Asian sulfate over the ocean is in the lower free troposphere (800-600 hPa), with a decrease in pressure toward land due to orographic effects. We calculate that 56% of the measured sulfate between 500-900 hPa over British Columbia is due to East Asian sources. We find evidence of a 72-85% increase in the relative contribution of East Asian sulfate to the total burden in spring off the northwest coast of the United States since 1985. Campaign-average simulations indicate anthropogenic East Asian sulfur emissions increase mean springtime sulfate in Western Canada at the surface by 0.31 µg/m 3 (∼30%) and account for 50% of the overall regional sulfate burden between 1 and 5 km. Mean measured daily surface sulfate concentrations taken in the Vancouver area increase by 0.32 µg/m 3 per 10% increase in the simulated fraction of Asian sulfate, and suggest current East Asian emissions episodically degrade local air quality by more than 1.5 µg/m 3 .
One year of aerosol particle observations from Alert, Nunavut shows that new particle formation (NPF) is common during clean periods of the summertime Arctic associated with attendant low condensation sinks and with the presence of methane sulfonic acid (MSA), a product of the atmospheric oxidation of dimethyl sulfide (DMS). The clean aerosol time periods, defined using the distribution of refractory black carbon number concentrations, increase in frequency from June through August as the anthropogenic influence dwindles. During the clean periods, the number concentrations of particles that can act as cloud condensation nuclei (CCN) increase from June through August suggesting that DMS, and possibly other oceanic organic precursors, exert significant control on the Arctic summertime submicron aerosol, a proposition supported by simulations from the GEOS-Chem-TOMAS global chemical transport model with particle microphysics. The CCN increase for the clean periods across the summer is estimated to be able to increase cloud droplet number concentrations (CDNC) by 23-44 cm -3 , comparable to the mean CDNC increase needed to yield the current global cloud albedo forcing from industrial aerosols. These results suggest that DMS may contribute significantly to modification of the Arctic summer shortwave cloud albedo, and they offer a reference for future changes in the Arctic summer aerosol.
Primary marine aerosol (PMA)-cloud interactions off the coast of California were investigated using observations of marine aerosol, cloud condensation nuclei (CCN), and stratocumulus clouds during the Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE) and the Stratocumulus Observations of Los-Angeles Emissions Derived Aerosol-Droplets (SOLEDAD) studies. Based on recently reported measurements of PMA size distributions, a constrained lognormal-mode-fitting procedure was devised to isolate PMA number size distributions from total aerosol size distributions and applied to E-PEACE measurements. During the 12 day E-PEACE cruise on the R/V Point Sur, PMA typically contributed less than 15% of total particle concentrations. PMA number concentrations averaged 12 cm À3 during a relatively calmer period (average wind speed 12 m/s 1 ) lasting 8 days, and 71 cm À3 during a period of higher wind speeds (average 16 m/s 1 ) lasting 5 days. On average, PMA contributed less than 10% of total CCN at supersaturations up to 0.9% during the calmer period; however, during the higher wind speed period, PMA comprised 5-63% of CCN (average 16-28%) at supersaturations less than 0.3%. Sea salt was measured directly in the dried residuals of cloud droplets during the SOLEDAD study. The mass fractions of sea salt in the residuals averaged 12 to 24% during three cloud events. Comparing the marine stratocumulus clouds sampled in the two campaigns, measured peak supersaturations were 0.2 ± 0.04% during E-PEACE and 0.05-0.1% during SOLEDAD. The available measurements show that cloud droplet number concentrations increased with >100 nm particles in E-PEACE but decreased in the three SOLEDAD cloud events.
Abstract. Detailed information on the size of ice-nucleating particles (INPs) may be useful in source identification, modeling their transport in the atmosphere to improve climate predictions, and determining how effectively or ineffectively instrumentation used for quantifying INPs in the atmosphere captures the full INP population. In this study we report immersion-mode INP number concentrations as a function of size at six ground sites in North America and one in Europe using the micro-orifice uniform-deposit impactor droplet freezing technique (MOUDI-DFT), which combines particle size-segregation by inertial impaction and a microscope-based immersion freezing apparatus. The lowest INP number concentrations were observed at Arctic and alpine locations and the highest at suburban and agricultural locations, consistent with previous studies of INP concentrations in similar environments. We found that 91 ± 9, 79 ± 17, and 63 ± 21 % of INPs had an aerodynamic diameter > 1 µm at ice activation temperatures of −15, −20, and −25 °C, respectively, when averaging over all sampling locations. In addition, 62 ± 20, 55 ± 18, and 42 ± 17 % of INPs were in the coarse mode (> 2.5 µm) at ice activation temperatures of −15, −20, and −25 °C, respectively, when averaging over all sampling locations. These results are consistent with six out of the nine studies in the literature that have focused on the size distribution of INPs in the atmosphere. Taken together, these findings strongly suggest that supermicron and coarse-mode aerosol particles are a significant component of the INP population in many different ground-level environments. Further size-resolved studies of INPs as a function of altitude are required since the size distribution of INPs may be different at high altitudes due to size-dependent removal processes of atmospheric particles.
Abstract.The Whistler Aerosol and Cloud Study (WACS 2010), included intensive measurements of trace gases and particles at two sites on Whistler Mountain. Between 6-11 July 2010 there was a sustained high-pressure system over the region with cloud-free conditions and the highest temperatures of the study. During this period, the organic aerosol concentrations rose from <1 µg m −3 to ∼6 µg m −3 . Precursor gas and aerosol composition measurements show that these organics were almost entirely of secondary biogenic nature. Throughout 6-11 July, the anthropogenic influence was minimal with sulfate concentrations <0.2 µg m −3 and SO 2 mixing ratios ≈0.05-0.1 ppbv. Thus, this case provides excellent conditions to probe the role of biogenic secondary organic aerosol in aerosol microphysics. Although SO 2 mixing ratios were relatively low, box-model simulations show that nucleation and growth may be modeled accurately if J nuc = 3 × 10 −7 [H 2 SO 4 ] and the organics are treated as effectively non-volatile. Due to the low condensation sink and the fast condensation rate of organics, the nucleated particles grew rapidly (2-5 nm h −1 ) with a 10-25 % probability of growing to CCN sizes (100 nm) in the first two days as opposed to being scavenged by coagulation with larger particles. The nucleated particles were observed to grow to ∼200 nm after three days. Comparisons of sizedistribution with CCN data show that particle hygroscopicity (κ) was ∼0.1 for particles larger 150 nm, but for smaller particles near 100 nm the κ value decreased near midway through the period from 0.17 to less than 0.06. In this environment of little anthropogenic influence and low SO 2 , the rapid growth rates of the regionally nucleated particles -due to condensation of biogenic SOA -results in an unusually high efficiency of conversion of the nucleated particles to CCN. Consequently, despite the low SO 2 , nucleation/growth appear to be the dominant source of particle number.
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