Jialei Zhu

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.

Significant concentration changes of chemical components of PM1 in the Yangtze River Delta area of China and the implications for the formation mechanism of heavy haze–fog pollution

Since the winter season of 2013, a number of persistent haze-fog events have occurred in central-eastern China. Continuous measurements of the chemical and physical properties of PM1 at a regional background station in the Yangtze River Delta area of China from 16 Nov. to 18 Dec., 2013 revealed several haze-fog events, among which a heavy haze-fog event occurred between 6 Dec. and 8 Dec. The mean concentration of PM1 was 212μgm(-3) in the heavy haze-fog period, which was about 10 times higher than on clean days and featured a peak mass concentration that reached 298μgm(-3). Organics were the largest contributor to the dramatic rise of PM1 on heavy haze-fog days (average mass concentration of 86μgm(-3)), followed by nitrate (58μgm(-3)), sulfate (35μgm(-3)), ammonium (29μgm(-3)), and chloride (4.0μgm(-3)). Nitrate exhibited the largest increase (~20 factors), associated with a significant increase in NOx. This was mainly attributable to increased coal combustion emissions, relative to motor vehicle emissions, and was caused by short-distance pollutant transport within surrounding areas. Low-volatility oxidized organic aerosols (OA) (LV-OOA) and biomass-burning OA (BBOA) also increased sharply on heavy haze-fog days, exhibiting an enhanced oxidation capacity of the atmosphere and increased emissions from biomass burning. The strengthening of the oxidation capacity during the heavy pollution episode, along with lower solar radiation, was probably due to increased biomass burning, which were important precursors of O3. The prevailing meteorological conditions, including low wind and high relative humidity, and short distance transported gaseous and particulate matter surrounding of the sampling site, coincided with the increased pollutant concentrations mainly from biomass-burning mentioned above to cause the persistent haze-fog event in the YRD area.

Mechanism of SOA formation determines magnitude of radiative effects

Significance Secondary organic aerosol (SOA) forms via a variety of processes and plays a key role in climate change and air quality. Recent measurements indicate that most SOA exists as an internal mixture with other aerosols. This study examines the radiative effect of using a mixing state for SOA that depends on the process of formation, based on an explicit mechanism for the chemical production of SOA. The radiative forcing of SOA in the future is estimated using this approach. A surprising result is that the contribution of SOA to radiative forcing increases substantially (becomes more negative) in the future even though the increase of its burden is slight. Secondary organic aerosol (SOA) nearly always exists as an internal mixture, and the distribution of this mixture depends on the formation mechanism of SOA. A model is developed to examine the influence of using an internal mixing state based on the mechanism of formation and to estimate the radiative forcing of SOA in the future. For the present day, 66% of SOA is internally mixed with sulfate, while 34% is internally mixed with primary soot. Compared with using an external mixture, the direct effect of SOA is decreased due to the decrease in total aerosol surface area and the increase of absorption efficiency. Aerosol number concentrations are sharply reduced, and this is responsible for a large decrease in the cloud albedo effect. Internal mixing decreases the radiative effect of SOA by a factor of >4 compared with treating SOA as an external mixture. The future SOA burden increases by 24% due to CO2 increases and climate change, leading to a total (direct plus cloud albedo) radiative forcing of −0.05 W m−2. When the combined effects of changes in climate, anthropogenic emissions, and land use are included, the SOA forcing is −0.07 W m−2, even though the SOA burden only increases by 6.8%. This is caused by the substantial increase of SOA associated with sulfate in the Aitken mode. The Aitken mode increase contributes to the enhancement of first indirect radiative forcing, which dominates the total radiative forcing.

Diagnostic Evalutation of Bromine Reactions on Mercury Chemistry

This model study about the influence of chemical reactants on the transport of mercury is part of an international mercury model inter-comparison (MMTF) coordinated by the EU-FP7 Research Project GMOS (Global Mercury Observation System). GMOS focuses on the improvement and validation of mercury models to assist establishing a global monitoring network and to support the implementation of the Minamata Convention. For the model inter-comparison, several global and regional Chemistry Transport Models (CTM) were used to simulate the influence of reactants on mercury oxidation. For this, gas and aqueous phase reactions of mercury with bromine were implemented into the models. As reactants, precalculated bromine concentrations were taken from the global bromine models P-TOMCAT and GEOS-CHEM. The modelled concentrations of oxidized mercury were compared to observations from GMOS measurement stations, and air craft campaigns. It was found that, even outside of polar regions, bromine plays an important role in the oxidation of mercury. Moreover, the chosen reactant influenced the vertical distribution of mercury in the atmosphere. While little difference was found for GOM concentrations at the surface level, the bromine reaction was able to explain the elevated concentration of GOM observed in the free troposphere.