Abstract
Abstract. Elucidating the relationship between characteristics of aerosol particles and optical absorption is important to deepen the understanding of atmospheric chemistry. Aerosol particles play significant roles in climate forcing via their optical absorption properties. However, the relationship between characteristics of aerosol particles and optical absorption remains poorly understood. Aerosol optical properties and morphologies were measured by a transmission electron microscope (TEM), cavity ring-down spectrometer (CRDS), a nephelometer and an Aethalometer in a urban site of Beijing from 24 May to 22 June. Five episodes were categorized according to the meteorological conditions and composition. The results showed that the clear episode (EP-2 and EP-4) featured as the low aerosol optical depth (AOD = 0.72) and fewer pollutants compared with haze (1.14) and fog (2.92) episodes and the particles are mostly externally mixed. The high Ångström exponent (> 2.0) suggests that coarse particles were scarcely observed in EP-2 due to the washout of a previous heavy rain, whereas they were widespread in EP-4 (Ångström exponent = 0.04), which had some mineral particles introduced from the north. In contrast, industry-induced haze (EP-1) and biomass-burning-induced haze (EP-5) were both affected by the south air mass. Compared with the EP-2 and EP-4, the AOD values and the size distribution of particles during EP-1 and EP-5 were much greater because of relatively high particle concentrations. All of the particles were classified into nine categories, i.e. S-rich, N-rich, mineral, K-rich, soot, tar ball, organic, metal and fly ash, on the basis of TEM analysis. In contrast to the EP-1, a large fraction of soot, which sticks to KCl, sulfate or nitrate particles, was detected during EP-5. Additionally, evident enhancement of light absorption was observed during the EP-5, which was mainly ascribed to both black carbon (BC) acceleration and other absorbing substances. However, soot was found mostly internally mixed with sulfate and nitrate during a soot fog episode (EP-3), resulting in evident enhancement of light absorption. The larger size distribution was likely to be caused by both hygroscopic growth and collision between particles during the aging. About 28 % of particles were internally mixed during the foggy days, which favoured the light absorption. The comparison of all the episodes provides a deeper insight into how mixing states influence the aerosol extinction properties and also a clue as to how to control air pollution in the crop burning seasons.
Highlights
Aerosol particles are ubiquitous in the troposphere and exert an important influence on global climate and the environment (Ramana et al, 2010)
Haze is usually defined as a weather phenomenon that lasts at least 4 h when the visibility is less than 10 km and relative humidity (RH) is lower than 80 % (Sun et al, 2006), while fog was characterized with a higher RH, larger than 90 %, according to the Chinese Meteorological Administration
The first episode (EP-1) was from 24 to 29 May, when a haze event occurred with the south wind bringing in the industrial pollution from the heavily polluted cities in the south, which conformed to the 3-day back trajectories shown in Fig. 2a, showing the air masses passing through Henan, Shandong, Hebei and Tianjin before arriving at the sampling site
Summary
Aerosol particles are ubiquitous in the troposphere and exert an important influence on global climate and the environment (Ramana et al, 2010). Once biomass burning particles are mixed with other atmospheric components during aging and transport, such as sulfate and dust, solar absorption is further amplified due to the formation of internally mixed particles (Ramanathan et al, 2005) Such mixtures of absorbing and scattering aerosols at the regional scale are referred to as “ABCs” (abbreviated from “atmospheric brown clouds”) (Ramanathan et al, 2007). Large uncertainties exist in estimates of the radiative forcing of haze particles because of the lack of detailed in situ measurements of the mixing state and the associated optical properties as a function of particle size and composition (Moffet and Prather, 2009) These uncertainties limit our ability to quantify the relative impacts of soot on climate, limiting our ability to make effective policy decisions. Back trajectory analyses suggest flow patterns consistent with long-range transport of agricultural smoke to the study site during periods when the sampling site was engulfed in severe haze and fog
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