|I thank the authors for considering my comments. However, I still find myself in disagreement with the conclusion regarding aerosol acidity. The stated intent of section 3.2.3 is to evaluate acidity because of its relevance to health impacts. However, as conceded by the authors, any observation of an apparently acidic aerosol is at odds with the observation of ammonium nitrate being present. To summarise the points raised by myself and the authors, the three possibilities of reconciling these observations are:|
1. Inaccurate calibration data
2. A high metal cation content
3. Nitrate partitioning into dilute aqueous droplets at high humidities
Personally, I still find possibility 1 to be highly plausible. While the instrument was cross-calibrated with a HRAMS, this sill only makes it as accurate as the HRAMS calibration, which having read the corresponding paper, I have the exact same concerns about. Possibility 2 could also be plausible because the ACSM does not quantify potassium, although if this was the case, I would expect the deviations from perceived neutrality to correlate with a marker for biofuel or coal burning (this could be done by colouring figure R5 with an appropriate marker, e.g. m60).
Possibility 3, on the other hand, I do not find at all plausible in view of figure R5. As far as I can tell, there is no dependency between relative humidity and apparent acidity except for the fact that the RH tends to be low when loadings are low, but this is probably an aspect of the meteorology and not a causal relationship. The apparent agreement noted at low concentrations is within the scatter of the points, which can easily be explained by the low signal-to-noise ratio of the ammonium measurement or possibly the influence of organic acids, which aren't included in the calculation. While there are points with a low NH4meas/NH4pred on figure R5b that correspond to RHs approaching 100%, there are also points on the exact same fit line that also correspond to RHs as low as 50%. At these relative humidities, the water content of particles is very low and this causes nitrate to partition to nitric acid in the event of an inbalance (this can be shown using e-AIM, http://www.aim.env.uea.ac.uk/aim/model3/model3b.php). In short, If possibility 3 were to be occurring, I would expect the behaviour to vary between 50% and 100%, but this simply isn't the case, according to figure R5b.
To return to the original point, if it is either possibility 1 or 2 that is occurring, then the logical conclusion would be that the ACSM (as operated here) is not a reliable measure of particle acidity. In this context, the NH4meas vs NH4pred is only really useful as a technical instrument diagnostic and as such, I still don't see the conclusion that the particles are acidic is supportable.