I presented this question to Dr. DuBois and here is his response:

In many cases PM2.5 is higher during

high pressure events due to low winds and little ventilation to transport

them away from the source area. Also if a subsidence inversion is present

above the area you can get limited mixing.

I will try to get more information...

I presented this question to Dr. DuBois and here is his response:

In many cases PM2.5 is higher during

high pressure events due to low winds and little ventilation to transport

them away from the source area. Also if a subsidence inversion is present

above the area you can get limited mixing.

I will try to get more information...

In short, high pressure can be associated with stable atmospheric conditions and low mixing height, whereas low pressure results in high mixing rate in swallowed boundary layer. That means, when pressure is low concentrations of particulate matters may decrease and vice-versa.

Hi @Sagar Parajuli,

Thanks for your reply here. AOD is based on Simplified high resolution MODIS Aerosol Retrieval Algorithm (SARA : http://www.sciencedirect.com/science/article/pii/S0034425713001417 http://www.sciencedirect.com/science/article/pii/S0034425714002636) and hourly PM2.5 concentrations from ground air quality stations. 

Hi Sipra Panda,

Many thanks for explanation - My question is that why AOD and PM2.5 has high correlation (R = 0.86) during surface low pressure - I want to know the reason of this high correlation - Also, during surface low pressure, concentrations of all the available PM2.5 measurements is higher than the recommended Air Quality Standard of WHO (25 mic.g/m^3). 

Generally low pressure conditions occur mostly during autumn, at least in my country (NL). Comparisons between AOD and PM2.5 were made here, finding higher corrrelations during autumn with "relatively well mixed conditions in which the presence of residual layers does not largely affect the AOD." Quote from: http://www.gaef.de/EAC2007/EAC2007abstracts/T20Abstractpdf/T20A011.pdf

Hi @Marcel Severijnen, Thanks for providing this information and I will consult your providing reference. 

did you look at the number of AOD data at hi and lo pressure situations? And this in relation to cloud-free conditions and also relative humidity that increases the AOD by the water uptake by the hygroscopic components: PM2.5 is a measure for the DRY aerosol.

Yes, I checked the number of observations for both high and low surface pressure:

For high surface pressure:

N = 37, AOD = 0.11 (min) - 0.87 (max), PM2.5 = 22 micg/m^3 (min) - 134 micg/m^3, R = 0.861, RH = 18% (min) - 83% (max), T = 4^o C (min) - 22^o (max), PBLH = 113m (min) - 757m (max) 

For low surface pressure:

N = 49, AOD = 0.14 (min) - 0.79 (max), PM2.5 = 27 micg/m^3 (min) - 118 micg/m^3, R = 0.860, RH = 33% (min) - 77% (max), T = 17^o C (min) - 30^o (max), PBLH = 50m (min) - 717m (max)

From the temp data it appears that the data are from different seasons and possibly (therefore) also with different sources for the aerosol.

What about aerosol in residual layers above the PBL?

Yes, data are for autumn and winter seasons of 2007 & 2008. I simulated PBL from WRF-ARW at 500 m resolution  and don't have information about aerosol residual layer. 

Are there lidar data available for your site? those reveal residual layers, but also whether your PBLH is the proper one.

Also: you should have a look at the model aerosol concentrations at levels above the PBL in the 2 situations to have a first impression why there are differences.

Finally: our experience is that when vertically interated aerosol and thus derived AOD is much less than the actual AOD it is a sure sign of residual aerosol layers (and proven by lidar data).

Hi, You may see the following links.

http://onlinelibrary.wiley.com/doi/10.1029/2005JD006996/full

http://www.tandfonline.com/doi/abs/10.3155/1047-3289.59.11.1358#.VFnlDzSUeyI

http://onlinelibrary.wiley.com/doi/10.1029/2006JD007918/full

http://onlinelibrary.wiley.com/doi/10.1029/2005JD006996/full

Atul

The question is specific not on AOD versus PM2.5 but a possible correlation depending on air pressure/synoptics

Dear Sir Harry, Unfortunately lidar data are not available.

Remains the question: do you have an estimate via your model of how much aerosol above the PBL

Then: your PBL height seems low?!