What is the impact of environmental conditions on indoor reactivity?

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. These environmental conditions (both indoors and outdoors) can impact species concentrations and hence overall reactivity. 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. For instance, photolysis rates are lower, surface to volume ratios are larger so that deposition is more important, and dilution is much slower compared to outdoors. In fact, the building parameters can be critical as discussed in the WG5 report. There is also more information on reactive species 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). 

Gas-phase reactions are directly dependent on the ACR (air change rate) and will have an impact only if the time scale of the reaction is faster than the ACR. If the mean residence time of an indoor chemical is long enough for chemical conversions, the build-up of reaction products (both short-lived and stable products) can be expected. The ACR will also control how quickly indoor emissions leave a building and also, how quickly pollutants generated outdoors can ingress indoors. Note that internal temperature variations can also affect the speed of gas-phase reactions.

Another important aspect is the attenuation of light through windows and the spectral characteristics of the indoor lighting, which together, will determine the indoor photolysis rates. The rate of OH formation through photolytic pathways indoors is influenced by photolysis rates, as well as the concentrations of the photolysed species. Very few characterizations concerning light distribution indoors and artificial light are currently available (see report WG1 and WG4 Techniques for measuring indoor radicals and radical precursors, p605.

The fate of radicals also depends on the NO-regime. At high NO concentrations, radicals are rapidly reformed and there is cycling between RO2, HO2 and OH, but at low NO concentrations, the radicals react with each other to form species that are then deposited on surfaces (such as peroxides). Therefore, the location of a building (trafficked street, urban background, rural) will determine the external and hence internal NO concentration, as NO typically ingresses from outdoors in the absence of internal combustion sources. The location will also affect indoor ozone concentrations, given outdoors is the major source of indoor ozone.

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. Reactions at the surface will depend on the type of surface, and also its condition (clean, soiled: covered by organic film, covered by a liquid film). The importance of the liquid-phase reactions will be dependent on the humidity in the room, the presence of species which may be hydrolysed or the potential of the species to be solubilized in the liquid. The partitioning gas/surface reactions will depend also on the surface/volume ratio. As an example, in the presence of occupants, more surfaces are available for deposition of ozone, limiting gas phase ozonolysis, but leading to enhanced emissions from people (see topic 3).

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

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. 

The related building and ancillary parameters must be measured to understand the their influence on 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, and simultaneously with, the chemical species, as some oxidants are formed by photolysis processes and photolysis fluxes are useful to quantify the production of oxidants. It is also critical that concentrations are measured outside the building, so the relative importance of indoor and outdoor sources and environmental parameters on indoor concentrations can be determined.