Shaocai Yu

Role of organic acids formic, acetic, pyruvic and / oxalic in the formation of cloud condensation / nuclei CCN : a review

Although it is believed that organic aerosols play a key role in cloud nucleation and make an . important contribution to the cloud condensation nuclei CCN population, their specific species remain poorly characterized. This paper reviews the current knowledge of organic acids mainly . formic, acetic, pyruvic and oxalic acids . Without specification, organic acids in this paper refer to these four organic acids in the gas and aerosol phases. This paper analyzes the extent to which organic acids act as CCN and compares the physical and chemical properties of organic acids with those of CCN. The results show that aerosol formate and acetate concentrations range from 0.02 to 5.3 nmol m y3 and from 0.03 to 12.4 nmol m y3 , respectively, and that between 34 to 77% of formate and between 21 to 66% of acetate are present in the fine fraction of aerosols. It was found . that although most 98-99% of these volatile organic acids are present in the gas phase, their concentrations in the aerosol particles are sufficient to make them a good candidate for CCN. The results also show that organic acids may make an important contribution to the formation of CCN in some special sources such as vegetation emissions and biomass-burning. Organic acids are . expected to contribute significantly to the estimates of indirect cloud-mediated forcing due to aerosols. q 2000 Elsevier Science B.V. All rights reserved.

A comparison of signals of regional aerosol‐induced forcing in eastern China and the southeastern United States

This paper compares the temperature change patterns and the signals of regional aerosol-induced forcing in eastern China and the southeastern US during the latter half of the twentieth century. Both regions show decreasing trends in the mean maximum temperature over recent decades and the cooling effect of Pinatubo volcanic aerosols. In contrast to the southeastern US, we found a slight overall warming trend in eastern China. Our analysis suggests that in addition to greenhouse warming effect, observed high concentrations of absorbing aerosols over eastern China during winter and spring might be one of the major reasons for the observed warming trend.

Attribution of the United States "warming hole": aerosol indirect effect and precipitable water vapor.

Aerosols can influence the climate indirectly by acting as cloud condensation nuclei and/or ice nuclei, thereby modifying cloud optical properties. In contrast to the widespread global warming, the central and south central United States display a noteworthy overall cooling trend during the 20th century, with an especially striking cooling trend in summertime daily maximum temperature (Tmax) (termed the U.S. “warming hole”). Here we used observations of temperature, shortwave cloud forcing (SWCF), longwave cloud forcing (LWCF), aerosol optical depth and precipitable water vapor as well as global coupled climate models to explore the attribution of the “warming hole”. We find that the observed cooling trend in summer Tmax can be attributed mainly to SWCF due to aerosols with offset from the greenhouse effect of precipitable water vapor. A global coupled climate model reveals that the observed “warming hole” can be produced only when the aerosol fields are simulated with a reasonable degree of accuracy as this is necessary for accurate simulation of SWCF over the region. These results provide compelling evidence of the role of the aerosol indirect effect in cooling regional climate on the Earth. Our results reaffirm that LWCF can warm both winter Tmax and Tmin.

Reinstate regional transport of PM2.5 as a major cause of severe haze in Beijing

Building upon fine particulate matter (PM2.5) data and the accompanying meteorological conditions in the fall of 2013, Guo et al. (1) conclude that local aerosol nucleation and growth dominantly contributed to severe haze in Beijing, whereas regional transport of PM2.5 played an “insignificant” role. Guo et al.’s conclusion is surprising because it not only requires an uncharacteristic aerosol formation mode for Beijing, but also goes against a broadly held view that “the entire eastern China region, or at least the North China Plain” should be looked at “as a single air basin” if Beijing’s air quality is to be improved (2). Instead of curtailing the emission of aerosol precursor gases in a broader region, Guo et al.’s (1) conclusion would advocate a campaign on limiting traffic and emission within and around the immediate vicinity of Beijing. Already, Beijing’s municipal government has increased urban traffic controls as its number one tactic in an effort to reduce PM2.5.* However, we believe that an in-depth scrutiny of the data makes science more accurate and … [↵][1]2To whom correspondence may be addressed. Email: shaocaiyu{at}zju.edu.cn or bao{at}lsu.edu. [1]: #xref-corresp-1-1

Direct radiative forcing and atmospheric absorption by boundary layer aerosols in the southeastern US: model estimates on the basis of new observations

In an effort to reduce uncertainties in the quantification of aerosol direct radiative forcing (ADRF) in the southeastern United States (US), a field column experiment was conducted to measure aerosol radiative properties and effects at Mt. Mitchell, North Carolina, and at an adjacent valley site. The experimental period was from June 1995 to mid-December 1995. The aerosol optical properties (single scattering albedo and asymmetry factor) needed to compute ADRF were obtained on the basis of a procedure involving a Mie code and a radiative transfer code in conjunction with the retrieved aerosol size distribution, aerosol optical depth, and diffuse-to-direct solar irradiance ratio. The regional values of ADRF at the surface and top of atmosphere (TOA), and atmospheric aerosol absorption are derived using the obtained aerosol optical properties as inputs to the column radiation model (CRM) of the community climate model (CCM3). The cloud-free instantaneous TOA ADRFs for highly polluted (HP), marine (M) and continental (C) air masses range from 20.3 to −24.8, 1.3 to −10.4, and 1.9 to −13.4 W m−2, respectively. The mean cloud-free 24-h ADRFs at the TOA (at the surface) for HP, M, and C air masses are estimated to be −8±4 (−33±16), −7±4 (−13±8), and −0.14±0.05 (−8±3) W m−2, respectively. On the assumption that the fractional coverage of clouds is 0.61, the annual mean ADRFs at the TOA and the surface are −2±1, and −7±2 W m−2, respectively. This also implies that aerosols currently heat the atmosphere over the southeastern US by 5±3 W m−2 on annual timescales due to the aerosol absorption in the troposphere.

A study of the aerosol radiative properties needed to compute direct aerosol forcing in the southeastern United States

To assess the direct radiative forcing due to aerosols in southeastern United States where a mild cooling is under way, an accurate set of data describing the aerosol radiative properties are needed. We report here aerosol optical depth (AOD) and diffuse-to-direct solar irradiance ratio (DDR) at three operational wavelengths (415, 500, 673 nm) determined by using Multifilter Rotating Shadowband Radiometers (MFRSR) at two sites (a mountain top site: Mount Gibbes, 35.78°N, 82.29°W, 2006 m mean sea level (msl); a valley site: Black Mountain, 35.66°N, 82.38°W, 951 m msl), which are separated horizontally by 10 km and vertically by 1 km. The characteristics AOD and DDR were determined from the field measurements obtained during 1995. It was found that the representative total AOD values at 500 nm at the valley site for highly polluted (HP), marine (M) and continental (C) air masses were 0.68 ± 0.33, 0.29 ± 0.19, and 0.10 ± 0.04, respectively. The fact that the ratio of the mean 1 km layer optical depth to total mean optical depth at 500 nm from the valley site was 71% indicates that the major portion of the atmospheric aerosol was located in the lowest 1 km surface boundary layer (SBL). There was a significant linear correlation between the DDR and the total AOD at both sites. A simple, fast, and operative search-graph method was used to retrieve the columnar size distribution (number concentration N, effective radius r eff , and geometric standard deviation σ g ) from the optical depth observations at the three operational wavelengths. The ground albedo, single-scattering albedo, and imaginary part of the refractive index are calculated using a mathematically unique procedure involving a Mie code and a radiative transfer code in conjunction with the retrieved aerosol size distribution, AOD, and DDR. It was found that N, r eff , and σ g were in the range of 1.9 x 10 to 1.7 x 10 4 cm -3 , 0.09-0.68 μm, and 1.12-2.70, respectively. The asymmetry factor and single-scattering albedo were in the ranges of 0.63-0.75 and 0.74-0.99 respectively. The ground albedo over the forested terrain and the imaginary part of refractive index were found to be in the range of 0.08-0.29 and 0.005-0.051, respectively.

Retrieval of aerosol properties from moments of the particle size distribution for kernels involving the step function: cloud droplet activation

Aerosol properties such as the number of particles that activate to form cloud drops and the mass contained within specified size ranges (as in the PM 2.5 and PM 10 regulatory standards) require integration over only part of the full size range of the particle distribution function (PDF) and may be formally expressed as integrals over kernels involving the Heaviside step function. Determination of these properties requires essentially that the size spectrum be partitioned into two (or more) portions, and poses a special challenge for aerosol modeling with the method of moments. To assess the ability of moment-based methods to treat kernels involving step functions, several algorithms for the estimation of aerosol properties associated with cloud activation have been evaluated. For 240 measured continental distributions employed here as test cases, the full size spectrum of the PDF was partitioned into three distinct portions based upon characteristic critical radii for activation in cumulus and stratiform clouds, and mass- and number-concentration metrics were evaluated for each portion. The first six radial moments yielded results accurate to within about 10% or better, on average, and the numbers of particles activated as cloud drops and the aerosol mass taken into cloud water were estimated to an accuracy of 5% or better. Of the moment-based approaches evaluated, the multiple isomomental distribution aerosol surrogate (MIDAS) (Wright, J. Aerosol Sci. 31 (2000) 1) technique performed best. Accurate results were also obtained with the randomized minimization search technique (RMST) (Yue et al., Geophys. Res. Lett. 24 (1997) 651; Heintzenberg et al., Appl. Opt. 20 (1981) 1308).

Performance and Diagnostic Evaluation of Ozone Predictions by the Eta-Community Multiscale Air Quality Forecast System during the 2002 New England Air Quality Study

Abstract A real-time air quality forecasting system (Eta-Community Multiscale Air Quality [CMAQ] model suite) has been developed by linking the National Centers for Environmental Estimation Eta model to the U.S. Environmental Protection Agency (EPA) CMAQ model. This work presents results from the application of the Eta-CMAQ modeling system for forecasting ozone (O3) over the Northeastern United States during the 2002 New England Air Quality Study (NEAQS). Spatial and temporal performance of the Eta-CMAQ model for O3 was evaluated by comparison with observations from the EPA Air Quality System (AQS) network. This study also examines the ability of the model to simulate the processes governing the distributions of tropospheric O3 on the basis of the intensive datasets obtained at the four Atmospheric Investigation, Regional Modeling, Analysis, and Estimation (AIRMAP) and Harvard Forest (HF) surface sites. The episode analysis reveals that the model captured the buildup of O3 concentrations over the northeastern domain from August 11 and reproduced the spatial distributions of observed O3 very well for the daytime (8:00 p.m.) of both August 8 and 12 with most of normalized mean bias (NMB) within [H11006]20%. The model reproduced 53.3% of the observed hourly O3 within a factor of 1.5 with NMB of 29.7% and normalized mean error of 46.9% at the 342 AQS sites.The comparison of modeled and observed lidar O3 vertical profiles shows that whereas the model reproduced the observed vertical structure, it tended to overestimate at higher altitude. The model reproduced 64 –77% of observed NO2 photolysis rate values within a factor of 1.5 at the AIRMAP sites. At the HF site, comparison of modeled and observed O3/nitrogen oxide (NOx) ratios suggests that the site is mainly under strongly NOx-sensitive conditions (>53%). It was found that the modeled lower limits of the O3 production efficiency values (inferred from O3-CO correlation) are close to the observations.

Using National Air Quality Forecast Guidance to Develop Local Air Quality Index Forecasts

The National Air Quality Forecast Capability (NAQFC) currently provides next-day forecasts of ozone concentrations over the contiguous United States. It was developed collaboratively by NOAA and Environmental Protection Agency (EPA) in order to provide state and local agencies, as well as the general public, air quality forecast guidance. As part of the development process, the NAQFC has been evaluated utilizing strict monitor-to-gridcell matching criteria, and discrete-type statistics of forecast concentrations. While such an evaluation is important to the developers, it is equally, if not more important, to evaluate the performance using the same protocol as the models intended application. Accordingly, the purpose of this article is to demonstrate the efficacy of the NAQFC from the perspective of a local forecaster, thereby promoting its use. Such an approach has required the development of a new evaluation protocol: one that examines the ability of the NAQFC to forecast values of the EPAs Air Qualit...

Searching for a regional fingerprint of aerosol radiative forcing in the southeastern US

Although aerosols have long been considered to exert a cooling influence on the regional climate due to direct and indirect radiative forcing, persuasive evidence of the response to this forcing has been lacking. Here, we analyze the regional patterns of climate change in the Southeast US during the period 1949–94 to search for a fingerprint of aerosol radiative forcing. The results show that direct and indirect radiative forcing of both natural (such as Pinatubo volcanic aerosols) and anthropogenic aerosols (such as those transported from the polluted regions of US) may be responsible for the regional cooling trend in the Southeast during the past 46 years. Lack of availability of long term measurements precludes a rigorous cause-and-effect analysis. Circumstantial evidence presented here amply justifies immediate establishment of a network of measurements of aerosol optical depth and cloud reflectivity in the southeastern US.