Measurements of light (C2-C5) non-methane hydrocarbons (NMHCs) from Ahmedabad are being made by PRL scientists to study their variability and relations with ozone and its other precursors using gas chromatography (Sahu and Lal 2006).

Later, this technique has been used to measure NMHCs from many different locations (Mt. Abu, Hissar, Kanpur, Nainital, Kolkata and from marine regions) by collecting air samples. A proton transfer reaction mass spectrometer (PTR-MS) is being employed for measurements of various ozone precursors such as NMHCs and VOCs including benzenoids and acetonitrile (a biomass burning tracer) at IISER Mohali. These observations, supported with back-air trajectories, showed that massive amounts of carcinogenic benzenoids are released from post-harvest paddy residue burning over northwest India, which aggravates urban smog at downwind urban centers. Subsequently a 300 % increase in ambient levels of VOCs due to wheat residue burning in northwest India was also documented (Sinha et al. 2014). Measurements of several key VOCs such as methanol, acetone, acetaldehyde, and isoprene are also made for the first time in India. The daytime ozone increase anti-correlates with acetaldehyde (Fig. 6), indicating that oxidation of acetaldehyde which occurs very rapidly produces acetyl peroxy radicals that are convert nitrogen monoxide to nitrogen dioxide, which on photolysis produces ozone rapidly. Similar measurements are now being made at PRL, Ahmedabad, and IITM, Pune, also.

A comparison of contribution of different NMHCs at Nainital, Haldwani/ Pantnagar, Kanpur, and Hissar shows nearly similar composition between Kanpur and Hissar, but different than those at Nainital or Haldwani/Pantnagar (Fig. 7) (Sarangi et al. 2016). The overall differences in the composition of NMHCs are suggested to be a manifestation of the relatively aged air sampled over Nainital as compared to the IGP sites having fresh emissions. Observations at Agra showed benzene and toluene to be most abundant, among BTX (Benzene, Toluene and Xylene), with an average value of 49 ± 1.8 and 36 ± 2.6 p.g m-3. Concentrations of p-xylene and o-, m-xylene are found to be 6.2 ± 1.4 and 16.1 ± 2.6 ^g m 3, respectively. Although the concentrations of xylenes are observed to be minimum among the BTX, p-xylene is found to be the most dominant contributor to ozone formation. Benzene being the most abundant and hazardous species among BTX showed minimum potential to ozone formation. Toluene was found to have second largest contribution to ozone formation (Singla et al. 2012). Additionally, O3/HNO3

Average diurnal variations in ozone and acetaldehyde at IISER, Mohali, in May, 2012 (Sinha—Personal communication)

Fig. 6 Average diurnal variations in ozone and acetaldehyde at IISER, Mohali, in May, 2012 (Sinha—Personal communication)

Fig. 7 Comparison of composition of different NMHCs at Kanpur, Hissar, Nainital, and Haldwani/Pantnagar

ratios range from 10 to 56, with an average value of 35.3 ± 2.6 (summer and winter) at Agra, which is mostly NOx-sensitive region and have also been confirmed at Pantnagar.

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