What is the role of Highly Oxygenated Molecules (HOM) indoors? 

Why is this important?

Highly Oxygenated Molecules (HOMs) can be rapidly produced following autooxidation of the peroxy radicals formed following oxidation of VOC compounds, and can make significant contributions to the secondary organic aerosol (SOA) mass. Although their formation had been observed in numerous locations outdoors (see review in WG2 report), HOM formation has now also been observed indoors in a museum (Pagonis et al., 2019). HOMs are highly multifunctional compounds, and their volatility is very low, hence their propensity to form SOA (Ehn et al., 2014). HOM species have been detected in both vapour and particle phases, the latter using the filter inlet for gases and aerosols (FIGAERO). However, HOMs are very under-studied in the indoor environment and we need to better understand their formation and behaviour, and their impact on IAQ, such as through formation of SOA. 

What do we already know?

SOA can be enhanced by the rapid formation of the low-volatility species HOMs, following oxidation of VOCs (Ehn et al., 2014; Bianchi et al., 2019; McFiggans et al., 2019; Pagonis et al., 2019). The HOMs result through auto-oxidation of the organic peroxy radicals (RO2) that are formed following oxidation of the parent VOC (Perakyla et al., 2020). When the RO2 radical lifetime is sufficiently long, auto-oxidation can be repeated several times and HOMs can be formed (Bianchi et al., 2019). The HOMs are formed by both OH- and/or O3-initiated oxidation (Berndt et al., 2016), of numerous indoor VOCs, including benzene, toluene, ethylbenzene, xylenes and terpenes. Ozonolysis is an efficient pathway to form HOMs, but reaction with OH radicals can also contribute, but with a smaller molar yield than the ozonolysis pathway (Bianchi et al., 2019).

HOMs are highly multifunctional compounds, and their volatility is expected to be very low (Ehn et al., 2014). However, the vapour pressure of HOM species remains uncertain (Kurten et al., 2016; Kurtén et al., 2018). Pagonis et al. (2019) investigated HOM formation and concentration in a museum following oxidation of limonene. HOM concentrations up to ~2.1 × 10− 5 μg m− 3 were noted, while aerosol mass concentration increased up to ~1.6 μg m− 3. The measurements could be reproduced by a model assuming a HOM yield of 11% following ozonolysis of limonene. Kruza et al. (2021) also assumed a similar yield to simulate experimental data for SOA formation following ozonolysis of a-pinene and a-terpineol.

It is clear that the formation of HOMs can shift the partitioning between gaseous and particulate pollutants in the indoor air. Because they are formed in the preliminary oxidation step, they form an effective route to produce SOA rapidly and in high quantities. However, there is still much we need to understand about their formation indoors (and outdoors) and under which conditions they are most important (see p11, WG2 final report). To aid this gap in our knowledge, we need measurements of HOMs indoors, along with those of VOCs and oxidants and for a range of typical indoor conditions and activities. 

What species should we measure?

HOMs consist of a large variety of species as discussed above. Whilst indoor measurements of any HOM species would be beneficial, most of the focus to date has been on relatively few terpene species and their propensity to form SOA through the HOM route (e.g. α-pinene), and less is known about the SOA potential of aromatics and longer-chain alkanes. Along with key VOCs, we would also need to measure oxidant and NOX concentrations to help interpret the HOM measurements. There are also additional parameters that could be measured to aid with model development, such as vapour pressure and the partitioning coefficients for HOMs between gas- and particle-phases. 

How should we measure these species?

The detection of HOMs has benefited from the recent advances in mass spectrometry with atmospheric pressure interface time-of-flight mass spectrometry (APi-ToF-MS) alone, or coupled with a nitrate ion chemical ionisation source (NO3-CI-APi-ToF-MS) generally used. HOM species have been detected in both the gas- and particle-phases, the latter using NO3-CI-APi-ToF-MS coupled with the filter inlet for gases and aerosols (FIGAERO) as described e.g. by Lee et al. (2014). Measurement techniques for HOMs are reviewed in the WG2 report, p11-12. Note that there are technical issues involving on-line MS-based approaches for HOMs, such as an absence of direct calibration and peak identification for some species. 

In addition to the measurements of the HOMs themselves, we would need:

ozone : WG4 work described in Part 5.2.1, p13 in Sampling and analysis techniques for inorganic air pollutants in indoor air

HOx/ROx : WG4 work described in Part 2.1.1-2.1.3, p6 for OH, 2.1.6 p10 for HO2,  2.2 p 11 for RO2 in Techniques for measuring indoor radicals and radical precursors

VOCs: refer to Mapping organic constituents and WG4 Full article: An overview of methodologies for the determination of volatile organic compounds in indoor air

NO2: WG4 work described in Part 5.3.1, p546 in Sampling and analysis techniques for inorganic air pollutants in indoor air

Particle composition: see chapter 8.3 of the WG4 report. 

Where should we measure these species?

Given the prevalence of HOM in the ambient atmosphere and the evidence in chamber studies of both OH- and O3-initiated auto-oxidation forming HOM species, it is widely expected that they will fulfil an important role in indoor chemistry and ideally should be investigated in the rich mixture of both biogenically- and synthetically-derived VOC in the indoor environment. Pragmatically, on-line measurements are likely to be limited to spaces where the instrumentation (large, bulky, noisy) can be housed easily in the first instance.