This document discusses reducing the pH of indoor aerosols to decrease virus transmission. It first develops a method to estimate the characteristic pH of indoor aerosols based on their salt composition. Indoor aerosols containing viruses typically have a pH above 4 due to salts like ammonium nitrate and chloride. The document then evaluates using acids like nitric acid, oxalic acid, and others to further lower the pH of indoor aerosols and investigated their solubility and ability to partition into the aerosol phase. Reducing the pH of indoor aerosols through targeted use of acids could help decrease virus infectivity and transmission.

1. Ποιότητα εσωτερικού χώρου: η επίδραση
της οξύτητας σωματιδίων σε κλειστούς
χώρους στην αερογενή μετάδοση
λοιμώξεων από το ιό της γρίπης και του
COVID-19
ONOMA:ΦΩΤΕΙΝΗ ΜΑΚΡΙΑΔΗ
ΕΠΙΒΛΕΠΩΝ:ΑΘΑΝΑΣΙΟΣ ΝΕΝΕΣ

2.  Introduction (Importance of aerosols and role in virus transmission).
 Goals of thesis
 Viruses and acidity
 Acidity of outdoor aerosols
 Ammonia and acidity
 Characteristic pH-Method
 Acidification of indoor aerosol-Migitation Method
 Conclusions
 Future Work
ΠΕΡΙΕΧΟΜΕΝΑ
2

3. INTRO - AEROSOLS
3
Pye, H. O. T., Nenes et al. (2020)

4. INTRO -AEROSOLS & VIRUS TRANSMISSION
4
Mira L. Pohlker et al.(2021)

5. INTRO – ENVIRONMENTAL FACTORS
5
Ahlawat, Ajit et al.(2022)

6. GOALS
6
1. Understand the acidity of aerosols in indoor environments
2. Understand the correlation of viruses and acidity
3. Estimation of aerosol pH indoors
4. Understand how ammonia affects acidity
5. See how can we reduce pH indoors to deactivate viruses

7. How RH and pH affects the viruses?
Viruses & Acidity
7
Ahlawat, Ajit et al.(2022); Nenes et al. (online presentations

8. Viruses & Acidity
8
Luo et al.(2022)

9. Acidity of outdoor aerosols
9
H. O. T. Pye et al.

10. Why is it difficult to measure aerosol pH?
1. Small sizes of the aerosol particles
2. Mixture of organic-inorganic compounds
3. Multiple equilibria between chemical species
Measuring Aerosol pH
Thermodynamic
models
liquid-phase activity coefficients
Equilibrium gas- particle
partitioning dynamic mass transfer of semi
volatile species
ALWC & pH
EPA
10

11. Indoor Sources
 Blood through the liver and eventually in the
urea
 Diffusion through skin,sweat and breath
 Pets
 Cleaning products
Ammonia and Acidity of Indoor Aerosol
11
 Colorless gas with strong odor
 Critical for the atmosphere
 Reacts with organic and inorganic acids found in
aerosol
 Indoor concentrations 10 to 70 ppb
 Outdoor concentrations 50 ppt to 5 ppb .
 Respiratory irritant and toxic at high levels (20 ppm
or higher) respiratory irritant and
toxic coughing, dizziness, nausea,
cardiovascular problems, asthma
NH3 increases pH
IAV
COVID

12. Industrial Environments Non-industrial Environments
 Air purifiers with activate carbon filters
 Zeolite-base products
 Ammonia free cleaning products
 Room ventilation
Ammonia removal methods
12

13. 1. Estimate the pH of expiratory aerosol in indoor environments.
2. Find a “characteristic pH” of each salt in the aerosol
Besides ammonia removal,what else can we do to reduce pH?
13
Beyond ammonia reduction
the pH of each salt in the aerosol when there aren’t any other salts present.
This may work for aerosol because the liquid water content in aerosols is
determined by the salts present and the relative humidity (chemical equilibrium).
Develop the concept using a large dataset from outdoor aerosol and apply to
indoor expiratory aerosol which contains sodium and ammonium salts with
chloride and nitrate.
3.See if the indoor aerosol can take low values of pH on their own
4. If not, try to develop mitigation strategies by deliberate acidification
(using semi-volatile acids).

14. 14
CHARACTERISTIC pH: METHOD
Datasets where total Na, sulphate, ammonia/ammonium, nitrate, chloride, calcium, potassium, magnesium),
temperature and relative humidity are known from:
• Cabauw, Netherlands, an environment that is very rich in ammonia and nitrates (hence less acidic
environment),
• a month-long intensive sampling in California (during the CalNex campaign),
• highly acidic summertime Southeastern US aerosol (during the SOAS campaign) and
• highly variable acidity aerosol in the northeast US (during the aircraft-based Winter campaign).
• 368 data points are available for RH = 0.4-0.45, 500 for RH 0.5-0.55, 588 for RH 0.6-0.65 ,444 for RH 0.7-
0.75 and 776 for RH 0.8-0.85.
Use ISORROPIA model (which our group has developed;
Kakavas et al., 2022)
1. pH calculations
2. Calculation of LWC associated with salts
(From ISORROPIA)
3. Multilinear regression
𝑝𝐻 = 𝛽1𝑊1 + 𝛽2𝑊2 + ⋯ +𝛽𝑖 𝑊𝑖
𝑤𝑖 → 1
𝛽𝑖 →characteristic pH
(i.e., the limiting pH when
water contributions from
other salts goes to zero)

15. CHARACTERISTIC pH: RESULTS
-2
-1
0
1
2
3
4
5
-2 -1 0 1 2 3 4 5
pH
regression
pH isolite
RH 0.6-0.65
Characteristic pH Standard Error t Stat P-value Lower 95% Upper 95%
W(NH4)2SO4 0.95 0.07 13.58 0.00 0.81 1.08
WNH4NO3 4.28 0.10 44.26 0.00 4.09 4.47
WLC -0.98 0.12 -8.21 0.00 -1.22 -0.75
W2H-SO4 -1.10 0.32 -3.47 0.00 -1.72 -0.48
WNa2SO4 2.91 0.38 7.61 0.00 2.15 3.68
Multiple R 0.95
R Square 0.91
Adjusted R Square 0.90
Standard Error 0.74
Observations 588
𝑝𝐻 = 0.95 ∙ 𝑊 𝑁𝐻4 2𝑆𝑂4
+ 4.28𝑊𝑁𝐻4𝑁𝑂3
− 1.10𝑊 2𝐻𝑆𝑂4
− 0.98𝑊𝐿𝐶 + 2.91𝑊𝑁𝑎2𝑆𝑂4
𝑡 − 𝑠𝑡𝑎𝑡 =
𝑒𝑠𝑡𝑖𝑚𝑎𝑡𝑒𝑑 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡
𝑠𝑡𝑎𝑛𝑑𝑎𝑟𝑑 𝑒𝑟𝑟𝑜𝑟 𝑜𝑓 𝑡ℎ𝑒 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡
15
OK

16. CHARACTERISTIC pH: RESULTS
-3
-2
-1
0
1
2
3
4
5
6
7
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Characteristic
pH
RH
(NH4)2SO4 NH4NO3 2H-SO4 LC Na2so4 NH4Cl
(NH4)2SO4 NH4NO3 2H-SO4 LC (NH4)3H(SO4)2 Na2SO4 NH4Cl
0.5 to 1 4 to 6 -1.5 to -0.7 -2 to -0.5 0.9 to 2 4 to 6
16
Indoor expiratory aerosols have:
 NH4NO3
 NH4Cl
 NaCl
High pH > 4
Virus stability is high.

17. FURTHER REDUCING pH INDOORS
17
Addition of a monoprotic acid that can set the pH to a low value
What can we do to reduce further the pH of indoor aerosols?
The source of the aerosol
acidity
Ka from ≈ 10 -5 M to ≈ 10 -2 M
& Ka ≈10 M
pH 1,2,3
LWC
0.1,1,10 μg m -3

18. FURTHER REDUCING pH INDOORS: ASSESSMENT
HA(gas) ↔ HA(liquid) (1)
[HA(liq)] = HHA ∙
pA
γHA
(2)
HA(liq) ↔ A−
(liq) + H+
(liq) (3)
Ka =
γΗ+ ∙ [H+
](liq) ∙ γΑ− ∙ A−
(liq)
γΗΑ ∙ HA(liq)
(4)
18
HA(liq) A−
(liq) H+
(liq)
Concentrations in
equilibrium(M)
C-x x x
equilibrium of an acid HA between the gas and aqueous phases
Mol l-1
• HHA:Henry’s constant M atm-1
• pΑ:partial pressure of the gas above the liquid
• γΗΑ :activity coeff.
Fixed pH value
• Ka :Dissociation constant M
•
γΗΑ
γΗ+γΑ−
=0.28
Mean value of activity coeff. for nitric acid predicted from ISORROPIA ,with the datasets used for regression

19. FURTHER REDUCING pH INDOORS: ASSESSMENT
19
CHA aerosol = CHA l + CA− l =
LWC
factor
HA liq + [A−
] liq (5)
ε =
CHA aerosol
CHA aerosol +CHA g
=
LWC factor ∙ HHA ∙ [H+
] liq +
HHA ∙ pA ∙ γΗΑKa
γH+γΑ−
LWC factor ∙ HHA ∙ [H+] liq +
HHA ∙ pA ∙ γΗΑKa
γH+γΑ−
+
pA
RT
(6)
HHA =
ε
(1 − ε)
∙
1
factor ∙ LWC ∙ R ∙ T ∙ H+
liq +
KaγΗΑ
γH+γΑ−
(7)
Mol m-3 air
• LWC :Liquid water
content, μg m-3 air
1
𝜌𝐻20
and ρΗ20 = 109 μg l-1
Partitioning
factor
Henry’s constant
M atm-1
Ctotal =
1
ε
∙
LWC
factor
x2
γΗ+γΑ−
KaγΗΑ
+ x 8
Plots of Henry’s constant and Ctotal

20. How do we know that our simplified equation is a good approximation?
FURTHER REDUCING pH INDOORS: ASSESSMENT
20
ε from ISORROPIA with the datasets vs ε from simplified equation for nitric acid
ε =
CHA aerosol
CHA aerosol +CHA g
=
LWC factor ∙ HHA ∙ [H+
] liq +
HHA ∙ pA ∙ γΗΑKa
γH+γΑ−
LWC factor ∙ HHA ∙ [H+] liq +
HHA ∙ pA ∙ γΗΑKa
γH+γΑ−
+
pA
RT
(6)

21. Nitric acid
 HA= 2.1x105 Μ atm-1
 Ka= 12 Μ at T=298K
RESULTS: Can HNO3 be used?
LWC(μg m-3
) pH ε Ctotal (mol m-3
) Ctotal (μg m-3
)
0.1 3 2% 4.16E-12 2.58E-02
4 20% 5.06E-14 3.14E-06
1 2 2% 4.16E-10 2.58E-02
3 20% 5.06E-12 3.14E-04
4 71% 1.41E-13 8.72E-06
10 1 2% 4.16E-08 2.58E+00
2 20% 5.06E-10 3.14E-02
3 71% 1.406E-11 8.72E-04
4 96% 1.00E-12 6.45E-05
21
Threshold: 5 mg m-3

22. Formic acid
 HA=880 M atm-1
 Kα= 1.8x10 -4 M
 For pH 1,2,3 and liquid water content (0.1,1,10
μg m-3) the partitioning factor is in the range of
10-8 ->exists in the gas phase
 Species with a Henry’s law coefficient lower
than ∼1000 M atm-1, partition strongly toward
the gas phase and are considered relatively
insoluble for atmospheric applications
RESULTS
Ka ≈10 – 4 M
22

23. Acetic acid
 HA= 400 M atm-1
 Ka= 1.75x10-5 M
 for pH 1,2,3 and liquid water content 0.1,1,10
μg m-3, the partitioning factor is in the range
of 10-9
 Henry’s law constants smaller than 400 M
atm, less than 1% of their mass is dissolved in
the aqueous phase of the aerosol28.
RESULTS
Ka ≈10 -5 M
23

24. Oxalic acid
 HA= 6.11x108 Μ atm-1
 Ka= 5.62x10-2 Μ at T=298K
OXALIC ACID: Can it effectively reduce pH?
LWC(μg m-3
) pH ε Ctotal (mol m-3
) Ctotal (μg m-3
)
0.1 2 3% 3.038E-11 0.00191
3 25% 3.98E-13 2.47E-05
1 2 26% 4.02E-11 2.50E-03
3 78% 1.30E-12 8.01E-05
10 2 78% 1.34E-10 8.32E-03
3 98% 1.03E-11 6.41E-04
24
Threshold: 1 mg m-3

25. Indoor viruses are sensitive to pH. So actions that reduce aerosol pH is
certainly important for virus infectivity.
To estimate the pH levels in indoor aerosols, we analyze a large dataset of
aerosol compositions from locations around the world and come up with a
parameterization of aerosol pH – “characteristic pH” that estimates the acidity
associated with specific salts that are formed in the aerosol.
The characteristic pH approach indicates that aerosols that are rich in
ammonium nitrate and chloride salts have pH above 4. Expiratory aerosol
containing viruses can contain these salts and sodium salts – which means that
pH is not expected to go below 4 in indoor environments (pH drops only if
sulfates somehow can go into the aerosol)
Ammonia reduction is part of a good strategy to reduce pH, but it’s not
sufficient based on the characteristic pH analysis.
Ιntroducing oxalic acid and nitric acid can significantly lower the pH (using
amounts that are much lower than stated safety levels).
CONCLUSIONS
25

26.  Improve the modelling approach by including activity coefficients for the
specific acids in the multicomponent solutions (currently we utilized activity
coefficients for HNO3/NO3 as reported from the ISORROPIA model).
 We considered only one carboxyl dissociation (oxalic acid has two stages,
although the first one is the strongest contributor to acidity)
The main source of uncertainty (activity coefficients) is not expected to vary
our estimations of required oxalic acid by more than a factor of 10, especially
since data to date on oxalic acid suggests that partitioning theory can work
reasonably well (Nah et al., 2018).
Thorough sensitivity analysis would be required to quantify these effects
 Dedicated experiments in the CSTACC chamber will be essential for model
validation and improvement.
FUTURE WORK
26

27. Σας ευχαριστώ πολύ.
ΕΥΧΑΡΙΣΤΙΕΣ
27

28. SUPPORTING INFORMATION
RH 0.4-0.45
-3
-2
-1
0
1
2
3
4
5
-2 -1 0 1 2 3 4 5
pH
regression
pH isolite
-3
-2
-1
0
1
2
3
4
5
-2 -1 0 1 2 3 4 5
pH
regression
pH isolite
RH 0.40-0.45 RH 0.50-0.55
28

29. -2
-1
0
1
2
3
4
5
6
-2 -1 0 1 2 3 4 5
pH
regression
pH isolite
-1
0
1
2
3
4
5
6
-1 0 1 2 3 4 5 6
pH
regression pH isolite
RH 0.70-0.75 RH 0.8-0.85
29

30. Equations
1. pH = −log10 aH+ = −log10
mH+
m−
γH+
aH+ is the activity of H+ in an aqueous solution on a molality basis
mH+ is the molality of H+ (mol k−1 , i.e., moles of H+ ions per kg of solvent, typically pure water
γΗ+ is the molal activity coefficient
m- = 1 mol kg−1 is the standard state (unit) molality used
2. pH±(Η, Χ) = −log10 mH+γ±,Η,Χ
approximation is based on mean molal ion activity coefficient of an H+ –anion pair in place, of γH+ ; i.e., γH+ ≈
γ±,H,X , where X is a monovalent anion such as HSO4− , NO3- or Cl−
3. pHF = −log10 mH+
assumption of γH+ = 1, which has been shown to introduce a relatively small uncertainty in pH – by 0.5 units
max – which is of the order of uncertainty for pH estimates for atmospheric aerosol
Equation used for our analysis
30

31. FURTHER REDUCING pH INDOORS: RANGES OF
TOTAL ACID NEEDED TO ACIDIFY AIR
Ka ≈10 -3 M Ka ≈10 -2 M
31

32. FURTHER REDUCING pH INDOORS: RANGES OF
TOTAL ACID NEEDED TO ACIDIFY AIR
Ka ≈10 -5 M Ka ≈10 – 4 M
32