Photochemical processes occurring at the air-sea interface

The surface of the ocean is enriched with organic compounds that play a paramount role in the mass transfer exchange of volatile organic compounds and particles between the ocean and atmosphere.
This interaction between the ocean and the atmosphere determines the air quality and climate issues. However, it remains the biggest enigma among the scientists.
The photochemical processing occurring on the ocean surface exhibit a substantial impact on ozone and SOA formation. 
The scientific knowledge about these processes is still in its infancy and there is still a huge discrepancy between the field measurements and the photochemical models.

This project goes a step further exploring the role on trace metals on light induced (photosensitized) heterogeneous processing of organic compounds on sea surface by gas-phase O3, NO2 and SO2 as a potential source of VOCs and hence, SOA formation in the marine boundary layer. The sea surface microlayer will be firstly collected and chemically characterized with the aim to prepare the synthetic proxy sea surface layers. Both, the real sea surface and the proxy sea surface layers will be then inserted in a vertical wetted wall flow tube reactor (Figure 1) to investigate kinetics, reaction products and mechanistic pathways of liquid phase organic products and VOCs formation from the O3, NO2 and SO2 light induced heterogeneous processes under laboratory conditions.

Figure 1

The emerged outcomes will shed new insights into the “missing source” of VOCs and SOA in the coastal region and benefit understanding the photochemical processes on the sea surface and oxidation processes in the atmosphere.

Light-induced heterogeneous chemistry on urban grime

Nitrous acid (HONO) is an important precursor of the hydroxyl radical (OH). Attributing sources of HONO during daytime is of great importance for the study of atmospheric oxidative capacity. However, recent field measurements in the urban areas using very sensitive HONO instruments have shown that daytime HONO concentrations are much higher than previously assumed and that the contribution of HONO to the OH radical formation was underestimated in the past. The scientific knowledge about these processes is still in its infancy and there is still a huge discrepancy between the field measurements and the photochemical models. This project will focus on light induced (photosensitized) heterogeneous processing of adsorbed organic material on urban surfaces by gas-phase NO2 as a potential source of daytime HONO in urban areas such as Guangzhou. The urban grime will be firstly collected on glass plates and glass beads over various collection time in direct sunlight and in the shade.

The glass plates will be then inserted in a horizontal flow tube photo-reactor (Figure 2) to investigate kinetics, reaction products and mechanistic pathways of HONO formation from the NO2 light induced heterogeneous processes under laboratory conditions. Meanwhile the collected glass beads will be tested under more realistic conditions in a smog chamber facility to differentiate between the two HONO sources: photolysis of adsorbed HNO3 versus light-induced heterogeneous reactions of NO2 with the organic content of the grime. The obtained results from the flow tube experiments and chamber simulations will be compared and implemented in the 0D model especially developed for this purpose to simulate the impact of renoxification processes on the urban air quality in Guangzhou. The studies will shed new insights into the “missing source” of HONO during daytime and benefit understanding the nitrogen chemistry and oxidation processes in the atmosphere. NOx, NO2 and NO concentrations are simultaneously measured by a chemiluminescence instrument (Eco Physics, model CLD 88p) hyphenated to a photolytic (metal halide lamp) converter (Eco Physics, model PLC 860). The gas phase HONO concentration are measured using a LOng Path Absorption Photometer (LOPAP, QUMA) (Figure 3).

Figure 2

Figure 3

Photochemical reactions in the aqueous phase (fog, cloud, aerosol deliquescent particles)

There is increasing evidence that aqueous-phase atmospheric photochemistry is an important source of secondary organic aerosols (SOA), but these processes are currently not adequately represented in atmospheric chemistry models.
This project explores the photosensitizing effect of dicarbonyls such as pyruvic acid, towards atmospherically relevant soluble organics in the aqueous phase. Compared to the direct photolysis of PA in dilute aqueous solution, the photolysis rate of PA is increased by three times in the presence of elevated concentrations (I = 5M) of the inert salt sodium perchlorate (NaClO4). Such a considerable enhancement is caused by a slight red-shift of about Δλ ~ 13 nm that is observed when passing from a dilute aqueous solution to a solution with I=5 M, which is very important when the absorption of sunlight UV radiation is involved. 

The photoinduced removal of PA in highly concentrated electrolyte solutions can proceed much faster than the reaction with OH radicals, a competing removal pathway for PA in the troposphere that influences aerosol composition and optical properties differently. This is, to our knowledge, the first report of a significant effect of the ionic strength on the rate of an atmospheric photochemical reaction. The phenomenon has important implications for the fate of PA in atmospheric aerosol deliquescent particles. The analysis of product formation by High-resolution Mass Spectrometry: Orbitrap Fusion Trihybridmass Spectrometer and by Fourier Transform-Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) further suggests the formation of higher molecular weight compounds upon irradiation of aqueous pyruvic acid at low ionic strength.

Indoor Atmospheric Chemistry: An Emerging Global Concern

The building science research community has largely been dominated by one dominant view:  What are the chemical emissions of building and furnishing materials, and what is the ventilation rate to remove these pollutants?  Indeed concerns about indoor formaldehyde and radon have been addressed for decades.  However, it is only in the past few years that it has become apparent that our indoor environment should be viewed as a highly complex, coupled chemical system, driven by not only emissions and ventilation but also by a rich set of chemical transformations.  Undoubtedly some of this chemistry is beneficial to our health by cleaning the air of toxic pollutants, whereas other processes may be making these pollutants more toxic still.  Just as the outdoor atmospheric chemistry community has focused in recent years on developing an understanding of specific environments, such as forests or Polar Regions, it is just now starting to address the environment in which we live 90% of our time.  In this manner the recently published Perspective article in Science will inform the readers of research questions of direct relevance to their lives. It also points out that this complex chemistry will become even more important as we live more and more of our lives indoors, i.e. increased indoor exposure will arise as societies industrialize, and as we live in tighter homes, air condition more, and protect ourselves from outdoor air pollution.