A Novel Method For Measurement Of Acidic Ultrafine Particles And Its Application

Atmospheric particulate pollution is a major public concern due to the fact that particulate matters are closely related to human respiratory health, visibility reduction, eco-environmental damage and global climate. The size of particles is directly linked to their potential for causing health problems. The adverse effects of particles with diameter less than10μm(PM10) and less than2.5μm (PM2.5) have been widely studied and well recognized in the past decades. Recently, more and more studies have found that much smaller particles such as ultrafine particles (<100nm in diameter)in the air may be more important than PM2.5and PM10to cause adverse health effects, because they normally contain trace elements and toxins and can lodge deep into the lungs and some may get into the bloodstream, owing to the high diffusion coefficients of the particles. Another fact is that more than90%of all airborne particles, both outdoors and indoors, when measured by number concentration, are generally found to be ultrafine particles。Therefore, people have high risk of exposureto these ultrafine particles which can affect both lungs and heart. Although growing epidemiological data indicates consistent and coherent associations between ambient ultrafine particles and health decrements such as mortality and morbidity, it is unlikely that all components of ultrafine particles are equally toxic. In fact, sulphuric acid and ammonium bisulfate, both strongly acidic, are important components of ultrafine particles in the air. Indeed, accumulated evidence strongly suggests that the number of acid-containing ultrafine particles is more closely correlated with total mortality, morbidity and hospital admissions for respiratory diseases. Environmental effects of acidic particle pollution include reduced visibility, environmental damage, and climate change. Thus, it is critical to be able to distinguish between the number of acidic ultrafine particles and the total number of ultrafine particles. It will provide useful data to air quality scientists, epidemiologists and policy makers to better understand the interplay among air pollution, climate change and human health. However, so far no reliable measurement techniques are available to obtain the number concentrations and size distributions of acidic ultrafine particles. Based on the reasons mentioned above, therefore, we utilized the physical and chemical properties of acidic ultrafine particles, developed a new method for measuring number concentrations and size distributions of acidic ultrafine particles in the atmosphere. The main contents and results were summarized as follows: (1) A method with the use of iron nanofilm detectors for enumeration and size measurement of acid aerosols is developed and refined. Standard sulfuric acid (H2SO4) or ammonium hydrogen sulfate (NH4HSO4) droplets and sulfuric acid-coated particles were generated and deposited on the detectors causing reaction spots. The dimensions of the reaction spots were examined with Atomic Force Microscopy (AFM) to establish the correlations between the diameter of the particle and the size of the reaction spot. To validate this method, field measurements were conducted from September to November,2010, at Tai Mo Shan in Hong Kong. The results indicated that the particle number concentrations obtained from the AFM scanning of the exposed detectors via scanning mobility particle sizer (SMPS) and electrostatic precipitator (ESP) collection were comparable to those derived from the SMPS+CPC (condensation particle counter) measurements (p>0.05). The average geometric mean diameter of particles at peak measured by the SMPS+CPC and the detectors scanned by the AFM was52.3±6.9nm and51.9±3.1nm, respectively, showing good agreement. It is suggested that the iron nanofilm detectors could be a reliable tool for the measurement and analysis of acidic particles in the atmosphere.(2) A diffusion sampler (DS) with iron nanofilm detectors was designed to effectively measure the number concentration and size distribution of airborne AUFPs in indoor and outdoor environments. The DS was made of stainless steel with a flat and rectangular channel with1.0mm height,50mm width and500mm length. The iron nanofilm detectors were deployed on rectangular recesses inside the sampler at three different locations along the length of the channel to collect the ultrafine particles. The exposed detectors were then scanned using an Atomic Force Microscope (AFM) to numerate and distinguish the AUFPs from the non-acidic UFPs. Prior to sampling, the semi-empirical equations for the diffusive deposition efficiency of particles at the different detector locations in the sampler were obtained based on theoretical diffusive mechanism and modified by the experimental data. It was found that the experimentally measured deposition efficiencies were all higher than the theoretical values for the designed DS; the stepwise deposition efficiency of particles in each size decreased with the increase in transport distance from the inlet; and the stepwise deposition efficiency of particles decreased with the increase in flow rate, showing negative power-law relationship at the first location. After calibration, the DS+AFM method and a commercially available online measurement system, i.e. SMPS+CPC, were simultaneously used in a one-month field measurement conducted at an urban site. Both methods showed very good agreement in terms of total particle number concentration and size. Therefore, it is reasonable to assume that the number concentration and size distribution of acidic particles estimated by the DS+AFM method were reliable. Moreover, we found that the highest fraction of acidic particles to the total particles was in the size range of5.5-30nm with a mean value of65%, while the lowest fraction was in the size range of100-200nm with an average of8%. The result indicated that most acidic particles at this urban site were ultrafine particles and relatively fresh, which were probably emitted from primary source(s) or formed by secondary new particle formations.(3) Based on the acidic particle number concentrations and particle size distributions during the one-month sampling, the causes and factors influencing acidic particle number concentration at this urban site were investigated. Northeastern monsoon prevailed during the sampling period. Apart from the particle number peaks appeared in traffic rush hours (i.e.08:00-09:00and17:00-18:00), a distinct peak of particle number concentrations in the afternoon (11:00-16:00) was observed during the sampling period. Concurrent measurement data of particle size distributions, ozone (O3) and proxy sulfuric acid (H2SO4) concentrations revealed that the afternoon peaks observed were likely due to new particle formation (NPF) via photochemical reactions. These NPF events were frequently observed under a clean and dry weather in Hong Kong. The occurrence of NPF was closely associated with high solar radiation (SR), low relative humidity (RH) and low condensation sink (CS) in the atmosphere. Besides the NPF events, we also found four nucleation mode particle burst events, typically with increased number concentrations of nucleation mode particles without growth to larger size particles. These burst events were generally accompanied by high-level primary air pollutants, i.e. sulfur dioxide (SO2), nitrogen oxide (NOx) and carbon monoxide (CO). The very different characteristics of the burst events from those of the NPF events indicated that these nucleation mode particle burst events were not caused by the photochemical reactions, but by the primary emission from the local combustion source(s).