Organic aerosols (OA) receive increasing attention due to their ubiquities in the atmosphere and their important roles in environmental issues related to global and regional climate, air quality, visibility degradation and human health effects. Secondary organic aerosols (SOA) usually take up a significant portion of OA. Until very recently, it has been generally accepted that the dominant formation pathway of SOA is the gas-to-particle partitioning of semi-volatile organic gases generated from oxidation of volatile organic compounds (VOCs) in the atmosphere. However, there has been increasing evidence in the last decade to suggest that in-cloud processing could also be a source for SOA.
In this thesis work, a one-box model incorporating multi-phase chemistry was developed and applied to investigate the SOA formation from major VOCs under conditions representative of the urban environment of Hong Kong. The model includes detailed gas- and aqueous-phase chemistry of major VOCs, mass transfer of species between gas and aqueous phases, emissions and depositions. The major VOCs included in the model were selected based on their abundances and chemical reactivities. Among the major VOCs, ethylene, isoprene, and toluene were examined in details in this thesis for their SOA forming potentials through in-cloud processing of their soluble oxidation products.
Ethylene is the simplest alkene and among the top five most abundant VOCs in Hong Kong. It has been overlooked in previous SOA modeling studies since its oxidation products are expected to be too volatile to partition onto particles. Ethylene reacts with OH radicals to form glycolaldehyde as one of its major products. Glycolaldehyde partitions into cloud water where it is oxidized to glyoxylic acid and oxalic acid and thereby contributing to SOA mass. With the inclusion of aqueous-phase chemistry, an SOA concentration of about 59 ng m-3
attributable to ethylene oxidation was predicted at the end of a five-day base case simulation in which the cloud period was set to last for 2 hours per day with a cloud water content of 0.3 g m-3
. The simulated ethylene-SOA mass corresponds to an SOA yield of ~ 0.3% and the estimated oxalic acid formed from ethylene accounts for ~ 10% of the ambient measured level.
The box model was then applied to study the isoprene-SOA formation in Hong Kong. Three gas-phase reaction schemes of isoprene, namely, RADM-E, GEOS-Chem, and MCMv3.1, were first compared for their model performance in simulating oxidants and major carbonyl products under low-, mid-, and high-NOx
scenarios. Results showed that GEOS-Chem significantly underestimated the formation of carbonyl species as a result of the key oxidation species presented as lumped species. RADM-E and MCMv3.1 have more consistent performances especially in high-NOx
regime. Results of a base case run coupled with RADM-E suggest that through cloud processing, an isoprene-SOA concentration of ~ 75 ng m-3
was obtained, contributing about 16% to the total in-cloud SOA simulated by the model. Isoprene-SOA formed through gas-to-particle partitioning is less than 5 ng m-3
since vapor pressure estimations showed that the oxidation products of isoprene are too volatile.
Toluene is the most abundant non-methane VOC in Hong Kong due to the strong emission sources such as solvent use and exhaust from gasoline vehicles. A base case simulation using the one box model estimated about 1.4 μg m-3
for the toluene-SOA, which could account for 10 ~ 30% of the total SOA observed in ambient Hong Kong. Gas-particle partitioning is the dominant formation pathway while aqueous processing also makes considerable contribution in forming in-cloud toluene-SOA (~ 50% of the total in-cloud SOA is generated by toluene).
A series of sensitivity tests were conducted for parameters including VOC emission rates, gas-phase photolysis rates, pH of cloud water, cloud water content, cloud period length, NOx
levels, temperatures, and estimated vapor pressures of condensable organic products. Cloud water content, cloud period, and species vapor pressures are found to be important factors impacting the amount of SOA produced. This implies that detailed treatment of cloud events using more advanced models would improve the prediction of SOA formed through the in-cloud processing pathway.
Simulations including all the major VOCs in local ambient environment indicate that (1) gas-particle partitioning pathway contributes 50 ~ 90% of the SOA formation while the significance of cloud processing pathway depends on different simulation conditions (cloud period, cloud water content, etc.); and (2) anthropogenic VOCs play more important roles in forming SOA due to their large emissions. Of the SOA formed through the cloud-processing pathway, VOCs from human activities account for around 80% of the total simulated organic mass in the base case scenario. The simulation result indicates that toluene dominates the SOA mass generated by the gas-particle partitioning pathway. However, it is noted that some biogenic VOCs have not been included in the model (e.g. sesquiterpenes) due to the little understanding of their reaction mechanisms. The inclusion of these biogenic VOCs would reduce the simulated relative contribution of anthropogenic SOA. The calculations also show that anthropogenic VOCs are responsible for approximately 85% of the SOA budget in Hong Kong. Hence, regulating VOCs emitted from industrial and commercial processes would help to reduce the SOA loadings in the ambient environment of Hong Kong.
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