Abstract
To clarify the aerosol optical properties under different pollution levels and their impacting factors, hourly organic carbon (OC), elemental carbon (EC), and water-soluble ion (WSI) concentrations in PM2.5 were observed by using monitoring for aerosols and gases (MARGA) and a semicontinuous OC/EC analyzer (Model RT-4) in Wuhan from 9 to 26 January 2018. The aerosol extinction coefficient (bext) was reconstructed using the original Interagency Monitoring of Protected Visual Environment (IMPROVE) formula with a modification to include sea salt aerosols. A good correlation was obtained between the reconstructed bext and measured bext converted from visibility. bext presented a unimodal distribution on polluted days (PM2.5 mass concentrations > 75 μg⋅m−3), peaking at 19:00. bext on clean days (PM2.5 mass concentrations < 75 μg⋅m−3) did not change much during the day, while on polluted days, it increased rapidly starting at 12:00 due to the decrease of wind speed and increase of relative humidity (RH). PM2.5 mass concentrations, the aerosol scattering coefficient (bscat), and the aerosol extinction coefficient increased with pollution levels. The value of bext was 854.72 Mm−1 on bad days, which was 4.86, 3.1, 2.29, and 1.28 times of that obtained on excellent, good, acceptable, and poor days, respectively. When RH < 95%, bext exhibited an increasing trend with RH under all pollution levels, and the higher the pollution level, the bigger the growth rate was. However, when RH > 95%, bext on acceptable, poor and bad days decreased, while bext on excellent and good days still increased. The overall bext in Wuhan in January was mainly contributed by NH4NO3 (25.2%) and organic matter (20.1%). The contributions of NH4NO3 and (NH4)2SO4 to bext increased significantly with pollution levels. On bad days, NH4NO3 and (NH4)2SO4 contributed the most to bext, accounting for 38.2% and 27.0%, respectively.
Highlights
In recent years, air quality deterioration and visibility impairment have occurred in many urban areas of China [1,2,3,4]
As the central city of central China, Wuhan is located in the eastern part of Hubei province, at the intersection of the Yangtze River and Hanshui
Wuhan is covered by rivers and lakes, and water accounts for about a quarter of the total area of the city
Summary
Air quality deterioration and visibility impairment have occurred in many urban areas of China [1,2,3,4]. Atmosphere 2020, 11, 17 than 2.5 μm), which has a great effect on visibility impairment, urban air quality, and human health [5,6,7]. PM2.5 in the atmosphere can reduce visibility through light extinction (including scattering and absorption of light) and plays an important role in the formation of haze in urban areas [10,11]. Bext is mainly contributed by (NH4 ) SO4 , NH4 NO3 , organic matter (OM), elemental carbon (EC), sea salt, coarse mass, and so on. Their relative contributions to bext vary with time and location.
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