What chemical species are responsible for new particle formation and growth in the Netherlands? A hybrid positive matrix factorization (PMF) analysis using aerosol composition (ACSM) and size (SMPS)
Abstract. Aerosol formation acts as a sink for gas-phase atmospheric species that controls their atmospheric lifetime and environmental effects. To investigate aerosol formation and evolution in the Netherlands, a hybrid positive matrix factorization (PMF) analysis has been conducted using observations from May, June, and September 2021 collected in a rural site of Cabauw in Central Netherlands. The hybrid input matrix consists of the full organic mass spectrum acquired from a time-of-flight aerosol chemical speciation monitor (ToF-ACSM), ACSM species concentrations, and binned particle size distribution concentrations from a scanning mobility particle sizer (SMPS). These hybrid PMF analyses discerned six factors that describe aerosol composition variations: four size-driven factors that are related to new particle formation and growth (F6, F5, F4, and F3), and two bulk factors driven by composition, not size (F2, F1). The smallest-diameter size factor (F6) contains ammonium sulfate and organics, and typically occurs during the daytime. Newly formed particles, represented by F6, are correlated with wind from the southwesterly-westerly, northerly, and easterly sectors that transport sulfur oxides (SOx), ammonia (NH3), and organic precursors to Cabauw. As the particles grow from F6 to F3, nitrate plays an increasing role, and the particle loading diurnal cycle shifts from daytime to a nighttime maximum. The inorganic ion balance and organics composition in the bulk atmosphere affects the chemical composition variation across factors and seasons. Changing ammonium-sulfate-nitrate equilibrium shifts inorganic species among factors, and greater organics availability makes secondary organic aerosol (SOA) more influential in summertime aerosol growth, principally due to volatility differences produced by seasonal variation in photooxidation and temperature.
Farhan R. Nursanto et al.
Status: final response (author comments only)
- RC1: 'Comment on egusphere-2023-554', Anonymous Referee #1, 24 Apr 2023
- RC2: 'Comment on egusphere-2023-554', Anonymous Referee #2, 13 May 2023
Farhan R. Nursanto et al.
Farhan R. Nursanto et al.
Viewed (geographical distribution)
Nursanto et al. utilize three months of observations collected in Cabauw, Netherlands, to investigate what aerosols control new particle formation (NPF). Using an Aerosol Chemical Speciation Monitor (ACSM) and scanning mobility particle sizer (SMPS) with positive matrix factorization (PMF), the authors found four distinct factors associated with the start and growth of new particles and two factors associated with background, large particles. The factors found by the authors were generally similar across the three different months. They generally found sulfate was associated with the beginning of an NPF event and nitrate was observed during the condensational growth. Further, organics were both associated with NPF and condensation. The authors also associate the events with different relative chemical composition of aerosol (organic rich, nitrate rich, sulfate rich, ammonium rich) and back trajectories for where the air masses originated.
Though this article is potentially of interest to the ACP community, and the authors have done a good job in setting up the scientific quesiton and premise, there are many technical aspects and analysis that either needs further discussion and/or evaluation, which are detailed below, prior to publication to ACP.
1) In general, much more details are needed in the measurements. There are many key details that are necessary in evaluating the science that have not been addressed. These include:
1.1) What size particle lens was used? It is not clear if a PM1 or PM2.5 particle lens was used, which is both important in the flow rates for the cyclone and the aerosol diameter cut-offs (further discussed below).
1.2) What was the flow rate throughout the system? Was there an external pull for the ACSM to reduce residence times? What type of tubing was used? Was the inlet heated or not? Any concern about temperature gradients between inlet outside and instrument inside?
1.3) Were the SMPS and ACSM on same or different inlets? If different, how far apart were the two inlets?
1.4) Was a drier used for either or both instruments?
1.5) More information needs to be included about the SMPS, as it is one of the key instruments. This includes type of DMA and CPC, resolution, type of column, software for analysis, type of neutralizer source, etc.
1.6) What are the limits of detection for everything? E.g., were any of the measurements at or below LOD for when trying to investigate NPF?
2) Some further discussion or details also needs to be included for PMF. This includes:
2.1) Why were the total mass concentration for the inorganics instead of their ions used?
2.2) Why was potassium used? It is generally related with surface ionization of the vaporizer. Evidence that the potassium was from aerosols and not surface ionization should be included in the SI.
2.3) Why were the 18 bins selected for the SMPS? Was this due to the software, or did this provide the optimal data for analysis? Would fewer bins be better or worse? Further, some more discussion about the weighting of the SMPS data and errors in the SI would be beneficial.
3) Looking at the PMF organic profiles for all seasons, many of the profiles look very similar and/or like they are split solutions. E.g., in Fig. 3, the mass spectra for organics for solution F6, F4, F2, and F1 look nearly identical, and there are mass spectra for organics that look nearly identical in the SI. Should these solutions be combined? Also, the profiles are generally surprising looking as they do not look like profiles expected for ambient aerosol. Since there can be a potential bias in the CO2+ signal from CV, was m/z 44 (and associated ions) downweighted the similar amount as is typical for SV or downweighted more? Inclusion of the time series of each profile and SMPS data associated with that factor in the SI would also be extremely beneficial here in further evaluating and understanding if each solution is unique and real.
4) One very large concern is what aerosol diameters are being observed with the ACSM. If it is using a PM1 lens, any aerosol below 40 nm is not observed, and aerosol between 40 and 70 to 100 nm is only fractionally observed (e.g., approximate linear growth in the amount of aerosol observed with diameter to 70 - 100 nm). However, if it is PM2.5 lens, the ACSM will only observe 100% of aerosol for diameter > 110 nm. Thus, any aerosol observed for most of the solutions/modes for NPF are very surprising. Since many of the figures show potential contribution of "large" particles (>100 nm diameter) showing small contribution to the factor, how much volume is the small, large particle, contribution adding? E.g., is the volume large enough that that is what is leading to the aerosol being observed by the ACSM?
5) Looking at the progession of the NPF with the ACSM data is very surprising and needs further discussions. Some specific questions that need to be addressed are listed below:
5.1) How does the composition shift entirely from sulfate to organics or sulfate/organics to nitrate between F6 to F5? What happened to the sulfate? Looking at the solutions, it appears that F6 --> F4 and may be F5 --> F3; however, as it is presented and discussed, it appears the NPF event goes from F6 --> F5 --> F4 --> F3.
5.2) Similarly, what happened to f(CO2)? Highly oxyenated material may be necessary for the initiation of NPF; however, it should not completely disappear as compounds with higher volatility, lower f(CO2) condenses onto the aeorosol.
6) Looking at the time series of SMPS number concentration vs time (Fig 5 & SI), it is not clear what has lead to some events being specifically selected as NPF and other times where there is what appears to be rapid particle formation not being selected as an NPF. For example, in Fig. 5, why was the third event selected as it appears it only went to 30-40 nms and stoppped but later times (after 5/30) not selected?
7) Fig 6 and associated figures in SI, it is surprising how the normalized mass spans what appears to be a larger time frame than the NPF event. E.g., Fig. S4 shows that the events are ~ 4 - 6 hours; however, looking at Fig. 6 (and associated figures), it seems that it takes a full 12 hrs to go from F6 --> F3. Clairification in how this figure/results are related to NPF needs to be further detailed.
8) Section S2. Further clarification needs to be added in this section to discuss the thermodynamics vs kinetics that may be controlling NPF and the aerosol composition in general. It is recommended that Weber et al. (2016) and Pye et al. (2020) are reviewed and incoporated in the discussions here, for the following reasons.
8.1) Are the values 0.99 and 0.98 statistically different, considering the overall uncertainties with the ACSM?
8.2) It is nearly impossible to say anything about aerosol acidity in the boundary layer just with charge balance calculated with the ACSM/AMS. E.g., it was not until the ammonium balance dropped below 0.65 could aerosol acidity be directly related to the charge balance measured on the AMS (ACSM) (Schueneman et al., 2021).
8.3) Though NO3 and SO4 would be with other cations, generally, both the cations and anions would be not easily observable due to the higher boiling point and the aerosol being more refractory. It would be recommended to say that both the cations and anions from these salts would be slowly detected and not "not detected." (Line 939 SI).
8.4) Line 945 - 950. This needs to be rephrased as both the association of sulfate with a base is both kinetically and thermodynamically controlled (see Weber et al., 2016 and Pye et al., 2020). Sulfuric acid will first react with a base (either ammonia or an amine) very quickly; then, it will more slowly form the ammonium sulfate or double-amine sulfate. E.g., > 100 ug m^-3 NH3 was estimated to be needed to make pure ammonium sulfate. Instead, it will be a combination of ammonium sulfate and bisulfate in the aerosol phase. Further, a combination of factors (temperature, relative humidity, ammonia, and ammonium) will play in the role to start having ammonium nitrate in the aerosol phase, which is best explained with a thermodynamic model. Even at "low" pH (~2), ammonium nitrate will be present even though the sulfate is not pure ammonium sulfate. Thus, it is not as straightforward that all the ammonia reacts with sulfate to form ammonium sulfate and the remainder then reacts with nitrate.
8.5) The terms "nitrate excess" and "sulfate-rich" also are hard to follow for the reasons discussed in 8.4.
9) What does an "orgnaic-rich" period mean, in that it was related to ammonium? Why was ammonium used to normalize and determine organic rich vs poor? Clarification in what this chemically means should be addressed.
1) Line 333, believe September should be fall instead of summer?
2) line 364, what is quiet NPF?
3) Sect 3.3 Title should be F6, F5, F4, F3 and not F7, F6, F5, F4
4) Line 225. A discussion about what happened to the sulfate and why it is suddenly poor in Sept should be included
Pye et al. The Acidity of Atmospheric Particles and Clouds. Atmos. Chem. Phys. 20, 4809 - 4888. doi:10.5194/acp-20-4809-2020, 2020.
Schueneman et al. Aerosol pH Indicator and Organosulfate Detectability from Aerosol Mass Spectrometry Measurements. Atmos. Meas. Tech. 4, 2237 - 2260. doi:10.5194/amt-14-2237-2021, 2021.
Weber et al. High Aerosol Acidity Despite Declining Atmospheric Sulfate Concentrations Over the Past 15 Years. Nature Geosci. 9, 282-285. doi:10.1038/ngeo2665