Indoor ultrafine particles (UFP, <100 nm) undergo aerosol processes such as coagulation and deposition, which alter UFP size distribution and accordingly the level of exposure to UFP of different sizes. This study investigates the decay of indoor UFP originated from five different sources: a gas stove and an electric stove, a candle, a hair dryer, and power tools in a residential test building. An indoor aerosol model was developed to investigate differential effects of coagulation, deposition, and ventilation. The coagulation model includes Brownian, van der Waals, and viscosity forces, and also fractal geometry for particles of >24 nm. The model was parameterized using different values of the Hamaker constant for predicting the coagulation rate. Deposition was determined for two different conditions: central fan on versus central fan off. For the case of a central fan running, deposition rates were measured by using a nonlinear solution to the mass balance equation for the whole building. For the central fan off case, an empirical model was used to estimate deposition rates. Ventilation was measured continuously using an automated tracer gas injection and sampling system. The study results show that coagulation is a significant aerosol process for UFP dynamics and the primary cause for the shift of particle size distribution following an episodic highconcentration UFP release with no fans operating. However, with the central mechanical fan on, UFP deposition loss is substantial and comparable to the coagulation loss. These results suggest that Received 1 July 2011; accepted 25 October 2011. Donghyun Rim's participation in this project was funded by the National Institute of Standards and Technology (NIST) through a US Intergovernmental Personal Act with the University of Texas at Austin. Michal Green participated in this study as a foreign guest researcher at the NIST while on sabbatical from the Israel Atomic Energy Commission; she contributed by collecting ultrafine particle data from consumer product tests. Jung-Il Choi is supported by Basic Science Research program (2011-0014558) and World Class University (WCU) program (R31-10049) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology. However, the manuscript does not necessarily reflect the views of these agencies and no official endorsement should be inferred.Address correspondence to Jung-Il Choi, Department of Computational Science and Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, South Korea. E-mail: jic@yonsei.ac.kr coagulation should be considered during high concentration periods (>20,000 cm −3 ), while particle deposition should be treated as a major loss mechanism when air recirculates through ductwork or mechanical systems.

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The sensitivity of nitrifying bacteria to acidic conditions is a well-known phenomenon and generally attributed to the lack and/or toxicity of substrates (NH 3 and HNO 2 ) with decreasing pHs. In contrast, we observed strong nitrification at a pH around 4 in biofilms grown on chalk particles and investigated the following hypotheses: the presence of less acidic microenvironments and/or the existence of acid-tolerant nitrifiers. Microelectrode measurements (in situ and under various experimental conditions) showed no evidence of a neutral microenvironment, either within the highly active biofilm colonizing the chalk surface or within a control biofilm grown on a nonbuffering (i.e., sintered glass) surface under acidic pH. A 16S rRNA approach (clone libraries and fluorescence in situ hybridizations) did not reveal uncommon nitrifying (potentially acid-tolerant) strains. Instead, we found a strongly acidic microenvironment, evidence for a clear adaptation to the low pH in situ, and the presence of nitrifying populations related to subgroups with low K m s for ammonia (Nitrosopira spp., Nitrosomonas oligotropha, and Nitrospira spp.). Acid-consuming (chalk dissolution) and acid-producing (ammonia oxidation) processes are equilibrated on a low-pH steady state that is controlled by mass transfer limitation through the biofilm. Strong affinity to ammonia and possibly the expression of additional functions, e.g., ammonium transporters, are adaptations that allow nitrifiers to cope with acidic conditions in biofilms and other habitats.Chemolithoautotrophic nitrifying bacteria, i.e., ammoniaoxidizing bacteria (AOB), catalyzing the first oxidation step of ammonia to nitrite and nitrite-oxidizing bacteria (NOB) completing the oxidation of the intermediate nitrite to nitrate are known to be sensitive to low pHs. Optimum growth occurs under neutral to moderately alkaline conditions (pH 7.5 to 8.0). In liquid pure culture, growth is usually restricted to a lower pH of 5.8 (AOB) or 6.5 (NOB) (62) and activity ceases typically below pH 5.5 (28, 31). The failure of AOB to cope with acidic conditions is thought to be primarily based on the unavailability of a substrate: with decreasing pHs, ammonia, the substrate of AOB (58), is increasingly protonated. Nitrite, the substrate of NOB, undergoes protonation to nitric acid, which disproportionates to nitrate and gaseous nitric oxide at low pHs (6). Furthermore, when present at elevated concentrations under low pHs, free nitric acid negatively affects the growth and activity of nitrifying bacteria (4).Despite these limitations, autotrophic nitrifying bacteria have been isolated from, or nitrifying activity has been demonstrated in, acidic environments, such as soils, activated sludge, and biofilms. Numerous nitrifying isolates have been obtained from soils with pHs around 4 (for a review, see reference 18 and references therein) to even as low as 2.5 (45). However, the majority of such isolates do not show nitrifying activity in acidic mineral medium (18). In contrast, autotrophic nitr...

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