C. S. Kiang

Aerosol radiative, physical, and chemical properties in Beijing during June 1999

Beijing experiences air pollution such that the sky overhead is gray much of the time even on cloudless days. In order to understand the cause of this problem, the aerosol light scattering coefficient σ_(sp) and absorption coefficient σ_(ap) were measured under dry conditions (instrumental relative humidity 1.0 μm), the submicron aerosol was responsible for ∼80% of the light scattering at 530 nm. The largest contribution to the PM2.5 aerosol mass was due to organic compounds, which accounted for ∼30% of the mass. The contributions of sulfate, ammonium, and nitrate to the PM2.5 mass concentration were ∼15%, 5%, and 8%, respectively. Mineral aerosol contributed ∼16% to the PM2.5 aerosol mass. These data show that combustion-related particles rather than wind-blown dust dominated the light extinction budget during June 1999.

Is ozone pollution affecting crop yields in China

Newly available data from non-urban locations in China along with regional model simulations suggest that ground-level ozone may be sufficiently high to affect Chinas winter wheat production. As non-urban ozone increases with industrialization, its effects on crops could hinder efforts to meet increasing food demands in the coming decades, in China.

Intercomparison of near real time monitors of PM2.5 nitrate and sulfate at the U.S. Environmental Protection Agency Atlanta Supersite

[1] Five new instruments for semicontinuous measurements of fine particle (PM2.5) nitrate and sulfate were deployed in the Atlanta Supersite Experiment during an intensive study in August 1999. The instruments measured bulk aerosol chemical composition at rates ranging from every 5 min to once per hour. The techniques included a filter sampling system with automated water extraction and online ion chromatographic (IC) analysis, two systems that directly collected particles into water for IC analysis, and two techniques that converted aerosol nitrate or sulfate either catalytically or by flash vaporization to gaseous products that were measured with gas analyzers. During the one-month study, 15-min integrated nitrate concentrations were low, ranging from about 0.1 to 3.5 μg m -3 with a mean value of 0.5 μg m -3 . Ten-minute integrated sulfate concentrations varied between 0.3 and 40 μg m -3 with a mean of 14 μg m -3 . By the end of the one-month study most instruments were in close agreement, with r-squared values between instrument pairs typically ranging from 0.7 to 0.94. Based on comparison between individual semicontinuous devices and 24-hour integrated filter measurements, most instruments were within 20-30% for nitrate (∼0.1-0.2 μg m -3 ) and 10-15% for sulfate (1-2 leg m -3 ). Within 95% confidence intervals, linear regression fits suggest that no biases existed between the semicontinuous techniques and the 24-hour integrated filter measurements of nitrate and sulfate;, however, for nitrate, the semicontinuous intercomparisons showed significantly less variability than intercomparisons amongst the 24-hour integrated filters.

A model study of the formation of cloud condensation nuclei in remote marine areas

A theoretical study is presented of the mechanisms by which freshly nucleated sulfuric acid particles can be grown to the size of cloud condensation nuclei (CCN) in remote marine areas. Three mechanisms are considered: (1) growth by the scavenging and subsequent aqueous phase oxidation of atmospheric SO2, (2) growth by coagulation, and (3) growth by scavenging of atmospheric sulfuric acid. While model calculations indicate that the first two mechanisms are too slow to represent a significant source of CCN in the remote marine atmosphere, scavenging of sulfuric acid can be an effective mechanism for transforming tiny sulfuric acid particles into CCN provided the concentration of preexisting CCN is less than some critical concentration. For conditions typical of the marine boundary layer with ambient SO2 concentrations of about 30 parts per trillion by volume (pptv), we estimate this critical concentration to be quite low (that is, about 1 cm−3). However, under conditions with enhanced SO2 the value of this critical concentration also increases and is estimated to reach a level of about 10 cm−3 when SO2 is equal to 100 pptv. Our calculations suggest that coupling between dimethyl sulfide (DMS) emissions and CCN production in the marine boundary layer can only exist when the existing CCN concentrations at a specific locale fall below this critical concentration.

Gas‐to‐particle conversion of tropospheric sulfur as estimated from observations in the western North Pacific during PEM‐West B

Aircraft observations during the Pacific Exploratory Mission in the western Pacific Ocean, phase B (PEM-West B), taken in February–March 1994, have been used to constrain a numerical model that calculates local concentrations of gaseous H2SO4 rates of homogeneous nucleation, and concentrations of newly formed, nanometer-sized particles. The data was selected from 13 flights over the western Pacific Ocean that covered an altitude range from the boundary layer (BL) to the upper troposphere (UT) and latitudes from 10°S to 60°N. The largest nucleation rates were calculated for the data from the flights over the temperate latitudes (λ>30°N). Within these latitudes, homogeneous nucleation rates averaged about 1–100 particles cm−3 s−1. Significantly smaller nucleation rates were calculated for the tropical (λ<20°N) and subtropical (20°N<λ<30°N) regions. In the tropics, average nucleation rates in excess of 10 particles cm−3 s−1 were limited to the UT. In the subtropics, large average nucleation rates in excess of 1 particle cm−3 s−1 were obtained in the BL and in the UT, and average rates of about 10−1 particles cm−3 s−1 were obtained for the rest of the troposphere. The relatively large nucleation rates calculated for the temperate latitudes could be largely attributed to the cold temperatures encountered in this region during the PEM-West B flights. For the data from the tropical and subtropical flights, little or no homogeneous nucleation was calculated for the average conditions encountered in the BL and midtroposphere (MT). Instead, significant nucleation was limited either to the UT or to several small-scale events. These enhanced nucleation events were generally characterized by spikes in relative humidity and low aerosol surface density. However, the strongest nucleation events, with homogeneous nucleation rates of about 10 particles cm−3 s−1, were associated with high concentrations of SO2, most likely as a result of pollution from the Asian continent. Our results imply that in regions in which homogeneous nucleation is dominated by small-scale fluctuations, approaches that attempt to infer nucleation rates using average or typical conditions will grossly underestimate the actual average rate of nucleation.

Estimated contribution of power plants to ambient nitrogen oxides measured in Atlanta, Georgia in August 1992

Abstract Since gasoline and coal vary in sulfur content, plumes from mobile and point sources can be identified by their characteristic sulfur dioxide to nitrogen oxides ratios. In this study this fingerprint ratio is used to estimate what percentage of the total ambient nitrogen oxides intercepted by a chemical measurement station is Atlanta is attributable to specific point sources. Emission estimates of nitrogen oxides from point and mobile sources were approximately equal for a 160 × 160 km grid centered around Atlanta. Using the present method, point sources represented 15% of the total ambient nitrogen oxides for the period at the measurement site. However, this average may be misleading because nitrogen oxides from point sources were found to be regularly affecting the station most probably through the process of fumigation, typically during the peak ozone producing hours of the day. For instance, on 31 August an ozone exceedenee day, a large percentage of the total ambient nitrogen oxides were from point sources at 3 p.m. (92%). On the other hand, 10 August showed no significant evidence of point source contribution, yet exceeded the ozone standard. The importance of fumigation to ozone production in Atlanta will be the subject of future work.