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
Authors: Bhowmik Himadri Sekhar , Shukla Ashutosh , Lalchandani Vipul , Dave Jay , Rastogi Neeraj , Kumar Mayank , Singh Vikram , Tripathi Sachchida Nand 
Published: May 2022

Abstract. Characterizing the chemical composition of ambient particulate matter (PM)
provides valuable information on the concentration of secondary species and
toxic metals and assists in the validation of abatement techniques. The
chemical components of PM can be measured by sampling on filters and
analyzing 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 24 h filter samples using ion chromatography (IC) were
compared with the online measurements of inorganics from an aerosol mass
spectrometer (AMS) with a frequency of 2 min. Also, the concentrations of
heavy and trace elements determined from 24 h filter samples using
inductively coupled plasma mass spectroscopy (ICP-MS) were compared with the
online measurements of half-hourly heavy and trace metal concentrations
from an Xact 625i ambient metal mass monitor. The comparison was performed over
two seasons (summer and winter) and at two sites (Indian Institute of
Technology Delhi (IITD) and Indian Institute of Tropical Meteorology, Delhi (IITMD)) which
are located in the Delhi National Capital Region (NCR), India, one of the most 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 the 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 into three 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 the 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,
the sampling strategy, variable concentrations of elements in blank filters, and the
digestion protocol for ICP-MS measurements.