January 8, 2010

Dear All,

The attached presentation shows data from the Barcelona 2009 DAURE campaign in reference to the NO3 CE correction.  The basic point is that there is likely particle size considerations with the AMS/SMPS comparison in addition to the likely increase of CE due to higher NO3 mass fractions.  

Slide Walkthrough:
Slide 2: The AMS and SMPS time traces are shown.  High MF NO3 periods are associated with high particle loadings in the Barcelona campaign.

Slide 3: SMPS/AMS calculated volume comparison shows stratification of the data roughly stratified by MF NO3 - this observation in many datasets is the basis for the empirical correction

Slide 4: Empirical CE correction for this dataset with the Juelich line superimposed.

Slide 5: Timeseries of AMS NO3 with co-located independent PM2.5 NO3 measurment showing excellent temporal correlation.

Slide 6: Scatter Plot of AMS measured NO3 vs PM2.5 NO3 colored by MF NO3.  This independent NO3 measurement indicates that there does appear to be a NO3 MF effect, but it is less pronounced than what is observed in the SMPS comparison.  The lower effect here could be due to size differences in the measurement as explained on the next few slides.

Slide 7: Size distributions from 2 periods of high MF NO3.  AMS size distribution shows characteristic shape of lens transmission effect at high sizes.  A comparison of AMS data converted to "volume" and dve using bulk density and dry SMPS calculated volume shows relatively poor agreement (see right hand side and next slide for "good" agreement).  Finally dry SMPS calculated volume distributions (scanned to 700 nm mobility diameter) do not start to go down indicating more volume is present at larger sizes.

Slide 8: If all comparisons looked this good, field campaign data analysis would be much easier.  At lower MF NO3, loadings are smaller, particles are smaller, SMPS volume and AMS volume distributions agree well withthe inherent assumptions.  


Key Points:

1. High MF nitrate associated with higher loadings and more importantly larger particles.
2. Comparison of AMS / SMPS volume needs to take into account both the scan range of the SMPS system as well as possible lens transmission cutoffs in the AMS
3. A comparison to a PM 2.5 co-located nitrate measurement indicates that it is likely both increasing CE AND size cut-off issues are happening.

Implication:
The empirical NO3 MF correction for CE, doesn't account for SMPS scanning range nor does it account for AMS lens transmission.  Depending on the scan range of the SMPS this empirical correction may not be appropriate for your dataset.  If your (dry) SMPS does not scan to higher diameter ranges (around 700 nm or so) then the empirical MF NO3 correction may be in part correcting for lower SMPS volume due to smaller scanning range, and not just for an increased CE from the NO3 mass fraction.

1.) Look at the high MF NO3 size distributions in both the AMS and the SMPS to see if there are potential scanning or lens transmission issues. (record the scan range, and include it in graphs of SMPS volume / AMS volume)
2.) If you have an independent NO3 measurement, compare to that.

Let us know if you have questions, and we can discuss how to integrate this the overall analysis of these datasets.

Cheers,

-pete and claudia