SENSITIVITY OF SULFURIC ACID CONCENTRATION TO RELATIVE HUMIDITY AND TO
INLET FLOW TEMPERATURE: PRILIMINARY RESULTS
K. NEITOLA1, D. BRUS1, T. PETÄJÄ2, M. SIPILÄ2, A-P. HYVÄRINEN1, H. LIHAVAINEN1
and M. KULMALA2,3
1
Finnish Meteorological Institute, Erik Palménin aukio, P.O. Box 503, FI-00101 Helsinki, Finland
2
3
Department of Physics, University of Helsinki, P.O. Box 64, FI-00014 Helsinki, Finland
Department of Applied Environmental Science, Stockholm University, Stockholm 10691, Sweden
Keywords: Sulfuric acid, CIMS, flow tube.
INTRODUCTION
Atmospheric aerosol formation consists of rather complicated sets of processes, the first of them is gas-toparticle nucleation which occurs naturally but might be also easily influenced by anthropogenic emissions
of gases such as SO2. It is generally accepted that sulfuric acid, as a gas-phase oxidation SO2 product by
the hydroxyl (OH) radicals, is a robust source of new particles and plays a central role in atmospheric new
particle formation (Weber et al. 1996, 1997; Kulmala, 2003). In number of field experiments (e.g. Weber
et al., 1996; Sihto et al., 2006; Riipinen et al., 2007) and also in some laboratory studies (e.g. Berndt et al.,
2005, Young et al., 2008, Sipilä et al., 2010) the rate of particle formation is not adequately explained by
binary classical homogenous nucleation (CNT), the theory greatly under predicts the observed nucleation.
At atmospheric conditions and also in some laboratory studies (Berndt et al., 2005; Sipilä et al., 2010) it
was observed that molecular clusters are activated and particle number concentration was found to have a
power-law dependency about 1-2, compared to CNT prediction that suggest exponents about 9
(Vehkamäki et al., 2002). This discrepancy is puzzling to atmospheric researchers more than one decade.
As a solution to this problem it has been suggested that other associate molecule as ammonia (Weber et
al., 1996) or organic acids (Zhang et al., 2004) may have a stabilizing effect on the clusters and allow
nucleation to occur at much lower concentrations of sulfuric acid than needed by CNT. The first in-situ
chemical ionization mass spectrometer (CIMS) atmospheric measurements of sulfuric acid in troposphere
were reported by Eisele and Tanner in 1993. Their study claims that the CIMS measurements are sensitive
to total acid without discrimination between free acid and monoacid hydrates, or even between free and
higher-order acid clusters and their hydrates. On the other hand Salcedo et al. in 2004 studied effect of
relative humidity on the detection of sulfur dioxide and sulfuric acid and found negative effect on the
sensitivity of the CIMS to SO2 and H2SO4 because water molecules form clusters with reactant and
product ions and thus shielding the molecules from being ionized. They claim that the effect can be
avoided by increasing the temperature.
METHODS AND RESULTS
In this study we performed series of measurements at several sulfuric acid concentrations and found strong
dependency on relative humidity. We also tested the effect of elevated temperature of the CIMS inlet flow
from 25 to 100°C. Three different setups were used in this study.
First we measured laboratory air with a heater at the inlet of the CIMS combined it with two DMPS setups
(HAUKE-DMA, length = 28 cm, UCPC TSI model 3776) with dry sheath air, one before and one after
heater. This was done to be able to estimate the amount of evaporated sulfuric acid vapor from the particle
phase when the sample flow was heated. Heater was used at the inlet to alter the initial RH (~ 20%).
The RH and temperature of the flow were measured with a humidity data processor (Vaisala HMI38) and
PT100 temperature probes, respectively. Unfortunately the number concentration of particles in the lab air
was always below 3000 #/cm3 and usually below 1000 #/cm3 with the mean size of the particle
distribution between 100 and 200 nm. The low number concentration caused high statistical error in
calculated volume distributions and within the measurement error there was no difference in the
distributions before and after the heating, meaning that we did not observe any effect of particle’s
evaporation on sulfuric acid concentration.
In the second setup we used temperature controlled saturator to generate sulfuric acid vapor. A particle
free dry compressed air was drawn through the saturator to saturate the flow with sulfuric acid vapor.
The flow from the saturator was then brought to a t-part through a Teflon line (6mm OD, length 1m)
where it was mixed with humidified particle free flow. Humidity of the flow was controlled by using one
or two humidifiers (Perma Pure model MH-110) with temperature controlled circulating water bath
(Lauda RC 6) filled with ultrapure water (Millipore, TOC less than 10 ppb, resistivity 18.2 MΩ.cm @25°C).
Four different flow rates (0.05, 0.2, 0.5 and 1.0 l/min) through the saturator were used to obtain different
orders of magnitude concentrations of sulfuric acid vapor and more sensitive adjustment was done by
altering the temperature of the saturator (ranging from 0 to 45 oC). The flow rate of the humidified flow
was usually around 7 l/min to ensure that the total flow was bigger than what CIMS was taking in as a
sample flow (~ 7 l/min). After the mixing the total flow was taken into the inlet where the heater with RH
and temperature measurements of the sample flow was. The resulting residence time to CIMS detector
was then 0.2 s after mixing. Two different sets of measurements were done with this setup, one with
different constant initial RH with heating the inlet flow at ten temperatures (ranging from room
temperature to 100 oC) and the other one with no heating and altering the RH with humidifiers. Heating
was programmed to measure at room temperature always in between different temperatures with a three
hour steps. Particle concentration was checked to be zero with UCPC (TSI model 3025).
Figure 1. Sulfuric acid concentration as a function of flow temperature at two different residence times.
Third setup was similar to the previous one but the mixing happened at a mixer in front of a temperature
controlled flow tube (length 2m, ID 6cm). The flow was drawn through the flow tube to the inlet of the
CIMS where the heater was used again. The resulting residence time for this setup was 40 s. Here were
also done two same sets of measurements, either altering the RH with heater, or with the humidifiers. The
particle concentration after the flow tube was also checked with UCPC (TSI model 3025) and it was found
to be zero.
The results are summarized in figure 1 and 2. Figure 1 shows sulfuric acid concentration as a function of
inlet flow temperature. It can be seen that elevated heater temperature has influence only at low
concentrations of sulfuric acid, where slight increase in concentration is observed starting from 80 to
100 °C. At higher concentrations starting from ~106 molecules/cm-3 and higher we observed decrease of
sulfuric acid concentration with increasing temperature of the inlet flow. The decrease can be explained
with higher diffusion losses in CIMS, the power-law fit to data has an exponent of 1.6 which is within the
experimental uncertainty close to diffusion coefficient temperature dependency (D~T1.75). Sulfuric acid
concentration as a function of relative humidity at two different residence times can be seen on figure 2.
The steep decrease about one order for high and about three orders for low concentrations of sulfuric acid
was observed when the flow was humidified from relative humidity 0 to 5 %. The increase of relative
humidity has stabilizing effect on sulfuric acid, the losses are smaller and sulfuric acid concentration is
rising again from 5 to 50 %. To provide quantitative explanation of this shielding effect more experiments
has to be performed and results have to be compared to another method for sulfuric acid concentration
determination such as method of trapping sulfuric acid in bubblers filled with hydroxide and subsequent
ion chromatography analysis.
Figure 2. Sulfuric acid concentration as a function of relative humidity at two different residence times.
ACKNOWLEDGEMENTS
The financial support by the Academy of Finland Centre of Excellence program (project no 1118615) and also
KONE foundation are gratefully acknowledged.
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