Influence of the Long-Range Transport of Siberian Biomass Burnings on Air Quality in Northeast China in June 2017

Biomass burning (BB) emits a large volume of trace gases and aerosols into the atmosphere, which can significantly affect the earth's radiative balance and climate and has negative impacts on air quality and even human health. In late June 2017, an intense BB case, dominated by forest and savanna fires, occurred in Siberia, and it affected the air quality of Northeast China through long-range transport. Here, multisatellite remote-sensing products and ground-based PM2.5 measurements are used to evaluate the influence of the Siberian smoky plume on Northeast China. The results show that the BB was intense at the early stage when the daily fire count and average fire radiative power exceeded 300 and 200 MW, respectively. The maximum daily fire count reached 1350 in Siberia, and the peak value of instantaneous fire radiative power was as high as 3091.5 MW. High concentrations of CO and aerosols were emitted into the atmosphere by the BB in Siberia. The maximum daily mean values of the CO column concentration and aerosol optical depth (AOD) increased by 3 × 1017 molec·cm2 and 0.5 compared with that during the initial BB stage. In addition, the BB released a large number of absorptive aerosols into the atmosphere, and the UV aerosol index (UVAI) increased by five times at the peak of the event in Siberia. Under the appropriate synoptic conditions and, combined with pyroconvection, the smoky plume was lifted into the upper air and transported to Northeast China, affecting the air quality of Northeast China. The daily mean values of CO concentration, AOD, and UVAI in Northeast China increased by 6 × 1017 molec·cm2, 0.5, and 1.4, respectively, after being affected. Moreover, the concentration of the surface PM2.5 in Northeast China approximately doubled after being affected by the plume. The results of this study indicate that the air quality of Northeast China can be significantly affected by Siberian BBs under favorable conditions.

Investigation of Policy Relevant Background (PRB) Ozone in East Asia

The concept of Policy Relevant Background (PRB) ozone has emerged in recent years to address the air quality baseline on the theoretical limits of air pollution controls. In this study, the influence of Long-range Transport (LRT) of air pollutants from North America and the effect of Stratosphere-Troposphere Transport (STT) on PRB ozone was investigated using GEOS-Chem coupled WRF-CMAQ modelling system. Four distinct seasons in 2006 were simulated to understand better the seasonal and geographical impacts of these externalities on PRB ozone over East Asia (EA). Overall, the LRT impact from North America has been found to be ~0.54 ppbv, while the maximum impacts were found at the mountain stations with values of 2.3 ppbv, 3.3 ppbv, 2.3 ppbv, and 3.0 ppbv for January, April, July, and October, respectively. In terms of PRB ozone, the effect of STT has enhanced the surface background ozone by ~3.0 ppbv, with a maximum impact of 7.8 ppbv found in the northeastern part of East Asia (near Korea and Japan). Springtime (i.e., April) has the most vital STT signals caused by relatively cold weather and unstable atmospheric condition resulting from the transition of the monsoon season. The simulated PRB ozone based on the mean values of the maximum daily 8-h average (MDA8) is 53 ppbv for spring (April) and 22 ppbv for summer (July). Up to ~1.0 ppbv and ~2.2 ppbv of MDA8 ozone were attributed to LRT and STT, respectively. Among the selected cities, Beijing and Guangzhou have received the most substantial anthropogenic enhancement in MDA8 ozone in summer, ranging from 40.0 ppbv to 56.0 ppbv.

Observational Analyses of Dry Intrusions and Increased Ozone Concentrations in the Environment of Wildfires

In this study, atmospheric dynamical processes, which govern the intensification of wildfire activity and the associated increase in low-level ozone concentrations, were studied using images, advanced products and vertical profiles derived from satellite observations. The analyses confirm that the influence of deep stratospheric intrusions, identified in the satellite water vapor imagery, on a fire-risk area contributes to the increase in fire activity. The depth of dry stratospheric intrusions, the associated synoptic evolution and the enhanced low-level ozone concentrations caused by vertical transport of stratospheric air and/or related to biomass burning emissions were analyzed using satellite measurements from SEVIRI, IASI and CrIS instruments, complemented with surface observations near the wildfires’ locations. It is shown that the spatial and vertical resolutions of these soundings provide a way of identifying areas of enhanced ozone downwind of wildfires. Influences of the upper-troposphere dynamics and the wind field evolution as factors of uncertainty and complexity in studying the ozone production from wildfire emissions are considered. The combination of satellite soundings and satellite estimations of fire radiative energy and WV imagery may contribute to better understand the ozone enhancement associated with stratospheric intrusion and wildfire emissions.

Clair JM, Tanner DJ, Ullmann K, Veres PR, Walega J, Warneke C, Washenfelder RA, Weibring P, Wisthaler A, Wolfe GM, Womack CC, Yokelson RJ. Ozone chemistry in western U.S. wildfire plumes

Wildfires are a substantial but poorly quantified source of tropospheric ozone (O₃). Here, to investigate the highly variable O₃ chemistry in wildfire plumes, we exploit the in situ chemical characterization of western wildfires during the FIREX-AQ flight campaign and show that O₃ production can be predicted as a function of experimentally constrained OH exposure, volatile organic compound (VOC) reactivity, and the fate of peroxy radicals. The O₃ chemistry exhibits rapid transition in chemical regimes. Within a few daylight hours, the O₃ formation substantially slows and is largely limited by the abundance of nitrogen oxides (NO_x). This finding supports previous observations that O₃ formation is enhanced when VOC-rich wildfire smoke mixes into NO_x-rich urban plumes, thereby deteriorating urban air quality. Last, we relate O₃ chemistry to the underlying fire characteristics, enabling a more accurate representation of wildfire chemistry in atmospheric models that are used to study air quality and predict climate.

Exploring the Constraints on Simulated Aerosol Sources and Transport Across the North Atlantic With Island-Based Sun Photometers

Atmospheric aerosol over the North Atlantic Ocean impacts regional clouds and climate. In this work, we use a set of sun photometer observations of aerosol optical depth (AOD) located on the Graciosa and Cape Verde islands, along with the GEOS-Chem chemical transport model to investigate the sources of these aerosol and their transport over the North Atlantic Ocean. At both locations, the largest simulated contributor to aerosol extinction is the local source of sea-salt aerosol. In addition to this large source, we find that signatures consistent with long-range transport of anthropogenic, biomass burning, and dust emissions are apparent throughout the year at both locations. Model simulations suggest that this signal of long-range transport in AOD is more apparent at higher elevation locations; the influence of anthropogenic and biomass burning aerosol extinction is particularly pronounced at the height of Pico Mountain, near the Graciosa Island site. Using a machine learning approach, we further show that simulated observations at these three sites (near Graciosa, Pico Mountain, and Cape Verde) can be used to predict the simulated background aerosol imported into cities on the European mainland, particularly during the local winter months, highlighting the utility of background AOD monitoring for understanding downwind air quality.

Fire Detection and Fire Radiative Power in Forests and Low-Biomass Lands in Northeast Asia: MODIS versus VIIRS Fire Products

Fire omission and commission errors, and the accuracy of fire radiative power (FRP) from satellite moderate-resolution impede the studies on fire regimes and FRP-based fire emissions estimation. In this study, we compared the accuracy between the extensively used 1-km fire product of MYD14 from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the 375-m fire product of VNP14IMG from the Visible Infrared Imaging Radiometer Suite (VIIRS) in Northeastern Asia using data from 2012–2017. We extracted almost simultaneous observation of fire detection and FRP from MODIS-VIIRS overlapping orbits from the two fire products, and identified and removed duplicate fire detections and corresponding FRP in each fire product. We then compared the performance of the two products between forests and low-biomass lands (croplands, grasslands, and herbaceous vegetation). Among fire pixels detected by VIIRS, 65% and 83% were missed by MODIS in forests and low-biomass lands, respectively; whereas associated omission rates by VIIRS for MODIS fire pixels were 35% and 53%, respectively. Commission errors of the two fire products, based on the annual mean measurements of burned area by Landsat, decreased with increasing FRP per fire pixel, and were higher in low-biomass lands than those in forests. Monthly total FRP from MODIS was considerably lower than that from VIIRS due to more fire omission by MODIS, particularly in low-biomass lands. However, for fires concurrently detected by both sensors, total FRP was lower with VIIRS than with MODIS. This study contributes to a better understanding of fire detection and FRP retrieval performance between MODIS and its successor VIIRS, providing valuable information for using those data in the study of fire regimes and FRP-based fire emission estimation.

Origin, Maintenance and Variability of the Asian Tropopause Aerosol Layer (ATAL): The Roles of Monsoon Dynamics

Using NASA MERRA2 daily data, we investigated the origin, maintenance and variability of the Asian Tropopause Aerosol Layer (ATAL) in relation to variations of the Asia Monsoon Anticyclone (AMA) during the summer of 2008. During May-June, abundant quantities of carbon monoxide (CO), carbonaceous aerosols (CA) and dusts are found in the mid- and upper troposphere over India and China, arising from enhanced biomass burning emissions, as well as westerly transport from the Middle East deserts. During July-August, large quantities of dusts transported from the deserts are trapped and accumulate over the southern and eastern foothills of the Tibetan Plateau. Despite strong precipitation washout, ambient CO, CA and dust are lofted by orographically forced deep convection to great elevations, 12-16 km above sea level, via two key pathways over heavily polluted regions: a) the Himalayas-Gangetic Plain, and b) the Sichuan Basin. Upon entering the upper-troposphere-lower-stratosphere, the pollutants are capped by a stable layer near the tropopause, advected and dispersed by the anticyclonic circulation of AMA, forming the ATAL resembling a planetary-scale "double-stem chimney cloud". The development and variability of the ATAL are strongly linked to the seasonal march and intraseasonal (20-30 days and higher frequency) oscillations of the Asian monsoon.

Air pollution over the North China Plain and its implication of regional transport: A new sight from the observed evidences

High concentrations of the fine particles (PM_2.5) are frequently observed during all seasons over the North China Plain (NCP) region in recent years. In NCP, the contributions of regional transports to certain area, e.g. Beijing city, are often discussed and estimated by models when considering an effective air pollution controlling strategy. In this study, we selected three sites from southwest to northeast in NCP, in which the concentrations of air pollutants displayed a multi-step decreasing trend in space. An approach based on the measurement results at these sites has been developed to calculate the relative contributions of the minimal local emission (MinLEC) and the maximum regional transport (MaxRTC) to the air pollutants (e.g., SO₂, NO₂, CO, PM_2.5) in Beijing. The minimal influence of local emission is estimated by the difference of the air pollutants' concentrations between urban and rural areas under the assumption of a similar influence of regional transport. Therefore, it's convenient to estimate the contributions of local emission from regional transport based on the selective measurement results instead of the complex numerical model simulation. For the whole year of 2013, the averaged contributions of MinLEC (MaxRTC) for NO₂, SO₂, PM_2.5 and CO are 61.7% (30.7%), 46.6% (48%), 52.1% (40.2%) and 35.8% (45.5%), respectively. The diurnal variation of MaxRTC for SO₂, PM_2.5 and CO shows an increased pattern during the afternoon and reached a peak (more than 50%) around 18:00, which indicates that the regional transport is the important role for the daytime air pollution in Beijing.

Use of multiple tools including lead isotopes to decipher sources of ozone and reactive mercury to urban and rural locations in Nevada, USA

Ambient air particulate matter (<2.5μm in diameter) samples were collected on two different filter types in 2014 and 2015 over 24h periods and analyzed for reactive mercury (gaseous oxidized mercury+particulate bound mercury) concentrations and lead isotopes to determine sources of pollution to three sites in Nevada, USA. Two sites were located on the western edge of Nevada (Reno, urban, 1370m and Peavine Peak, rural, high elevation, 2515m); the third location was ~485km east in rural Great Basin National Park, NV (2061m). Reactive mercury samples were collected on cation exchange membranes simultaneously with lead samples, collected on Teflon membranes. Lead isotopic ratios have previously identified trans-Pacific lead sources based on the 206/207 and 208/207 lead ratios. Influence from trans-Pacific air masses was higher from March to June associated with long-range transport of pollutants. Spring months are well known for increased transport across the Pacific; however, fall months were also influenced by trans-Pacific air masses in this study. Western North American background ozone concentrations have been measured and modeled at 50 to 55ppbv. Median ozone concentrations at both rural sites in Nevada were within this range. Sources leading to enhancements in ozone of 2 to 18ppbv above monthly medians in Nevada included emissions from Eurasia, regional urban centers, and global and regional wildfires, resulting in concentrations close to the USA air quality standard. At the high elevation locations, ozone was derived from pollutants being transported in the free troposphere that originate around the globe; however, Eurasia and Asia were dominant sources to the Western USA. Negative correlations between reactive mercury and percent Asian lead, Northern Eurasia and East Asia trajectories indicated reactive mercury concentrations at the two high elevation sites were produced by oxidants from local, regional, and marine boundary layer sources.

Influence of contemporary carbon originating from the 2003 Siberian forest fire on organic carbon in PM2.5 in Nagoya, Japan

In May 2003, high concentrations of organic carbon (OC) in PM2.5 were measured in Nagoya, a representative metropolitan area in Japan. To investigate the influence of possible forest fires on PM2.5 in Japan via long-range aerosol transport, the radiocarbon ((14)C) concentrations of PM2.5 samples from April 2003 to March 2004 were measured. (14)C concentrations in total carbon (TC) from May to early June showed higher values than those in other periods. The OC/elemental carbon (EC) ratios from May to early June were also significantly higher than the ones in other periods. In addition, OC concentrations from May to early June were typically high. These results indicate that the abundant OC fraction from May to early June in Nagoya consisted predominantly of contemporary carbon. Furthermore, simulations of diffusion and transport of organic matter (OM) in East Asia showed that abundant OM originating from East Siberia spread over East Asia and Japan in May and early June. Backward air mass trajectories from this time frame indicate that the air mass in Nagoya likely first passed through East Siberia where fire events were prevalent. However, the backward trajectories showed that the air mass after early June did not originate mainly from Siberia, and correspondingly, the (14)C and OC concentrations showed lower values than those from May to early June. Therefore, the authors conclude that contemporary carbon originating from the forest fire in East Siberia was transported to Nagoya, where it significantly contributed to the high observed concentrations of both OC and (14)C.