Impact of surface ozone interactions on indoor air chemistry: A modeling study

An INdoor air Detailed Chemical Model was developed to investigate the impact of ozone reactions with indoor surfaces (including occupants), on indoor air chemistry in simulated apartments subject to ambient air pollution. The results are consistent with experimental studies showing that approximately 80% of ozone indoors is lost through deposition to surfaces. The human body removes ozone most effectively from indoor air per square meter of surface, but the most significant surfaces for C6 -C10 aldehyde formation are soft furniture and painted walls owing to their large internal surfaces. Mixing ratios of between 8 and 11 ppb of C6 -C10 aldehydes are predicted to form in apartments in various locations in summer, the highest values are when ozone concentrations are enhanced outdoors. The most important aldehyde formed indoors is predicted to be nonanal (5-7 ppb), driven by oxidation-derived emissions from painted walls. In addition, ozone-derived emissions from human skin were estimated for a small bedroom at nighttime with concentrations of nonanal, decanal, and 4-oxopentanal predicted to be 0.5, 0.7, and 0.7 ppb, respectively. A detailed chemical analysis shows that ozone-derived surface aldehyde emissions from materials and people change chemical processing indoors, through enhanced formation of nitrated organic compounds and decreased levels of oxidants.


| INTRODUCTION
Many sources contribute to indoor air po ution Indoor contaminants can originate outdoors ie ozone O 3 nitrogen oxide NO x particuate matter PM however there are a so significant sources indoors 1 The main indoor air po utant sources are indoor activities such as cooking eg NO x PM 2 smoking such as forma dehyde HCHO PM 3 or c eaning eg terpenes An increasing y active area of research for indoor air po ution is that driven by emissions from interna materia s such as carpets painted wa s or furniture Species can be emitted direct y from a surface primary po utants but a so fo owing gas phase transformations or interactions at surfaces secondary po utants as shown in Figure Furthermore these processes form an effective means of removing air po utants prone to deposition that is ozone from indoor air and consequent y the concentrations of these species are much ower indoors than outdoors 8 Surface deposition depends on two main processes the transport of ozone to the surface and the uptake to the surface Transport to the surface is determined by the thickness of the surface boundary layer. 9 The rate of ozone that is removed from indoor air ozone oss is proportiona to the indoor ozone concentration air exchange rate surface area with a characteristic deposition ve ocity different for each surface materia and a tota vo ume of indoor space 10 Indoor surfaces range from high y reactive carpet to poor y reactive g ass The deposition rate for materia s ike carpet is most y imited by externa mass transport whi e deposition to g ass is typica y imited by surface reaction kinetics Ozone is one of the most reactive compounds indoors and in the absence of indoor sources most y originates outdoors 11 Once indoors it can undergo a number of oss processes depending on the conditions but deposition usua y dominates Porous and f eecy surfaces such as carpets and soft furniture are important sinks of ozone and are a so ab e to form a wide range of higher C 6 and above a dehydes 12 The age of the materia is a so significant because emission rates tend to be higher for new materia s and reduce as a materia becomes o der Morrison and Nazaroff termed this process ozone aging Ozone can oxidize the avai ab e unsaturated bonds in a surface coating over time eading to decreasing ozone uptake and a so decreasing emission rates of secondary po utants from this source There is a significant difference between the tempora evo ution of primary and secondary po utants indoors Emission of primary VOCs tends to dec ine at a predictab e rate and genera y reduces to ower eve s within a year The formation rate of secondary po utants is more pro onged as ozone uptake and consequent surface processing to produce secondary po utants can continue for severa years Products of these reactions inc ude a dehydes and ketones and secondary organic aeroso s SOA 19 Furthermore secondary po utants from surface production can be more damaging for human hea th than the primary emissions causing asthma and pu monary infections 20 and thus warrant further investigation One surface receiving increasing attention indoors is the human body Humans are an important sink for ozone in the indoor environment The chemica s that constitute human skin can be c assified as wax esters g ycero s fatty acids squa ene esters and stero s and contain unsaturated carbon bonds which readi y react with ozone 21 Fo owing such reactions a wide range of secondary products can be formed inc uding a dehydes ketones acids and SOA some of which are known to be harmfu to hea th In the absence of comprehensive indoor air measurements indoor air po utant concentrations can be simu ated using a detai ed indoor air chemistry mode Such mode s can provide insight into mechanisms that inf uence chemica processing which is essentia to understand the fundamenta science and hence app y appropriate mitigation strategies The aim of this study is to deve op an existing mode to inc ude ozone derived surface emissions and probe the imp ications of these emissions on indoor air composition In particu ar ozone deposition onto surfaces and resu tant secondary po utant formation in a simu ated apartment is investigated Different surface types are considered such as soft furniture painted wa hard furniture wooden f oor ino eum countertop and human skin We quantify secondary po utant concentrations generated from ozone reactions with these surfaces as we as use the mode to investigate the impact of ozone initiated surface emissions on chemica processing and pathways in indoor air.

FIGURE
Primary and secondary VOC formation fo owing surface deposition of ozone through oxidation processes and gas phase transformations The INDCM considers a sing e we mixed environment and assumes that the concentration of each species is ca cu ated according to Equation : where C i C o is the indoor outdoor concentration of species υ d its deposition ve ocity A the surface area indoors V i the vo ume of air in the indoor environment λ r the air exchange rate between indoors and outdoors AER f the bui ding fi tration factor E i the indoor emission rate for species i, and R ij the reaction rate between species i and j.
For this study the INDCM has been deve oped to consider ozone deposition onto different types of surface as we as emissions of higher a dehydes fo owing surface interaction The ozone oss rate to a surface is ca cu ated according to Equation where F s …n is the ozone deposition f ux to the surface from to n number of surfaces υ dO 3 is the ozone deposition ve ocity to a surface A s is the surface area tota area of a specific surface type V i is the tota vo ume of the indoor environment The emission of the surface products was ca cu ated using Equation 30 : where E sec …n is the re evant secondary product emission rate from to n number of surfaces Y is the a dehyde yie d of the emitted po ut- The products detected comprised unsaturated a iphatic a dehydes ketones and carboxy ic acids so we have inc uded in our study those a dehydes that are a ready represented in our mode nonana decana and oxopentana OPA Distribution of reported ozone deposition ve ocity onto different indoor surfaces considering number of measurements n for each The box and whisker p ot shows the minimum median and maximum va ues in cm s .
As the data for a dehyde yie ds for wooden materia s were not avai ab e direct emission rates were ca cu ated using iterature data for different types of wooden materia s 33 | Base case scenario We consider an apartment in Mi an for this mode ing study using typica and heatwave summer conditions Mi an is a high y po uted city due to traffic emissions and poor dispersion Summer ozone concentrations are often high in Mi an but during the heat wave that occurred in Europe in summer concentrations were exceptiona y high for severa days Tab e Given that high outdoor ozone concentrations ead to high indoor ozone concentrations Mi an was considered a good study site for reactive indoor chemistry as shown through previous studies However we a so considered more typica summertime conditions through an identica apartment in Seou which has much ower outdoor ozone concentrations in summer Tab e Note that Seou is a study ocation as part of the CAPACITIE project which provided funding for this study We assumed a typica apartment size using data proposed by Tae      Sensitivity test resu ts the change in concentrations of C 6 C 10 a dehydes in the apartment in Mi an for typica summer conditions re ative to base ine conditions  Rate of production ana ysis for the major rates of reaction for a mode run with ozone deposition and no oxidation derived a dehyde emissions figures in bo d and with ozone deposition fo owed by ozone driven a dehyde surface production norma font in units of 10 mol cm s MT denotes monoterpene Red arrows denote radica initiation processes and b ue arrows are termination processes with green arrows representing radica propagation Note that RO 2 inc udes RCO 3 acety type peroxy radica s eg CH 3 CO 3 as we as a ky peroxy radica s RO 2 eg CH 3 O 2 conditions in Mi an Given that heat waves may become more frequent in future with c imate change 69 indoor ozone derived surface a dehyde emissions may a so increase Among a the tested surfaces the human body was shown to be the most efficient in terms of removing ozone from indoor air per square meter However when interna ozone driven emissions of adehydes are considered soft furniture and painted wa s become more important owing to their arger surface areas in a typica bui ding Ozone initiated emissions from the human body can be important in sma er areas of a house eg a bedroom at nighttime when concentrations of various carbony species can become significant An important conc usion from our study is that inc usion of oxidation derived surface emissions from surfaces and or peop e within a detai ed chemica mode profound y affects chemica processing Ozone driven surface emissions dep ete oxidants increase the importance of radica production from a dehyde photo ysis indoors and shift formation of products toward nitrated organic carbon species Even though indoor surfaces can be quite different in their initia reactivity aging and soi ing of surfaces may make indoor surfaces more simi ar than different over time 70 In a study of five homes Wang and Morrison 38 showed that o der carpet was ess reactive than new carpet but that kitchen countertops tended to remain reactive regardess of age and that this was probab y due to continuous app ication of cooking oi s and or c eaning agents Occupants a so add to the reactivity of surfaces via desquamation skin shedding and transferring skin oi s to surfaces 68 This process of s ow oxidation of the origina surfaces or surface fi ms and continuous deposition of reactive organic materia may resu t in indoor surfaces especia y upward facing horizonta surfaces having simi ar chemica properties Therefore mode s wi benefit from more extensive fie d measurements of ozone surface reactivity deposition ve ocity and product yie ds in occupied homes as we as information on surface interactions for indoor poutants other than ozone Lee SC Li WM Ao CH Investigation of indoor air qua ity at residentia homes in Hong Kong case study Atmos Environ Lin HH Ezzati M Murray M Tobacco smoke indoor air po ution and tubercu osis a systematic review and meta ana ysis PLoS Med.
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