the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Aug 2026
Fine Particle Liquid Water and Acidity over North American Cities during the Summer of 2023
Abstract. Fine aerosol liquid water content (ALWC) and acidity (pH) are co-determined, pH primarily reflects the ratio of hydrogen-ion concentrations in air (H+air) to ALWC. Inorganic ions dominate H+air and often ALWC, whereas organic aerosol (OA) mainly adds water. Added OA water, however, shifts gas–particle partitioning of semi-volatile species, altering H+air itself, and thus pH and the aqueous-phase processes it governs. We characterize PM1 ALWC and pH over four North American cities using airborne AEROMMA measurements (June–August 2023), including periods influenced by aged wildfire smoke with high OA but little effect on inorganic species. ALWC and pH were predicted with ISORROPIA-Lite, which includes OA water, and evaluated against measured partitioning of NH3–NH4+ and HNO3–NO3-. Predicted ammonia partitioning agreed with observations (R2 > 0.75, within ~±10 %), whereas nitrate was systematically over-predicted by ~27 %. Outside smoke, inorganic ions dominated ALWC despite being a minor mass fraction; within smoke, organic water dominated (45–65 %). Across all cities, pH remained low and varied little (1.5–2.5, 10th–90th percentile). Particle-phase fractions were 0.17–0.56 for NH4+ but only 0.1–0.22 for NO3-, giving nitrate less redistribution and pH-buffering capacity than NH4+. Including OA water raised pH during smoke by at most 0.62 units and improved HNO3–NO3- agreement. Summertime PM1 acidity thus remains persistently low and thermodynamically stabilized across diverse composition regimes, with wide-ranging implications for regulatory, environmental, and human-health impacts.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
(12581 KB) - Metadata XML
-
Supplement
(5535 KB) - BibTeX
- EndNote
Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-4960', Anonymous Referee #1, 24 Sep 2026
-
RC2: 'Comment on egusphere-2026-4960', Anonymous Referee #2, 10 Oct 2026
The authors used AEROMMA 2023 airborne measurements and ISORROPIA-Lite, including organic-aerosol-associated water, to characterize PM1 aerosol liquid water content (ALWC) and pH across several North American urban and rural regions under low/no-smoke and aged-smoke conditions. The model is evaluated against measured NH3/NH4+ and HNO3/NO3⁻ partitioning. The main conclusions are that summertime PM₁ pH is persistently low and near-invariant (≈1.5–2.5), that nitrate partitioning is systematically overpredicted by ~30%, that OA water dominates ALWC under smoke but only modestly raises pH, and that the East-West nitrate gradient is mass-driven rather than acidity-driven.
The topic is important, the dataset is unique, and the coupled inorganic–organic thermodynamic analysis is valuable. The manuscript is generally well organized and the supplement is extensive. I recommend acceptance after the authors have addressed the following comments:
Major comments
1. AMS mass/density correction
The comparison of total PM₁ mass (AMS + rBC) to UHSAS volume yields a slope of 2.1 g cm-3, implying AMS mass may be ~50% high relative to a nominal density of 1.4 g cm-3. The authors choose not to correct the AMS data because nitrate partitioning agreement is better with uncorrected inputs. However, I find the reasoning a little circular: using uncorrected data because it improves model-measurement agreement could compensate for other biases. Also, the UHSAS volume uncertainty is large, so the density estimate is not definitive. That uncertainty needs to be propagated. I think independent comparisons, e.g., AMS ions versus PILS ions (if available), would help. At minimum, present the key ALWC and pH results with density-corrected AMS inputs in the supplement and discuss whether the main conclusions change.
2. OA water parameterization and internal-mixing assumption
The model uses constant κOA=0.1 and ρOA=1.3 g cm-3. Table S4 shows that values derived from AMS O:C/H:C/f(CO2) are consistently higher (e.g., κOA up to ~0.15). Under smoke, OA contributes 45-65% of ALWC and raises pH by up to 0.62 units. If κOA is higher, the OA water contribution and pH increase could be larger. I feel that (if possible) the authors should run sensitivity tests using composition-derived κOA and ρOA. Show how much ΔpH changes under smoke.
3. Uncertainty in pH invariance and East–West pH difference
The paper claims pH is near-invariant and that the East-West pH difference is only ~0.2 units. However, no systematic uncertainty estimate for mean pH is provided. Given the nitrate bias, RH uncertainty, OA parameterization, and activity-coefficient issue, the 0.2-unit difference may not be significant. I think the authors should provide uncertainty bounds on mean pH for each region/condition and a statistical test or clear statement about whether the East–West difference is robust. This is important to support their claim that the historical nitrate gradient is not an acidity gradient.
Minor comments
4. Abstract vs. text discrepancy: abstract says nitrate overpredicted by ~27%; p. 21, Sect. 3.2.1 says ~30%.
5. Fig. 5 caption: Should define “unique event periods”
Citation: https://doi.org/10.5194/egusphere-2026-4960-RC2
Viewed
| HTML | XML | Total | Supplement | BibTeX | EndNote | |
|---|---|---|---|---|---|---|
| 253 | 185 | 63 | 501 | 96 | 62 | 66 |
- HTML: 253
- PDF: 185
- XML: 63
- Total: 501
- Supplement: 96
- BibTeX: 62
- EndNote: 66
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
Mohan et al. presents observations of airborne particulate matter composition and its role towards aerosol liquid water (ALW) and aerosol acidity from a recent airborne campaign across the United States and Canada. Mohan et al. found that even though there were differences in composition and ALW, the aerosol acidity, or pH, remained relatively constant across all the urban areas across the sampling regime. They determined that that combination of temperature and relative humidity, impacting the partitioning of of ammonia and nitric acid to ammonium and nitrate. The paper is extremely well written and the findings are important for improving our understanding emissions and controls on aerosol acidity, ALW, and particulate matter mass.
After the authors address the following comments, the paper will be ready for ACP:
1) Please reference the author's guidelines for ACP (https://www.atmospheric-chemistry-and-physics.net/policies/guidelines_for_authors.html). The conclusion currently does not adequately address the guidelines.
2) Methods. The following aspects need to be addressed:
a) Is ram heating assumed for the aerosol to have it drop below 40% RH for collection efficiency?
b) What calibrations were conducted for the AMS and how frequently? If assumed RIEs are used, especially for ammonium and sulfate, how much uncertainty does that introduce for the calculations?
c) What RIE was assumed for organics?
d) What flow rates and residence times for all instruments? How were they time sync? Was 1 s or 1 min data used? If 1 min, how was the data averaged?
3) Throughout the paper, it is assumed that the sulfate, nitrate, and ammonium are the inorganic ions. Prior work has demonstrated that from forest fires, a large fraction of sulfate is organic (https://doi.org/10.5194/acp-22-15603-2022) and at low nitrate mass concentration, a larger fraction of it is organic instead of inorganic (https://doi.org/10.5194/amt-15-459-2022). Furthermore, the authors discuss the inorganic aerosol as ammonium sulfate and ammonium nitrate, assuming(?) full neutralization of sulfate at the pHs calculated. Both are surprising. Have the authors looked at the NO-to-NO2 ratio to determine if there was any organic nitrate? Have the authors looked at high resolution data for organic sulfate ions or the potential ratios discussed in previous work (10.1021/acs.est.9b00884)? Did ISORROPIA really solve the sulfate to be pure ammonium sulfate instead a mixture of ammonium sulfate and ammonium bisulfate, which is what would be more expected at the lower pH?
4) Currently, the authors are assuming internally mixed aerosol and minimal hysteresis impacts. As the wildfire smoke is being transported to the urban areas, is there indication that the wildfire smoke is fully mixing with the urban pollution or if the urban and wildfire aerosol are different sizes? How does internal vs external mixing impact the assumptions? Further, the authors go into detail about how sulfate is more sensitive to RH than nitrate until high RH, but that is looking at the instantaneous RH the particulate matter was observed at at the time. It is understood that metastable assumption includes some aspects of hysteresis, e.g., RH does not have to be above deliquescence point for sulfate and/or nitrate to start uptaking water and growing in size. However, does the aerosol experiencing a difference / drying out impact this interpretation of compositional relationship to RH / ALW?
Minor
Fig 9. The binning of the data and having lines drawn between the binned points is confusing for interpretation. How is the data binned? Does a line have to be drawn across the data points?