Measurement report: Sulfuric acid nucleation and experimental conditions in a photolytic flow reactor

Abstract

Abstract. Nucleation rates involving sulfuric acid and water measured in a photolytic flow reactor have decreased considerably over a time period of several years. Results show that the system – flow reactor, gas supplies and lines, flow meters, valves, H2SO4 photo-oxidant sources – has reached a baseline stability that yields nucleation information such as cluster free energies. The baseline nucleation rate is punctuated by temporary bursts that in many instances are linked to cylinder changes, delineating this source of potential contaminants. Diagnostics were performed to better understand the system, including growth studies to assess H2SO4 levels, chemiluminescent NO and NOx detection to assess the HONO source, and deployment of a second particle detector to assess the nanoparticle detection system. The growth of seed particles shows trends consistent with the sizes of nucleated particles and provides an anchor for calculated H2SO4 concentrations. The chemiluminescent detector revealed that small amounts of NO are present in the HONO source, ∼ 10 % of HONO. The second condensation-type particle counter indicates that the nanoparticle mobility sizing system has a bias at low sulfuric acid levels. The measured and modeled nucleation rates represent upper limits to nucleation in the binary homogeneous system, H2SO4-H2O, as contaminants might act to enhance nucleation rates and ion-mediated nucleation may contribute. Nonetheless, the experimental nucleation rates, which have decreased by an order of magnitude or larger since our first publication, extrapolate to some of the lowest rates reported in experiments with photolytic H2SO4. Results from experiments with varying water content and with ammonia addition are also presented and have also decreased by an order of magnitude from our previous work; revised energetics of clusters in this three-component system are derived which differ from our previous energetics mainly in the five-acid and larger clusters.

Semiempirical H2SO4–H2O Cluster Standard Gibbs Reaction Energies from Nucleation Experiments: Improved Temperature Dependence

“…Two variations of the cluster thermodynamics were also used to simulate particle number densities for comparison with the experimental results. Using the Gibbs energies obtained from Hanson et al (denoted Hanson_2021) produces number densities that fall within the scatter of the measurements, perhaps a factor of 3 larger than the average of the experimental results. The third set of simulated particle number densities is obtained using an altered SAWNUC approach (denoted SAWNUC_alt).…”

“…Experimental data are shown for both the UCPC and nanoMPS systems from early March 2022; the large scatter observed at low N p for the nanoMPS is due to extremely low count rates. Predicted number densities from simulations using thermodynamics obtained from the standard SAWNUC model, from Hanson et al, and the altered SAWNUC model are shown for 52% relative humidity.…”

“…Measured particle number densities N p are generally investigated as a function of one experimental parameter, such as the amount of HONO. A substantive difference from ref is the addition of an electric field (several hundred V/cm) near the exit to scavenge charged particles (previously postulated as the reason for discrepancies between the two sampling methods).…”

“…The experimental apparatus and results have been described previously. , In short, a 120 cm long glass flow reactor with a 5 cm inner diameter is operated at ambient pressure with a plug-flow residence time of about 40 s. The flow reactor is jacketed to maintain a constant temperature and surrounded with UV lights to facilitate the photo-oxidation of SO 2 to H 2 SO 4 , which is initiated by the photolysis of HONO (∼365 nm). Nucleation putatively occurs in the binary system, after which particles have sufficient time to grow several nanometers in diameter by means of H 2 SO 4 and H 2 O uptake.…”

“…Two-dimensional (2D) laminar flow simulations of the photochemistry and subsequent cluster chemistry within the flow reactor yield particle number densities for comparison to experiment. , Convection, reaction, diffusion, and wall loss are incorporated into a continuity differential equation that is solved using finite volumes with an explicit Euler approach. Cluster coagulation is generally not included, justified by the relatively high H 2 SO 4 levels and very low cluster concentrations of our experimental conditions, but has been crudely incorporated to appraise the conditions under which it may become significant.…”