pH is a fundamental aerosol property that affects ambient particle
concentration and composition, linking pH to all aerosol environmental
impacts. Here, PM
Ambient aerosol particles affect human health and climate (Lim et al., 2012;
IPCC, 2013), and have many other environmental effects. Particle pH is linked
to all of these by altering the fundamental aerosol properties of particle
mass and chemical composition. For example, some important pathways leading
to secondary organic aerosol (SOA) formation from biogenic volatile organic
compounds (VOCs), such as isoprene and
Particle pH is linked to adverse health impacts, both directly and indirectly. Synergistic adverse health effects have been observed between ozone and acidic aerosols (Last, 1991; Enami et al., 2008) and epidemiological studies have reported adverse health outcomes associated with strong aerosol acidity (Koutrakis et al., 1988; Thurston et al., 1994; Dockery et al., 1996; Raizenne et al., 1996; Gwynn et al., 2000; Lelieveld et al., 2015). Low pH increases the solubility of transition metals, such as iron and copper (Meskhidze et al., 2003; Oakes et al., 2012; Longo et al., 2016; Fang et al., 2017), which have been linked to aerosol toxicity through aerosol oxidative effects (Ghio et al., 2012; Verma et al., 2014; Fang et al., 2015, 2017). Metal mobility also affects nutrient distributions with important impacts on photosynthesis productivity (Duce and Tindale, 1991; Meskhidze et al., 2003; Nenes et al., 2011; Ito and Xu, 2014; Myriokefalitakis et al., 2015; Li et al., 2017), carbon sequestration and ocean oxygen levels (Ito et al., 2016).
Due to limitations with direct particle pH measurement techniques, fine
particle pH has often been indirectly inferred from aerosol composition based
on ion balances or cation-anion molar balances, e.g., ammonium to sulfate
(NH
This work adds to our investigation of particle pH in differing locations and
under different emission characteristics. We have reported that ground-level
pH in the southeastern US is 0.9
Aerosol and gas measurements were conducted on the California Institute of
Technology campus in Pasadena, California (34.140582
PILS-IC: PM AMS: PM PALMS: single aerosol composition and size for diameters
0.15–3
QC-TILDAS: gas-phase NH NI-PT-CIMS: gas-phase HNO
pH is defined as the negative logarithm of the hydronium ion
(H
In previous studies, the effect of
ISORROPIA-II was run assuming particles were “metastable” with no solid
precipitates (H
The model was also run in “forward” mode, which calculates the gas–particle
equilibrium partitioning concentrations based on the input of total
concentration of a species (i.e., gas
The predicted gas- or particle-phase semi-volatile compounds can be compared
to measurements for validating the thermodynamic calculations. Possible
partitioning pairs for ISORROPIA-II are HNO
Using gas–particle phase partitioning to constrain particle pH can be
complicated by the presence of multiple phases within the particle, which may
distribute inorganic species amongst multiple phases, each with their own
water activity and hence inorganic concentration. Lab studies show that
liquid–liquid phase separations are always observed at O : C (organic
aerosol atomic O to C ratio)
In running ISORROPIA-II to predict pH and semi-volatile species partitioning,
it is also assumed that the particles are internally mixed, that pH does not
vary with particle size (i.e., bulk properties represent the overall aerosol
pH), and that the ambient aerosols and gases are in thermodynamic
equilibrium. For the WINTER study, which included measurements over coastal
and marine areas, we found that PM
Time series for various measured parameters during CalNex are shown in
Fig. 1. CalNex
CalNex campaign time series of meteorological conditions
(
PM
Measurements of semi-volatile particle (NO
PM
Number fraction of sea salt particles with observable
nitrate signals and mass fraction of sea salt particles to total mass in two
size ranges, 0.15–1 and 1–2.5
We assess predicted pH from the thermodynamic model by comparing predicted
and measured gas–particle partitioning of NH
Comparisons of predicted and measured HNO
Comparison of predicted PM
Predicting the bulk pH of PM
Intercomparisons of predicted and measured gas–particle
phase partitioning for PM
To examine the effects of sea salt components on the thermodynamic
predictions, we compared the observed to measured partitioning of PM
An analytical calculation of HNO
Comparison of measured
PM
Diurnal profiles of predicted pH, LWC, and measured
Diurnal profiles for the last week of CalNex of predicted
pH and LWC, and measured
PM
The bias between ISORROPIA-predicted and observed nitrate partitioning may be
a result of several causes. Since the
The sampling systems for HNO
Another possible cause of the
Comparisons between different studies for particle pH, major inorganic ions and gases and meteorological conditions. All pH values are from ISORROPIA-II run in forward mode. The campaign acronyms other than CalNex stand for Southern Oxidant and Aerosol Study (SOAS), Southeastern Nexus of Air Quality and Climate (SENEX), and Wintertime Investigation of Transport, Emissions, and Reactivity (WINTER).
A comparison of pH and related statistics in five field studies is summarized
in Table 2. The campaigns are CalNex, SOAS, SENEX, and WINTER, all conducted
in the continental US. Also included are results from a study in the eastern
Mediterranean (the campaign acronyms are given in Table 2). The SOAS
(ground-based) and SENEX (aircraft-based) studies provide an interesting
contrast with CalNex, that is, between the southeastern and southwestern US
in summertime. WINTER aircraft data add the dimension of seasonal variation
(summer versus winter). The eastern Mediterranean data provide a remote
European (Crete) and urban (Athens) perspective, and a case where air masses
were known to be impacted by biomass burning (BB). All pH values in Table 2
were calculated by ISORROPIA-II in forward metastable mode, but only the US
data (SOAS, WINTER, CalNex) used gas–particle phase partitioning to
constrain and verify the pH prediction for all the data analyzed. Lack of
NH
Comparing LA (CalNex) to the other summertime measurements in the eastern US
(SOAS, SENEX), Table 2 shows that the LA ground site had uniquely higher
NO
To better understand the relationship between NO
Analytically calculated S curves of
Consider the nitrate partitioning case,
S curves have significant utility for understanding how
pH of PM
PM
The study mean (
Single-particle analysis showed that 85 % by number of sea salt particles
in the 1 to 2.5
The CalNex data provide unique contrast to pH reported in other regions and
demonstrates the complex interactions between pH and emissions. They also
support the general application of
SO
The data
were collected during the 2010 CalNex ground campaign in LA and open to the
public at the NOAA website,
The authors declare that they have no conflict of interest.
The Georgia Tech researchers were funded through National Science Foundation (NSF) grants AGS-0931492 and AGS-0802237. This work was also supported by the NSF under grants AGS-1242258 and AGS-1360730, which supported our participation in the SOAS and WINTER campaigns. We wish to thank Aikaterini Bougiatioti for sharing the eastern Mediterranean data included in Table 2 and Raluca Ellis and Jennifer Murphy for use of the gas-phase ammonia data. P. L. Hayes and J. L. Jimenez were partially supported by NSF grant AGS-1360834. We thank Amber Ortega for providing the residence time in the AMS inlet. The NOAA work was supported by the NOAA Air Quality and Climate Research programs. Edited by: A. Kiendler-Scharr Reviewed by: two anonymous referees