a bandh is called, some pollutants may increase; this is because in the absence of public transport, more personal vehicles ply on city roads.
The Karnataka SPCB has taken a significant step in generating air quality data in the breathing zone of people at a height of 1.5 m. This is different from monitoring ambient air quality at six m height, which does not capture direct exposure to toxic and harmful pollution.
The only other city that is known for taking significant steps to assess urban air quality is Kolkata. Here, the West Bengal SPCB has conducted studies to generate specialised data and information on unregulated pollutants like air toxins; the board has also carried out its own inventory assessment.
Delhi has set up “Open Path” system-based monitoring (differential optical absorption spectroscopy technique) and also established LIDAR (laser based ‘Light Detection And Ranging’ technique)-based air quality forecasting system during Common Wealth Games, 2010. Air quality forecasting is also being done with the use of weather monitoring and use of atmospheric chemistry transport forecasting model that provides 24 hour advance forecast of air pollutant levels.
Graph: Snapshot of DPCC real time website depicting PM2.5 data for previous 24 hours for July 03
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