What controls the gas phase radical budget indoors?

Radicals have an important impact on reactivity and chemical processing indoors. Therefore, to understand indoor air chemistry, you need to understand the radical budgets and which processes control radical formation and loss and under which conditions. For more information, see the WG1 summary report, (Part 1. Chemical transformations, Brief summary of the state-of-the-art).

Laboratory studies, indoor field measurements and modelling studies have investigated various aspects of indoor radical budgets (see the WG1 editorial, and in the WG1 summary report (Part 1. Chemical transformations, Brief summary of the state-of-the-art and references within)). The hydroxyl (OH), chlorine (Cl) and nitrate (NO3) radicals are all important oxidants indoors. The OH radical and can be formed through the ozonolysis of unsaturated VOCs (volatile organic compounds), as well as through photolysis of precursors such as HONO (nitrous acid) and to a lesser extent, of HOCl (hypochlorous acid). Ozonolysis reactions are also a direct source of HO2 (hydroperoxy) and RO2 (organic peroxy) radicals, as well as Criegee intermediates.

The presence of photocatalysers or air cleaners can also generate high levels of OH radicals indoors. HO2 and RO2 radicals can be formed through oxidation of VOCs. Peroxyacetyl nitrates (RCOOONO2), produced outdoors can be thermally decomposed and could potentially generate RO2 radicals when transferred into warmer indoor environments. Nitrate radicals have been shown to form indoors under combustion conditions, but their concentration is unlikely to be high under most typical indoor conditions. Cl radicals can be formed following cleaning activities. See the WG1 summary report and references within (Part 1. Chemical transformations, Brief summary of the state-of-the-art,p6).

Radical concentrations are controlled by the balance between precursor species and their reactants in the indoor environment. As well as radical concentrations (OH, HO2, RO2, Cl, NO3), there is a need to measure those of their precursors O3, HONO, HCl, HOCl, ClNO2, and the concentrations of their key reactants (VOCs, NOX) in order to understand radical budgets. For more information, see the WG1 editorial, the WG1 summary report (Part, 1. Chemical transformations, Brief summary of the state-of-the-art) and the list of oxidants and their precursors (WG3 table indoor_oxidants_precursors_gas). Outdoor concentrations of NOX, O3 and VOCs also need to be measured to understand the role of ingress from outdoors.

• 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. Note that no instrument is yet available for speciation of RO2 (with the possible exception of CH3O2) but some instruments are under development (based on mass spectrometry). OH reactivity can be a useful measure for understanding total formation and loss processes.

• Cl: No technique is currently available, so precursors should be quantified and models used. 

• NO3: WG4 Techniques for measuring indoor radicals and radical precursors

• Ozone : WG4 work described in Part 5.2.1, p542 in Sampling and analysis techniques for inorganic air pollutants in indoor air

• HONO: WG4 work described in Part 2.3, p13 in Techniques for measuring indoor radicals and radical precursors

• HCl, HOCl, ClNO2 : WG4 work described in Part 2.4 and table 5 p596, 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

• Nitrogen dioxide : WG4 work described in Part 5.2.1, p546 in Sampling and analysis techniques for inorganic air pollutants in indoor air

Ideally in a range of building types, though some of these instruments are challenging to use indoors, with demanding power, space, ventilation and noise considerations. Ideally, all of the instruments would be co-located as the radicals have a very short lifetimes and there may be concentration gradients across indoors spaces (especially near windows). NO3 is only likely to be present in appreciable quantities where NO2 and O3 are both present at high concentrations and with low lighting conditions, given the propensity of NO3 to photolyse at visible wavelengths. Chlorinated compounds will be most prevalent during cleaning activities, or in swimming pools. 

We also suggest that the related building and ancillary parameters are measured to understand the measured concentrations of the chemical species (WG5 : List of parameters to be measured in ALL buildings, D11). Again, these ideally need to be measured adjacent to the chemical species as some oxidants are formed by photolysis processes and photolysis fluxes are useful to quantify the production of oxidants.