B. L. Zhuang

The interactions between anthropogenic aerosols and the East Asian summer monsoon using RegCCMS

An online coupled regional climate-chemistry model called RegCCMS is used to investigate the interactions between anthropogenic aerosols and the East Asian summer monsoon (EASM) over East Asia. The simulation results show that the mean aerosol loading and optical depth over the region are 17.87 mg/m2 and 0.25, respectively. Sulfate and black carbon (BC) account for approximately 61.2% and 7.8% of the total aerosols, respectively. The regional mean radiative forcing (RF) is approximately −3.64, −0.55, and +0.88 W/m2 at the top of the atmosphere for the total aerosol effect, the total aerosol direct effect, and the BC direct effect, respectively. The surface direct RF of BC accounts for approximately 31% of the total RF of all aerosols. Because of the total aerosol effect, both the energy budgets and air temperature are considerably reduced in the region with high aerosol loadings, leading to decreases in the land-ocean air temperature gradient in summer. The total column-absorbed solar radiation and surface air temperature decrease by 8.4 W/m2 and 0.31 K, respectively. This cooling effect weakens horizontal and vertical atmospheric circulations over East Asia. The wind speed at 850 hPa decreases by 0.18 m/s, and the precipitation decreases by 0.29 mm/d. The small responses of solar radiation, air temperature, and atmospheric circulations to the BC warming effect are opposite to those of the total aerosol effect. The BC-induced enhancement of atmospheric circulation can increase local floods in south China, while droughts in north China may worsen in response to the BC semidirect effect. The total aerosol effect is much more significant than the BC direct effect. The East Asian summer monsoon becomes weaker due to the total aerosol effect. However, this weakness could be partially offset by the BC warming effect. Sensitivity analyses further indicate that the influence of aerosols on the EASM might be more substantial in years when the southerlies or southwesterlies at 850 hPa are weak compared with years when the winds are strong. Changes in the EASM can induce variations in the distribution and magnitude of aerosols. Aerosols in the lower troposphere over the region can increase by 3.07 and 1.04 µg/m3 due to the total aerosol effect and the BC warming effect, respectively.

Characteristics of atmospheric mercury deposition and size-fractionated particulate mercury in urban Nanjing, China

A comprehensive measurement study of mercury wet deposition and size-fractionated particulate mercury (HgP) concurrent with meteorological variables was conducted from June 2011 to February 2012 to evaluate the characteristics of mercury deposition and particulate mercury in urban Nanjing, China. The volume-weighted mean (VWM) concentration of mercury in rainwater was 52.9 ng L −1 with a range of 46.3–63.6 ng L −1. The wet deposition per unit area was averaged 56.5 μg m −2 over 9 months, which was lower than that in most Chinese cities, but much higher than annual deposition in urban North America and Japan. The wet deposition flux exhibited obvious seasonal variation strongly linked with the amount of precipitation. Wet deposition in summer contributed more than 80 % to the total amount. A part of contribution to wet deposition of mercury from anthropogenic sources was evidenced by the association between wet deposition and sulfates, as well as nitrates in rainwater. The ions correlated most significantly with mercury were formate, calcium, and potassium, which suggested that natural sources including vegetation and resuspended soil should be considered as an important factor to affect the wet deposition of mercury in Nanjing. The average Hg P concentration was 1.10± 0.57 ng m−3. A distinct seasonal distribution of HgP concentrations was found to be higher in winter as a result of an increase in the PM 10 concentration. Overall, more than half of the Hg P existed in the particle size range less than 2.1 μm. The highest concentration of Hg P in coarse particles was observed in summer, while Hg P in fine particles dominated in fall and winter. The size distribution of averaged mercury content in particulates was bimodal, with two peaks in the bins of < 0.7 μm and 4.7–5.8 μm. Dry deposition per unit area of Hg P was estimated to be 47.2 μg m −2 using meteorological conditions and a size-resolved particle dry deposition model. This was 16.5 % less than mercury wet deposition. Compared to Hg P in fine particles, HgP in coarse particles contributed more to the total dry deposition due to higher deposition velocities. Negative correlation between precipitation and the Hg P concentration reflected the effect of scavenging of Hg P by precipitation.

An agricultural biomass burning episode in eastern China: transport, optical properties and impacts on regional air quality

Agricultural biomass burning (ABB) has been of particular concern due to its influence on air quality and atmospheric radiation, as it produces large amounts of gaseous and aerosol emissions. This paper presents an integrated observation of a significant ABB episode in Nanjing, China during early June 2011, using combined ground-based and satellite sensors (MODIS, AIRS, CALIPSO, and OMI products). The time-height distribution, optical properties, sources and transport of smoke, as well as its impacts on air quality are investigated. Lidar profiles indicate that the smoke aerosols are confined to the planetary-boundary-layer (PBL) and have a depolarization ratio of less than 0.08. The aerosol optical depths (AOD) increase from 0.5 to 3.0 at 500-nm while the extinction-related Angstrom exponent (EAE) increases from 1.1 to 1.6 at the wavelength pair of 440-870 nm. The single-scattering-albedo (SSA) becomes lower at 670-1020 nm following the ABB intrusion, and particularly shows a decreasing tendency between wavelengths of 440 to 1020 nm. The absorption Angstrom exponent (0.7) is smaller than 1.0, which may indicate the aged smoke particles mixed or coated with the urban aerosols. Surface PM10 and PM2.5 show a dramatic increase, reaching hourly mean of 800 µg/m3 and 485 µg/m3, respectively, which results in a heavy air pollution event. The stagnant and high-moisture weather provides favorable conditions for the aerosols to accumulate near the surface. CALIPSO observations also illustrate that the large-scale aerosols are primarily present in the PBL and transported to the ocean; but some dense smoke plumes are misclassified as cloud or polluted dust. By comparing with the observations, we found that the WRF-Chem model captured the accumulation and downwind transport of surface PM2.5 from 20:00 on June 2 to 10:00 on June 3 (Phase-1), but showed a dramatic underestimate from 20:00 on June 3 to 4 June (Phase-2) when dense aerosols are present. Such a discrepancy in the model is associated with the improper vertical apportion of transported smoke and atmospheric diffusion conditions when comparing with the observed aerosol and wind profiles. In addition, the model simulations indicate that the transported smoke can contribute to 50-70% of the ground-level PM2.5 in Nanjing.