Zhonghui Huang

Geospace magnetic field responses to interplanetary shocks

We perform a statistical survey of geospace magnetic field responses, including the geosynchronous magnetic field and the sudden impulses on the ground, to interplanetary shocks (IP shocks) between 1998 and 2005. The magnitude of the geosynchronous magnetic field (dB(z)) responses to IP shocks depends strongly on local time, which peaks near the noon meridian; however, the relative magnitude of the responses depends only weakly on local time. These results are similar to those obtained from the statical study of the responses to solar wind dynamic pressure pulses. However, negative responses (where dBz is negative) were sometimes observed in the nightside of the magnetosphere even though the IP shocks always caused increases in the solar wind dynamic pressure, a new phenomenon not widely reported in the literature. Our analysis shows that similar to 75% of negative responses in the midnight sector are associated with southward interplanetary magnetic field. For a moderately compressed magnetosphere, the amplitude of the geosynchronous response dBz could be determined by the average value of the background local magnetic field. As the magnitude of the upstream solar wind dynamic pressure increases, the rate of response increases correspondingly. The dBz at the geosynchronous orbit near local noon and the amplitude of sudden impulses (dSYM-H) on the ground are highly correlated.

Response of the magnetic field in the geosynchronous orbit to solar wind dynamic pressure pulses

We do a statistical survey of solar wind dynamic pressure (P-d) pulses and geosynchronous magnetic fields observed between 1998 and 2005. In geomagnetic quiet times with D-st > -50 nT, we find 111 solar wind dynamic pressure pulses which produce geosynchronous magnetic field responses. These responses are often observed by two or three GOES spacecraft at different local times in geosynchronous orbit. The magnitudes of the geosynchronous magnetic field changes (dB(z)) have a peak near the noon meridian, similar to the results obtained in the study of the response of the geosynchronous field to the large and sharp solar wind dynamic pressure variations. However, the relative change of the geosynchronous magnetic field dBz/AV-B-z (where AV-B-z is the average of the geosynchronous magnetic field Bz observed during the response to the pressure pulse) depends weakly on the local time; thus the change of B-z(dB(z)) is proportional to the average field (AV-B-z). As the magnitude of the relative change of solar wind dynamic pressure (dP(d)/P-d) increases, the rate of geosynchronous magnetic field variation increases correspondingly. These results imply that the magnitude of the geosynchronous magnetic field response could be determined by AV- B-z. In addition, the interplanetary field orientation does not affect the response significantly. Using an MHD code which models the global behavior of the solar wind-magnetosphere-ionosphere system, we reproduce the main characteristics of the observations.

Ambient air benzene at background sites in China's most developed coastal regions: exposure levels, source implications and health risks.

Benzene is a known human carcinogen causing leukemia, yet ambient air quality objectives for benzene are not available in China. The ambient benzene levels at four background sites in Chinas most developed coastal regions were measured from March 2012 to February 2013. The sites are: SYNECP, in the Northeast China Plain (NECP); YCNCP, in the North China Plain (NCP); THYRD, in the Yangtze River Delta (YRD) and DHPRD, in the Pearl River Delta (PRD). It was found that the mean annual benzene levels (578-1297 ppt) at the background sites were alarmingly higher, especially when compared to those of 60-480 pptv monitored in 28 cities in the United States. Wintertime benzene levels were significantly elevated at both sites (SYNECP and YCNCP) in northern China due to heating with coal/biofuels. Even at these background sites, the lifetime cancer risks of benzene (1.7-3.7E-05) all exceeded 1E-06 set by USEPA as acceptable for adults. At both sites in northern China, good correlations between benzene and CO or chloromethane, together with much lower toluene/benzene (T/B) ratios, suggested that benzene was largely related to coal combustion and biomass/biofuel burning. At the DHPRD site in the PRD, benzene revealed a highly significant correlation with methyl tert-butyl ether (MTBE), indicating that its source was predominantly from vehicle emissions. At the THYRD site in the YRD, higher T/B ratios and correlations between benzene and tetrachloroethylene, or MTBE, implied that benzene levels were probably affected by both traffic-related and industrial emissions.

Effects of the interplanetary magnetic field on the twisting of the magnetotail: Global MHD results

We used the global magnetohydrodynamic (MHD) simulation to investigate effects of the interplanetary magnetic field (IMF) on the twisting of the magnetotail. It is shown that the cross section of the magnetotail is elongated along a certain direction close to the IMF orientation. The elongated direction twists with the IMF orientation, magnitude, and the distance away from Earth, and the quantitative relationship has been given. In addition, the current sheet has a similar twisting behavior as the magnetotail magnetopause, with a smaller twisting angle. Our simulated results fall within the range that people have deduced from observations.

Sources of C2–C4 alkenes, the most important ozone nonmethane hydrocarbon precursors in the Pearl River Delta region

Surface ozone is becoming an increasing concern in Chinas megacities such as the urban centers located in the highly industrialized and densely populated Pearl River Delta (PRD) region, where previous studies suggested that ozone production is sensitive to VOC emissions with alkenes being important precursors. However, little was known about sources of alkenes. Here we present our monitoring of ambient volatile organic compounds at four representative urban, suburban and rural sites in the PRD region during November-December 2009, which experienced frequent ozone episodes. C2-C4 alkenes, whose total mixing ratios were 11-20% of non-methane hydrocarbons (NMHCs) quantified, accounted for 38-64% of ozone formation potentials (OFPs) and 30-50% of the total hydroxyl radical (OH) reactivity by NMHCs. Ethylene was the most abundant alkene, accounting for 8-15% in total mixing ratios of NMHCs and contributed 25-46% of OFPs. Correlations between C2-C4 alkenes and typical source tracers suggested that ethylene might be largely related to vehicle exhausts and industry activities, while propene and butenes were much more LPG-related. Positive Matrix Factorization (PMF) confirmed that vehicle exhaust and liquefied petroleum gas (LPG) were two major sources that altogether accounted for 52-62%, 58-77%, 73-83%, 68-79% and 73-84% for ethylene, propene, 1-butene, trans-2-butene and cis-2-butene, respectively. Vehicle exhausts alone contributed 32-49% ethylene and 35-41% propene. Industry activities contributed 13-23% ethylene and 7-20% propene. LPG instead contributed the most to butenes (38-65%) and substantially to propene (23-36%). Extensive tests confirmed high fractions of propene and butenes in LPG then used in Guangzhou and in LPG combustion plumes; therefore, limiting alkene contents in LPG would benefit regional ozone control.

Primary particulate emissions and secondary organic aerosol (SOA) formation from idling diesel vehicle exhaust in China

In China diesel vehicles dominate the primary emission of particulate matters from on-road vehicles, and they might also contribute substantially to the formation of secondary organic aerosols (SOA). In this study tailpipe exhaust of three typical in-use diesel vehicles under warm idling conditions was introduced directly into an indoor smog chamber with a 30m3 Teflon reactor to characterize primary emissions and SOA formation during photo-oxidation. The emission factors of primary organic aerosol (POA) and black carbon (BC) for the three types of Chinese diesel vehicles ranged 0.18-0.91 and 0.15-0.51gkg-fuel-1, respectively; and the SOA production factors ranged 0.50-1.8gkg-fuel-1 and SOA/POA ratios ranged 0.7-3.7 with an average of 2.2. The fuel-based POA emission factors and SOA production factors from this study for idling diesel vehicle exhaust were 1-3 orders of magnitude higher than those reported in previous studies for idling gasoline vehicle exhaust. The emission factors for total particle numbers were 0.65-4.0×1015particleskg-fuel-1, and particles with diameters less than 50nm dominated in total particle numbers. Traditional C2-C12 precursor non-methane hydrocarbons (NMHCs) could only explain less than 3% of the SOA formed during aging and contribution from other precursors including intermediate volatile organic compounds (IVOC) needs further investigation.

Spatiotemporal patterns and source implications of aromatic hydrocarbons at six rural sites across China's developed coastal regions

Aromatic hydrocarbons are important anthropogenic precursors of tropospheric ozone and secondary organic aerosols. Here we measured ambient aromatic hydrocarbons from March 2012 to February 2014 at six rural sites in Chinas developed coastal regions. On average, benzene (B) comprised > 50% of total benzene (B), toluene (T), ethylbenzene (E), and xylenes (X) (BTEX) at sites in the Northeast China Plain (NECP) or in the North China Plain (NCP), whereas T, E, and X accounted for > 77% of total BTEX at sites in the Yangtze River Delta (YRD) and the Pearl River Delta in the south. BTEX at the northern sites was significantly correlated (p < 0.01) with combustion tracer-carbon monoxide (CO) but weakly correlated with traffic marker-methyl tert-butyl ether (MTBE), suggesting that their main sources were coal and biofuel/biomass burning with substantially elevated B levels during the winter heating period. In contrast, BTEX at the southern sites originated mainly from traffic-related and/or industrial emission sources, as indicated by the poor correlations with CO but highly significant (p < 0.01) correlations with MTBE and tetrachloroethylene, an industrial emission tracer. The B/CO emission ratios from measurement agreed within a factor of 2 with that of a previous widely used emission inventory of China, but the T/CO ratio at the NECP site and the o-X/CO ratio at the NCP site were 29% and 38% of that in the inventory, respectively; the E/CO and X/CO ratios at the YRD site were 3.2–3.5 fold that in the emission inventory.

Real-time monitoring of respiratory absorption factors of volatile organic compounds in ambient air by proton transfer reaction time-of-flight mass spectrometry

Respiratory absorption factors (AFs) are essential parameters in the evaluation of human health risks from toxic volatile organic compounds (VOCs) in ambient air. A method for the real time monitoring of VOCs in inhaled and exhaled air by proton transfer reaction time-of-flight mass spectrometry (PTR-TOF-MS) has been developed to permit the calculation of respiratory AFs of VOCs. Isoprene was found to be a better breath tracer than O2, CO2, humidity, or acetone for distinguishing between the expiratory and inspiratory phases, and a homemade online breath sampling device with a buffer tube was used to optimize signal peak shapes. Preliminary tests with seven subjects exposed to aromatic hydrocarbons in an indoor environment revealed mean respiratory AFs of 55.0%, 55.9%, and 66.9% for benzene, toluene, and C8-aromatics (ethylbenzene and xylenes), respectively. These AFs were lower than the values of 90% or 100% used in previous studies when assessing the health risks of inhalation exposure to hazardous VOCs. The mean respiratory AFs of benzene, toluene and C8-aromatics were 66.5%, 70.2% and 82.3% for the three female subjects; they were noticeably much higher than that of 46.4%, 45.2% and 55.3%, respectively, for the four male subjects.

Fast screening compositions of PM 2.5 by ATR-FTIR: Comparison with results from IC and OC/EC analyzers

Chemical speciation of fine particles or PM2.5 collected on filters is still a costly and time-consuming task. In this study, filter-based PM2.5 samples were collected during November-December 2013 at four sites in Guangzhou, and the major components were fast screened (~7min per filter sample) by Attenuated Total Reflectance (ATR)-Fourier Transform Infrared Spectroscopic (FTIR) in comparison with that measured by Organic carbon/Element carbon (OC/EC) analyzer and Ion Chromatography (IC). The concentrations of nitrate, ammonium, sulfate, primary organic carbon (POC) and secondary organic carbon (SOC) measured by OC/EC and IC analyzers were better correlated with their infrared absorption peak heights at 1320cm-1 for nitrate, 1435, 3045 and 3215cm-1 for ammonium, 615cm-1 for sulfate, 690, 760 and 890cm-1 for POC and 1640 and 1660cm-1 for SOC respectively, during polluted days (PM2.5>75μg/m3) than during clean days (PM2.5≤75μg/m3). With the evolution of a haze episode during our field campaign, the concentrations of the major PM2.5 components displayed consistent variations with their infrared absorption peak heights, suggesting ATR-FTIR could be a fast and useful technique to characterize filter-based PM2.5 compositions particularly during pollution events although cautions should be taken when PM2.5 levels are low. Notably, elevated PM2.5 mass concentrations occurred with enhanced ratios of [NO3-]/[SO42-] and [NH4+]/[SO42-], implying that nitrogenous components play vital roles in the PM2.5 pollution events in the study region.