Influence Of PM_2.5 Major Components On Aerosol Acidity And Light Extinction In The Pearl River Delta Region

Organic matter（OM）, elemental carbon（EC）, sulfate（SO42-）, nitrate（NO3-） and ammonium（NH4+） are the main components of fine particles（PM₂.5）. PM₂.5 and their major components derived from variety of natural and anthropogenic sources, i.e. emitted by combustion and industrial processes, and formed from a series of complicated gas-particle reactions in the atmosphere. PM₂.5 has a severe effect on human health and is implicated in ecosystems and regional climate. With national economy developing by leaps and bounds, many city clusters in China currently suffer severe air pollutions problems caused by fine particles. During these episodes, ambient PM₂.5 levels were well above the Air Quality Guidelines of World Health Organization（WHO）, resulting in adverse effects to public health and daily life. Facing with the increasingly grim air pollution problems, Chinese government enacted new national ambient air quality standards in 2012, which for the first time include PM₂.5（NAAQS, GB3095-2012）. The new standard was took the lead in the implementation in the three economically relevant and densely populated city clusters, i.e. the North china Plain（NCP）, the Yangtze River Delta（YRD）, and the Pearl River Delta（PRD） region.The PRD region is one of the most economically dynamic areas in China, which occupies only 0.5% of the national land but contributes to about 10% of the national gross domestic products, and is home to around 10% of the population. This region has been through severe air pollution problems in the past years. For instance, 144 hazy days were found in Guangzhou, the major city in the PRD region, in 2004. Local government has carried out a series of control measures on the emission reduction, such as the phasing out of small coal-fired power generation unites, the establishment of stricter emission standards for industrial boilers and vehicles, and improvments in the quality of vehicle fuel. Therefore, the air pollution in this region was alleviated obviously in recent years, the annual hazy days of Guangzhou was decreased to about 50 days in 2013. A systematic, long-term investigation into the variations in the mass concentration of PM₂.5 and major components, as well as their impacts on aerosol acidity and light extinction, which will be useful in the formulating and implementing of the control measures for air pollutant in the PRD region, and also of value to other Chinese city clusters. Hence, in this study, 24 h PM₂.5 filter samples were collected from a background site in the PRD region in fall-winter of 2007-2013. The main results were listed below:1. The mass concentration of PM₂.5 decreased from 113 ± 8 μg m-3（average ± 95% Confidence Interval, the same as follows） in 2007 to 79 ± 8 μg m-3 in 2011, with the reduction rate of 8.6 μg m-3 yr-1（p < 0.01）. During fall-winter of 2007-2011, the average concentrations of OM, SO42-, NO3-, NH4+ and EC were 37 ± 2, 18 ± 1, 10 ± 1, 7 ± 0.4 and 4 ± 0.3 μg m-3, respectively. They constituted 79 ± 12% of the PM₂.5 mass. Specifically, OM accounted for the largest proportion（38 ± 8%） of PM₂.5, then SO42-（19 ± 6%） and NO3-（10 ± 4%）.2. In general, the reduction of PM₂.5 was mainly due to the decrease of OC and SO42- in fall-winter of the PRD region. OC and SO42- showed the decreasing rate of 1.1 and 1.7 μg m-3 yr-1, respectively（p < 0.01）. By contrast, NO3- displayed an increasing rate of 0.8 μg m-3 yr-1（p < 0.05）. NH4+ and EC did not present significant reduction trend.3. The concentrations of SO2 decreased much more rapidly than SO42-, with a decreasing rate of 10.2 μg m-3 yr-1（p < 0.01）. Our data showed that each 1 μg m-3 reduction in SO2 only caused a 0.2 μg m-3 decrease in SO42-. The faster reduction of SO2 than SO42- was associated with the co-effect of several reasons, which made the conversion efficiency of SO2 to SO42- to be more rapid. Firstly, the series of effective control measures made the SO2 emission decreased over this region. Next, in recent years, PM₂.5 concentrations have significantly decreased in the PRD region, resulting in enhanced solar radiation and atmospheric oxidizing capacity in the lower troposphere. In addition, due to the aerosol acidity decreased, the solubility of SO2 was enhanced in the particle phase and certain oxidation processes were accelerated. On the other hand, the NOx concentrations increased more rapidly than those of the NO3-, with a decreasing rate of 6.7 μg m-3 yr-1（p < 0.05）. Specifically, every 1 μg m-3 increase in NOx concentration resulted in a 0.12 μg m-3 increase in NO3-concentration in fall-winter of the PRD region. This phenomenon was most likely due to increased power generation and vehicle numbers, which offset efforts to control coal-fired power plants, and NOx was converted into NO3- and other nitrogen compounds.4. In fall-winter of 2007-2012, the average daily concentrations of total acidity（[H+]total） and in-situ acidity were 123 ± 13 and 34 ± 6 nmol m-3, respectively. [H+]total and [H+]in-situ significantly decreased at a rate of-32 ± 1.5 nmol m-3 yr-1 and-9 ± 1.7 nmol m-3 yr-1, respectively（p < 0.05）. With the influence of steady concentration of liquid water content（LWC） in these years, p H showed an increasing rate of 0.3 ± 0.1 yr-1（p < 0.05）. The descending trend of aerosol acidity was attributable to the fact that the decreasing rate of [H+]total due to the decrease of SO42-exceeded the increasing rate caused by the growth of NO3- in these years.5. The annual average [H+]total, [H+]in-situ, [HSO4-] and LWC on hazy days were 0.9-2.2, 1.2-3.5, 0.9-2.0 and 2.0-3.0 times those on non-hazy days, respectively. The average proportion of [H+]in-situ in [H+]total increased from 19 ± 4% in non-hazy days to 27 ± 11% in hazy days, while the proportion of [HSO4-] did not show obvious discrepancy during the two periods. The large amount of LWC appeared in the hazy period helped to dissociate more in-situ H+, which was not caused by the dissociation of HSO4-, but probably by the secondarily formed water-soluble organic/inorganic acids.6. The significant logarithmic relationship was found between R（i.e. [NH4+] /（2× [SO42-] + [NO3-]）） and [H+]in-situ（p < 0.05）. R values of 0.6（i.e. [H+]in-situ = 83 nmol m-3） could be treated as the threshold for different acidity of aerosol in the fall-winter of the PRD region. In hazy days, OM was affected by the degree of aerosol acidity. The mass concentrations of OM showed significant enhancement with the increase of [H+]in-situ during hazy period（p < 0.05）. The average difference for OM between low and high [H+]in-situ on hazy days reached 20 μg m-3. The mainly reason was as follows: During fall-winter in the PRD region, the acid-catalyzed SOC formation on hazy days could be more significant when aerosol acidity was larger than 83 nmol m-3.7. The average visibility significantly increased from 12 ± 1 km in 2007 to 22 ± 4 in 2013（p < 0.01）, with a rate of 1.1 km yr-1. At the same time, the increasing rate for the annual value of 75 th percentile was two times higher than that for the value of 25 th percentile（1.5 km yr-1 vs. 0.5 km yr-1）. The emission control measures in the PRD region indeed made the good visual range of people to be even better, while the days with low visibility was not significantly improved in recent years.8. The visibility decreased with the increasing mass concentration of PM₂.5 in function of power for a certain RH range, and showed a faster decreasing rate with higher RH. In the dry condition（23% ≤ RH < 60%）, the threshold mass concentration of PM₂.5 corresponding to the visibility of < 10 km was 138 μg m-3. When the RH was high（80% ≤ RH < 90%）, the threshold PM₂.5 concentration corresponding to visibility < 10 km was 74 μg m-3. The daily standard for PM₂.5 in NAAQS（75 μg m-3） could guarantee the disappearance of hazy episode in fall-winter of the PRD region.9. During fall-winter of 2007-2013, the daily average light extinction coefficient（bext） was 505 ± 29 Mm-1. The chemical budget of bext for organic matter（OM）, ammonium sulfate（AS）, ammonium nitrate（AN）, sea salt（SS） and light absorbing carbon（LAC） was 227 ± 13, 127 ± 9, 65 ± 7, 60 ± 8 and 26 ± 2 Mm-1, respectively. OM accounted for the largest proportion of bext, which was 46 ± 2%. AS and AN constituted 26 ± 1% and 12 ± 1% of bext. During the whole sampling time, bext showed a significant reducing rate of 14 Mm-1 yr-1（p < 0.05）. Meanwhile, the growth trend of hygroscopic growth factor（f（RH）） offset the contribution of AS and AN for reducing bext.