Indoor acids and bases William W Nazaroff1 and Charles J. Weschler2,3 1 Department of Civil and Environmental Engineering, University of California, Berkeley, CA, USA 2 Environmental and Occupational Health Sciences Institute, Rutgers University, Piscataway, NJ, USA 3 International Centre for Indoor Environment and Energy, Technical University of Denmark, Lyngby, Denmark Correspondence: William W Nazaroff, Civil and Environmental Engineering Department, University of California, Berkeley, CA, USA. Email: [email protected] Abstract Numerous acids and bases influence indoor air quality. The most abundant of these species are CO2 (acidic) and NH3 (basic); building occupants are important sources. Other prominent inorganic acids are HNO3, HONO, SO2, H2SO4, HCl and HOCl. Prominent organic acids include formic, acetic and lactic; nicotine is a noteworthy organic base. Sources of N-, S-, and Cl- containing acids can include ventilation from outdoors, indoor combustion, consumer product use, and chemical reactions. Organic acids are commonly more abundant indoors than outdoors, with indoor sources including occupants, wood, and cooking. Beyond NH3 and nicotine, other noteworthy bases include inorganic and organic amines. Acids and bases partition indoors among the gas phase, airborne particles, bulk water, and surfaces; relevant thermodynamic parameters governing the partitioning are the acid-dissociation constant (pKa), Henry’s law constant (KH), and the octanol-air partition coefficient (Koa). Condensed-phase water strongly influences the fate of indoor acids and bases and is also a medium for chemical interactions. Indoor surfaces can be large reservoirs of acids and bases. This eXtensive review of the state of knowledge establishes a foundation for future inQuiry to better understand the roles of acids and bases influencing human health, preservation of cultural artifacts, and protection of sensitive equipment. Practical Implications • Acids and bases are major components of indoor air, potentially influencing health risks, perceived air quality, and material damage. • Buildings are partially protective when outdoor air is the dominant source, as is the case for sulfur dioXide and aerosol strong acidity. • Human emissions lead to higher indoor than outdoor concentrations for carbon dioXide, ammonia, and a broad suite of organic acids, amines and amino acids. • Wood emissions, cleaning with bleach and smoking lead to higher indoor than outdoor concentrations for formic and acetic acid, hypochlorous acid, and nicotine, respectively. • In typical indoor environments, excepting CO2, the mass of acids and bases sorbed to surfaces and in condensed-phase water is commonly much larger than the co-occurring mass in air. Keywords: Ammonia, Carbon dioXide, Chemistry, Sources, Surfaces, Water 1 1. INTRODUCTION The chemical composition of indoor air influences its healthfulness as well as its suitability for preserving cultural artifacts and protecting sensitive electronic eQuipment. As measurement technologies have improved, our understanding of the compleXity of indoor air has grown. A striking feature of the atmosphere in general and of indoor environments in particular is the steep increase in the number of chemical species of potential interest as the minimum quantifiable concentration diminishes. In the atmosphere, the number of chemical species present at a level of 0.1% or higher is only four: N2, O2, Ar, and H2O. Decreasing the minimum level of concern to one part per million adds only a few components, such as CO2 and CH4. However, when the threshold for concern is set at a part per billion or a part per trillion, the number of constituents rises to hundreds or thousands of species. These numerous species exhibit a broad range of chemical properties and pose diverse health-risk and material-damage concerns. Even at relatively low fractional abundance, some chemical components may significantly influence the attributes of indoor air. Thinking about the vast number of molecules in a given macroscopic air volume can help to establish perspective. Consider, for eXample, that adults inhale an average of 15 m3 or about 600 moles of air daily. This daily quantity of inhaled air corresponds to almost 4 ´ 1026 molecules. Even at the small fractional abundance of one part per trillion, the daily number of molecules of a trace species inhaled could be nearly 400 trillion. In part because of the large number of compounds of potential interest, it is scientifically valuable to categorize species according to key properties. One prominent eXample is the grouping of organic compounds into categories based on volatility, i.e., very volatile organic compounds, volatile organic compounds, and semivolatile organic compounds.1 Such a grouping allows for more efficient identification and treatment of important physicochemical processes governing the sources, dynamic behavior, and fates of indoor-air constituents than would be possible using a purely chemical-by-chemical approach. This review is concerned with two broad and interrelated categories of chemicals occurring in indoor environments: acids and bases. We are guided principally by the Brønsted-Lowry conceptualization, in which a key feature of an acid is its tendency to donate a proton when in aqueous solution; the key complementary feature of a base is to accept a proton. The review’s scope is specifically restricted to compounds that can be found in indoor air, considering gaseous species and also species primarily associated with airborne particles. The review aims to be thorough but does not aspire to be comprehensive. We do intend to include all major classes of acids and bases that occur indoors with substantial eXploration of specific eXamples within these major classes. The indoor environments of concern are those that are normally occupied and of the types in which people spend much time, including but not limited to residences, schools, and offices. As much as possible, our review approach is strongly grounded in physical science and aims to be incisively critical. We synthesize and report measured concentrations. We are particularly interested in processes that govern such concentrations, including characterizing sources and 2 associated emission rates; factors influencing the dynamic behavior; fates; and conseQuences. Depending on the relative abundance of condensed-phase water indoors and key physicochemical properties of the chemical compounds, aQueous-phase processes can strongly influence the airborne concentrations of acids and bases indoors as well as altering the pH of indoor water. Although there is a deep and eXtensive history of interest in indoor acids (especially) and bases, until now there has not been a systematic and thorough review of the state-of-knowledge for these important chemical classes. As early as the 1850s, MaX von Pettenkofer used indoor abundance of carbonic acid (as gaseous CO2 was then called) to determine the level of ventilation reQuired to achieve good indoor air.2 In the middle of the 20th century, sulfur dioXide emerged as an important urban air pollutant, and studies were undertaken to better understand the eXtent of protection provided by being indoors.3,4 Later, as urban and regional air pollution concerns began to focus on particulate matter, a specific interest emerged in the role of aerosol strong acidity as a potential cause of adverse health effects. Several studies were undertaken in the late 1980s and 1990s to better understand indoor concentrations and associated eXposures of acidic aerosols.5,6 Long-term awareness that acidic pollutants can damage cultural and historic materials has been documented by Baer and Banks.7 Corrosion of metals in indoor environments in relation to acid gases and other pollutants was already studied in the early 1970s.8,9 During the past decade, strong new research interest has emerged concerning indoor acids and bases. One dimension has been some evidence, although not yet conclusive, that eXposure to excessive carbon dioXide levels indoors can impair cognitive performance.10 This concern is but one eXample of a broad array of issues regarding how occupants influence indoor air quality, including through the acidic and basic species they generate, such as the fatty acids in skin oils.11 Following parallel advances in outdoor atmospheric chemistry, a new area of focus indoors is the class of compounds that are water soluble organic gases, of which acids are a major subcategory.12 In addition, instruments that have advanced the study of outdoor atmospheric chemistry are now beginning to be applied indoors. Advanced technologies, such as high-resolution time-of-flight chemical ionization mass spectrometry (CIMS), aerosol mass spectrometry (AMS), and semivolatile thermal-desorption aerosol gas chromatography (SV- TAG) are permitting new aspects of indoor air Quality to be probed, reflecting their capabilities for sensitive measurement with fast time response combined with strong levels of chemical specificity. Recently published studies with such instruments are providing new insights in many aspects of indoor air Quality, including the sources, abundances, and dynamic behaviors of indoor acids and bases.13-15 The body of this review is divided into three main sections. The first considers water in indoor environments. An important topic in its own regard, only certain aspects of indoor water have been well-addressed in prior studies. For this review, it is an important subject because of the strong two-way