Abstract. Water-soluble inorganic ions such as ammonium, nitrate and sulfate are
major components of fine aerosols in the atmosphere and are widely used in
the estimation of aerosol acidity. However, different experimental practices
and instrumentation may lead to uncertainties in ion concentrations. Here,
an intercomparison experiment was conducted in 10 different laboratories
(labs) to investigate the consistency of inorganic ion concentrations and
resultant aerosol acidity estimates using the same set of aerosol filter
samples. The results mostly exhibited good agreement for major ions
Cl−, SO42-, NO3-, NH4+ and K+.
However, F−, Mg2+ and Ca2+ were observed with more variations
across the different labs. The Aerosol Chemical Speciation Monitor (ACSM)
data of nonrefractory SO42-, NO3- and NH4+ generally correlated very well with the filter-analysis-based data in our
study, but the absolute concentrations differ by up to 42 %. Cl− from
the two methods are correlated, but the concentration differ by more than a
factor of 3. The analyses of certified reference materials (CRMs)
generally showed a good detection accuracy (DA) of all ions in all the labs,
the majority of which ranged between 90 % and 110 %. The DA was also
used to correct the ion concentrations to showcase the importance of using
CRMs for calibration check and quality control. Better agreements were found
for Cl−, SO42-, NO3-, NH4+ and K+
across the labs after their concentrations were corrected with DA; the
coefficient of variation (CV) of Cl−, SO42-, NO3-,
NH4+ and K+ decreased by 1.7 %, 3.4 %, 3.4 %, 1.2 % and
2.6 %, respectively, after DA correction. We found that the ratio of anion
to cation equivalent concentrations (AE / CE) and ion balance (anions–cations) are not good indicators for aerosol acidity estimates, as the
results in different labs did not agree well with each other. In situ
aerosol pH calculated from the ISORROPIA II thermodynamic equilibrium model
with measured ion and ammonia concentrations showed a similar trend and good
agreement across the 10 labs. Our results indicate that although there are
important uncertainties in aerosol ion concentration measurements, the
estimated aerosol pH from the ISORROPIA II model is more consistent.
