Preprints
https://doi.org/10.5194/amt-2021-191
https://doi.org/10.5194/amt-2021-191

  28 Oct 2021

28 Oct 2021

Review status: this preprint is currently under review for the journal AMT.

Inter-comparison of online and offline methods for measuring ambient heavy and trace elements and water-soluble inorganic ions (NO3, SO42−, NH4+ and Cl) in PM2.5 over a heavily polluted megacity, Delhi

Himadri Sekhar Bhowmik1, Ashutosh Shukla1, Vipul Lalchandani1, Jay Dave3, Neeraj Rastogi3, Mayank Kumar4, Vikram Singh5, and Sachchida Nand Tripathi2 Himadri Sekhar Bhowmik et al.
  • 1Department of Civil Engineering, Indian Institute of Technology Kanpur, Kanpur, India
  • 2Department of Civil Engineering and Centre for Environmental Science and Engineering, Indian Institute of Technology Kanpur, Kanpur, India
  • 3Geosciences Division, Physical Research Laboratory, Ahmedabad, India
  • 4Department of Mechanical Engineering, Indian Institute of Technology Delhi, New Delhi, India
  • 5Department of Chemical Engineering, Indian Institute of Technology Delhi, New Delhi, India

Abstract. Characterizing the chemical composition of ambient particulate matter (PM) provides valuable information on the concentration of secondary species, toxic metals and assists in the validation of abatement techniques. The chemical components of PM can be measured by sampling on filters and analysing them in the laboratory or using real-time measurements of the species. It is important for the accuracy of the PM monitoring networks that measurements from the offline and online methods are comparable and biases are known. The concentrations of water-soluble inorganic ions (NO3, SO42−, NH4+ and Cl) in PM2.5 measured from the 24 hrs filter samples using ion chromatography (IC) were compared with the online measurements of inorganics from aerosol mass spectrometer (AMS) with a frequency of 2 mins. Also, the concentrations of heavy and trace elements determined from the 24 hrs filter samples using inductively coupled plasma mass spectroscopy (ICP-MS) were compared with the online measurements of half-hourly heavy and trace metal’s concentrations from Xact 625i ambient metal mass monitor. The comparison was performed over two seasons (summer and winter) characterized by their different metrological conditions at IITD and during winter at IITMD, two sites located in Delhi, NCR, India, one of the heavily polluted urban areas in the world. Collocated deployments of the instruments helped to quantify the differences between online and offline measurements and evaluate the possible reasons for positive and negative biases. The slopes for SO42− and NH4+ were closer to 1:1 line during winter and decreased during summer at both sites. The higher concentrations on the filters were due to the formation of particulate (NH4)2SO4. Filter-based NO3 measurements were lower than online NO3 during summer at IITD and winter at IITMD due to the volatile nature of NO3 from the filter substrate. Offline measured Cl was consistently higher than AMS derived Cl during summer and winter at both sites. Based on their comparability characteristics, elements were grouped under 3 categories. The online element data were highly correlated (R2 > 0.8) with the offline measurements for Al, K, Ca, Ti, Zn, Mn, Fe, Ba, and Pb during summer at IITD and winter at both the sites. The higher correlation coefficient demonstrated the precision of the measurements of these elements by both Xact 625i and ICP-MS. Some of these elements showed higher Xact 625i elemental concentrations than ICP-MS measurements by an average of 10–40 % depending on the season and site. The reasons for the differences in the concentration of the elements could be the distance between two inlets for the two methods, line interference between two elements in Xact measurements, sampling strategy, variable concentrations of elements in blank filters and digestion protocol for ICP measurements.

Himadri Sekhar Bhowmik et al.

Status: open (until 13 Jan 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on amt-2021-191', Anonymous Referee #1, 19 Nov 2021 reply

Himadri Sekhar Bhowmik et al.

Himadri Sekhar Bhowmik et al.

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Short summary
An inter-comparison of laboratory and in-situ measurements of ambient inorganic ions and heavy and trace metals were studied at two heavily polluted sites in Delhi-NCR, India, during warmer and cooler seasons to quantify and evaluate the possible reasons for positive and negative biases. We elaborately discussed the reasons for differences in measurements. The findings highlight the measurement methods' accuracy and implement the particular type of measurements as needed.