We just don’t know enough about either population exposures or exposure-response relationships to make a satisfactory risk assessment.”Scientific knowledge about this subject improved considerably during 1985-1995 and has continued to advance during the past few decades. We can now outline major features of the system, such as what are the causes, nature, and levels of atmospheric aerosol strong acidity; and what are the relationships among indoor concentrations, outdoor concentrations, and personal exposures. But measurements remain challenging, even for stationary sites sampling outdoor air. And so Lippman’s caution retains much of its validity today, more than three decades later.The largest data set regarding particle strong acidity was acquired as part of a respiratory health study conducted at 24 outdoor sites in the United States and Canada. Samples were collected over 24-h periods every second day for one year. Measured parameters included “ozone, particle strong acidity, sulfate, and mass … In 20 of the communities, sulfur dioxide, ammonia, nitric acid, nitrous acid,hydroponic tables canada and particulate nitrate were measured.”Table 22 reproduces the annual and summertime mean concentrations of particle strong acidity. The grand average across all 24 sampling sites was 28 nmol/m3 for the annual period and 44 nmol/m3 for the summer.
The studied sites were more highly concentrated in areas expected to have elevated acidity. The different averages among the geographic clusters illustrate large-scale spatial variability, with annual averages of 41 for the “sulfate belt,” in the “transport region,” for the “West coast” sites, and 6 for the “background” sites. The data reported in Table 22 were acquired using a sampling system developed by Koutrakis et al.For determining particle strong acidity, particles larger than 2.1 µm in diameter are excluded by means of an inlet impactor. The sampled air then passes through two denuders in series to remove acidic gases and ammonia. Fine particles, collected on a Teflon filter downstream of the denuders, are extracted after sampling and analyzed for strong-acid pH.From the pH measurement result and the sampling conditions, particle strong acidity is determined, in units of nmol of H+ per m3 of air sampled. Two essential features of the measurement method should be highlighted. First, removing coarse particles from the sampling stream allows the acidity of fine particles to be isolated from the potential neutralizing contributions of basic minerals associated with coarse airborne particles. Lipfert et al. cautioned that, “an aerosol sampling device that combines small acidic particles with larger basic particles … may yield misleading information with respect to biological responses.”Second, the use of denuders avoids artifact generation that would result from acidic and basic gases interacting with previously collected particles or with the filter material itself. That particle pH varies with size and that small particles tend to be more acidic has been demonstrated in several studies. For example, using a cascade impactor, Ludwig and Klemm determined the size-dependent acidity of aerosol particles at three locations in Bavaria, Germany. They reported that, “the in situ pH’s were calculated as pH 1 … 2 for these [fine] particles at all sampling sites.
Coarse particles were only slightly acidic, with a mean in situ pH 5.5 … 6.5.” Fang et al. assessed the pH of size-segregated aerosol particles sampled from roadside and urban sites in Atlanta, GA.Their assessment used a thermodynamic model applied to measured ionic species. Quoting a key result, “sulfate was spatially uniform and found mainly in the fine mode, whereas toxic metals and mineral dust cations were highest at the road-side site and in the coarse mode, resulting in fine mode pH < 2 and near neutral coarse mode.”The large-scale pattern of aerosol strong acidity is mainly controlled by the respective spatial distribution of the key precursors. On a regional scale, atmospheric sulfate concentrations are relatively uniform owing to the combined effects of numerous emission sources of SO2 , atmospheric mixing prior to secondary atmospheric production of sulfate from the oxidation of SO2, and relatively slow removal of sulfate from the atmosphere. Being a primary pollutant, ammonia exhibits a spatial pattern more closely associated with the pattern of source emissions, which tend to be more concentrated in urban areas and in rural areas with intensive agricultural activity as contrasted with more remote rural environments. Brook et al. describe the “Canadian Acid Aerosol Measurement Program,” with sampling conducted over three years at 10 sites. They reported that “acidities were lower in areas where the fine particle acidity experienced greater neutralization from reaction with ammonia. This included the major urban centres and areas with greater amounts of agricultural activity, as in rural southern Ontario.”Suh et al. studied the spatial variability of aerosol strong acidity in and around Philadelphia. They reported that, “outdoor sulfate concentrations were uniform within metropolitan Philadelphia; however, aerosol strong acidity concentrations varied spatially. This variation … was related to local factors, such as the NH3 concentration.”Interpreting results from a measurement campaign conducted in three sites in Pennsylvania during the summer of 1990, Liu et al. reported that “aerosol acidity was found to be lower in the urban area than the semi-rural areas.
Ammonia levels were higher at the urban site than in the semi-rural environments, probably due to the higher population density at the urban site.”The respective balances between atmospheric sulfate and ammonia levels is believed to be responsible for the observation that fine particles in the air in and around Beijing, China, are much less acidic than in North America and Europe. Liu et al. studied the pH of fine particles in Beijing during selected haze episodes occurring during late 2015 and 2016. Using a thermodynamic model to interpret measurements of particle-phase ions and precursor gases, they reported that, “Fine particles were moderately acidic, with a pH range of 3.0-4.9 and an average of 4.2 … Excess NH3 and high aerosol water content are responsible for the relatively lower aerosol acidity.” They reported remarkably high levels of aerosol water content during the haze episodes, up to several hundred µg m-3 . Ding et al. describe a more extensive investigation of the pH associated with PM2.5 particles in Beijing. Among the features that would support such an emphasis would be the lower aerosol water content levels during cold weather. A year-long study in Detroit, MI, using a sampler with an open-faced filter, showed seasonal variation in aerosol strong acidity with highest values during the summer: 39 nmol/m3 , 15 nmol/m3 , 13 nmol/m3 , and 3 nmol/m3 . An intensive study of aerosol strong acidity during summer months in Toronto separately assessed concentrations during daytime and overnight . Averaging first across the monitoring sites and then across the three years,microgreen rack for sale the levels were somewhat higher during daytime hours than overnight . With regional variation, the overall global trend has been a decrease in anthropogenic SO2 emissions over the period 1990-2015. As a result, one might expect substantial shifts in aerosol strong acidity. However, while there is agreement that SO2 emissions and atmospheric sulfate levels are decreasing, there isn’t a consensus about the consequences for acidity. Using a modeling approach, Murphy et al. report that, “steep increases in pH and the gas fraction of NHx are found as NHx:SO4 varies from below 1 to above 2.”They state that, “regions of the world where the ratio of NH3:SO2 emissions is beginning to exceed 2 on a molar basis may be experiencing rapid increases in aerosol pH of 1-3 pH units.” On the other hand, focusing on a rural area in the southeastern US, and combining experimental observations with modeling interpretation, Weber et al. conclude that, “the reductions in aerosol acidity widely anticipated from sulfur reductions, and expected acidity-related health and climate benefits, are unlikely to occur until atmospheric sulfate concentrations reach near pre-anthropogenic levels.”During the period of most intensive study of aerosol strong acidity, which centered on the decade 1985-1995, several investigations reported indoor conditions and/or personal exposures. Key findings are presented here, in approximate chronological order. Spengler et al. provided one of the earliest reports substantially concerned with acidic aerosols indoors and associated exposures.
They stated that “acidic aerosols occurring indoors are assumed to originate from outdoors.” They also reported that, “indoor gaseous ammonia concentration is expected to be higher compared to outdoors since it is produced by people, pets, and household products.” In considering exposures, they stressed the importance of micro-environmental conditions and time-activity patterns, highlighting, for example, that “children are more likely to be outdoors during the day, particularly in the summer.” They combined micro-environmental measured and modeled concentrations with time-activity patterns to estimate means and percentiles of the distribution of exposures to aerosol strong acidity for children. For Portage, Wisconsin, the annual average exposure concentration for aerosol H+ so determined was 7.6 nmol/m3 , with variation among averages between 1.2 nmol/m3 for winter and 18 nmol/m3 for summer daytime conditions. Corresponding results for Steubenville, Ohio, were 24 nmol/m3 for the annual average, 5.1 nmol/m3 for winter average, and 55 nmol/m3 for summer daytime average. This report highlighted the finding that atmospheric acidic aerosols could be elevated episodically: “measurements made in Kingston, TN, and Steubenville, OH, resulted in 24-h H+ ion concentrations exceeding 100 nmol/m3 more than 10 times during summer months.”An important conclusion from their investigation is that, “children engaged in summertime outdoor activities can experience H+ doses comparable to effects levels reported in human clinical studies.” Brauer et al.undertook the first direct experimental study of personal exposure to particle strong acids. Sampling was carried out in the Boston metropolitan area for 24 days during the summer of 1988. Two volunteers were each outfitted with two personal sampling systems similar to that described in Koutrakis et al.In each case, one sampler was operated continuously to collect a 24-h total exposure. For one subject, the second sampler was turned on only when outdoors; for the other subject, the second sampler was turned on only when indoors. Separate stationary samplers were used to measure aerosol strong acidity at a central monitoring site outdoors and overnight in three residences. The authors reported that “personal exposures to aerosol strong H+ were slightly lower than concentrations measured at the stationary site due to the neutralization of acidic particles and their incomplete penetration into indoor environments.”Using the same type of sampling system, indoor and outdoor concentrations of aerosol strong acidity were sampled in 11 homes in the Boston area during late winter and summer .In this study, the indoor/outdoor ratio of fine-particle strong acidity had geometric mean values of 0.48 in the summer and 0.36 in the winter. The mean ± standard deviation indoor H+ concentration was 2.4 ± 1.8 nmol/m3 in the winter and 8.8 ± 4.8 nmol/m3 in the summer. The authors reported that, “Indoors, we found a large available excess of NH3, which apparently coexisted at times with particle acidity.”Liang and Waldman measured indoor aerosol strong acidity at three institutional sites in New Jersey: a child care facility, a nursing home, and a home for the elderly. Simultaneously, outdoor sampling was conducted at a nearby central station and at the home for the elderly. Sampling was conducted during a six-week period, June-July 1989. The sampling train included a particle impactor that excluded particles larger than 2.5 µm and a denuder to remove gaseous ammonia. Sampling was conducted for 12-h daytime periods at all three indoor and both outdoor sites. Nighttime samples were also collected indoors at the elderly home and at both outdoor sites. The number of samples collected varied between 28 and 41 for each combination of conditions. Table 23 reproduces the mean and 90th percentile values of H+ concentrations reported by Liang and Waldman. The authors concluded that, “75% of the daily dose of aerosol acidity for the elderly was due to indoor exposures” and that “these data suggest that indoor settings are protective, but children may still be at risk from summertime acidic aerosol exposure, depending on their activities outdoors.”Suh et al. studied indoor, outdoor, and personal exposure to aerosol strong acidity in Uniontown, PA130 and in State College, PA. The Uniontown study focused on 24 children with monitoring conducted during summer 1990.