The sources of reactivity indoors

Reactivity in indoor environments is initiated by the presence of oxidants and leads to secondary products. These secondary products tend not to be identified by emission studies and/or regulation, both of which focus on the primary emissions from particular products. However these secondary species can have an impact on indoor air quality and are often more harmful to health than the primary emissions. There are different components of indoor reactivity: in the gas phase, on indoor surfaces and between mixtures of gases, particles and oxidants. The relative importance of these different components needs better quantification and ideally, for a range of environmental 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 reactivity (see a recent review here). Some processes occuring indoors are similar to those observed in the atmosphere, such as deposition onto surfaces, particle formation and photolysis reactions (WG2 final report). However, the physico-chemical boundary conditions indoors are quite different compared to that encountered outdoors, e.g. photolysis rates are lower. There is more information in the WG1 editorial, and in the WG1 summary report (Part 1. Chemical transformations, Brief summary of the state-of-the-art and references within).

In the gas-phase, the chemical transformation processes of indoor gaseous trace constituents may arise from hydroxyl (OH), chlorine (Cl) and nitrate (NO3) radical-initiated oxidation processes or from the ozonolysis of unsaturated VOCs (volatile organic compounds). The main source of ozone indoors in the absence of specific equipment (e.g. photocopiers, laser printers) is from outdoors through ventilation and/or infiltration. Ozonolysis reactions produce stable products, mainly carbonyl compounds, and are also a direct source of OH, HO2 (hydroperoxy) and RO2 (organic peroxy) radicals, as well as Criegee intermediates. OH can also be formed indoors by the photolysis of HONO (nitrous acid) and to a lesser extent, of HOCl (hypochlorous acid). HONO is formed through combustion and surface interactions indoors, whilst the main source of HOCl is through bleach use.

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. See the WG1 summary report and references within (Part 1. Chemical transformations, Brief summary of the state-of-the-art,p6).

Heterogeneous processes taking place indoors are numerous and involve interaction between the gas phase and solid or liquid phases. Of particular relevance for indoors, are wall surfaces or furniture, which can be covered by a water layer or soiled by organic films such as skin oil deposits. For more information, see the WG1 summary report and references within (Part, 1. Chemical transformations, Brief summary of the state-of-the-art, p7).

As the gas phase reactivity is linked to the presence of oxidants, there is a need to measure oxidant concentrations (Ozone, OH, HO2, RO2, Cl, NO3), those of their precursors  HONO, HCl, HOCl, ClNO2, and the concentrations of their key reactants (VOCs) in order to understand chemical processing in the gas-phase. 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).  In terms of species to measure around surface reactivity, more information can be found here in this table: Table WG3 aging_reactive. Outdoor concentrations of NOX, O3 and VOCs also need to be measured to understand the role of ingress from outdoors.

• 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. 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

• 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 p 596, 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

• Surface characterisation : WG4 : A review of microbial and chemical assessment of indoor surfaces

• Particles: WG4: Full article: Particulate matter indoors: a strategy to sample and monitor size-selective fractions

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.