the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Aug 2026
Source-dependent organic aerosol hygroscopicity in eastern China: observational evidence for conditional enhancement under biomass-burning influence
Abstract. Aerosol hygroscopicity is a key property governing aerosol water uptake and aerosol–cloud interactions, yet the hygroscopicity of organic aerosol (OA) remains poorly constrained because of its chemical complexity and diverse emission sources. Here, OA hygroscopicity (κOA) was retrieved from comprehensive field observations at a rural site in eastern China using humidified light-scattering measurements and ZSR-based closure analysis. κOA exhibited pronounced temporal variability during winter, ranging from nearly zero to 0.49 with a mean value of 0.11 ± 0.11. Although OA was generally highly oxidized, κOA was only weakly correlated with the O:C ratio, indicating that bulk oxidation state alone cannot explain its variability. Instead, κOA increased systematically with biomass-burning influence. Atmospheric aging substantially enhanced κOA under biomass-burning-dominated conditions, whereas highly oxidized OA under weak biomass-burning influence remained weakly hygroscopic. These findings demonstrate that the hygroscopic response of OA to atmospheric aging is fundamentally source dependent and indicate that source-dependent parameterizations provide a more physically realistic framework than conventional oxidation-based approaches for representing OA hygroscopicity in atmospheric models.
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Status: open (until 17 Sep 2026)
- RC1: 'Comment on egusphere-2026-4178', Anonymous Referee #1, 24 Aug 2026 reply
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RC2: 'Comment on egusphere-2026-4178', Anonymous Referee #2, 25 Aug 2026
reply
This manuscript presents a comprehensive investigation of aerosol hygroscopicity at a rural site in eastern China. By combining humidified light-scattering measurements, particle size distributions, and chemical composition with a Mie-theory-based inversion and ZSR closure approach, the authors provide a useful framework for constraining ambient κOA and investigating its controlling factors. The results provide interesting insights into the source-dependent response of OA hygroscopicity. The manuscript is generally well written, I have a few suggestions before the manuscript can be accepted for publication in ACP.
- Lines 270–274: OA hygroscopicity (κOA) is derived from κinv using the ZSR mixing rule and therefore depends on the retrieved κinv, the assumed hygroscopicity parameters, and the estimated volume fractions of the individual aerosol components. Uncertainties in these quantities may become particularly important when the residual term used to derive κOA is small. Please discuss the sensitivity of κOA to the major uncertainties and assumptions involved in Eq. (11).
- Line 175 – 185: The relative contribution of biomass burning to BC fbb is derived assuming fixed AAE values of 1.0 and 2.0 for fossil-fuel and biomass-burning emissions, respectively. I suggest the authors discuss this uncertainty using fixed AAE.
- Line 313–319: The authors show that κinv systematically decreases with increasing RH. While in Line 239–242, the κinv retrieved over 80–90% RH is used to represent bulk aerosol hygroscopicity and derive κOA. Given the observed RH dependence of κinv, I suggest the authors to assess whether the choice of RH interval affects the derived κOA and its main relationships with source and oxidation indicators.
- Line 218–219 and 234–235: The Mie calculation of B_sp,dry accounts for the angular response of the Aurora 3000 nephelometer. Please clarify whether the angular response of the Aurora 1000 nephelometer was similarly considered when calculating B_sp,wet.
- Line 245: “a simply proxy” should be “a simple proxy”.
- Line 270: “The consistent between...” should be “The consistency between...”.
- Figure 5 caption: “20% percentiles” should be “20-percentile intervals”.
- Figure 3: The legend for O:C and fbb is unclear. Please revise it to improve readability.
Citation: https://doi.org/10.5194/egusphere-2026-4178-RC2
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- 1
This manuscript investigates the temporal variability and controlling factors of organic aerosol hygroscopicity (kOA) at a rural site in eastern China using humidified scattering measurements, aerosol size distributions, chemical composition for a closure study approach. The study addresses an important topic for understanding aerosol–cloud interactions and the representation of organic aerosol hygroscopicity. The manuscript presents an interesting result: bulk oxidation state alone does not adequately explain the observed variability in kOA, while biomass-burning influence is positively associated with kOA and appears to modify the relationship between oxidation state and hygroscopicity. The combination of statistical analysis and the January pollution episode provides useful observational evidence for this interpretation. Overall, I find the study scientifically interesting and suitable for publication after minor revision.
Specific comments:
1. better quantify kOA uncertainty. kOA is derived indirectly from bulk aerosol hygroscopicity using the ZSR framework and assumed hygroscopicities of inorganic components and refractory material. Since kOA is subsequently used as the central variable throughout the paper, it would be useful to provide at least a quantitative estimate of its uncertainty. For example, the observation size ranges of PNSD, ACSM, scattering growth, and AE are different, how this influence your analysis. During the drying process, volatile OA may loss, how much of this could influence the analysis of bulk kOA.
2. kOA is estimated by the k_inv subtracting other known k_components, as in eq.11. While, the direct impact of other components has been considered with this equation, the indirect impacts could also influence the kOA estimation, such as co-condensation. Previous studies have reported high impact of co-condensation on hygroscopic growth associated with chloride and nitrate particles. Please discuss how this may impact the uncertainty of kOA estimate.
3. The manuscript retrieves κ over several RH intervals but uses 80–90% RH as the representative value. Please briefly demonstrate that the main κOA–fbb relationship is not sensitive to the selected RH interval.
4. Fig.3. Around Jan. 22nd, there seems to be a gap of data in vol. frac. and just in vol. frac., and still with successful retrieval of kOA. Could you explain a bit more about this.